Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

I'm plagued by oscillations.

  1. Dec 6, 2006 #1
    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: Dec 6, 2006
  2. jcsd
  3. Dec 6, 2006 #2

    berkeman

    User Avatar

    Staff: Mentor

    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?
     
  4. Dec 6, 2006 #3
    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.
     
  5. Dec 7, 2006 #4

    NoTime

    User Avatar
    Science Advisor
    Homework Helper

    Proper sheilding is probaby the issue.
    You need to keep the amp output from being radiated back to the input.
     
  6. Dec 7, 2006 #5

    berkeman

    User Avatar

    Staff: Mentor

    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.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?