Class C amplifier for FM - hobby/test circuit

[archjeb]

Folks,

Playing around with building a VHF Class C amplifier and using this schematic as a baseline http://electronics-diy.com/1W_Universal_RF_Amplifier.php

After building this per the schematic, I'm not getting any gain!
If I add some biasing to the base of the transistor with a 20K resistor off the supply V+ side (basically biasing at 0.7v with the diode there), then I'm seeing a 15dBm gain, which is ok (not great) but ok. Transistor gets a little hot...so I probably need a heatsink on it; but at least its amplifying now.

I don't see based on my experimenting and tweaking of the design, how this amplifier will work "as is" on the schematic with no biasing on the base of the transistor? With the minor modification, it works with some gain (15dBm); but without that, I get no amplification.

Any amplifier Elmers out there that could provide some insight and some enhancements to this? I'm surprised that this schematic is scattered around the internet and it doesn't work. Maybe I'm missing something here?

Thanks,

PS> I know this is only going to get you around 1watt here. I'm trying to build a basic transmitter to experiment because I'm trying to go beyond the 'go to Ham Radio Outlet' and buy a radio; but trying to build the pieces from scratch to learn. I have an exciter in front of this with about 20mW of output to generate the FM signal to feed to this.

-J

 

[nomadradio]

A class C amplifier like this will exhibit an abrupt turn-on threshold. Until the input signal at the base of the transistor begins to exceed about 7/10 of a Volt peak-to-peak the transistor will remain switched off. Nothing comes out so long as the input signal remains below this threshold. And this is what defines class C for a bipolar transistor. All the base current comes from partially rectifying the drive carrier. The Base-to-Emitter junction will only have current flow when the drive signal reaches the turn-on threshold of around 2/3 of a Volt. Takes a bit more to get any real drive into the transistor. Usually more like a couple of Volts peak-to-peak.

Adding a source of DC bias current to the transistor base now causes current to flow into the base full time. This in turn causes collector-to-emitter current to flow with or without an input signal. This zero-signal collector current is dissipated in the transistor as heat. This makes the transistor sensitive to lower levels of input signal, but makes it less efficient. Throws more heat for the same level of output than it would without the bias current.

You will only get 1 Watt out of this circuit if you have enough drive level. To make it work with less input power, you need that bias current. And that will reduce the efficiency. The transistor will be working at its max temperature before you reach that 1-Watt level if you use DC base bias.

Cool to see you investigating this amplifier at the basic level.

Rock on!

73

 

[archjeb]

Thanks for the explanation; this makes sense.
I guess I could add a first stage amp to drive the second if I didn't want to bias the base of that transistor to improve efficiency?

What about impedance matching? Should I consider an LC network on the input so I'm around 50ohms? This is assuming that my exciter is at 50 ohms (which it is).

My next phase of the project is to build a low pass filter on the output to clean things up a bit.

Thanks again.

-J