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Fully differential transient analysis waveform drifting

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wenadinho

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Hi, currently I'm implementing a miller compensation fully differential amplifier with continuous time CMFB. The ac response shows that the system is stable. In the closed loop, i used capacitor feedback to set the closed loop gain of the system. the DC bias feedback is set using pseudoresistor(diode connected PMOS).
The problem comes when I tried to simulate transient analysis. The output waveform tends to saturate after a several hundred ms of simulation time. My question is, has anyone encountered the same problem, does the problem related to the wrong compensation or bad CMFB design.
Thanks
 

Is your circuit a integrator? If so, it will be saturate.
 

wenadinho said:
Hi, currently I'm implementing a miller compensation fully differential amplifier with continuous time CMFB. The ac response shows that the system is stable. In the closed loop, i used capacitor feedback to set the closed loop gain of the system. the DC bias feedback is set using pseudoresistor(diode connected PMOS).
The problem comes when I tried to simulate transient analysis. The output waveform tends to saturate after a several hundred ms of simulation time. My question is, has anyone encountered the same problem, does the problem related to the wrong compensation or bad CMFB design.
Thanks
Click to expand...

if you doubt the problem related to the wrong compensation or bad CMFB design. Why not do a loop stability analysis to check it?

Thanks
 

hi :your circuit and compencation are right,your integrator has saturated.you should check the conditions that you set.
 

Hi,

I've checked, the result applies not only to fully differential topology but also to the single ended one. The system is as shown in the picture. The ac closed loop response is also shown.
The open loop response simulation shows that the OTA is stable.
Assuming that the closed loop cutoff is 1 kHz. When performing transient analysis by applying an input signal @ 100 Hz (i.e. within the pass band) the output is stable with respect to DC ground. However, by applying an input signal of 20 kHz (i.e. stop band) the output tends to saturate after several hundred ms.

Btw, the system was taken from Reid Harrison paper.
 

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