A rather strange op amp, take a look

福尔摩擦啊

A rather strange op amp, take a look [Copy link]

 

I don't quite understand this op amp. There are a few things I don't quite understand. Please give me some advice:
1. I don't quite understand the negative feedback. How to calculate the gain
? 2. What is the role of the two capacitors in series at the reverse input
? 3. What is the role of a resistor and capacitor in parallel at the non-inverting input?
4. What is Q5 for?

bigbat

1. R126 is a feedback resistor and forms a differential amplifier circuit with R119. R127, R128 and R142 are input bias voltages used for zero adjustment. R122 is a series current limiter. But it is a bit too big!

2. Capacitors C91 and C93 are connected in series to eliminate the polarity of the capacitors.

3. R119 and C90 are voltage stabilizers.

gmchen

This is a multi-feedback low-pass filter.

The two series connections are capacitors, as mentioned above.

The capacitor at the non-inverting terminal provides AC bypassing.

The transistor is used to expand current.

alianyyj

1. The low-pass large capacitor is equivalent to a short circuit: the op amp output Vout (point A) = -Vin* { [R127+R127*R142/R128] || R126} /R122, which is to amplify the input by 1-3 times.

2 Input limit +/-0.7,

3. VIN>0, the output of the op amp pin 6 is less than 0, D17 is cut off, Q5 is turned on and current is expanded, which can provide current to the collector load; VIN<0, whether this diode and transistor are turned on, what is the role of the diode in this circuit? I don't understand it. Please help me, seniors

福尔摩擦啊

bigbat published on 2020-3-18 12:04 1. R126 is a feedback resistor and R119 form a differential amplifier circuit. R127, R128 and R142 are input bias voltages for zero adjustment. R122 is a series current limiter. ...

I read that one capacitor is usually used for coupling. What is the purpose of using two here?

福尔摩擦啊

alianyyj published on 2020-3-18 14:11 1. The low-pass large capacitor is equivalent to a short circuit: the op amp output Vout (point A) = -Vin* { [R127+R127*R142/R128] || R126} /R122, which means the input...

I understand the first two points you mentioned, but I don't quite understand the third point. Please help me.

福尔摩擦啊

gmchen posted on 2020-3-18 12:27 This is a multi-feedback low-pass filter. The two series are capacitors, as mentioned above. The capacitor at the in-phase end provides AC bypass. ...

Can Q5 be used as a shooter?

gmchen

Forfrio posted on 2020-3-18 21:00 Can Q5 be used as a follower?

Q5 is used as a follower only in the negative half cycle of the output, and D17 is turned on in the positive half cycle of the output. Therefore, the load carrying capacity of this circuit is different in the positive and negative half cycles of the output. I don't know why it is designed like this.

gmchen

Forfrio posted on 2020-3-18 21:00 Can Q5 be used as a follower?

In addition, R127, R128 and R142 form a T-shaped network. According to the star-delta transformation, it can be equivalent to a resistor from the right end of R122 to the ground, a resistor from vout to the ground, and a large resistor between vout (R142 sliding end) and the right end of R122. The resistance values of these three equivalent resistors are related to the position of the sliding end of R142. Usually, the purpose of this circuit is to use a small resistor to obtain a large equivalent resistance in order to achieve a large voltage gain. Therefore, the gain of this circuit is related to the position of the sliding end of R142.

在学习的路上

Posted by For Friction on 2020-3-18 20:57 I read that one capacitor is usually used for coupling. What is the purpose of using two here?

The capacitance of non-polar capacitors cannot be made large. If 100uF is used in the circuit, only polarized ones can be selected. Using two, connected in positive and negative phases, should be to eliminate the influence of the polarity of the capacitor.

xiaxingxing

gmchen posted on 2020-3-19 09:33 Forfried posted on 2020-3-18 21:00 Can Q5 be used as a shooter follower? In addition, R127, R128 and R142 form a T-shaped network...

Hello, what is the function of C96 in the figure? Is it the resistor connected in parallel with the feedback resistor to eliminate high-frequency self-excitation and phase compensation? Thank you!

gmchen

xiaxingxing posted on 2020-3-19 21:05 Hello, what is the function of C96 in the figure? Is it the resistor connected in parallel at both ends of the feedback resistor, used to eliminate high-frequency self-excitation and phase compensation?

This circuit is a bit weird. If you ignore the parameters of the resistors and capacitors, it is a multi-feedback low-pass filter just from the circuit form. However, the capacitance of the two capacitors (one is C96, the other is C91 in series with C93) is too different. The capacitance of C96 is too small, and some distributed capacitances may have the same order of magnitude; the capacitance of C91 in series with C93 is too large, and usually active filters do not use electrolytic capacitors because their capacitance and Q value are very unstable. So it is hard to say.