Why does my Opamp's gain increase with frequency?

Thread Starter

notreallyamaster

Joined Mar 14, 2016
4
Hi there,

I just built up a two-stage operational amplifier circuit using 0.18 um CMOS process in LTspice. When making an AC analysis, I was expecting that the output gain would decrease as the frequency increases, but instead, the gain increases with frequency.

You can see the image of the circuit and bode plot here: http://imgur.com/RlHr1FX
The 0.18 um CMOS library file is here: http://www.ee.iitm.ac.in/~nagendra/cadinfo/tsmc018.lib

Am I doing something wrong?

Thanks!
 

crutschow

Joined Mar 14, 2008
24,125
I suspect the parasitic circuit capacitances are causing some positive feedback at those high frequencies, which shows up as an increase in gain.
If you probe the various circuit nodes you may be able to determine where this is happening.
 

Thread Starter

notreallyamaster

Joined Mar 14, 2016
4
No, you just simulated.
Not that this is a good answer, but you must consider who's doing the plotting.
Yes, I only simulated it :)

I cannot view the image very well, as I am on a phone right now, but have you plotted both the magnitude and phase on the bode plot?
When I click on a node, LTspice automatically plots both magnitude and phase.

I suspect the parasitic circuit capacitances are causing some positive feedback at those high frequencies, which shows up as an increase in gain.
If you probe the various circuit nodes you may be able to determine where this is happening.
Thank you for the answer. I just probed various circuit nodes as you said, and this happens on almost every node.
 
Which curve on you plot is the magnitude and which is the phase?

Looking at the graph with no other input I would have guessed the top line was the magnitude and the bottom line was the phase. I have been wrong before...
 

Thread Starter

notreallyamaster

Joined Mar 14, 2016
4
Yes, the solid line is magnitude.

I think I found out the solution for this issue. Adding a compensation capacitor of 10pF between Vout and source pin of M5 resolved the issue, now my gain is decreasing as the frequency increases as I was expecting. However, my purpose was to design an op amp that has an open-loop gain of 5000 and this one's gain is around 1000. I have been trying different width and length parameters but I haven't managed to see a gain of 5000 yet. But this is not the question I asked on the topic title, so I guess I have to find it myself :)

Thanks...
 

ErnieM

Joined Apr 24, 2011
8,011
Achieving a gain of 5000 all in one amp is very tough.

Achieving a gain of 70.7 in each of two amps is much simpler.

That is what I did when designing a hi frequency amp with a min gain of 1,000, though hi frequency and hi gain opens another can of worms.
 
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