Solving the Mystery of Unstable Amplifier Performance

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Discussion Overview

The discussion revolves around the performance issues of an RF amplifier (AMMC-6220) that exhibits gain spikes at high frequencies. Participants explore potential causes, including the amplifier's stability, input matching, and the overall system configuration, while seeking advice and theories to resolve the issue.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes the occurrence of gain spikes at specific high-frequency locations, which vary in amplitude between different units of the amplifier.
  • Lowering the voltage to the amplifier eliminates the spikes, while increasing it exacerbates the issue, despite the bias voltage being within specifications.
  • Another participant suggests that the amplifier may be unconditionally stable, but the overall system might not be, indicating a potential interaction causing the spikes.
  • There is a proposal to conduct an S-parameter sweep to analyze the system's response and identify any oscillations or resonances that could be contributing to the problem.
  • Participants discuss the importance of isolating the power supply and ensuring the stability of the RF device feeding the amplifier.
  • One participant notes the need to check how other S-parameters are impacted and questions the stability of the RF device itself.

Areas of Agreement / Disagreement

Participants express varying opinions on the cause of the gain spikes, with some suggesting the amplifier's stability while others point to the overall system configuration. No consensus is reached on the exact source of the issue.

Contextual Notes

The discussion includes references to specific measurements and configurations that may not have been fully detailed, such as the complete setup of cascading amplifiers and the effects of various components in the signal path.

lennybogzy
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Gentlemen and ladies,

I have an RF device and an in-line amplifier (specifically an AMMC-6220 - http://www.datasheetcatalog.com/datasheets_pdf/A/M/M/C/AMMC-6220.shtml). Certain units develop gain spikes at the high-freq band. They are in very specific frequency locations, and vary in amplitude from unit to unit.

Lowering the voltage to the amp eradicates the spikes. Raising it has the opposite effect. The bias voltage is well within spec.

Another thing: playing with the input match to the amplifier has an effect on the spikes. However, the AMMC-6220 is (supposed to be) an unconditionally stable amplifier. This shouldn’t happen, assuming that what I’m seeing is oscillation.

I’m stumped, and can’t do much except keep replacing this relatively expensive amplifier. The manufacturer doesn’t know what’s going on, nor do engineers a lot smarter than I. I can’t even seem to isolate the problem to a bad lot of amps. Any advice, theories, ideas, words of emotional support, would be appreciated.
 

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lennybogzy said:
Gentlemen and ladies,

I have an RF device and an in-line amplifier (specifically an AMMC-62220). Certain units develop gain spikes at the high-freq band. They are in very specific frequency locations, and vary in amplitude from unit to unit.

Lowering the voltage to the amp eradicates the spikes. Raising it has the opposite effect. The bias voltage is well within spec.

Another thing: playing with the input match to the amplifier has an effect on the spikes. However, the AMMC-6220 is (supposed to be) an unconditionally stable amplifier. This shouldn’t happen, assuming that what I’m seeing is oscillation.

I’m stumped, and can’t do much except keep replacing this relatively expensive amplifier. The manufacturer doesn’t know what’s going on, nor do engineers a lot smarter than I. I can’t even seem to isolate the problem to a bad lot of amps. Any advice, theories, ideas, words of emotional support, would be appreciated.

Could you post a link to that AMMC-62220 amplifier? I'm having trouble finding it with Google. What is the RF device that feeds the amplifier? Where does the output of the amp go? If you go straight into the input of the spectrum analyzer with coax and no other feeds to other devices, do you still get the same results?
 
Thanks for the reply and I apologize for the typo. It's an AMMC-6220 and the datasheet can be found here http://www.datasheetcatalog.com/datasheets_pdf/A/M/M/C/AMMC-6220.shtml

The problem is the amp is part of a series of cascading amps, so I would have to tap directly onto the trace after this amp. There are a few attenuators, an equalizer, some diodes, some other amps and various filters along the way. I will have to give more details on Monday.
 
Any way you can get an S-parameter sweep?
 
attached to original post
 
The amp may be unconditionally stable but your total system may not be.

I can't see the frequency range on your plot, but I'd hook up a DC to 30ghz spectrum analyzer and look for an oscillation that depends on the bias voltage.

The second spike on your chart looks to me like what happens when the network analyzer sweeps past a resonance.

You need to set up your analyzer to do a gain-phase plot and start looking for ways that the output is leaking back to the input or bias supply.
 
lennybogzy said:
attached to original post

My bad, didn't see that. Is this a sweep of the rf device and the amp? If that's the case, is your rf device stable? Also, how are the other S-parameters impacted?

I'm going to agree with Antiphon, and I definitely would check on the isolation of your power supply/amps/inputs.
 

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