Design a FET-Based Amplifier

[biomed_dude]

I need to design a FET-based amplifier with three stages. The first stage must be a differential amplifier with an active load and simple current source. The second stage must be a Common Source Amplifier with the third stage being a Push-Pull Amplifier. How can I design this to get a CMRR > 80 dB and a GBW of >10^5?

 

[BobK]

Buy an opamp for about a quarter.

Bob

 

[biomed_dude]

It's a class assignment to design our "own" op amp.

 

[BobK]

I thought so. So this thread should be moved to "homework help".

Bob

 

[(*steve*)]

Firstly, do you understand what these three stages are?

Then perhaps you need to consider what is going to have the major effect on the CMMR (which stage)?

What will affect the CMMR in this stage, and how can you maximise it?

Next look at the GBW (do you know what this means?) What will affect this and how do you maximise it?

Is there any conflict (i.e. are the changes to maximise one negatively impact the other)?

 

[biomed_dude]

I have implemented a design (with the first two stages) like this using BJT's but am a lot less familiar with FET's. I am also unfamiliar on how to design a push-pull amplifier using FET's.

 

[(*steve*)]

Push-pull with mosfets is a little more tricky than with BJT.

See here.