# Distortion output operation amp

#### Kelvin289

Joined Apr 12, 2022
3
Hi guys,
I'm trying some ways to optimize the output of operation amp by setting bias voltage and change the type PNP but the same result occur.
Could you give me some advices?
Thanks
Kelvin

#### Ian0

Joined Aug 7, 2020
6,316
If you vary the input voltage keeping the frequency constant, does the glitch stay at the same point in time (but at a different voltage), or does it occur at the same voltage level (but at a different point in time)?
I note that there seems to be a small downwards step in the input voltage at the same time.

This circuit should have unity gain (or about 0.95), so why are the input and output signals of opposite polarity?

#### Kelvin289

Joined Apr 12, 2022
3
If you vary the input voltage keeping the frequency constant, does the glitch stay at the same point in time (but at a different voltage), or does it occur at the same voltage level (but at a different point in time)?
Yes,the more the frequency is increased,the bigger the glitch is.However,at low freq (around 10khz),the glitch is too small to see.The signal output is nice at low freq.
I note that there seems to be a small downwards step in the input voltage at the same time.
No,-24V and +24V is standing still.
This circuit should have unity gain (or about 0.95), so why are the input and output signals of opposite polarity?
I've just change the gain to see it clear.It's not about the amplitude,it's about the glitch waveform at the higher and higher freq.
The truth is that I've tried to change the PNP frequency transition ,it look better but I can not solve the problem.
Thanks

#### Ian0

Joined Aug 7, 2020
6,316
Definitely a small step on the yellow trace corresponding to the glitch one the blue trace.
Are you sure that is the correct circuit diagram? The diagram is of a non-inverting circuit with no voltage gain. The ‘scope traces are for an inverting circuit with a gain of 3.

#### Kelvin289

Joined Apr 12, 2022
3
Definitely a small step on the yellow trace corresponding to the glitch one the blue trace.
Are you sure that is the correct circuit diagram? The diagram is of a non-inverting circuit with no voltage gain. The ‘scope traces are for an inverting circuit with a gain of 3.
Ah,I got your point,cause this is the inverting operational output,I supplied to the inverting input,and applied gain.
However,at Q7 the signal input at base is nice,the signal output at emitter is distorted

#### Ian0

Joined Aug 7, 2020
6,316
I suspect that it occurs when Gm doubling stops.
When both NPN and PNP output transistors are conducting there is twice as much Gm (forward transconductance) in the output stage. When one of the pair stops conducting, the Gm suddenly halves.
When you complete the feedback loop it will probably go away.

#### MrChips

Joined Oct 2, 2009
27,165
The little glitch shown on the oscilloscope trace does not appear to be distortion to me.
I don't know what is causing it but it looks like an artifact to me.

#### MisterBill2

Joined Jan 23, 2018
13,168
Really, there is quite a bit of distortion toward both peaks on the output. What happens if you reduce the amplitude a bit by reducing the input? Also, that point on the output waveform is where I have seen bursts of oscillation on some other amplifiers. I am not able to explain that part, though.

AND, the different orientation of that Q17 section makes the circuit harder to follow, at least for me.

How does the output look when it is 28 volts peak to peak??

Also, you are running the amplifier with a very large amount of negative feedback. What is the motivation for that??