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Monday, 20 June 2011

Not low enough?

I've been testing the Low Level Amplifiers from my last post. There is something very strange going on! The first amp, from the RadCom article, seems to have some pretty good gain:

The input signal is at 14.14 MHz and an amplitude of about 300mV, here the output is clearly looking OK and has an amplitude of about 17V pk-to-pk. That's a gain of 20 log (17/0.3) which is about 35dB - exactly on the nail with the design gain. However, as soon as I hook another scope probe on the input to compare the two signals, the circuit seems to oscillate and some of the gear in the shack goes a bit nuts. The signal generator display starts to ramp up and down frequency as does the voltage display on my bench PSU, I suspect there's just too much RF flying about. This is the ouput signal with no scope probe on the input:

and this is the display with both probes attached:
Looking at the second circuit, the one from here:

The whole thing seems much more stable but I haven't got anything like the gain I expected. Also the transistors are running hot, hot, hot!

The output is only about 3V pk-to-pk which is only 20 log (3/0.3) - 20dB gain.

Here are the two circuits as they sit under test:

Confusing, egh?

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