Re: an analysis of opamp , need you help
lavitaebelle said:
This is a 3 stage amplifier with class AB control loop and nested miller compensation.
1. What is the gain of every stage
Gain of first stage=gm1(rin2||R1) (gm1=Ic/Vt,rin2=βVt/Ic)
Gain of second stage=gm2ro2 (ro2 is the cascode impedance seen at nodes E and F)
Gain of third stage=gmN4.rout when NMOS is active and gmP3rout when PMOS is active (rout is the output impedance of the final stage).
2. What is the polse and zeros
There are 4 poles-input of input diff pair, input to second stage input diff pair, output of second stage folded cascode stage and output of final output stage.
The only high impedance node among the first three poles is the output of the folded cascode. Hence there needs to be some pole splitting for compensation.
C3,R9 and C3,R8 do the job.
There is an additional nested compensation present in the form of C2 and R8.
3. How to calculate its slew rate
I am not sure but I think it is I1/C2+I2/C3
4. What is the function of N3 P2
They are part of the class AB control loop. They determine bias to nodes E and F and are responsible to fix minimum current in the output stage during class AB operation.
5. How to ensure the opamp is stable with 500PF cap load
Do proper pole splitting using the present compensation. Make use of the zeros.
I am soryy , I didn't give the enough information
Ra ,Re,Rout are the output r of node A ,E ,Out ...
Ra ≈ R1 = 17k
Re= Rf=1meg ( ?? )
Rout = 100k
Gq1=Gq2 =150u
Gq3=Gq4 = 210u
Gp3=0.7m Gn4=0.8m
1. The second stage gain is ?
A2 = Gq3 * ( (R2+R3) / (1/Gq7+R2+R3) ) * Rf
The term of (R2 +R3) / (R2 +R3+1/Gq7) is current divider ,for R2 and R3 is not high enough and 1/Gq7 , 1/Gq8 is not low enough .
A3 = (Gp3 + Gn4) * Rout
2.
The poles is ?
P1= 1 / (R1 * A2 * A3 * C2)
P2 = 1/ (R1 * C1)
P3= ? P4=?
"Hence there needs to be some pole splitting for compensation.
C3,R9 and C3,R8 do the job. "
I don't know 'pole splitting' , so May you say more about pole spllitting . And Why C3 ,R9 , R8 are not pole
splitting for the output node the 3rd stage .
Should the first zero be equal to the second pole ?
3. The slew rate ?
4. N3 and P2 is to increase the transfer rate for the high Cap at
node E and F by positive feedback .
5. May you be good enough to say more about the 5rd question ?