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# Why is this opamp circuit not amplifying over entire range?

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#### amic

##### Member level 5
Problem Statement – Why is this opamp circuit not amplifying over the entire range?
VDD = 3.3 V, IBIAS = 1uA; ICMR = 0V-2.4 V, Output CMR = 0.2V – 3.1V

With the opamp that I designed (which gives unloaded gain of 80 dB and loaded (RL =10K; CL = 12pF), the dc input is amplified till only 550mV above which output saturates at around 1.9V as shown in fig 1.

What could be the reason..?

Re: Why is this opamp circuit not amplifying over entire ran

It would be helpful of you post the schematic of the opamp

Re: Why is this opamp circuit not amplifying over entire ran

pseudockb said:
It would be helpful of you post the schematic of the opamp

THE SCHEMATIC IS ATTACHED IN THE WORD DOC IF YOU CAN DOWNLOAD IT.
I WILL TRY TO POST THE IMAGE SOON.
THANKS

Re: Why is this opamp circuit not amplifying over entire ran

Ideal opamp has maximum swing at output that is between -ve and +ve supply voltages. But, real opamps not. So, your circuits has real opamp and output voltage can not reach 3.3V, it stops at ~1.9V.
If you want increase output voltage range, just increase supply voltage or use rail-to-rail opamp.

Re: Why is this opamp circuit not amplifying over entire ran

happy2005 said:
Ideal opamp has maximum swing at output that is between -ve and +ve supply voltages. But, real opamps not. So, your circuits has real opamp and output voltage can not reach 3.3V, it stops at ~1.9V.
If you want increase output voltage range, just increase supply voltage or use rail-to-rail opamp.

Output CMR is 0.2 to 3.1 V ...if u look at the specs i gave :!:

Re: Why is this opamp circuit not amplifying over entire ran

Here is a procedure to follow to find more clues.

1. Plot on the same graph the voltages at the amplifier + and - inputs.

2. Rewire the amplifier as a voltage follower (output to - input) and do a sweep of the + input.

3. Rewire the amplifier as an inverting type with the + input biased to half the supply voltage. Plot the sweep graph again.

From these clues we might find the reason.

Re: Why is this opamp circuit not amplifying over entire ran

I think that you don't have big enough current capacity on output. You use 498u to suply feedback, and I don't think that your amplifier can suply more. That's why you enter in saturation mode when you try to raise voltage at output (biger voltage means biger current).
For solution I have to see topology of amplifier.

Regards

amic
I agree with happy2005.
as i know, there are 2 type of opamp [from point of view output topology], full swing (or rail to rail) and normal type.
in normal type maximum output voltage are ≈[VCC - (1.5-3)] Volt.
and minimum output voltage are ≈[VEE + (0-1)] volts.
and they are unbalanced.

regards,
Davood.

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