Power control of Class D Rf AMP

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The discussion centers on designing a Class D low power RF amplifier that can control RF output without altering the input signal, specifically aiming for 200mW at 50 MHz. Participants clarify that Class D amplifiers are not typically used at RF frequencies and that controlling output power usually requires pulse width modulation or varying the supply voltage, which impacts efficiency. The conversation shifts to exploring alternative RF amplifier topologies that minimize power waste while achieving the desired output, with Class C amplifiers being suggested for their efficiency in handling continuous wave signals. The complexities of the input signal, which is a digitally modulated data stream, further complicate the design, as it cannot be modified. Ultimately, the need for an efficient RF amplifier that accommodates the specific modulation scheme without introducing distortion remains the primary focus.
  • #31
Okay, so why do the architects of the system I'm suffering with use the term class D?
There is a fixed 50 MHz source that is being turned on and off (OOK) at the data rate. I have seen the schematic on a white board. Its two or four fets in series on a die whose power supply is controlled by the programmable switcher I told you about... I've heard the term Class D power osc flung around as well...
 
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  • #32
Probably because they started with a square wave generator which technically would be class D and filtered the hell out of it. Class C is closely related to class D but C is made to run at a resonant frequency. There is a parallel tank circuit in place of the collector resistor. I have no idea why they would run all the way out to the last amplifier stage with a square wave. Good engineering practice would be to filter as soon as possible in the signal chain. I guess I wouldn't be too surprised if what you have is actually class C but for whatever reason is mis-named.
 
  • #33
So you have a On-Off-Keyed 50MHz signal switched on and off at 1MHz and you want a adjustable gain amplifier that is as efficient as possible to step up the power?

A fixed gain amplifier won't work for you because you have found it necessary to vary the output of your current amplifier by adjusting its power supply voltage, correct?

I'm looking at the diagram for the Class C amplifier on wikipedia. Would varying the value of the collector resistor change the gain of that circuit?
 

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