Question about Fmax (max oscillation frequency)

[Joe]

Hello, I have a quick question about the maximum oscillation
frequency.

I was reading earlier in a paper that the maximum oscillation
frequency of a transistor is defined as the frequency in which the
powergain is 0 (i.e. input power = output power).

I was just wondering why a transistor was considered oscillating if
it's output power is less than it's input power?

I've read many papers explaining the Rollett stability factor (K) and
the Available Power Gain (Ga) but nowhere have I found an explanation
of why I transistor is "considered" oscillating below fmax.

As well does anyone know of a decent paper or resources that can
explain this phenomenom to me?

Thanks in advance

 

[Jeffrey Turner]

Joe wrote:

Hello, I have a quick question about the maximum oscillation
frequency.

I was reading earlier in a paper that the maximum oscillation
frequency of a transistor is defined as the frequency in which the
powergain is 0 (i.e. input power = output power).

I was just wondering why a transistor was considered oscillating if
it's output power is less than it's input power?

I've read many papers explaining the Rollett stability factor (K) and
the Available Power Gain (Ga) but nowhere have I found an explanation
of why I transistor is "considered" oscillating below fmax.

As well does anyone know of a decent paper or resources that can
explain this phenomenom to me?

Thanks in advance

The power gain goes down with frequency, so you have it backwards, the
output power is above the input power at lower frequencies. If the
transistor won't work as an amplifier it can't oscillate as any
oscillations would eventually damp out. You can get stable
oscillation at the unity gain frequency, unstable oscillation below it
and damped out oscillation (that will die out) above that frequency.

--Jeff

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