unity gain differential amplifier output

[simplsoft]

Hello

I am simulating a fully differential OTA. The setup is shown in the figure with the output wave form. I have connected the op-amp in unity gain negative feedback but the circuit is showing a gain. I have applied 200mV differential input. the output should also be 200mV but I am getting 400mV. I cant understand whats wrong with my circuit. Anyone who can help me in this?





In output waveform red one is the input and yellow is the output.

 

[frankrose]

Where is the feedback of the 1st stage? Check the output signal of that.

 

[ZoOneR]

Hi,
Your input is not 200mV, it's 400mV. You have 200mV on Vip and -200mV on Vin. Since this is a differential input Vip-Vin is 400mV.
If the load is the OTA to be tested connected in unity gain feedback path (this means the load acts as a resistor of value 1/gm), the the output voltage will be: Vo=gm*Vin/gm=Vin. So the first circuit doesn't need a feedback path.


Sorry, I did a mistake. I thought that the 200mV comes from the Vsin sources, but it comes from the DC source.
We can't tell what's wrong in this case, unless you show us how the OTA schematic.

 

[simplsoft]

When I am simulating a single OTA I am getting 410mV at the output when I am using it with negative feedback (two OTAs) I am getting 770mV at the output of first OTA why is it so I couldnt understand. My supply voltage is 800mV and DC output voltage should be at the mid of the supply voltage (400mV) to get maximum swing. But when I am connecting it with second OTA I am getting 770m V at the output instead of 410mV.

 

[BigBoss]

Current consumption 185.2nA .. ??

 

[simplsoft]

Yes. I am operating the transistors in weak inversion

 

[Audioguru]

Your schematic and waveform images are negatives. Then I cannot see them unless it is dark in my computer room. Instead why not make then normal positives?