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Help needed - Unstable amplifier

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mateusbatera

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Hi there,

I purchased this antenna amplifier, which is supposed to work between 1MHz and 800MHz. I made some stability tests connecting the amplifier output to a spectrum analyzer and leaving the input open. It turned out that I see a strong peak around 700 MHz, meaning that the amplifier is oscilating. Varying the gain control causes a small frequency shift in the peak. Any ideas of what is causing the oscilation? What is the best way to track the root of the problem?
 

FvM

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Any ideas of what is causing the oscilation?
- bad design
- defective part or assembly fault

I guess you know about the principle ways to turn an amplifier into an oscillator?

Did you check, if it's still unstable with an input load, either 50 ohm termination or an antenna wire?
 

mateusbatera

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I guess you know about the principle ways to turn an amplifier into an oscillator?
All I know is that oscilation occurs when there is some sort of positive feedback in the circuit. However, I don't know what I need to do to avoid this behavior.
 

FvM

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I don't know what I need to do to avoid this behavior.
The standard measure is to return the device to the vendor. For the limited options you have to use it anyway, I already made a suggestions.
 

LvW

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...................
I made some stability tests connecting the amplifier output to a spectrum analyzer and leaving the input open. It turned out that I see a strong peak around 700 MHz, meaning that the amplifier is oscilating.
Why did you test with an open input? Try to terminate it correctly or simply use it as desired (antenna connected) and check if it operates correctly.
 

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