Why the coil of class C amplifier should not needed to tune using trimmer?

[Willen]

Why the inductor (at collector to power supply) of class C RF amplifier (VHF) should not needed to tune using trimmer? Does this inductor need to be very accurate on its value? I think not because the same amplifier can handle different frequencies like 88MHz to 108MHZ without tuning. (inductor of class A amplifier need to tune using trimmer.) Make me clear please!

 

[ronsimpson]

There are many different amplifier designs.

Some have LC resonant on the collector. This needs the trimmer. The gain of this amplifier is high at the resonant frequency and low at other frequencies. It is important to have low gain at 2x, 3x 4x. And low gain at DC.

Some amplifiers have only the inductor on the collector. This is a wide band amplifier. It is easy because there is no adjustment. It is not high quality because it amplifies harmonics. It is not good to transmit on many frequencies. If you are transmitting 10mW at 100mhz then no cares that you are transmitting 2mW at 200mhz. In a radio transmitter that is transmitting 100kW you can not transmit 20kW at 200mhz and 1kW at 300mhz and 2kw at 400mhz etc.

 

[Willen]

...may i know the purpose of these inductors in Class C vhf rf amp?
-Base to ground
-Collector to out
-Collector to power supply

 

[ronv]

Here are a couple of write ups that may help, but basically the base to ground sets the DC operating point (off), but has little effect on the input frequency.

Collector to power supply supplies current when the transistor is off to "fill in" the 1/2 cycle that the transistor is off.

The output filter matches the output to the load and reduces unwanted harmonics.

https://www.electro-tech-online.com/custompdfs/2012/11/322Lecture22.pdf

https://hem.passagen.se/communication/pa313.html

 

[audioguru]

My Mod4 FM transmitter uses a class-A output RF amplifier transistor. Its collector has an LC tank that is a high impedance at resonance but the antenna connected to it is only 75 ohms then the Q of the tank is very low. If it is tuned for a peak at 100MHz then the transmitter still works fairly well at 88MHz and at 108MHz.

A class-C RF output transistor will be the same but it needs a higher input level.