Please help me figure out how the following circuit can get these results.

sunboy25

Please help me figure out how the following circuit can get these results. [Copy link]

Can any expert help me with this problem? I want to know how the formula for drawing circles was derived. I have been working on it for a week but cannot get a result.

maychang

You have determined that the capacitive reactance of the two capacitors is small and can be ignored when the frequency is high enough. Then the maximum and minimum gains are the gains when the wiper end of potentiometer R4 is at both ends of the potentiometer.

sunboy25

maychang posted on 2020-11-29 10:32 You have already determined that the capacitive reactance of the two capacitors is very small and can be ignored when the frequency is high enough. Then the maximum and minimum gain is the potentiometer R4 sliding end located at both ends of the potentiometer...

The problem is that I can't calculate the result in the book because I don't understand the node formula very well.

gmchen

You have already posted two posts on this question! I feel a little confused, so I will help you solve the problem.

First, assume that both capacitors are equivalent to short circuits. As Mr. Zhang on the second floor said, the circuit has now degenerated into only resistance, and the sliding end of the potentiometer R4 is located at one of its two ends.

Second, notice that there is an assumption in the original text, that is, R4 is very large, so it is approximately an open circuit. At this time, the circuit is further degenerated to only R1, R3 and R5.

By finding the gain in this simplified circuit, we can obtain the results in the book.

sunboy25

gmchen posted on 2020-11-29 15:38 You have already posted two posts on this question! I really feel a little confused, so I will help you solve the confusion. First, assume that both capacitors are equivalent to short...

So, for the ft frequency of the second red circle part, do we need to write the transfer function with capacitors and then find the poles?

gmchen

sunboy25 posted on 2020-11-29 17:19 Then does the ft frequency of the second red circle part need to include the capacitor to write the transfer function and then find the poles?

You can use the transfer function as you wish, but if after adopting the various approximations mentioned above, the only branch that is actually related to frequency is R3C2, and all other branches are either resistors or can be ignored, then the second formula is also easy to come by.

sunboy25

gmchen posted on 2020-11-29 21:33 You can use the transfer function, but if you use the previous approximations, the only branch related to frequency is R3C2, ...

Why can you analyze the circuit so clearly? Is it because you have learned circuit analysis? Your level is really too high.

gmchen

sunboy25 posted on 2020-11-29 22:46 Why can you analyze the circuit so clearly? Is it because you have learned circuit analysis? Your level is really too high.

You should have learned circuit analysis, but usually they don't teach the approximate estimation methods like the ones in this circuit. If you list all the equations and solve them according to the solution method in circuit analysis, it will be a problem whether you can finish it on two A4 sheets. But with the approximate estimation method mentioned above, the problem is solved easily. The difference here seems to be experience, but in fact it is a deep understanding of the circuit. In this kind of circuit, the result of the strict solution is not necessary for design and application, and the approximate solution is completely sufficient in practice.

sunboy25

gmchen posted on 2020-11-30 07:56 You should have learned circuit analysis, but usually the approximate estimation methods like those in this circuit are not taught. If this circuit is completely analyzed according to the circuit analysis...

This is the difference between a master and a rookie!