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Using unconditional stable mmic amplifiers in parallel feedback oscillators.

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Terminator3

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Hello! Is it good idea to use amplifier stable mmic as basic element of parallel feedback oscillator?
Good transistors have high frequency capablities, and in wrong design they oscillate regardless of feedback, at some high frequency, i guess where K is far below -1 and some feedback on transistor leads is high. What about mmics?

MMICS has very good features, as:
1. high gain
2. unconditional stable (so frequency determined only by parallel feedback)
3. internally matched
4. cheap

Problems:
1. is it enough to have high gain to start oscillation?
2. some MMICS have 2 or even 3 active devices inside, connected in series, is it ok?
3. many MMICS have no S-parameters, S21 must be found by experiment
 

Manuel Maritescu

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Hello,

I have used in projects for RF design, an Colpitts(Clap) oscillator(434/315 MHz). The schematic containts as basic element a special transistor( P-select)....It has good gain, but is very hard to find a good and stable matching. But it works at aprox -10dBm, and 7mA current consumtion. As i know , using an mmic is too expensive, and there are problems like stability that are the same in both cases.(I think also the mmic has an transistors in the final stage).

So , my sugestion is to use an transistor,to have a good oscillator.

Good Luck

BR
 

biff44

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1. is it enough to have high gain to start oscillation?

Of course not. You need to simultaneously have a round trip gain >0 dB, AND a Round trip Phase shift near 0 degrees. Then when the oscillation occurs and the amplifiers saturate, you need to phase shift to not move too much to make a stable oscillation possible.

Having multiple gain stages cascaded does not stop an oscillation from potentially happening. However, since each gain stage may impart a changing phase shift due to temperature/aging/supply voltage...it does not increase the oscillators repeatability. And since those additional gain stages are probably running fully saturated, they can add significant phase noise to the microwave signal.
 

Terminator3

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nice, i push helped button later, my phone cant do it. thank you all.
 

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