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why current mirror ? for CMOS

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tomhive

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hi guys,
I do we need current mirror topology for CMOS and not for MESFET & PHEMT. what is the basic reson.
thanks
 

I guess, whatever may be the technology (read BJT, MOS etc.) current mirrors are needed/used. They are the basic blocks and one will prefer using them. And if they are not used in any technology, then it must be because it's not possible to have good current mirrors in that technology (this is just a guess from my knowledge of 'current mirrors usage in analog').
 

Simple answer, make all MOSFETS in saturation region and get quasi-constant transconductance.
 

See Sedra&Smith 4th edition Chapter 6 Differential and Multistage Amplifiers

Channel length modulation in MESFET is greater than MOSFET because λ between 0,1 and 0,3 V^-1

We need another techniques different from current mirror to attain high output resistance, from the semiconductor device.

For example:

In GaAs - depletion MESFET we have a implicit channel Vt<0

**Simple current source: MESFET with VGS=0 , rout is the problem

**Cascode of another MESFET to the simple current souce, to increase output impedance

**Increasing the output resistance with a bootraping circuit that put voltage gain between drain and source approached to 1 (Miller Theorem).

And so on
 

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    tomhive

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teteamigo said:
See Sedra&Smith 4th edition Chapter 6 Differential and Multistage Amplifiers

Channel length modulation in MESFET is greater than MOSFET because λ between 0,1 and 0,3 V^-1

We need another techniques different from current mirror to attain high output resistance, from the semiconductor device.

For example:

In GaAs - depletion MESFET we have a implicit channel Vt<0

**Simple current source: MESFET with VGS=0 , rout is the problem





**Cascode of another MESFET to the simple current souce, to increase output impedance

**Increasing the output resistance with a bootraping circuit that put voltage gain between drain and source approached to 1 (Miller Theorem).

And so on
Click to expand...



hi ,
thanks 4 repaly, r we using current mirror to just increase the out put resistance.but a good Amp should have high in put resistance..
i am confused
cleare me please...
waiting..
thanks
 

In MESFET we need current source (active load, high resistance), to help for voltage gain, like current mirror in CMOS.


If we put a high resistor at drain (enhnancement NMOS) or collector (NPN BJT), the semiconductor device need low gain (gm proportional to the bias [drain|collector] current, to remain in active region.



With a active load, we obtain a bias (wichever gain and a correspondent incremental output resistance (high)) .
 
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