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# Class-C amplifier, how it behaves under the presense of many signals?

#### neazoi

HI, I have a simple BJT class-C amplifier in mind, which is able (as a class-C amp) to amplify only the peak points of a sine signal, when they reach a certain level at the BJT base.

If a powerful input signal appears at it's base it should amplify it's edges (peak points). The LC in the collector usually restores the remaining waveform from these amplified peaks.
At the time interval when the peaks are amplified, what will happen to such an amplifier if a second lower level signal is presented at it's input?

Will this low level signal amplified along with the high level one (the one that "triggers" the BJT at these time intervals)?
Will this low level signal not amplified at all, no matter if the BJT is "triggered" by the high level one at these time injtervals?

Depends on the Phase relationship between those two signals.

### neazoi

Points: 2
Depends on the Phase relationship between those two signals.
Interesting. Please explain that a bit.
Do you mean that if the two signals are in the same frequency and of opposite phase, the low level one will be canceled?
Do you mean that if the signals are on different frequency, the relative phase of one to the other has some effect on that?

I am interested in signals that are both in the same and at different frequencies.

What will happen to that low level signal, at the time the class-C amplifier is "triggered" by the peaks of the high level signal? Will it be cancelled or amplified at that small time interval or not?

You are basically asking one signal to switch the other signal on or off depending on it's amplitude so you have a crude mixer. The output will be a mixture of the two (or more) signals and exactly what comes out will depend on their amplitudes, frequency and phases. Class-C will inevitably produce distortion and for low level signals be inefficient as it uses signal power to provide bias current instead of getting it from the power supply.

Brian.

### neazoi

Points: 2
You are basically asking one signal to switch the other signal on or off depending on it's amplitude so you have a crude mixer. The output will be a mixture of the two (or more) signals and exactly what comes out will depend on their amplitudes, frequency and phases. Class-C will inevitably produce distortion and for low level signals be inefficient as it uses signal power to provide bias current instead of getting it from the power supply.

Brian.
All right, perfect, thanks Brian.
I use the class-C amp as a crude way to amplify high level signals and (up to now as thought) to ignore low level ones.
In practice it works, but if you connect this to an antenna and you do not limit the gain, it produces all shorts of distrortion, essentially noise.
If you have an input preselector, things are much more usable and you have to increase the gain much more to notice distortion. Essentially you limit the input signals to the amplifier that way and you do not allow it to distort that easily.

Up to now, I have not found another practical way to differentiate high level signals from low level ones on HF.
Any ideas (you know I like discrete if possible) are appreciated!

Interesting. Please explain that a bit.
Do you mean that if the two signals are in the same frequency and of opposite phase, the low level one will be canceled?
Do you mean that if the signals are on different frequency, the relative phase of one to the other has some effect on that?

I am interested in signals that are both in the same and at different frequencies.

What will happen to that low level signal, at the time the class-C amplifier is "triggered" by the peaks of the high level signal? Will it be cancelled or amplified at that small time interval or not?
If those two signals are at same frequency, the phase relationship will play a role.
The signals are at different frequencies and if you combine two signals at a common node, this will be amplified unless the signal level rest in below the threshold of C-Class amplifier.
This is a time domain problem.

Points: 2