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楼主 |
发表于 2019-8-24 16:47
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音频功率放大器设计手册第六版 audio_power_amplifier_design_6e，page227
The Folded-cascode VAS
The folded-cascode configuration shown in Figure 8.23 is not just another variation on the push-pull VAS driven by a single input stage. It is usually called the foldedcascode configuration, because Q4, Q5 are effectively cascoding the collectors of input stage Q2, Q3. The distinguishing characteristic of this configuration is that the two transistors Q4, Q5 are common base stages and their emitters are driven from the collectors of input stage Q2, Q3. Q1 is a cascode transistor for the collector of Q4 (sort of a cascode of a cascode); it is not an essential part of the folded-cascode concept. The current output of Q1 is bounced off the V-rail by the currentmirror Q6, Q7 and provides the lower part of the push-pull drive to the VAS stage Q5, Q7.
The really important point about this approach is that it is not a three-stage architecture; it is a two-stage architecture with input stage and VAS combined in one stage. The current variations in the input pair are passed unamplified to the VAS output by the folded-cascode transistors Q4, Q5, which give no current gain. These relatively small current variations have to do the best they can to generate a high voltage gain although subject to non-linear loading from the output stage.
This is bound to lead to a lack of overall open-loop gain. The only obvious way to increase it is to raise the impedance seen at this point by putting a unitygain buffer between the input/VAS and the output stage. This approach can work well in opamps, but there seems to be little advantage in a power amplifier.
We are already using more transistors than in a threestage Blameless amplifier. Another objection is that there is no obvious way to apply the Miller dominant pole compensation that is so very useful in linearising a conventional VAS.
While the folded-cascode configuration has been used extensively in opamps (in opamp usage the two resistors R1, R2 are normally replaced by constantcurrent sources), it has only rarely been applied to audio power amplifiers, and I have no practical experience with it.
I am not sure if any commercial amplifiers have been built using the folded-cascode structure, but at least one such design has been published for amateur construction by Michael Bittner and the circuit values here are based on that.
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发表于 2019-8-24 17:39
就说吗，电路从原理分析不可能的事。
