How to analyze this kind of circuit? I need your advice. It can be paid.

福尔摩擦啊

How to analyze this kind of circuit? I need your advice. It can be paid. [Copy link]

 

It feels very complicated, I don't know how to analyze it, please give me some advice, it's paid

maychang

The signal goes from point A to AMPS (the signal is input from point A and output from point AMPS), which is just an ordinary amplifier. Judging from the parameters of each component, it is estimated to work in the audio range, including multiple amplitude-frequency correction circuits.

However, the amplifier is overly complex and could be simplified.

maychang

U65 is a common-mode amplifier, and its gain in the low frequency band is roughly (R137/R139)+1, which is quite small.

U66 plus Q3 and Q4 is an inverting amplifier. Q3 and Q4 are complementary push-pull circuits, and their voltage gain is approximately 1. The amplifier with a voltage gain of approximately 1 formed by Q3 and Q4 is cascaded with the inverting amplifier formed by U66, and it is still an inverting amplifier with a small voltage gain, but with a large current gain, which can drive heavier loads.

gmchen

The first stage amplifier (U69) is a low-pass filter with gain, and the passband (low frequency) gain is R137/R139+1=2.3.

The second-stage amplifier (U66) and Q3Q4 form a push-pull power output amplifier, and the low-frequency gain is -R124/(R131+R135)=-2.55.

U68 is the static operating point stabilization feedback circuit of the above power output amplifier.

gmchen

The low-pass filter formed by the first-stage amplifier (U69) is relatively complex. It has three poles and one zero. If R135 and C103 are considered, there are as many as four poles. When analyzing, we consider that the resistor values in the circuit are similar, but the capacitors differ by several orders of magnitude, so it can be roughly considered that the feedback network at the inverting input of the op amp constitutes its main pole. In that case, the main pole frequency is 1/(2*pi*R137*C106), about 2.5kHz. The second pole is determined by R135, R131 and C103, about 320kHz. The zero frequency is 1/(2*pi*(R137||R139)*C106), about 6kHz. These zero poles roughly determine the amplitude-frequency characteristics of this filter.

gmchen

For the analysis of the output amplifier, U68 can be ignored for now. The output of U66 directly drives Q3 and Q4. Q1 is a constant current source, providing a conduction current for D11 and D8. The saturation voltage drop of these two diodes prevents Q3 and Q4 from crossover distortion. Due to the deep negative feedback, the gain of this stage (from U69 to the left side of R104) is -R124/(R131+R135).

gmchen

U68 is the static operating point stabilization feedback circuit of the above power output amplifier, and its working process is as follows:

The voltage at the output terminal (left side of R104) is sampled and compared with the set value (center sliding terminal of R117). The comparison result is filtered by C102 and C110, and the DC component is taken out and fed back to the in-phase input terminal of U66. If the DC component at the output terminal (left side of R104) increases due to some reason, the output of U68 will decrease, and the voltage at the in-phase input terminal of U66 will also decrease, causing its output to decrease. Vice versa.

In summary, the function of this circuit is to try to maintain the output DC level unchanged. The DC level of the output end can be adjusted by adjusting R117.

福尔摩擦啊

maychang posted on 2019-12-9 14:55 The signal goes from point A to AMPS (the signal is input from point A and output from point AMPS), which is just an ordinary amplifier. Judging from the parameters of each component, it is estimated to work in audio...

Yes, it is a signal processing circuit. Please help me analyze the function of each part of the circuit.

福尔摩擦啊

gmchen published on 2019-12-9 19:06 U68 is the static operating point stabilization feedback circuit of the power output amplifier mentioned above. Its working process is as follows: Sampling the voltage from the output end (left side of R104), ...

Can you give me contact information?

福尔摩擦啊

gmchen posted on 2019-12-9 18:48 The low-pass filter formed by the first-stage amplifier (U69) is relatively complex. It has three poles and one zero. If R135 and C103 are considered, there are as many as four poles...

Can you teach me how to analyze a circuit?

gmchen

Posted by For Friction on 2020-2-10 13:46 Can you teach me how to analyze circuits?

This is quite difficult and cannot be explained in just a sentence or two.

My reply was an approximate analysis. From an analytical point of view, approximate analysis can grasp the main contradiction, but it requires the analyst to have more experience. The opposite approach is to list the node equations and solve the equations directly. That approach can be completed as long as you have learned circuit analysis, but it is more cumbersome.

maychang

Posted by For Friction on 2020-2-10 13:19 Yes, it is a signal processing circuit. Please help me analyze the function of each part of the circuit

From the 4th floor to the 7th floor, Teacher Gmchen has explained it in detail. Read it carefully several times and you will definitely gain something.

FYzj

I read the analysis carefully, but I still don't understand it. It seems that my level is too low.

gmchen

To fully understand the analysis on the 5th post, you should have the following basics: have learned circuit analysis, have learned op amp circuits, can write the voltage transfer function of amplifiers composed of op amps, and know the frequency characteristics of common RC circuits (preferably common active filters). The first two are required courses for undergraduate students majoring in electronics, whether the third one has been learned depends on whether you are a school or a teacher, and the fourth one is purely your own accumulation.

gmchen

The 6th and 7th floors are analyses of the push-pull power amplifier on the right side of the first floor diagram. To fully understand this analysis, the basics are: having studied analog circuits, understanding and being familiar with transistor amplifier circuits and operational amplifiers, understanding the role of feedback and being able to analyze various feedback circuits.