Troubleshooting Oscillations in an IF Amplifier

In summary, the speaker is experiencing issues with an IF amp that constantly oscillates when the monolithic crystal filter is connected to the base. Despite trying a simple L match, the problem persists and the gain decreases after adding a 3.3 pf cap to provide negative feedback. The speaker suspects improper shielding as the cause and suggests adding a ferrite bead in series with the base of the transistor as a potential solution. However, the speaker does not have schematics and is unsure why the oscillation occurred in the first place.
  • #1
waht
1,501
4
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?
 
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  • #2
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?
 
  • #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.
 
  • #4
Proper sheilding is probaby the issue.
You need to keep the amp output from being radiated back to the input.
 
  • #5
waht said:
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.
 

What causes oscillations?

Oscillations can be caused by a variety of factors, such as external forces, energy input, or feedback loops. It is important to identify the specific cause in order to effectively address the issue.

How do I measure oscillations?

Oscillations can be measured using various instruments, such as a oscilloscope, accelerometer, or frequency meter. The specific measurement method will depend on the type of oscillation and the desired accuracy.

What are the effects of oscillations?

The effects of oscillations can vary depending on the system in which they occur. However, some common effects include instability, reduced efficiency, and mechanical stress. In extreme cases, oscillations can lead to system failure.

How can I reduce oscillations?

Reducing oscillations typically involves identifying and addressing the underlying cause. This can include adjusting the system parameters, adding damping elements, or implementing control measures to counteract the oscillations.

Can oscillations be beneficial?

Yes, in some cases oscillations can be beneficial. For example, they are essential for many biological processes and are used in many technological applications, such as clocks and radio signals. However, excessive or uncontrolled oscillations can have detrimental effects.

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