I'm plagued by oscillations.

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I'm working on an IF amp and whatever I do it starts oscillating.

A monolithic crystal filter 10.7 MHz, about 330 ohm is fed into a nicely designed npn transistor 20 dB amp, with bypass caps all over the sucker. It is in common emmiter configuration. The amp responds nicely to a test signal, but when I touch one end of the filter to the base, the amp starts oscillating at 10.7 MHz. I have to disconnect it from power to kill oscillation.

I tried a simple L match, but has no affect. So somehow the amp is getting a positive feedback.

Is there anyway to fix the problem?
 
Last edited:

berkeman

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Can you post a picture and the schematic? Is it a kit, a homebrew, a work design, or working with an existing circuit from a TV or something?
 
1,442
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Problem fixed, I put a 3.3 pf cap across the collector and the base to provide some more negative feedback. Oscillation stopped but the gain went down, grrr.

Don't have schematics, it's just a normal amp, I'm just baffled as to why that happened.
 

NoTime

Science Advisor
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Proper sheilding is probaby the issue.
You need to keep the amp output from being radiated back to the input.
 

berkeman

Mentor
54,308
4,717
Problem fixed, I put a 3.3 pf cap across the collector and the base to provide some more negative feedback. Oscillation stopped but the gain went down, grrr.

Don't have schematics, it's just a normal amp, I'm just baffled as to why that happened.
Instead of increasing the Miller capacitance like that, try adding a ferrite bead in series with the base of that transistor. If the transistor is a through-hole device, you can often just string a bead onto the base lead.
 

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