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# differential couple + common source

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#### Alundra81

##### Newbie
Hi, I'm trying to project this amplifier. I have to choose resistor values to have a voltage gain of 60 dB. VCC=10 V

The first stage is a differential couple, so the voltage gain is nothing but

A1=gm(r04||r07)

the second stage is a common source amplifier so

A2=gm(r08||(r02+R3))

I can ignore R3 (for small signals) if I will put R3<<r02:

A2=gm(r08||r02)

Now substituting values I found that:

A1=1.49/sqrt(ISD1)
A2=-1/sqrt(ISD2)

now I have to assign those currents.
I don't know which would be a good choice, maybe setting A1=sqrt(1000)
and A2=sqrt(1000)? or maybe ISD2=ISD1? or?
And what about the voltage in Q1 and Q2 gates? I could write some equations, like

VSG1=sqrt(ISD1/kp)+|VTp|
VSG2=sqrt(ISD2/kp)+|VTp|
VG2=VCC-VSG2-R3*ISD2=[R4/(R2+R4)]VCC
VG1=VCC-VSG1-R1*ISD1

I can assign VG1 so that resistances have commercial values, but simulating
with LTSpice it doesn't work. There is always a bias voltage at the output that makes
Q2 turn off. I don't know how to care about it. OK, VSD>VSG-|VTp| but I would need the output voltage value using the complete expression of ID. Impossible: too many variables...
have you any hint for me please? I spent a lot of time thinking about it, and now I'm so confused..

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