运放做的非平衡--->平衡电路（http://sound.westhost.com/project87.htm）
--------------------------------------------------------------------------------Mk II Transmitter
This transmitter is quite a bit more complex than the Project 51 version, but this is the price one pays for higher performance. The input is unbalanced, and has an input impedance of 11k. This must be driven from a low impedance source (such as an opamp's output) or performance will be degraded.
With the values shown, the circuit has a gain of 6dB when measured from the input, and between +Out and -Out. R13 and R14 are not absolutely essential in this version, but are recommended. They enforce a balance on the circuit, and prevent the possibility of "latch-up" where the outputs get stuck to a supply rail. This is extremely unlikely with the values shown, but the precaution is worth the very minor effort.
Figure 3 - Active Balanced Transmitter
If either output is shorted to ground by connection to an unbalanced input, the output voltage is only 0.4dB less than when operating in fully balanced mode. When one output is shorted, the feedback path to the other opamp is removed, so it provides (almost) the full swing that would normally be available between both opamps. This is the way a transformer (without centre tap) works, so the behaviour of this design is much closer to that of a transformer than the "standard" balanced output circuit.
1 Apr 2002 - I tested the circuit shown using 1458 dual opamps and 5% resistors. If the circuit is reliable and shows no bad habits with very basic opamps (basically dual 741s) and ordinary carbon resistors then I know that it will work when you use better components (however, see note below). Indeed, my test version is both stable and surprisingly accurate, despite the lowly parts used to test the circuit's operation. Because the crossed feedback paths are reduced from the optimum (by virtue of making R5 and R9 1.2 times the "correct" value), the overall stability and frequency response is much less dependent on the component values and opamp quality. I was able to verify that even using 741 type opamps, frequency response is less than 1dB down at 75kHz.
Somewhat surprisingly, output impedance is 100 ohms, and not the 200 ohms one would expect - there are 100 ohm resistors in series with each output after all. The crossed feedback paths make the difference here, and both simulation and measurement confirm that this is the case.
Although the above circuit does work exactly as described, in general it cannot be recommended for normal use. With high speed opamps, the circuit can (and likely will) oscillate, especially if there is a capacitive load (such as a cable) connected to the outputs. While it has been used commercially, the results have often been less than successful, and few commercial products use the scheme.
While it is a very interesting arrangement, it actually fails to emulate a transformer anywhere near as well as might be imagined. The PCB version of the balanced transmitter does not use this scheme, for the simple reason that stability cannot be assured. In addition, the impedance balance is sensitive to component tolerance, and impedance balance is far more important than signal balance.
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Construction Hints
Both the transmitter and receiver circuits require at least 1% tolerance resistors, or common mode rejection will be unacceptable. Even with 1% tolerance, the worst case rejection is only 40dB, and if you can use your multimeter to match the resistors to closer tolerance this will improve the performance.
Although the transmitter and receiver are shown with (mainly) 33k and 10k resistors respectively, these may be changed if desired. Any value between 10k and 100k could be used, but remember that higher value resistors create more thermal noise. R5 and R9 in the transmitter are approximately 1.2 times the other resistors - the next E12 value up. For example, if you elected to use 22k resistors throughout, then R5 and R9 would be 27k. Also remember that for the transmitter's input, the impedance is 1/3 of the resistor value used - 10k resistors would therefore give an input impedance of about 3k.
Both circuits require a balanced +/-12V or +/-15V supply (Project 05 or similar power supply), and it must be free of noise. Make sure that 100nF ceramic caps are placed between the supplies as close as possible to the supply pins of the opamps. This is especially important if you use high speed opamps. 学习了 利用运放做同相和反相差分放大
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