Why is this opamp circuit not amplifying over entire range?

[amic]

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..?

 

[pseudockb]

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

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

 

[amic]

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

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

 

[happy2005]

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.

 

[amic]

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.

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

 

[flatulent]

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.

 

[zajbanlik]

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

 

[Davood Amerion]

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.
for more information you can refer to internal circuit of your opamp.

regards,
Davood.