Auto-zero technique questions

[AMSA84]

Alright.

One last question, I think.

If I want to simulate the open loop gain aand the stability from the amplifier, I should have the switches included in the circuit? Biased correctly so that one can take into account their interference?

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I have forgotten another question.

If I use the moscap in series with the switch I find the offset higher when compared to when I don't use the moscap.

Since with it I am having a higher offset voltage, should I remove it? Normally literature says to add those caps, but I find them increasing my offset voltage.

 

[AMSA84]

An old circuit topology isn't necessarily bad. It's up to your decision if it's still useful.

I wanted to point out that recent chopper stabilized amplifiers are using a different topology. I suggest to pay attention to intermodulation problems of topologies that directly chop the signal path like the shown Razavi circuit. If the comparison of historical with modern chopper topologies confuses you, just ignore my posts.

Hi FvM,

I would like to ask you something.

Since you have mentioned that this auto-zero technique is very old, can you tell me some papers where they have used this kind of topology? I was wondering where this kind of circuits are useful (not the chopper amplifier or CDS one). From my understanding, since this topology/system is based on a sampling method, I was wondering where this kind of circuit is useful.

Best regards.

 

[Warpspeed]

I have seen circuits that dated back to the 1930's that used valves and vibrating mechanical contacts to chop dc signals. Its a very old technique.

If you wanted to amplify millivolt dc voltages from a sensor, and the only amplifier you have is a vacuum tube, it was state of the art technology for the time.

 

[AMSA84]

But nowadays where this kind of circuit could be applied?

 

[Warpspeed]

Its obsolete technology because there are now semiconductor switches to replace mechanical contacts, and semiconductor amplifiers to replace high voltage vacuum tubes.

The circuit techniques such as chopping/switching dc signals and amplification multiplexing and demultiplexing signals have not changed in principle, just in the hardware used to do it.

 

[AMSA84]

Yes, I know that but what I meant to ask was where the auto-zero amplifier (with the recent semiconductors of course) are used? Not referring to the chopper amplifier or CDS amplifier, only the switched one.

 

[Warpspeed]

Probably not at all in recent designs, because the latest techniques offer performance advantages with fewer potential problems.

It all depends on what you are actually trying to do. Some applications such as switching mixers actually require the signal path to be switched directly in order to work. Stabilizing an amplifier can now be done around the amplifier without breaking the signal path, which is a much more elegant solution.

But you may want to periodically zero out an integrator for example, so you use whatever circuit topology suits your need.

The old original classic chopper amplifier circuit may be ideal if you wish to convert a varying dc voltage to a varying amplitude square wave for some special purpose. Its just one more tool to have in the box of design tricks.