melkord
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What is the correct operating region of MOSFETs which work as current steering devices?
I have a circuits looks like below.
The feature that I would like to have is I can vary VC+ and VC- as wide as possible while keeping the the current (ID8 and ID17) varying as linear as possible.
ID8 and ID17 are used to bias a Gilbert Cell-based VGA.
I could consider M8 and M17 as linear potentiometers. This leads to an idea of putting both in triode.
But I keep questioning myself why not in saturation then.
I made two simulations hoping that it would help me to reason about this circuit.
In one simulation, both M8 and M17 are in saturation. In another simulation, both are in triode.
I could not find any useful conclusion except that in saturation, the output resistance of M8 and M17 are higher. This is obvious.
How this can relate to the intention of having a current steering circuit remains opaque to me.
I mean, as long as I keep the VC variation <2Vov which is the lower limit between these 2 cases, triode and saturation looks the same except the output resistance (and intrinsic gain).
In publication like this one, VC- is connected to the ground. I assume that makes M17 works in triode.
But it does not state it explicitely nor explain the reason.
I appreciate it if someone can point any fallacy that I made or provide any reference about this.
------------------------------------------------------------------------------------------------------------------------------------
in triode
in saturation
picture from the publication I mentioned above.
I have a circuits looks like below.
The feature that I would like to have is I can vary VC+ and VC- as wide as possible while keeping the the current (ID8 and ID17) varying as linear as possible.
ID8 and ID17 are used to bias a Gilbert Cell-based VGA.
I could consider M8 and M17 as linear potentiometers. This leads to an idea of putting both in triode.
But I keep questioning myself why not in saturation then.
I made two simulations hoping that it would help me to reason about this circuit.
In one simulation, both M8 and M17 are in saturation. In another simulation, both are in triode.
I could not find any useful conclusion except that in saturation, the output resistance of M8 and M17 are higher. This is obvious.
How this can relate to the intention of having a current steering circuit remains opaque to me.
I mean, as long as I keep the VC variation <2Vov which is the lower limit between these 2 cases, triode and saturation looks the same except the output resistance (and intrinsic gain).
In publication like this one, VC- is connected to the ground. I assume that makes M17 works in triode.
But it does not state it explicitely nor explain the reason.
I appreciate it if someone can point any fallacy that I made or provide any reference about this.
------------------------------------------------------------------------------------------------------------------------------------
in triode
in saturation
picture from the publication I mentioned above.