power level of unmodulated vs modulated carrier

[ssbothwell KJ6RSG]

Hi guys. while doing some series modulation tests (which i just posted about in the QRP AM challenge thread) i decided to try amplifying a low level modulated signal with a 100mW AB amplifier stage i have been working on.

i used an HP3312A function to produce and modulate the carrier signal. when i insert an unmodulated carrier into my amplifier stage i get a nice 2.25VRMS (100mW) output. if i turn on the modulator function on the function generator then i get a nice looking modulated signal but the output drops to 1.29VRMS (33mW).

the amplifier draws about the same amount of current when the signal is modulated (160mA when unmodulated and 170mA when modulated) yet the output power is reduced significantly.

if i turn up the amplitude on the modulated carrier to try to compensate then the waveform starts clipping at around 1.8VRMS.

i then tried skipping the amplifier and just looking at my function generator output on the 'scope. if i set the carrier to 1VRMS and then turn on the modulation function the output drops down to 530mV.

why does this happen and will it always be the case that an amplifier will produce a weaker carrier when it is modulated?

[W7TFO]

Your amplifier needs to be able to make 4 times the unmodulated carrier output to handle a fully modulated carrier in linear service.  There is a LOT of power in those sidebands!

Sounds like your amp is just run out at the requirements.

73DG

[kb3ouk]

With a linear amplifier, as you add modulation to the signal the efficiency increases. The amp is running at it's lowest efficiency when amplifying a dead carrier and at it's highest when amplifying a modulated signal. The input power will remain constant, whether the signal has modulation or not. What changes is the amplifier's efficiency. But the input has to be high enough to allow for enough power to amplify a fully modulated signal. So if you want your amp to amplify a carrier to 100 mw, and have enough power to amplify the modulation you put to that carrier, which would be a total output of 400 mw, the amp has to have an input of around 500 mw. That would give it a 20% efficiency at dead carrier and 80% at full modulation. If your amp was designed to put out no more than 100 mw, then that's why the carrier reduced with modulation. It needs headroom for the peaks so the only way for that to happen is to have a low carrier.

[ssbothwell KJ6RSG]

thanks for the explanations. i had heard that amplifiers can only run out 1/4 the output when amplifying an AM signal but i thought it was versus SSB not CW.

in my experiments i have only recently gotten to the point where i have been attempting to modulate signals and amplify them so this is the first time i have experience the reduction in power first hand.

could you guys recommend a suitable transistor for 400mW output in class A or AB and close to 27dB of gain? could i use an IRF510?

[WA1GFZ]

When you look at an unmodulated carrier on a scope then apply 100% modulation the modulation peak will be twice the voltage of the carrier and go down to zero at the negative peak. Since the voltage is double and the output Z is constant the current also doubles so that is why the amp has to make 4 times the power. Yes the efficiency goes up at the positive peak.