I am trying to design a simple RF amplifier for a
1MHz AM signal (for low-power transmission). My modulator (JFET-based) outputs ~
800mV peak-to-peak. My power supply is 9V, and I want near the largest possible amplification without distortion (class A operation) - my modulator outputs a reasonably-clean sine with next to 0 harmonic distortion, so I might avoid filtering.
This is what I came up with:
As I understand, voltage gain is
Rc/Re which is 6.8 here. That should mean an output of
Vout = 800mV*6.8 = 5.44V, reasonably big and there is still a big room for non-precise Q point setting. I picked those two resistor values based on what I have at hand.
I want to set the Q-point so the output is at
4-5V. That should be at
0.74-0.59 mA. To enable a more precise setting, the lower resistor (
1.7k) is a
As I understand, the BC238 should be capable to this amplification at this frequency, the datasheet has a 85MHz transition frequency at
0.5mA, so it should still have a bandwidth of
f' = f / 6.8 = 12.5 MHz.
I built up this circuit on a piece of copper board I cut lands into with miniature powertool with a diamond cutting bit. I used an old analog signal generator to provide the test signal, set it to
~800mV 1MHz (rather imprecise due to it's analog nature, sadly), and monitored the output with a scope. With a 100MHz 10x probe on the 1x setting I got
~700mV, and on the 10x I got
~300mV. I am not sure what is the real output voltage (
In any case, the resulting signal is smaller than the expected output. I understand that internal capacitances in the transistor start to attenuate the signal at higher frequencies, but this drastic effect was unexpected for me. I feel like I might have overlooked something simple.
I have already tried adding a small (
6nF-1u, the later was an electrolyt cap) cap between ground and emitter. That kind-of fixed the gain problem (both on 1x and 10x probe settings) but distorted the signal highly, the output more resembled a sawtooth than a sine.