Voltage amplifier.....

[Rcy]

I have designed a folded cascode opamp and need to convert it to a inverting amplifier with a gain of 10.... I'am getting a closed loop gain of 10 when i put the a resistor(R=1K) in series with my input voltage source and feed back resistance (Rf=200K).....The designed folded cascode opamp(without feedback has an output resistance of 0.9Mega ohm when calculated).... so is my resistance value justifiable???

 

[erikl]

is my resistance value justifiable???

Of course not! With such high output resistance you should use Rf=10Meg and R=1Meg for a gain of -10 .

 

[ic98]

actually since it was interesting to me, i calculated the accurate voltage gain formula for the circuit with an opamp in inverting config and R2 as feedback res and R1 as the resistor between the input and the noninverting opamp input node.
you can see that in terms of transcondutances below. assume we model the opamp as an OTA with a transconductance (gm) current source in parallel with an output resistance of ro (shown as go=1/ro)

for the denominator if we assume go<<(gm-g1), and for the numerator, on the other hand, if we assume (gm-g1)>>(g1+g2)..
in brief, since g2 is usually less than g1, while gm>>g1 and go<<gm, we reach the well known formula Av=-(R2/R1) *simple one
you see, against the common guess that says, for having a gain formula close to simple one we should have an opamp output resistance very less than R2, two obtained conditions show we should consider the value of gm.. generally if gm is very greater than go and g1,g2, we don't need to be concerned about the relation between R1, R2, and ro vaules!
however, you can check the gain for values that disregard the conditions.

---------- Post added at 13:17 ---------- Previous post was at 12:42 ----------

I have to correct my writings; the first condition as for denominator should be corrected as (go/(gm-g1))<<(g2/(g1+g2)) that can be rewritten as:
go<<(gm-g1)(1/(1+Av_ideal)), where Av_ideal point to absolute value of circuit ideal gain (-R2/R1)
here it's evident that if we have large value for (R2/R1) ratio, then reaching to this condition is more difficult in some cases.
noting the second condition simply we need to: ro>>Av_ideal/gm