Common emitter bandidth and probing

by Sasszem   Last Updated August 01, 2020 11:25 AM - source

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:

schematic

simulate this circuit – Schematic created using CircuitLab

Reasoning:

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 10k pot.

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 (700mV or 3V).

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.

PS: 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.

So questions:

  • what is / how to find my real output voltage?
  • how can I increase the maximum working frequency of this amplifier?
  • is there a simple but fatal design flaw in my circuit?


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