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question regarding microwave amplifier stability

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Member level 3
Jun 3, 2002
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I'm trying to design an amplifier for maximum gain. The problem is that the amplifier is not stable at the design frequency which is 5.5 GHz. It is only stable from 1.4 to 4.0 GHz. The transistor operates from 100 MHz to 6 GHz.
I managed to stabilize it in the entire range except at low frequency (from 100 to 400 MHz). I designed alot of matching networks and still not stable at low frequencies. I was using resistors to load both the input and output or transmission lines with stubs.

Do you think that the transistor is suitable to be an oscillator or what?



This is a common problem

You need to draw the stability circles on a Smith Chart to see what load and source impedances will work. This should be done at all frequencies. The utility program AdLab from has a two port program that does this from the s parameter file for your transistor. It is well worth paying the shareware fee to get the additional features on the two port program which will allow the effects of feedback in these calculations. You can run the program for free for something like 20 times before it quits totally.

It will also tell you what is the maximum stable gain and other things. The usual way to stabilize a transistor stage is to put a resistance from output to input to reduce the gain sllightly. You also have to put a capacitor in series with it to not short out the bias. It is also helpful to put a small inductor in series to remove the feedback at higher frequencies. You can cut and try the values of these components to make the stage unconditionally stable at all frequencies.

I haven't heard that there have special transistors specially for oscillators because it is not stable outside its target frequency band.

What is your applications - 100MHz to 6GHz? Different application has different method for stabilization.


yes, you have to plot the stability chart in smith chart to check whether the input or output that causes such oscillation. once determine the culprit, use resistor loading technique at that port with suitable filter to kill the oscillation.

Use the proper matching network (a high pass in this case) so there is no gain at the low frequency.
Just because the K is unstable does not mean the amp will oscillate.
Remember that *not un-conditionally stable* does not mean *un-conditionally un-stable*.


If I understand well, your design freq. is 5.5 GHz and your design is not stable in the freq range of 100 - 400 MHz.
In this case the problem freq. range is about a decade lower than the design frequency. In this case it is very easy to "place" resistors (shunt or series) to get the design stable in a manner that in practical terms the resistors does not affect the one decade higher 5.5 GHz range.

thanks for you all.
The problem is that i cannot use resistor for stabilization because the professor told us they don't have resistors for GHz applications.

Resistors ?

I have not understood if the requirement of the not available resistors is a must from the professor or if it is due to the fact the resistors are not phisically existing in your workplace.
If it is the second choice, use simply 0805 SMD resistors, they are good until about 6-7 GHz.


ayhz2002 said:
thanks for you all.
The problem is that i cannot use resistor for stabilization because the professor told us they don't have resistors for GHz applications.

Seems that there have some different views between school and practical case. There won't be any resistors or even some chip capacitors mentioned that it can be used in about 6GHz. But it is really the case, this component has already been used thousand of times. The most important point should be the RF layout to minimize the lead inductance or other stray effect to made in predictable manner.

Use an inductive feed back on the emitter (source) side of your amp.
Increasing the inductance on the emitter (source) will stabilize the low frequency portion of your amp.
This can be done using an actual inductor or simply increase the length of the GND trace of the emitter (source). Too much inductance will make the amp unstable at high frequency.


i dont know the circuit of your design, but i think you could decrease the q factor of your bias circuits.

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