opamp osciltions cause problem

Thread Starter

yef smith

Joined Aug 2, 2020
836
Hello , i have designed a driver,At first glanse it amplified great the signal how ever when i probed R1 resistor as shown below on my lab scope with DC input , i saw oscilations.
Now when i did step responce of the circuilt as shown below i see the oscilations.
I also know from analog design course that i can try and see the AC responce.
LT1028 is -1 stable ,how does it influence the connection between the stages so i will not get oscilations.
What could cause instability in such configuration?
Thanks.
https://www.analog.com/media/en/technical-documentation/data-sheets/1028fd.pdf
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LowQCab

Joined Nov 6, 2012
4,304
Your Input-Stages when multiplied together, create a Voltage-Gain of ~400X,
I'm not at all surprised that You have Noise problems.

This may, or may not be, an "Oscillation" problem,
it may simply be amplifying ambient Electrical-Noise.

Why do You need so much Gain ?

R-1 is a Feedback-Resistor, why do You care about the Waveform on it ?

After looking more carefully ..........
You need an Op-Amp that can deliver more Current.
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Papabravo

Joined Feb 24, 2006
21,313
Why are you using lossy integrators and what is your damping ratio. You don't really expect an opamp to follow that edge, a 1 nanosecond rise time – do you?
 

MrAl

Joined Jun 17, 2014
11,723
Hello , i have designed a driver,At first glanse it amplified great the signal how ever when i probed R1 resistor as shown below on my lab scope with DC input , i saw oscilations.
Now when i did step responce of the circuilt as shown below i see the oscilations.
I also know from analog design course that i can try and see the AC responce.
LT1028 is -1 stable ,how does it influence the connection between the stages so i will not get oscilations.
What could cause instability in such configuration?
Thanks.
https://www.analog.com/media/en/technical-documentation/data-sheets/1028fd.pdf
View attachment 312133
View attachment 312134

View attachment 312135

View attachment 312136
Hello,

There could be several things happening.

First, what are the specs for your coil L1.
An idea inductor can cause problems because there is no damping present. All real inductors have at least some series resistance (ESR). You should add some resistance and see if that helps. I do not know what kind of coil you are going to use in real life, but if it is high power maybe you can start with something like 0.05 Ohms or even 0.1 Ohms or even higher depending on the coil type. That will add some damping that occurs in real life.

Did you post your .asm circuit yet? If you do others here can do the simulation also and see what they can find out.
 

Thread Starter

yef smith

Joined Aug 2, 2020
836
Hello, the ASC circuit is attached.
I have this circuit printed in real life.
do you see some serius errors in the design in stability.
putting two inverter stages one after the other?
or on the third stage to input from the plus although LT1028 is minus stable as shown below?
are these fatal errors?
Thanks.
1704819880969.png
 

Attachments

LvW

Joined Jun 13, 2013
1,774
Oscillations (in your case: heavy ringing) can occur only when there is a closed loop that tends to create positive feedback.
* In this respect, both opamps with a capacitor in the feedback loop are non-critical.
* However, the opposite is true for the opamp with an inductor in the loop. In your circuit, the L1-R4 combination is a classical lowpass which - together with the real-opamp-inverter - will cause a phase shift of 180deg at a certain frequency.
Taking the phase inversion of the inverter into consideration, we have positive feedback.
 

Thread Starter

yef smith

Joined Aug 2, 2020
836
Hello,So non of the "problems" i saw in post seven are not problems?
in real life its not an inductor its an inductor wit some wires.
how do you propose me canceling this ringing i get when probing R1?
maybe i should tryuse the OVER-COMP leg and connect capacitor between it and GND at the last opamp?
is this a valid way?
i cannot change much the circuit because its already manufactured.
LTSPICE simulation file is attached.
https://www.analog.com/media/en/technical-documentation/data-sheets/1028fd.pdf
1704821707854.png1704821621307.png
 

Attachments

MrAl

Joined Jun 17, 2014
11,723
Hello,So non of the "problems" i saw in post seven are not problems?
in real life its not an inductor its an inductor wit some wires.
how do you propose me canceling this ringing i get when probing R1?
maybe i should tryuse the OVER-COMP leg and connect capacitor between it and GND at the last opamp?
is this a valid way?
i cannot change much the circuit because its already manufactured.
LTSPICE simulation file is attached.
https://www.analog.com/media/en/technical-documentation/data-sheets/1028fd.pdf
View attachment 312240View attachment 312239
Hello,

The inductor L1 and resistor R4 form a low pass filter. However, it is directly in the feedback path.
There is a rough rule that whatever is in the feedback path of an op amp creates the dual of what it actually is. Namely, the low pass filter becomes a high pass filter when combined with the op amp, but it could be even worse than that. It could be more like a differentiator.

I'll take a closer look at this and get back here.
 

MisterBill2

Joined Jan 23, 2018
19,459
Where did that circuit come from?? I have seen that scheme once before, only it used an LM301 op-amp. think resistor R1 should be a higher resistance value. Certainly there is a lot of feedback, far more than is needed. That part of the circuit does not look right.
 
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LvW

Joined Jun 13, 2013
1,774
..................
how do you propose me canceling this ringing i get when probing R1?
maybe i should tryuse the OVER-COMP leg and connect capacitor between it and GND at the last opamp?
is this a valid way?
Did you hear about "input compensation"?
You can improve stability by using a series R-C combination directly between both input terminals of the last opamp.
However, the cross-over frequency of this RC combinaton must be matched to the "critical" frequency region of the last stage.
This region can best be identified using loop gain simulation for this stage only.
 

MrAl

Joined Jun 17, 2014
11,723
Hello,

The inductor L1 and resistor R4 form a low pass filter. However, it is directly in the feedback path.
There is a rough rule that whatever is in the feedback path of an op amp creates the dual of what it actually is. Namely, the low pass filter becomes a high pass filter when combined with the op amp, but it could be even worse than that. It could be more like a differentiator.

I'll take a closer look at this and get back here.
Hello again,

I took a closer look and found that if we do a purely theoretical analysis the circuit would look like a sort of differentiator. That means allowing the gain of the op amp to go toward infinity. If the gain is not allowed to go too high, the circuit looks stable, but may still be a sort of high pass filter which means the higher the frequency the higher the output. The only limitation then is the bandwidth of the op amp itself.
[Note the above was an analysis of the last stage only with U1 as that is the most suspect stage that might cause a problem]
 
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LowQCab

Joined Nov 6, 2012
4,304
1)
What is the full description of the Load-Inductor ?
( including the DC-Resistance measurement )

2)
What function does the Load-Inductor provide when Current flows in it ?

3)
Why is the Load-Inductor in series with the Feedback-Loop in your Circuit ?
Is this because You require very fast response to Input-Changes ?
or, is this to provide very linear Current-Changes for Input-Voltage-Changes ?

4)
What do You expect the Response-Delay-Time to be
between an input change, and a matching Current change in the Inductor ?

5)
What would happen if the Current-change in the Inductor was
delayed by some arbitrary amount of Time relative to an Input-Voltage change ?

6)
Is your choice of Power-Supply-Voltages flexible, or is it fixed at plus/minus 15-Volts ?

Most of the Geniuses in these Forums will easily get deep into discussing Circuit-Theories for days on end.
But I assume that your Circuit has a particular job that it needs to perform,
and that, the particular method of obtaining a satisfactory end result is what is most important.
All of the above questions must be answered completely, and then analysed to determine the
most reasonable approach to take to achieve your desired End-Goal.
Trying to "fix" the first approach to this problem may not ever work satisfactorily,
a completely different approach may need to be taken to obtain the desired End-Goal.

We still don't know what your desired End-Goal is.
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MisterBill2

Joined Jan 23, 2018
19,459
Once again I am asking where this amplifier circuit came from, because it is quite unusual in using the power source terminals of an op-amp IC as drivers for output transistors. It is MUCH simpler than the original version that has been published in books by a trusted publisher. Thus the motivation for asking about the source.
And the fact remains that the feedback voltage is quite large, and the arrangement is quite different from most audio amplifiers.
 

MrAl

Joined Jun 17, 2014
11,723
Once again I am asking where this amplifier circuit came from, because it is quite unusual in using the power source terminals of an op-amp IC as drivers for output transistors. It is MUCH simpler than the original version that has been published in books by a trusted publisher. Thus the motivation for asking about the source.
And the fact remains that the feedback voltage is quite large, and the arrangement is quite different from most audio amplifiers.
Hi,

It's a very old technique you do not see too often anymore, but it is a valid technique.
 

Thread Starter

yef smith

Joined Aug 2, 2020
836
My circuit has a very simple goard i put DC voltage of 10mV and i get 20mA on the inductor.
pure DC signal.
My goal is to learn how to investigate stability issues of a circuit i have built
I can do LTSPICE sinulations to learn how to tune the circuit into stability.
There could be many issues, maybe i have noisy power supply .
Maybe i have bad decoupling circuit.i need to learn how to investigate in real life.

suppose i have oscilations on my scope instead of DC, how can i know the phase margin situation from it?
looking at the AC responce i cant see anywhere a point where i have 0dB and 180 phase(barkausen condition)

Thanks.

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LowQCab

Joined Nov 6, 2012
4,304
"" My goal is to learn how to investigate stability issues of a circuit i have built ""

Your Circuit is almost guaranteed to be unstable with an Inductor in series with the Feedback-Loop.
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Thread Starter

yef smith

Joined Aug 2, 2020
836
Hello, could you please answer the last part of post 17,i need to know how to see the phase margin situation based on the oscilation i get on the scope, THE AC responce barkausen condition.
 

LowQCab

Joined Nov 6, 2012
4,304
How to design a Circuit with Feedback involves following certain well known "Rules".

One of those Rules is to minimize any Inductance in the Feedback-Loop,
and to minimize any Capacitive-Load on the Inverting-Input of an Amplifier.

You have, ( purposefully ???? ), installed a large Inductance into the Feedback-Loop,
this will always create a "Non-Linear-Response" from the Amplifier.
This "Non-Linear-Response" can not be "undone" or "compensated" to zero.
It's effect can only be partially "reduced".

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