need some help with understanding and designing op amp

[savulescucostin]

Hello everybody,

I am asking you for some help with the understanding and designing a diferential operational amplifier with 2 amplifying stages. The first stage must be diferential and use nMOS transistors and the differential stage must use a folded cascode. I actually designed the circuit with some help from someone and I have attached it, but I need to fully understand it and answer questions about it. The circuit must also fulfil the following specifications:

-first stage: differential with nMOS transistors
-differential stage load must be of folded cascode type
-amplification must be 90dB
-band B must be 30MHz
-Miller compensation for stability must be with 60 degree phase reserve
-PSRR more than 80 dB
-capacity(load) used must be C=5pF
-maximum 3.3V power supply



So if someone could please describe in detail the circuit that I have uploaded, where is the Input and Output, what is the use of all the stages, how does the circuit actually work and how do I fulfil all the specifications.
I thank you in advance, all the best!

 

[LvW]

You have "designed the circuit" without knowing where the input and output is?

 

[savulescucostin]

I wasn't the one who designed it actually, somebody did it for me but without any explications.

 

[dkace]

Well,
let's start from basics:

What is a "first stage" in an OpAmp?Why we use two identical MOST instead of one?

Can you reply on this?

Then we will see where it really operates or not.
D.

Added after 2 minutes:

Further more, on this design there are a couple of mistakes that makes it nonoperational. But let's focus to the first step.

What we need the OpAmp for?

 

[invaforce]

-first stage: differential with nMOS transistors
-differential stage load must be of folded cascode type
-amplification must be 90dB
-band B must be 30MHz
-Miller compensation for stability must be with 60 degree phase reserve
-PSRR more than 80 dB
-capacity(load) used must be C=5pF
-maximum 3.3V power supply



I think He is just kidding

 

[savulescucostin]

I don't really know, the task said that the op amp should have 2 aplification stages.
I actually need this for a school project, so it's basicaly theoretical, I must simulate this circuit with a modelling software called Mentor if you are familiar with it. So one of the teachers gave me that scheme and I must know to explain it and answer questions about it.

 

[dkace]

So, as I post previously, you need to understand what an OpAmp is.

Do you understand why we choose a differential stage instead of a single stage?

D.

 

[savulescucostin]

Not really. Please explain

 

[dkace]

OK, let's try :

AN Operational Amplifier is a differential input amplifier.

That means that we need to Amplify the DIFFERENCE between two signals.

If you follow the OpAmp theory you will find several topologies ( non inverting, inverting, adder, integrator etc) that are based to this difference amplification.

Next thing to investigate is which branch is the inverting and which is the non inverting and why. Can you answer this one?

 

[dkace]

Start from a single amplifier;

A Transistor ( MOS or BJT) is Gate/Base biased in order to amplify the input signal
in the Gate/Base.

Amplification output is taken out from the Drain of the MOS or collector for the BJT.

Understand by solving the transistor equation that describe its behavior why this is happening.

Then understanding the differential stage of the Opamp is easy.

D.