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Help: How to decrease mag(S22) when mag(S22)>1

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Hi, all

I'm designing a low noise amplifier working @5GHz, however, the stability is a big big problem to me, can anyone suggest me something to read??

And, The magnitude of S22 of the amplifier is larger than 1, how am I gonna do??please help me out!!

Thank you.
 

This book gives a good idea of handling stability problem. You can find this book on this website.

Practical RF Circuit Design for Modern Wireless Systems Vol. 2 - Active Circuits and Systems
 

stability circles, K factor, max stable gain,... look at the data sheet and read up on amp design. If all else fails, trade off NF and gain for stability with feedback or lossy components.
 

"how am I gonna do??"

Quick, build a 5 GHz oscillator!

If you really want an amplifier instead,
1) make sure you have a good ground connection simulated (multiple vias, no dc stabilizing resistors between device and ground, etc)
2) try putting series resistors or shunt resistors at either the devices input, output, or both. The resistors might limit the impedance the device sees, and keep that impedance out of the region that generates an S22>1.
 

have you tried putting an inductor in the emitter to increase stability?

At that freqeuncy you normally use the transistor lead inductances themselves.

Hope this helps.

Spook
 

Plot the stability circles and then find the nearest circle to the center of the smith chart and then add either series or parallel resistance that will have a value whic cuts the vicinity of the circle...
This is most basic method to obtain stability.
In that case , the vicinity of the neares circle twon't be smaller than resistance value that you have already added.
 

Hi,

you can add resistance (or inductance) in series with emiter (source). This effectively build up feedback that can be adjusted to make device stable. Feed back can be made frequency selective by adding shunt/series capacitors to this source reactance, so you can adjust device stability only in the band of interest.

flyhigh
 

to bigboss
could you explain your method to improve output stability in detail??? i need it very much! thanks!!
what range S22 lie in is reasonable??
 

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