How can I assure that mosfet of second gain stage is in saturation region? (Two stage amp case)

[dkdlek93]

How can I make M6 to operate in saturation region? (Two stage amp case)
And How do I determine D-S voltage of M6?

Figure below shows a case in which the voltage of node 1 is 87mV
so it does not operate in the saturation region.

 

[crutschow]

You need a circuit with DC negative feedback from the output to the bias stage to insure it stays in the saturation region (half-way between the supply voltage and ground, I assume).

 

[dkdlek93]

You need a circuit with DC negative feedback from the output to the bias stage to insure it stays in the saturation region (half-way between the supply voltage and ground, I assume).

Do you mean cmfb?

 

[frankrose]

Simple negative feedback.
OpAmp without feedback network is just a comparator, like above it is used.
CMFB is used in fully differential circuits, above example is single-ended.

 

[BigBoss]

\[ \frac {W}{L} \] Ratio is weird of the PMOS Transistors for given \[V_{DD}\]

 

[crutschow]

Do you mean cmfb?

I mean DC feedback from the output to some point in the MOSFET bias stage that adjusts the bias to maintain the output DC level at 1/2 the supply voltage.
Sometimes a differential stage or opamp is used for that.

If you look at some examples of MOSFET output audio amps you will see how it's done.

 

[sutapanaki]

You want to use this circuit as an opamp. Opams work with negative feedback. More specifically, this is a single ended opamp and you must have a feedback from the output to the negative input. Otherwise you can't even think that you will get a good operating point if you just simulate it open loop. So, the short answer to your initial question is - you can't guarantee that the second stage transistors, each one of them, will be in saturation unless you use feedback. You can actually guarantee with good certainty that if you don't use feedback, one of those output transistors will not be in saturation.

Something else I notice in your circuit - M6 and M7 are not sized quite well with respect to each other. It is good if you can make the ratio of the widths of M7 to M5 2x smaller than the ratio of the widths of M6 to M4. For that, it will be much better if the ratio of M6 to M4 is an even number rather than odd, as it is now.