fixing double inverting series two stage opamp amplifier

Thread Starter

yef smith

Joined Aug 2, 2020
756
Hello , i have done a two stage amplifier as shown below.
however there is a problem becase its a double inverter so there is stability problem.
could you recommend a better two stage congiguration which whould not cause staboility problems?
maybe something where on the second stage we go into the plus instead of the minus?
Thanks.
1707594026881.png
 

Thread Starter

yef smith

Joined Aug 2, 2020
756
i was just told that double inverter is not the best configuration,stability wise.
i know that its the phase margin that counts but if you could recommend a configuration where there will not be double invertion, so i could try and impelement it?
Thanks.
 

AnalogKid

Joined Aug 1, 2013
11,055
There is nothing inherently unstable about a double-inverter circuit. Why do you think yours is unstable? Anything can oscillate if the layout is poor, or if there is no power supply decoupling, etc.

Yes, a double-inverter circuit - analog or digital - is a common oscillator configuration, but that is by design, not a law of nature. Note that an intentional oscillator requires loop feedback, and your schematic has none.

ak
 

Thread Starter

yef smith

Joined Aug 2, 2020
756
Hello , In general when we test stability of a feedback circuit we need to open the loop .
However here i have two loop feedback circuits one after the other there is no common loop.
how do i measure phase margin for such a case?
 

AnalogKid

Joined Aug 1, 2013
11,055
OR - it is a case of a little learning being not necessarily dangerous, but misleading. One of the conditions necessary for oscillation is 360 degrees of phase shift in the total circuit loop. That is not the same as 360 degrees of phase shift without global feedback.

The most important thing about any rule of thumb is knowing when *not* to use it. This probably is going to irritate someone, but in my experience that is one of the differences between a technician and an engineer.

ak
 

Ron H

Joined Apr 14, 2005
7,063
LTSpice provides an example simulation of how to measure gain and phase margin. When you look at the circuit, you can see that it has feedback. Phase and gain margin only apply to closed loop (feedback) Phase and Gain Margin simulation.pngcircuits. Yours has no overall feedback.
You CAN use this technique to measure the margins of the individual inverting stages.
 

Thread Starter

yef smith

Joined Aug 2, 2020
756
so if i have to stages of amplifiers back to back how do i know is they are stable?
by what method should i use to asses stability?
Thanks.
 

michael8

Joined Jan 11, 2015
415
How does that datasheet answer a question about compound amplifier stability?

The datasheet is the first thing I look at after looking at an entry on allaboutcircuits.com to see
what the nominal expected environment of the IC in question is and to be able to relate
that to the specific (or guessed as usual) circuit of the thread starter. It seemed to me
that stability in a physical circuit with a fast opamp would depend on how it was assembled
and bypassed not just on the "schematic" of the query circuit. So it was just a brief
glance (and leaving a pointer for others so they could skip the datasheet search).

Since you ask, I decided to look at the datasheet again to see what they said abou bypasses.
And found page 15 talking about stability... I didn't see any notes on bypasses.
 

Thread Starter

yef smith

Joined Aug 2, 2020
756
Hello, What is the meaning of gain -1 stable, gain of +1 stable for an opamp as shown below?
Does it somehow says that the input signal needs to connect on a + or minus to the opamp so it will be stable?
Thanks.
1707737391419.png
 

crutschow

Joined Mar 14, 2008
34,459
What is the meaning of gain -1 stable, gain of +1 stable for an opamp as shown below?
It means they designed the two amps with the maximum gain-bandwidth that is stable down to a inverting gain of 1 for the LT1028, and a non-inverting gain of 1 for the LT1128.

Loop stability gets worse as the feedback increases when the closed-loop gain is reduced, and a non-inverting gain of 1 is the worst-case for loop-stability, since the feedback is 1.
But the feedback gain is 1/2 for an inverting gain of one allows a higher gain-bandwidth while still being stable (the gain-bandwidth point where the phase margin is at least 45°).
Look at the data sheet's gain-phase graphs, and under "Frequency Response" for more information on this.

So the result is that the difference in the two amps is the gain-bandwidth for the LT1128 can be higher than the LT1028 for stable operation at those two gain settings (below):
Edit: Of course, it not apparent, it should be noted that both will be stable at an inverting gain of one or greater.

1707752520398.png
 
Last edited:

Ron H

Joined Apr 14, 2005
7,063
It means they designed the two amps with the maximum gain-bandwidth that is stable down to a inverting gain of 1 for the LT1028, and a non-inverting gain of 1 for the LT1128.

Loop stability gets worse as the feedback increases when the closed-loop gain is reduced, and a non-inverting gain of 1 is the worst-case for loop-stability, since the feedback is 1.
But the feedback gain is 1/2 for an inverting gain of one, allowing a higher gain-bandwidth while still being stable (the gain-bandwidth point where the phase margin is at least 45°).
Look at the data sheet's gain-phase graphs, and under "Frequency Response" for more information on this.

So the difference in the two amps, is that the gain-bandwidth for the LT1128 can be higher than the LT1028 for stable operation at those two gain settings (below):

View attachment 315066
Great explanation!
 

Thread Starter

yef smith

Joined Aug 2, 2020
756
Hello,i put LT1128 into -1 stable configuration, however i dont see this opamp going unstable.
i dont see the opamp going unstable :)
where am i going wrong in testing stability of LT1128 stability +1?
Thanks.

1708093092268.png
 

BobTPH

Joined Jun 5, 2013
8,996
For one thing, the gain of that circuit is -300, not -1 or +1.

Also, instability means sensitivity to perturbations, such as random noise, which will not be the same in simulation as in a physical circuit.
 

Thread Starter

yef smith

Joined Aug 2, 2020
756
Hello, I have made the following configuration as you can see i get a barchausen instability.
but its not stable at 115.6MHz.
Is the a way i could put a filter to eliminitate osciltions?
Thanks.

1708094105889.png
 
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