# 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:

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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