problems for differential amplifier

Hello everyone, I'm new for circuit design.I meet some problems and hope someone could help me with them.
Now I am planning to design a readout circuit for microbolometer(thermoelectric device).

For the first stage,a symmetry MOS bridge as constant current is choosed to bias two microbolometer(same parameters)
and generate two differential voltage as one of them absorbs heat.
then the two voltage is amplified by the simple differential MOS amplifier
next,the output current is amplified and integrate

so far, the simulation result for every single stage is fine but after combining the three stages,
an offset appears and makes some MOS out of saturation region.
the voltage for bridge and differential pair is not symmetry even there's no thermal absorption
how could I do ?

I know one solution is to connect a symmetry load to the opposite side of differential pair(dummy load)
but not sure what actually works.

I've also considered using constant current buffered direct injection (CCBDI) configuration.
that is, another MOS differential pair to increase the circuit symmetry and achieve feedback mechanism
but I feel difficult to give the W/L ratio for each MOS especiallly the dummy differential pair.

As I know, differential amplifier allows only very small differential input voltage,
but how does the output of differential amplifier 1 as the input of differential amplifier 2?
how does it work and all keep all MOS in saturation region ?

lots of words is written because I attempt to describe more detail for my situation

Thank you

Anything you can do to use symmetry on differential output, such as loads, current mirrors, CCBDI, will help reduce offsets.

Use a balanced Wheatstone bridge config with an Instrument Amplifier IC. They come in all kinds, but are designed for small signals, low offset, high impedance.

SunnySkyguy said:

Anything you can do to use symmetry on differential output, such as loads, current mirrors, CCBDI, will help reduce offsets.

Use a balanced Wheatstone bridge config with an Instrument Amplifier IC. They come in all kinds, but are designed for small signals, low offset, high impedance.

Click to expand...

But if I need to design a circuit for CMOS process and apply real IC from foundry,
how could I do?

thank you

- Calculate realistic offset voltages.
- Design feedback amplifiers as found in analog design text books.

For lowest offsets, you may want to refer to autozero/chopper amplifier concepts.

FvM said:

- Calculate realistic offset voltages.
- Design feedback amplifiers as found in analog design text books.

For lowest offsets, you may want to refer to autozero/chopper amplifier concepts.

Click to expand...

thank you so much