# Why S22 curve is like this

1. ## Why S22 curve is like this

Hi, I am working on a triple stack amplifier. I just baised it, all transistors are in saturation, and wanted to check S22. The resistances are negative. What could be wrong. I am not sure If I can share the netlist outside for this technology. Also Is there any particular method (Like defined formulas) to provide how much bias voltage at the input of all the three transistors (Say M1 is the bottom transistor to which I give signal. Bottom M1 -> M2-> M3 (Top)) such that all of them will be in saturation. I know how to choose the gate voltage at M1 but I am not exactly sure how much gate voltage tp be given to M1 and M2. Thanks

Anyone know any other good book for stacked transistor amplifier other than Millimeter-Wave Power Amplifiers by Jaco du Preez, Saurabh Sinha

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2. ## Re: Why S22 curve is like this

Originally Posted by circuitslave
Hi, I am working on a triple stack amplifier.
I just baised it, all transistors are in saturation,
and wanted to check S22.
The resistances are negative.
What could be wrong.
Not wrong at all.
Simply due to Y12.

Calculate Yout by hand from your small signal model of MOSFET.

3. ## Re: Why S22 curve is like this

I want to have positive resistances. I am wondering how to make it possible

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4. ## Re: Why S22 curve is like this

You amplifier may be unstable..

5. ## Re: Why S22 curve is like this

Originally Posted by BigBoss
You amplifier may be unstable..
Yes it is, but If I have to keep degenerative inductor it needs to a huge value. I tried with small inductance values but still it is unstable.

6. ## Re: Why S22 curve is like this

Originally Posted by circuitking
Yes it is, but If I have to keep degenerative inductor it needs to a huge value. I tried with small inductance values but still it is unstable.
There are methods to stabilize an amplifier such as connecting a resistor at input and/or at output.A negative feedback may also stabilize the amplifier..

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7. ## Re: Why S22 curve is like this

Originally Posted by circuitslave
I want to have positive resistances.
I am wondering how to make it possible
Surely learn and understand following.