CMOS OpAmp Design

[[email protected]]

Hi,
I was looking for a CMOS OpAmp circuit which doesnt need need any
special Common Mode Feedback circuit for stabilizing the bias point.
Anybody has come across a circuit like that ?

Please let me know
regards
Jean

 

[MooseFET]

Hi,
I was looking for a CMOS OpAmp circuit which doesnt need need any
special Common Mode Feedback circuit for stabilizing the bias point.
Anybody has come across  a circuit like that ?

Take a look at some of the CMOS op-amps that RCA made back in the
stone age. They had a very low transistor count.

 

[[email protected]]

Hi,
I was looking for a CMOS OpAmp circuit which doesnt need need any
special Common Mode Feedback circuit for stabilizing the bias point.
Anybody has come across  a circuit like that ?

Please let me know
regards
Jean

Do you mean fully differential CMOS opamp?

 

[[email protected]]

Yes,
I was looking for fully differential design.

Jim,
which is the textbook you are talking about ? GHLM ?

regards
Jean

 

[[email protected]]

Yes,
I was looking for fully differential design.

Jim,
which is the textbook you are talking about ? GHLM ?

regards
Jean

Unless you have something really lame such as accurate resistor loads
and a diff pair driven by an accurate bias current, I don't see any
way to do such a design without common mode feedback. Basically, to do
such a circuit without common mode feedback, you need to be able to
look at the circuit and predict the bias conditions, specifically the
voltage at the high impedance nodes (gain nodes).

You can do common mode with switch cap, but that doesn't mean you
don't have common mode feedback.

 

[[email protected]]

As "miso" points out, I don't think a full differential OpAmp can be
done without a common-mode loop; though I think (without putting
pencil to paper) that you _may_ be able to do it if you have a ground
pin.  (But I think that can be argued as a variation of a common-mode
loop.)

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Even with a ground pin, I don't see a way to establish a common mode
at the outputs of the op amp without some sort of feedback, with the
one exception I stated in my previous post, i.e. well known current
and resistor loads. High impedance nodes tend to go to a rail without
feedback.

I suspect the original poster isn't phrasing the question properly.