Opamp design for amplifying input common mode voltage

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Vabzter

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Hi all,
Please refer the following diagram of two stage opamp..Vdd = 1.8V, Vbias = 1.234V, Cload = 35pF



When I give 900mV as input to both input terminals the output is 900mV and all the transistors are in saturation.
My problem is:
1When I connect it in non inv. config. with resistive feedback(Rf = 1K, R1 = 1K) I get 100mV out if input is 1V..Not working as amplifier..
2. How to find the offset voltage of opamp in cadence
Thanks in advance
BR
Vabzter
 

Re: Opamp design help

1> You cannot use a CMOS OPA as a Bipolar one!
as I know, different with Bipolar OPA, CMOS OPA have low current drive capability, it can not drive a 1K resistor! usually we use capacitor (such as mos gate) as a loading. maybe you can find detail in Paul Gray's book.
 

Re: Opamp design help

Hi butterfish
Thanks for your comments..That means the feedback network must be capacitve?
BR
Vabzter
 

Re: Opamp design help


it should have a high input impedence, means you loading should match with the drive capability.
 

Opamp design help

Vabzter
Your output stage has a large output resisitor , and thus you get a large DC gain. The feedback resistor is paralleled with your opamp's output resisitor, thus the output resisitor is significantly reduced and the dc gain lost much.
 

Re: Opamp design help

Hi,
I want to design an opamp which will amplify from 1.2V to 1.7V. Is a simple two stage opamp as above sufficient for this application?
BR
Vabzter
 

Opamp design help

Vabzter
1.2v to 1.7v,power supply voltage or input common mode voltage?
 

Re: Opamp design help

paladinzlp said:
Vabzter
1.2v to 1.7v,power supply voltage or input common mode voltage?
Click to expand...
Hi,
Its input common mode voltage..the supply is at 1.8V
BR
Vabzter
 

Re: Opamp design help



Connect your opamp output to a gate of a Nmos. Use the source of Nmos to drive the resistive load. To drive resistive load, u need a low out impedance. Only source of Nmos can provide u 1/gm impedance which is very low. And also make sure the Nmos able to source that much of current to the resistive load.
 
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