转帖《A类的最新定义》

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The Last Word on Class A
原文见http://www.aikenamps.com/ClassA.htm


First of all, what is a class A amplifier?

A class A amplifier is defined as one which is biased to a point where plate current in all the output devices flows for the entire 360 degrees of an input cycle, at the full, unclipped output of the amplifier.  This is typically done by biasing the output stage halfway between cutoff and saturation, with the plate load impedance to an appropriate value that gives maximum undistorted output power.  This is the least efficient method of amplification, because the output devices are dissipating maximum power with no input signal.
For audio amplification, a class A amplifier can be either single-ended or push-pull.  Now, you might be thinking, how can a push-pull amplifier be class A?  Doesn't one side amplify half the waveform and the other side amplify the other half?  Isn't this why we use a phase splitter?  These are common misconceptions.  You can, indeed have a true class A amplifier that operates in push-pull mode.  Amplifier class has absolutely nothing to do with output stage topology.   If the output tubes on either side of a push-pull pair are biased in class A (halfway between cutoff and saturation), then the current in each side will still flow for the full 360 degrees of the input cycle, just in opposing directions.  As one tube's current increases from the midpoint, or idle, bias current, the other tube's current is decreasing by an equal amount.  The output transformer sums these oppositely-phased currents to produce the output waveform in the secondary winding.  As one side reaches saturation, the other side reaches cutoff, just as they would in a single-ended class A amplifier.  Neither side cuts off at the full, unclipped output power of the amplifier. The output power of a push-pull class A amplifier is exactly twice the output power of a single-ended class A amplifier operating under the same conditions of plate voltage, bias, and effective load impedance.

Another misconception is that of cathode biasing.  The method of biasing has nothing to do with the class of operation.  You can have a fixed-bias class A amplifier or a cathode-biased class AB amplifier, or vice-versa.   The presence of a cathode bias resistor and bypass capacitor is not an indication of class A operation.

There are several advantages to push-pull class A amplification.  First, the bias current for each side is flowing in opposite directions in the primary of the output transformer, so they effectively cancel each other out.  This lack of static, DC offset current in the output transformer means that the core can be made smaller, because it requires no air gap to prevent core saturation from the static DC offset current.  A single-ended class A amplifier output transformer is huge compared to a push-pull class A amplifier of the same power level.   The air gap required to prevent core saturation drastically reduces the primary inductance, so the transformer must have a larger core and more windings to achieve the same primary inductance and the same -3dB lower frequency cutoff point.   Second, a push-pull class A amplifier output stage will have inherent rejection of power supply ripple and noise.  This is because the power supply signal is "common-mode", i.e., it is amplified by each side equally, but since each side is out of phase, it cancels in the output.

The main disadvantage of push-pull class A amplification over single-ended class A, is the necessity for a phase splitter stage to generate the oppositely-phased drive signals.  Another "disadvantage", in terms of guitar amplification, is that even-order harmonics generated in the output stage are canceled out in a push-pull output stage (hi-fi guys consider this a great advantage, by the way!).   This does not mean that the push-pull amplifier generates no even order harmonics, however, because even-order harmonics generated in the preamp stages are amplified by the output stage and will pass right through to the output.  Only those even-order harmonics generated in the output stage itself are canceled.

What is a class B amplifier?
A class B amplifier is one in which the grid bias in all output tubes  is set at cutoff, i.e., no plate current flows in the absence of an input signal.  Plate current only flows when a signal is present, and only flows for exactly half, or 180 degrees, of the input cycle.
For audio amplification purposes, a class B amplifier must operate in push-pull mode, because each output device only amplifies half the input signal, and the output would be fully clipped on one side if operated single-ended.  The important thing to remember is that, even though the current in one side is fully off, or "clipped on one side", the output waveform is not clipped at all, because the other tube has taken over the job of reproducing it's half of the waveform.  Clipping of the output stage only occurs when both tubes are at their respective, and opposite, limits of saturation and cutoff.

The advantage of class B operation is it's efficiency, which is far greater than class A, because the average dissipation of the output devices is much lower, because they are biased normally "off", and only dissipate power during half the input cycle. The limiting factor in output power is the average dissipation of the output devices.  If the average dissipation can be decreased, more output power can be obtained. The disadvantage of class B operation is a large amount of "crossover distortion", which occurs when one tube of the push-pull pair cuts off and the other turns on.  The characteristic curves of a tube are not perfectly linear and symmetrical, so the "handoff" between the two sides results in a short time at the zero crossing where there is distortion.  This crossover distortion looks like a notch, or flat spot, in the sine wave as it crosses the zero axis.

How is class AB defined?
A class AB amplifier is one in which the grid bias is set so that plate current flows for more than half, but appreciably less that the full 360 degrees of the the input cycle, again measured at the full, unclipped output of the amplifier.   This increase in idle bias current over class B operation keeps the tubes on a small amount at all times, resulting in reduced crossover distortion, because it keeps the tubes out of the highly nonlinear region near cutoff.  Unless the idle bias is set too close to class A operation, efficiency gains similar to class B operation can be obtained, without the unwanted crossover distortion. This is the most popular class of operation for medium to high powered guitar amplifiers.
Finally, can a single-ended amplifier be class AB or class B?  The simple answer is, yes. Many RF (radio-frequency) amplifiers are single-ended class B or class C (current flows for less than 180 degrees of an input cycle).  However, for audio amplification, these are of little use.  Technically speaking, you can have a a class AB single-ended amplifier, which was biased away from the linear portion of the curves, but that amplifier would (hopefully!) be rated at the unclipped output power, so plate current would be flowing at all times at the max undistorted output power.  This would stretch the definition a bit, particularly since the amp would undoubtedly be able to be driven into asymmetrical cutoff (where us guitar players like to hang out!) which would effectively be class AB single-ended operation, because the amp is not biased in the middle of the transfer curves and is capable of being driven into cutoff for a portion of the input cycle.  As long as the manufacturer isn't rating the amplifier for its output power in this clipped state, the amplifier would normally be called class A, single-ended.

What about class A2, AB2, and B2?
The numerical suffix appended to the class designation indicates whether or not grid current flows in any appreciable portion of the cycle.   A "1" suffix indicates no grid current flows, while a "2" suffix indicates grid current flows for some part of the cycle.   Class A2/AB2/or B2 requires a very low impedance, transformer-coupled or DC-coupled driver stage.   The standard AC-coupled phase inverter or single-ended driver stages used in nearly all guitar amplifiers will not allow grid current flow, so they are class A1/AB1/B1 amplifiers.
The advantage of class A2, AB2, or B2 is the complete lack of "blocking distortion", or transient intermodulation distortion.  The disadvantage is the extra complexity of the output stage required to source current to drive the output tube grids into the positive region.

Which is better, class A or class AB?
From a guitar amplification standpoint, neither class of operation is necessarily better, they are just different. You shouldn't get too hung up on the "class A" designation, because most of the push-pull amplifiers that are supposed to be class A aren't really class A at all, they are just cathode-biased, non-negative feedback class AB amplifiers. Operating class is not the reason for the tonal differences between these amplifiers.
The cathode biasing and lack of negative feedback is one of the main differences between the Vox clones and the Marshall/Fender style stuff. The typical Marshalls and Fenders used a fixed-bias output stage with negative feedback from the output back to the phase inverter input, while the Vox clones use a cathode-biased output stage and no global negative feedback.  In addition, the output tubes and preamp stage/phase inverter configurations contribute greatly to the tonal signature of these amplifiers.
Cathode biasing vs. fixed biasing
In a cathode-biased amplifier, the bias voltage is developed across a cathode resistor that is bypassed with a big electrolytic capacitor. In a class AB amplifier, as the current through the tube increases, the average voltage across the cathode resistor changes, which modulates the plate current, creating a bit of "sag" and a dynamic change in the harmonic structure of the note that changes while playing. This occurs because the plate current in a class AB amplifier is not continuous for the entire AC cycle. The tube goes into cutoff for a portion of the cycle, which means that the average DC level of the signal on the cathode will shift, changing the operating point of the tube, with the resulting dynamic tonal changes. The average value of a sine wave is zero, but the average value of a clipped sine wave, such as occurs when the plate current is cut off for some percentage of time, is not zero. The current in a true class A amplifier is constant, so it doesn't exhibit this bias shift, unless driven to clipping, where all bets are off. This is why a cathode bypass cap is not necessary in a true class A push-pull output stage - the plate currents are equal and out of phase, unless there is an imbalance in the output transformer, the output tubes, or the drive signals (it is a good idea to use one anyway, for these reasons). The fixed-bias amplifier maintains the bias at a more constant level, so it doesn't have the constantly changing operating point that varies with the output level.
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The effect of global negative feedback
The use of global negative feedback does several things: it flattens and extends the frequency response, it reduces distortion generated in the stages encompassed by the feedback loop, and it reduces the effective output impedance of the amplifier, which increases the damping factor. All of these things affect the tone in some manner.
The flattened, extended frequency response obviously changes the tonal character by removing "humps" in the output stage response and producing more high and low end frequencies. The distortion reduction makes the amp sound cleaner and more "hi-fi", up to the point of clipping. Perhaps the main difference for the "feel" is the increased damping factor produced by the negative feedback loop. The decreased effective output impedance causes the amp to react less to the speakers. A speaker impedance curve is far from flat; it rises very high at the resonant frequency, then falls to the nominal impedance around 1kHz, and again rises as the frequency increases. This changing "reactive" load causes the amp output level to change with frequency and changes in speaker impedance (a dynamic thing that changes as the speakers are driven harder). Global negative feedback generally reduces this greatly. This can be good or bad, depending upon what you are looking for.

Negative feedback makes the amp sound "tighter", particularly in the low end, where the speaker resonant hump has the most effect on amplifier output. This is better suited for pristine clean playing or a tight distorted tone, while a non-negative feedback amp has a "looser" feel, better suited to a bluesy, dynamic style of playing. The other disadvantage of a negative feedback amplifier is that the transition from clean to distorted is much more abrupt, because the negative feedback tends to keep the amp distortion to a minimum until the output stage clips, at which point there is no "excess gain" available to keep the feedback loop operating properly. At this point, the feedback loop is broken, and the amp transitions to the full non-feedback forward gain, which means that the clipping occurs very abruptly. The non-negative feedback amp transitions much more smoothly into distortion, making it better for players who like to use their volume control to change from a clean to a distorted tone.

There is an output stage topology that is kind of in between, called "ultralinear" operation. This uses local negative feedback to the screen grids of the output stage by means of a tapped output transformer primary. This increases the damping factor and makes the amp a bit tighter without the use of a global negative feedback loop (you can use global negative feedback with ultralinear output stages, but you may not like the tone as much). The Dr. Z Route 66 amplifier uses an ultralinear output stage. There is also a triode output stage, which has even higher damping factor than ultralinear, but some players feel that it sounds too "compressed" and midrangey, while others like it.  Part of the reason for the midrange emphasis is the increased input capacitance of triode mode over pentode mode because of the Miller effect, which in effect, multiplies the grid to plate capacitance by the gain of the tube.  This increased capacitance rolls off the high frequencies.

Does true class A operation require any particular current or bias point?
True class A operation does not have to be above any particular current rating or dissipation. It depends on the tube type, the power supply voltage, the reflected impedance, and the required operating point. However, in general, when a class A power amplifier is designed, the bias point is chosen to correspond with the spot on the plate curves at the intersection of the load line, the plate voltage, and the  maximum dissipation curve that gives maximum symmetrical swing in both directions before clipping. This means that the tube is biased right at maximum plate dissipation, which is okay, because the dissipation is maximum at idle in a class A amplifier, and does not increase with applied signal, as it does in a class AB or class B amplifier (it actually decreases to a minimum at full power). This is not to say that that is the only current and voltage that will work. If you lower the plate voltage by 100V, you will find another "optimum" spot where these lines intersect. If you change the reflected load impedance, you will find yet another optimum spot. There is, however, an upper limit on the voltage that can be applied where you can no longer bias for symmetrical swing about the idle point without exceeding the plate dissipation ratings. This is the limiting voltage for that tube in true class A operation *at the max recommended tube ratings*. If you choose to run the tube over ratings, as is the case in some amplifiers, you can bias the tube to a point that is running class A, but is above the maximum dissipation curve. Although this seems to work with some tubes, it is not a recommended practice.
This holds true for both single-ended and push-pull designs. In push-pull class A, the bias point and plate supply voltage is the same as for single-ended, but there is a phase inverter and a center-tapped transformer, which are used to increase power and reduce distortion (even-order harmonics are canceled, and power supply hum is canceled in a balanced push-pull amp). Power is twice that of single ended (for a two-tube push-pull vs. a single tube single-ended, etc.).

To get a better feel for this, take a set of plate curves for a given tube, and draw a load line representing the reflected impedance (it has a slope corresponding to the negative reciprocal of the reflected load impedance, and passes through the intersection of the bias current and plate voltage lines), and draw a curve representing the plate dissipation (it will be a parabolic shape, with each point equal to the current that corresponds to the plate dissipation divided by the plate voltage). The load line should just touch the plate dissipation curve at the selected plate voltage (for max power out - if you want less than max power, it can be below the dissipation curve). The current corresponding to this point will be the required bias current, and the dissipation will be maximum at that point. All tube signal swings will occur on the load line (assuming a purely resistive load - reactive loads generate elliptical load lines), so you can find the plate voltage swing for a given grid voltage swing, and you will see that you will have to either change the plate voltage or the reflected load impedance, or both, in order to get the optimum class A bias point. Don't forget that the actual plate voltage swings both above and below the supply voltage, and the center of the swing is the actual plate supply voltage. This is kind of confusing at first, because it isn't intuitive that you could get a 400V peak with only a 250V supply (i.e., a swing from 100V to 400V, centered around 250V). The "extra" voltage comes about because of the nature of how the output transformer works.

Does biasing at max dissipation guarantee class A operation?
Just because you are biased at max dissipation does not mean you are class A! You must be in the region where the voltage swing is symmetrical and biased in the center of the range, where plate current flows for all unclipped output. Biasing to a high voltage and low plate current whose product equals the maximum plate dissipation might not allow this, because, although you are at max plate dissipation, the bias point is such that plate current will flow for an appreciably less time on the negative signal swing (cutoff) than it will on the positive signal swing (saturation), and *no* load line can be found that will allow symmetrical swing, or it will be in such a non-linear portion of the curves as to be unusable. This is because the plate voltage is too high, and the max allowable current without exceeding dissipation limits is too low. The same thing can occur on the other end of the scale, where you can reduce the plate voltage to a point that the max dissipation current will exceed the maximum allowable plate or cathode current ratings of the tube. There is an optimum area of the curves that will become apparent when you start drawing load lines and picking bias points. It is a bit of an iterative process, so the tube manufacturers make it easy for you by listing typical class A operating conditions in the data sheets.
In theory, you can take a class AB push-pull amplifier and convert it to class A push-pull operation, *however*, you would, in nearly all cases, have to reduce the plate voltage to be able to bias the tubes into the class A region, because the whole reason for going to class AB is to get higher power, so the plate voltage is run higher and the idle current lower than what is allowed in class A. Once again, you have to look at the plate curves for the particular tube to determine where the allowable class A region is. If you simply bias a class AB amp to max dissipation at idle, you will find that as you apply a signal, the tubes will dissipate more power, and they will start to glow a lovely cherry red color, and something will croak. In addition, the power supply and/or output transformer may not be able to handle the extra current required for true class A operation, so, unless you know the ratings of the trannies, it is best not to attempt this, even if you lower the supply voltage.

Are those class A amplifiers I see advertised really class A?
There is much debate raging in the marketplace about "class A" amplifiers, and whether or not they are truly class A, or just class AB amplifiers unscrupulously marketed to the unsuspecting public as "class A".   The truth is that most, if not all, are in reality cathode-biased, non-negative feedback class AB amplifiers, contrary to what the manufacturer's literature may say.
What is the difference, then, and why is it a problem for so many people?
The fundamental problem is in how class AB is defined, and how people interpret it.  The people who say a class AB amp is "class A at lower volumes" are technically wrong, but for the right reasons.  If you were to define class A as being only conduction for a full 360 degree phase angle, you would be correct.  However, there is more to the definition of amplifier classes than that.
The defining factor in a determining whether or not an amplifier is class A, class AB, or class B *has* to be made at the full output before clipping, otherwise, the class definitions have no meaning whatsoever.  It is indeed, a very black and white thing, and depends on the bias point on the characteristic curves, and the load line, among other things.

If, at the full undistorted output, the plate current flows in each tube for a full 360 degrees of the input conduction cycle, the amplifier is class A.  However, if the amplifier is biased such that the plate current cuts off for an appreciable time during each cycle at this full undistorted output power, it is then a class AB amplifier.  If it is biased such that each side is in cutoff for half the input cycle, it is a class B amplifier.  Note that cutoff does not mean that the output of the amplifier is clipped, or distorting.  Cutoff refers to plate current cutting off on one side of a push-pull pair for a portion of the cycle, while the other side continues to function.  The output waveform is still a clean, unclipped sine wave, because the transformer sums the two "halves" of the input signal into one composite signal. Effectively, one tube amplifies the "upper half" and the other tube amplifies the "lower half".  This is done to provide higher efficiency and greater output power.   In a class AB amplifier each tube amplifies a bit more than half the signal, in order to reduce the distortion that occurs at the zero crossings of the waveform, which is called "crossover distortion".

Here is where the problem comes in:  because a class AB amplifier is biased so that the plate current flows for the entire cycle at lower output levels (which is done to reduce crossover distortion),  many people claim it is a "class A amplifier at lower volumes".  This is simply not true. It is operating in conditions *similar* to class A, but is not a class A amplifier by any means.  It is still a class AB amplifier, no matter what you choose to call it.

Now, what are the differences, you might ask? Well, for one, the Class AB amplifier is biased in a more non-linear portion of the characteristic curves, which means it has more distortion than a true class A amplifier.  Also, the efficiency will be greater than is theoretically possible with a class A amplifier at these levels.  There is a very real difference in tone and operating conditions between a true class A 10W amplifier running at say, 1W, and a 10W class AB amplifier running at 1W.  Same output level, same overall power level, *but* a different class of operation, different amount of distortion, different efficiency, *and* a different tone, even though neither one of them is in cutoff for any portion of the output cycle at that low level.  This is due to the bias point differences and load line differences. The differences become even more apparent when the amplifiers are run at their full undistorted output power.  The true class A amplifier will have no crossover distortion, while the class AB amplifier will.   The average plate current for the true class A amplifier will not change, or will change very little, from idle to full output power, while the average plate current in a class AB amplifier will increase dramatically.  This will lead to "sag" in the power supply that doesn't exist in the true class A amplifier, which again results in a tonal change.

As you can see, there is indeed such a thing as a "true class AB" amplifier, just as there is a "true class A" amplifier, and the class definitions are not at all ambiguous, except to those who don't understand them, or choose to ignore them for marketing advantage.

One more thing: What if you push the class A or class AB amplifier into clipping? Does it then become a class AB/ B, C, or D amplifier?  No, of course not.  It is simply the same class amplifier it was to begin with, but driven into clipping.   A class A amplifier driven to clipping is still a class A amplifier by definition. This is why amplifier classes are defined the way they are.  Otherwise, the class designations would have no meaning.  Any amplifier can be driven beyond it's limits into a fully-clipped square wave output (unless it is limited), but that doesn't make it a class D switching amplifier, now does it?

Which one to buy?
The bottom line is this: don't worry about whether an amp is "class A" or not.  If you are interested in details, find out if it is cathode-biased or fixed-biased, and whether is uses global negative feedback or not, whether it uses a pentode, triode, or ultralinear output stage, and what type of output tubes are used. These parameters will give an idea of the "feel" of the amp, but in the end, you still must play the amp and use your ears to tell you which one is best suited for your playing style. Don't make a decision based on technical specs alone, you may miss out on a great-sounding amplifier!

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Copyright © 2000, 2001, 2002 , 2003, 2004 Randall Aiken.  May not be reproduced in any form without written approval from Aiken Amplification.

Revised 10/17/04

安安

金星文。

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转帖 有关A类放大器的定义的 一段及译文                                                        

                                    The Last Word on Class A
First of all, what is a class A amplifier?

A class A amplifier is defined as one which is biased to a point where plate current in all the output devices flows for the entire 360 degrees of an input cycle, at the full, unclipped output of the amplifier.  This is typically done by biasing the output stage halfway between cutoff and saturation, with the plate load impedance to an appropriate value that gives maximum undistorted output power.  This is the least efficient method of amplification, because the output devices are dissipating maximum power with no input signal.
For audio amplification, a class A amplifier can be either single-ended or push-pull.  Now, you might be thinking, how can a push-pull amplifier be class A?  Doesn't one side amplify half the waveform and the other side amplify the other half?  Isn't this why we use a phase splitter?  These are common misconceptions.  You can, indeed have a true class A amplifier that operates in push-pull mode.  Amplifier class has absolutely nothing to do with output stage topology.   If the output tubes on either side of a push-pull pair are biased in class A (halfway between cutoff and saturation), then the current in each side will still flow for the full 360 degrees of the input cycle, just in opposing directions.  As one tube's current increases from the midpoint, or idle, bias current, the other tube's current is decreasing by an equal amount.  The output transformer sums these oppositely-phased currents to produce the output waveform in the secondary winding.  As one side reaches saturation, the other side reaches cutoff, just as they would in a single-ended class A amplifier.  Neither side cuts off at the full, unclipped output power of the amplifier. The output power of a push-pull class A amplifier is exactly twice the output power of a single-ended class A amplifier operating under the same conditions of plate voltage, bias, and effective load impedance.


                                                    A类的最新定义

首先，什么是A类放大器？

A类放大器定义为输出器件的静态阳极电流设在某个水平使之输入信号的每一个360°周期得以完全没有削波的输出的放大器，其典型的做法是输出级偏置在截止和饱和的中间，驱动适当的阻抗的阳极负载获得最大的不失真输出功率，这是最低效率的放大，由于输出器件在无输入信号时有最大的功耗。
对于音频放大，A类放大器既可以是单端也可以是推挽。现在，你可能会想，推挽放大器怎么能够是A类？不是一边放大一半波形另一边放大另一半波形吗？这不就是我们使用分相器的原因吗？这些都是误解。你可以，真的可以有运作在推挽模式的真正A类放大器，放大器的类别绝对与输出级的布局无关。如果输出的电子管推挽对的两边都偏置在A类（在截止和饱和的中间），这样在输入周期的360°都一直有电流流动，只是方向相反。当一个真空管对于中点的电流增加，即静态偏置电流，则另一管的电流减少相同的数量，这输出变压器把这相反相位的电流合起来在次级绕组产生输出波形，当一边达到饱和则另一边达到截止，正如它们在单端A类放大器工作的那样，在整个过程两边都不会截止而放大器输出不削波功率。运作在相同的阳极电压、偏流和有效负载的情况下推挽A类放大器的输出功率是单端A类的精确的2倍。

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Another misconception is that of cathode biasing.  The method of biasing has nothing to do with the class of operation.  You can have a fixed-bias class A amplifier or a cathode-biased class AB amplifier, or vice-versa.   The presence of a cathode bias resistor and bypass capacitor is not an indication of class A operation.

There are several advantages to push-pull class A amplification.  First, the bias current for each side is flowing in opposite directions in the primary of the output transformer, so they effectively cancel each other out.  This lack of static, DC offset current in the output transformer means that the core can be made smaller, because it requires no air gap to prevent core saturation from the static DC offset current.  A single-ended class A amplifier output transformer is huge compared to a push-pull class A amplifier of the same power level.   The air gap required to prevent core saturation drastically reduces the primary inductance, so the transformer must have a larger core and more windings to achieve the same primary inductance and the same -3dB lower frequency cutoff point.   Second, a push-pull class A amplifier output stage will have inherent rejection of power supply ripple and noise.  This is because the power supply signal is "common-mode", i.e., it is amplified by each side equally, but since each side is out of phase, it cancels in the output.

The main disadvantage of push-pull class A amplification over single-ended class A, is the necessity for a phase splitter stage to generate the oppositely-phased drive signals.  Another "disadvantage", in terms of guitar amplification, is that even-order harmonics generated in the output stage are canceled out in a push-pull output stage (hi-fi guys consider this a great advantage, by the way!).   This does not mean that the push-pull amplifier generates no even order harmonics, however, because even-order harmonics generated in the preamp stages are amplified by the output stage and will pass right through to the output.  Only those even-order harmonics generated in the output stage itself are canceled.


另一个误解是有关阴极偏置，阴极偏置和运作类别没有关系，你可以设定一个A类的放大器或阴极偏置AB类的放大器，或反之亦然。阴极偏置电阻和旁路电容的出现不是A类放大器的标志。
对于推挽A类放大器有几个好处：第一，在输出变压器的初级两边的偏置电流的流向是相反的因此它们实际上互相抵消，这样在输出变压器没有静态的直流漂移意味着磁芯可以做得较小，因为它不需要气隙以防止由于静态直流漂移导致的磁芯饱和。一个单端A类放大器的输出变压器比一个同样功率水平的推挽A类放大器的输出变压器要大得多。为防止磁芯饱和气隙大为降低了初级电感，因而变压器必须有巨大的磁芯和更多的绕圈以获得一样的初级电感
和一样的-3dB低频转折点。
第二，一个推挽A类放大器输出级将具有固有的抑制电源纹波和噪声的本质，这是因为电源提供的信号是“共模”，也就是它是由每边等量放大，然而由于两边是反相的，它在输出端被抵消。
和单端A类相比这推挽A类 放大器主要缺点是需要一个分相器级以产生相反相位的驱动信号。另一个“缺点”，在吉他放大方面，是在输出级产生的偶次谐波在推挽输出级被抵消（发烧友认为这有很大好处，顺便提及！）。然而，这并不意味着推挽放大器不产生偶次谐波，因为在前级产生的偶次谐波被输出级放大并输出，仅仅是由输出级它自己产生的偶次谐波被抵消。

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最新定义？哪一个年代的啊？

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<最新定义？哪一个年代的啊？>

文章题目如此，
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复制在这里《Revised 10/17/04》

我心已死

首先，什么是A类放大器？

A类放大器的定义是：一个是有偏见的一个地步板目前在所有的输出设备流入整个360度的投入周期，充分， unclipped输出的放大器。这通常是这样做的偏置输出级中间截止和饱和度，与板负载阻抗到适当的值，使输出功率最大undistorted 。这是最有效的方法，扩增，因为输出设备的最大功率耗散没有输入信号。
对于音频放大，一个A类放大器可单端或推挽。现在，你可能会想，怎么能推挽放大器是A类？没有一方放大波形和一半的另一面扩大的另一半？这难道不是我们为什么使用分相器？这些都是常见的误解。您可以，确实有一个真正的A级放大器，工作在推挽模式。放大器级绝对没有任何关系与输出级拓扑。如果输出管两侧的推拉对有偏见的A级（中间截止和饱和度） ，那么，目前在各方面仍然会流全面360度的投入周期，公正，反对的方向。作为一个管的电流增大的中点，或闲置，偏置电流，其他管目前正在减少的同等数额。输出变压器汇总这些生地阶段产生的电流输出波形中的二次绕组。作为一方达到饱和，另一方达到临界，就像他们将在一个单端A类放大器。任何一方都切断充分， unclipped输出功率的放大器。输出功率的推挽A级放大器正是两倍的输出功率的单端A类放大器的操作相同的条件下板电压，偏见和有效负载阻抗。

另一个误解是，阴极偏置。偏置的方法没有任何关系的一类作业。你可以有一个固定的偏见A级放大器或阴极偏见的AB类放大器，反之亦然。在场的情况下阴极偏置电阻和旁路电容不能说明A类作业。

有几个好处推挽A类放大。首先，偏置电流为每一方都在流动方向相反的主要输出变压器，所以他们有效地相互抵消。这种缺乏静态，直流偏移电流输出变压器的核心手段，可更小，因为它不需要空气间隙，以防止核心饱和从静态直流偏移电流。一个单端A类放大器的输出变压器是巨大相比，推挽A级放大器相同的功率水平。气隙要求，以防止核心饱和度大幅度降低了初级电感，因此，变压器必须有一个较大的核心，更绕组实现同样的初级电感和同三分贝较低频率分界点。第二，推挽A级放大器输出阶段将拥有固有拒绝电源纹波和噪声。这是因为电源供应器的信号是“共同模式” ，即它是扩增同样每一方，但由于每方的阶段，取消的产出。

的主要缺点推挽放大了A类单端A类，是有必要的一个阶段分流阶段产生相反相驱动信号。另一种“劣势” ，在吉他放大器，是偶数阶谐波产生的输出级是取消了在推挽输出级（高保真人认为这是一个很大的优势，通过了道路！ ） 。这并不意味着推挽放大器甚至为了不产生谐波，但是，因为即使次谐波产生的前置阶段的扩增输出阶段，将通过传递正确的输出。只有那些即使次谐波产生的输出级本身的取消。

什么是B类放大器？
B类放大器是一个网格偏见所有输出管定为截止，即没有车牌目前流动在没有输入信号。板目前流动时，只存在一个信号，只有流动的一半，或180度，输入周期。
音频放大的目的， B类放大器必须在推挽模式，因为每个输出设备的一半只有放大输入信号，输出将完全截断一方如果经营单端。对重要的事情要记住的是，尽管目前在一方完全关闭，或“截断的一方” ，输出波形不是擦在所有的，因为其他管接管工作的音响它的一半波形。裁剪输出阶段只发生在这两个管在各自，相反，限制饱和和截止。

的优势， B类作业的效率，这是远远大于A级的，因为平均损耗的输出装置要低得多，因为他们是有偏见的正常“关闭” ，而且只有在散热的一半投入周期。制约因素是在输出功率的平均损耗的输出设备。如果平均损耗可降低，更多的输出功率可以得到。的缺点B级操作大量的“交越失真” ，这发生在一个管推挽对切断和其他打开。特性曲线的管不完全的线性和对称，这样的“切换”双方之间的结果在很短的时间在过零那里失真。这交越失真看起来像一个档次，或扁平点，在正弦波作为穿越零轴。

AB类是如何界定？
A级AB放大器是一个网格偏置设置，使板当前流动的一半以上，但略微低于360度的全面的输入周期，再次测量充分， unclipped输出的放大器。这增加闲置偏置电流超过B级的管操作上保持少量在任何时间，从而减少交越失真，因为它使管出高度非线性区域附近截止。除非闲置偏见是太接近A级操作，提高效率类似B级行动可以得到，但不想要的交越失真。这就是最流行的一类手术中高功率吉他放大器。
最后，可以单端放大器是AB类或B类？答案很简单，是的。许多的RF （无线电频率）放大器是单端的B级或C级（目前流量小于180度的投入周期） 。但是，音频放大，这些是没有多大用处。从技术上讲，你可以机管局AB类单端放大器，这是有偏见的距离线性部分的曲线，但放大器将（希望！ ）是在unclipped额定输出功率，所以目前的车牌将流动在所有在时代的最大undistorted输出功率。这将延伸一点的定义，特别是因为功放无疑将能够驶入不对称截止（如我们的吉他像挂了！ ）这将有效地将AB类单端工作的，因为功放不怀偏见在中东的转移曲线，并有能力推动到截止的一部分投入周期。只要制造商不是评价的放大器，其输出功率在此截断的国家，放大器通常被称为A级，单端。

什么阶级素A2 ， AB2型，和B2 ？
数值后缀附加到指定表明阶级与否电网目前的流量在任何可观的一部分周期。一个“ 1 ”后缀表明当前流动没有网格，而“ 2 ”后缀表明当前流动的网格部分的恶性循环。类A2/AB2/or素B2需要一个非常低阻抗，变压器耦合或直流耦合驱动的阶段。标准的AC耦合三相逆变器或单端驱动阶段使用的几乎所有的吉他放大器将不允许电网电流的流动，所以级A1/AB1/B1放大器。
的优势，一流的素A2 ， AB2型，或B2是完全没有“阻断失真” ，或瞬态互调失真。缺点是额外的复杂性，需要输出级电源电流驱动输出管网的积极地区。

这是更好的， A类或AB类？
从一个吉他扩增的角度来看，无论是阶级的运作必然是更好的，他们只是在不同的。你不应该太挂的“ A级”称号，因为大多数推挽放大器，被认为是A级其实并不是真正的A级了，他们只是阴极偏见，非负AB类放大器的反馈意见。业务类是没有理由的音调差异放大器。
阴极偏压和缺乏负反馈是一个主要的分歧氧化钒克隆和马绍尔群岛/护舷风格的东西。典型的马绍尔群岛和挡泥板采用了固定偏置输出级负反馈输出回到三相逆变器的输入，而克隆使用氧化钒阴极偏置输出的阶段，没有全球性的负反馈。此外，输出管和前置阶段/三相逆变器的配置大大有助于音调签署这些放大器。
阴极偏置与固定偏置
在阴极偏置放大器，偏置电压是发达国家之间的阴极电阻是绕过一个大电解电容器。在AB类放大器，因为目前通过管的增加，平均电压，阴极电阻变化，从而调节板目前，建立一个有点“凹陷”和动态变化的谐波结构的变化，同时注意到，玩。这是因为该板块目前在AB类放大器是不连续的整个交流周期。管进入截止的一部分周期，这意味着平均水平的直流信号对阴极将转移，转变经营点的管，由此造成的动态声调的变化。的平均价值，正弦波是零，但平均价值被截断的正弦波，如发生时，板电流切断一些的时间百分比，不是零。目前在一个真正的A级放大器不断，因此它不具有这种偏见的转变，除非驱动剪报，所有的赌注都赶走。这就是为什么阴极旁路第没有必要在一个真正的A级推挽输出级-板电流都是平等的阶段了，除非有一种不平衡的输出变压器，输出管，或驱动信号（它是一个好主意，用一个总之，这些原因） 。固定偏置放大器偏置保持在一个更稳定的水平，因此它不具有不断变化的经营点，随产量水平。

我心已死

的影响，全球负反馈
利用全球负反馈的几件事：它flattens和扩展频率响应，减少失真产生的阶段所包含的反馈回路，它降低了有效输出阻抗的放大器，这增加了阻尼因子。所有这些都影响了基调以某种方式。
为平地，扩展频率响应明显改变音调特征，消除“拱”在输出级的反应和生产更多的高，低端的频率。减少失真使功放更清洁和更健全的“高保真” ，直至点剪报。可能是主要的区别是“感觉”是增加阻尼因子所产生的负反馈回路。减少的有效输出阻抗的放大器的原因作出反应较少的发言。一位发言者阻抗曲线远远持平;上升非常高的谐振频率，然后下降到标称阻抗约1kHz时，再次上涨的频率增加。这种变化的“反应”负荷导致功放输出水平的变化与频率变化，音箱阻抗（动态改变的内容作为扬声器驱动更难） 。全球负反馈普遍降低，这就大大。这可能是好还是坏，这取决于你在找什么。

负反馈使放大器声音“更严格” ，尤其是在低端，那里的一位发言者谐振驼峰拥有最影响放大器输出。这是更适合播放或质朴清洁紧张歪曲基调，而一个非负反馈放大器有一个“宽松”的感觉，更好地适应bluesy ，动态式的比赛。其他不利的负反馈放大器的是，过渡到清洁，以歪曲更突然，因为负反馈往往使功放失真到最低限度，直到输出级剪辑，此时没有“多余的增益”可随时向反馈回路正常运作。在这一点上，反馈环被破坏，以及安培过渡到全面的非反馈着增益，这意味着剪报发生非常突然。非负反馈放大器的过渡更加顺利地进入失真，使它更好地为球员谁喜欢用自己的音量控制来改变一个干净，以一个扭曲的基调。

有一个输出级拓扑是种之间，所谓的“ ultralinear ”作业。这种利用当地的负反馈到屏幕上电网的输出级通过利用输出变压器初级。这增加了阻尼因子，并提出了更严格的功放有点不使用全球负反馈回路（您可以使用全球负反馈输出与ultralinear阶段，但你可能不喜欢的调子多） 。博士66 ž路线使用ultralinear放大器输出级。还有一个三极管输出级，甚至更高的阻尼因子比ultralinear ，但一些球员认为，这听起来太“压缩”和midrangey ，而另一些喜欢它。部分原因是中端的重点是增加输入电容三极管模式超过pentode模式，因为米勒的影响，因为这实际上，乘以发车板电容的增益管。这增加电容推出了高频率。

是否真正的A级作业需要任何特定的电流或偏见点？
真正的A级操作不必上述任何特定的额定电流或功耗。这取决于管式，供电电压，反映阻抗，以及所需要的工作点。不过，一般而言，当一类功率放大器的设计，有偏见的一点是选择符合的位置上板曲线的交叉点上的载重线，板电压，最大功耗曲线，使最大对称摆动前两个方向的剪报。这意味着管是有偏见的权利在最大板散热，这是好的，因为散热是最大闲置在A级放大器，并且不增加应用的信号，因为它在AB类或B类放大器（它实际上减少到最低限度满负荷） 。这并不是说，这是唯一的电流和电压，将工作。如果您降低板电压100V的，你会发现另一种“最佳”的地方这些线路相交。如果你改变了反映负载阻抗，你会发现又一个最佳位置。但是，有一个上限的电压，可套用在这里您可以不再为对称摆动偏见的闲置点不超过板耗散评级。这是限制电压该管的真正运作* A级的最高评级*.管建议如果您选择运行管的评级，如在一些放大器，您可以偏见管这一点正在运行A级，但首先是最大的散热曲线。虽然这似乎与一些管，这不是一个推荐的做法。
这也适用于这两个单端和推挽式设计。在推挽A级，偏差点和板供电电压是一样的单端，但是有一个三相逆变器和一个中心，利用变压器，用于提高功率，减少失真（偶数阶谐波是取消，和电源哼声取消平衡推挽放大器） 。功率的两倍单端（为双管推挽与单管单端，等等） 。

为了更好地感受到这一点，采取了一系列板曲线某一管，并提请载重线代表反映阻抗（它有一个斜坡相应的消极互惠的反映负载阻抗，并通过交叉的偏置电流和电压线路板） ，并提请代表曲线板耗散（这将是一个抛物线形状，每个点相当于目前对应于板耗散除以板电压） 。载重线应该触摸板耗散曲线在选定板电压（最大功率为了-如果你想小于最大功率，可以在下面的耗散曲线） 。目前相应的这一点是需要偏置电流，以及最大耗散将在这一点。全电子管的信号会出现波动的载重线（假设一个纯粹的电阻负荷-无功负荷产生椭圆载重线） ，这样你就可以找到板电压摆幅为给定格电压摆幅，你会看到，你将有可能改变板电压或反映负载阻抗，或两者兼施，以取得最佳的阶级偏见点。不要忘记，实际板电压摆幅两个以上和以下的供电电压，和中心的摆动是实际板供电电压。这是一种混乱，第一，因为它不是直观的，你可以得到伏峰值只有250供应（即摇摆不定的100V到伏，围绕250 ） 。在“额外的” ，因为电压来的性质如何输出变压器厂。

是否偏置保证在最大耗散A级操作？
因为你是有偏见的最大耗散并不意味着你是A级！您必须在该地区的电压摆幅是对称的和有偏见的中心范围内，在当前流动板块的所有unclipped输出。偏置的高电压和低板目前其产品等于最大板耗散可能不会允许这种情况，因为，尽管你是最大板耗散，偏差一点是，目前将流板的相应时间更少的消极信号挥杆（截止）相比，将在积极的信号摆幅（饱和） ，和*没有*载重线可发现，将允许对称的摆动，或将在这样一个非线性部分曲线作为将无法使用。这是因为板电压过高，最高不超过允许的电流损耗限制太低。同样的事情会发生在另一端的规模，在那里你可以减少板电压这一点的最大耗散电流将超过允许的最大板或额定电流阴极管。存在一个最佳的曲线领域将变得很明显当你开始画载重线和采摘偏见点。这是一个有点反反复复的过程，因此，管制造商可以很容易为您列出典型的A级作业的状况，在数据表。
从理论上讲，你可以采取AB类推挽放大器和转换为A级推挽操作， *但是* ，你将在几乎所有情况下，不得不减少板电压能够偏见管到A类地区，因为整个的理由去AB类是为了获得更高的权力，因此，板块运行电压较高，电流低于闲置是什么答：在课堂上允许再次，你必须看看板曲线特别是管，以确定允许的情况下A类地区。如果您只是偏见1 AB类放大器最大耗散在空闲，你会发现，当您申请一个信号，管子将消散更多的权力，他们将开始辉光一个可爱的樱桃红色，并有将沙哑。此外，电力供应和/或输出变压器可能无法处理额外当前需要真正的A级作业，所以，除非您知道该评级的trannies ，最好不要尝试此动作，即使您降低电源电压。

这些A级放大器我看到广告真的A级？
有很多的争论在市场上的“ A类”放大器，以及它们是否是真正的A级，或AB类放大器不择手段销售给公众的信任为“ A级” 。事实是，大多数，如果不是全部的话，是在现实中的阴极偏见，非负反馈AB类放大器，相反的是制造商的文学可以说。
的区别是什么，那么，为什么它的问题，如此多的人？
根本问题是如何AB类的定义，以及人们如何解释。谁的人说， AB类放大器是“ A级较低卷”在技术上是错误的，但合适理由。如果你是A级确定为唯一的传导全面360度相位角，你会是正确的。然而，有更多的定义，放大器班比。
的决定性因素在确定是否放大器是A类， AB类或B类*已*应在充分输出裁剪前，否则，类定义是毫无意义的。这的确是一个非常黑色和白色的东西，而且取决于偏置点的特征曲线，以及载重线，除其他外。

如果在充分undistorted输出，板目前流动在每个管进行全面的360度输入传导周期，放大器级答：但是，如果放大器的偏见等，该板块目前切断了可观的时间在每个周期在此充分undistorted输出功率，它是那么AB类放大器。如果是有偏见的，例如，每一个方面是在截止的一半投入周期，这是一个B类放大器。请注意，截止并不意味着输出放大器被截断，或扭曲。截止目前指板切断一方的推挽对一个周期的一部分，而其他方面继续发挥作用。输出波形仍然是一个干净， unclipped正弦波，因为变压器汇总两个“半”的输入信号成一个复合信号。实际上，单管放大的“上半部”及其他管扩大了“降低一半。 ”这样做是为了提供更高的效率和更大的输出功率。在AB类放大器放大每个管量的一半多一点儿的信号，以减少失真发生在零通道的波形，这是所谓的“交越失真” 。

这是那里的问题有：由于AB类放大器的偏见，使板当前流动的全过程在较低的产出水平（这是为了减少交越失真） ，许多人声称这是一个“ A级放大器在较低的数量“ 。但事实却并非如此。这是在类似的条件* *为A级，但不是A类放大器的任何手段。它仍然是一个AB类放大器，无论您选择调用它。

现在，有什么区别，你可能会问？嗯，一个原因是， AB类放大器是偏见更非线性部分的特征曲线，这意味着有更多的失真比真正的A级放大器。此外，效率将大于理论上是可能的A类放大器在这些级别。有一个非常现实的差异，语气和操作条件之间的一个真正的A级10W的放大器运行也就是说， 1W的，和一个10W的AB类放大器在1W的运行。相同的输出水平，总功率水平相同，但* *不同类别的行动中，不同数额的失真，不同的效率， *和*不同的音调，即使其中一人既不是在截止的任何部分输出周期这一较低的水平。这是由于偏见点载重线的分歧和差异。差异变得更加明显时，放大器是运行在他们充分undistorted的输出功率。真正的A级放大器，也没有交越失真，而AB类放大器将。目前的平均板的真正A级放大器将不会改变，或将改变很少，从零至满输出功率，而平均板块目前在AB类放大器，将大大增加。这将导致“凹陷”的电源供应，不存在真正的A类放大器，而这又产生一个音调的变化。

正如你所看到的，确实存在这样的事，作为一个“真正的AB类”放大器，就像有一个“真正的A类”放大器，以及一流的定义是不模糊的，除了那些谁不明白它们，或者选择忽略他们的营销优势。

还有一件事：如果你把A类或AB类放大器到裁剪？它随后成为AB类/的B ， C或D类放大器？不，当然不是。这只不过是同一类放大器，这是开始，但驶入剪报。 A类放大器驱动，以剪报仍是A类放大器的定义。这就是为什么放大器类的方式定义他们。否则，一流指定不会有任何意义。任何放大器可驱动超越它的限制到一个完全截断方波输出（除非它是有限的） ，但是，这并不使D类开关放大器，现在不是吗？

其中购买？
底线是：不必担心是否安培是“ A级”或没有。如果您有兴趣的细节，找出是否是阴极偏见或固定偏见，以及是否是利用全球负反馈与否，是否使用了pentode ，三极管，或ultralinear输出阶段，什么类型的输出管使用。这些参数将一个想法的“感觉”的放大器，但最终，你仍然必须发挥功放和使用你的耳朵告诉你哪一种最适合您的演奏风格。不要做出决定的基础上的技术规格单，您可能会错过一个伟大冠冕堂皇放大器！

ch639827608

<吃饱了撑的
不就是个名称嘛，如果大家都认可，把A类和D类的定义相互倒过来用也未尝不可>

其实，阁下也可以来个09年的最最新定义，为论坛好些为计算A类功率而迷惘的网友，定义明确了，计算就轻而易举了。

《http://bbs.hifidiy.net/viewthrea ... p;page=7&extra=
发一贴非常晕的功率计算贴大家帮看看。我做了快70斤的功放只有7。2W？。》

ch639827608

软件翻译虽然省事，但错误很多。

我心已死

对啊   不过比一个个翻好

ch639827608

《对啊   不过比一个个翻好》

是省事不是好。
或者可以寻找更好的翻译软件。

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这是最后一段的人工翻译，如用软件翻译后修改，还不如直接翻译。我试过用软件翻译后进行修改结果比直接翻译还慢。

《将购买哪一种？

底线是这个：不用考虑放大器是否是A类，如你对详细的参数有兴趣，看看它是阴极偏置还是固定偏置，和是否是使用了大环负反馈，是否它使用了五极管、三极管或超线性输出级，和使用什么类型的输出真空管。这些参数将给你关于放大器听感的一个轮廓，但最终，你仍然必须实际运作放大器并用你的耳朵听哪一种重播类型最适合你。不要单纯以技术参数来做决定，你将因此而遗漏掉某一部极为好声的放大器。》

hl8999696

字太多！！

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这里http://sound.westhost.com/class-a.htm有一篇

<Class A Amplifiers - A Brief Explanation > (A类放大器－一个简要的解释）


Let's have a quick look at some of the power amp "classes", so we have all the info:

Class-A Output device(s) conduct through 360 degrees of input cycle (never switch off) - A single output device is possible. The device conducts for the entire waveform in Figure 1

Class-B Output devices conduct for 180 degrees (1/2 of input cycle) - for audio, two output devices in "push-pull" must be used (see Class-AB)

Class-AB Halfway (or partway) between the above two examples (181 to 200 degrees typical) - also requires push-pull operation for audio. The conduction for each output device is shown in Figure 1.

Class-C Output device(s) conduct for less than 180 degrees (100 to 150 degrees typical) - Radio Frequencies only - cannot be used for audio! This is the sound heard when one of the output devices goes open circuit in an audio amp! See Figure 1, showing the time the output device conducts (single-ended operation is assumed, and yes this does work for RF).

When I first wrote this article, I had completely forgotten about the Quad "Current-Dumping" amp, which uses a low power "good" amplifier, with a push-pull Class-C type amp to supply the high currents needed for high power. Although these enjoyed a brief popularity, they seem to have faded away. I was reminded of their existence by an article by Douglas Self ("Class Distinction", in the March 1999 issue of Electronics World ), in which he quite rightly points out that the current-dumper is (at least in part) Class-C.

Class-D Quasi-digital amplification. Uses pulse-width-modulation of a high frequency (square wave) carrier to reproduce the audio signal - although my original comments were valid when this was written, there have been some very significant advances since then. There are some very good sounding Class-D amplifiers being made now, and they are worthy of an article of their own.


Copyright &copy; 1999-2005 Rod Elliott
Page Last Updated 02 Apr 2005

ch639827608

<定义来定义去有什么意义？
事实只有一个，模拟功放末级静态电流高的声音更好。
这是功放管的特性决定的，靠玩定义、玩线路都不能绕过功放管自身的特性。>

我不是已经说过，有些网友觉得算A类的功率不太明确，所以转贴此文。

99824201

看不懂