Different opamp gains using different analysis

[batmanbeginz]

Hi all,

I am new to analog design. I have created an op-amp schematic and tried to get open loop gain by 2 ways:
(Simulation uses cadence virtuoso)

1) Transient analysis: applied a ramp pulse of 0->VDD to +ve input [with a dc value of VDD/2] and kept -ve input to Vdd/2, then plotted Vout vs V+ and took its derivative, this gave me 1300

2) DC analysis: applied a volt of 0->VDD at +ve volt and kept -ve input to VDD/2. Processed Vout as in step 1 and got ~ 50

I cannot believe what makes the 2 gains so different and which is correct ? Voltage supply is from 0->VDD.

Can someone help in this regard ?
Thanks a lot

 

[ashugtiwari]

as far as i understood the question, here is the answer:
1. both 1 and 2 are same circuits i.e. op-amps configured as comparator except for the input type, both are step inputs except for one has discontinuous change from 0 to Vdd and another has ramp (continuous) change.
2. Almost most of op-amps have very high open loop gain, for eg. uA741 has open loop amplification rate of min 20V/mV and typical 200V/mV. so, even a small diff in voltage will drive op-amp to its rail. Im not sure how did u actually calculated two gains, the only thing i can think of op-amp with no rail to rail o/p.
3. Also, if u use circuit in negative feedback configuration then only the gain changes based on ur feedback component values.
4. In comparator mode, gain is never considered as parameter for app design.

Hope that helps.

 

[ustligen1]

Hi, Batman

I think both your methods are not very intuitive since you are a freshman. Try to simulate AC gain first and then you can try DC gain and transient response.

1. Make sure you have biased the circuit correctly.

2. Put one ac signal superpositioned at your input with a ac magnitude of 1

3. Do ac simulation and read out the output at low freq.

Since your circuit may have some offset, the ac gain you read out may not be the maximum value. But it doesn't matter since you are at the very beginning. Try CMFB then.

Justin