[sdiy] Yet Another Daft CMOS Question

[ASSI]

On Montag, 19. Juli 2004 18:40, Scott Gravenhorst wrote:
> So I discover in a practical way, the I cannot use a FET device as a
> VCR for controlling large signals.  I need up to 3 volts to do what I
> want to what a CMOS linear amp (powered at 5 volts), but what if I
> use really a low voltage supply?  Spec for CD4xxx says 3 to 18 volts,
> but I'm quite sure that this means for use as logic devices, not as
> linear amps.

I would suspect a device that is speced for 3V...18V supply to have a 
threshold voltage around 0.7V...1.2V - this doesn't give you much 
headroom for controlling the gates when the supply gets really low. You 
could try to work in weak inversion regime (i.e. just around the 
threshold voltage), but it's hard to bias the transistors correctly and 
keep it that way over temperature (and you don't have enough 
transistors available to pull the sort of stunts that are done in an 
integrated circuit for this purpose). For "normal" operation the 
practical limit is about 2*Vth for the supply.

Linearizing MOSFET circuits is usually done with cascodes or telescopic 
cascodes, which requires a higher supply depending on the height of the 
cascode stack. This would work well even in a discrete circuit if it 
weren't for the large number of current mirrors. The principle can 
still be applied if you feed the MOSFET from a current source rather 
than a voltage. The supply via resistors proposed earlier in this 
thread is somewhat similar.

Operation at low voltage can also be achieved by compressing the input 
via the subthreshold characteristic so that the current is proportional 
to the input variable, but the voltage varies only logarithmically, 
process the current and then decompress at the end (log-domain 
circuits). I'm not sure this is practical as a discrete circuit, as 
both the compression/decompression and the interaction with the actual 
filter require close matching between transistors. The same principle 
can be applied for the quadratic part of the MOSFET characteristic, 
leading to sqrt-domain circuits.


Achim.
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