Solving the Mystery of Unstable Amplifier Performance

In summary: If you're not getting oscillations with just the amp, you might have to go back and check all of those other parts as well. Attached is a plot of the gain-phase response of an AMMC-6220 connected to a network analyzer. The oscillations can be seen when the analyzer sweeps past a resonance.
  • #1
lennybogzy
94
0
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.
 

Attachments

  • gainspikes.JPG
    gainspikes.JPG
    34.9 KB · Views: 460
Last edited:
Engineering news on Phys.org
  • #2
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?
 
  • #3
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.
 
  • #4
Any way you can get an S-parameter sweep?
 
  • #5
attached to original post
 
  • #6
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.
 
  • #7
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.
 

What causes an amplifier to have unstable performance?

An amplifier can have unstable performance due to several factors, including incorrect circuit design, poor component quality, thermal effects, and electromagnetic interference.

How can I identify and troubleshoot unstable amplifier performance?

To identify and troubleshoot unstable amplifier performance, you can use a variety of tools such as oscilloscopes, spectrum analyzers, and signal generators. You can also perform a thorough inspection of the circuit and its components, check for any loose connections or damaged components, and eliminate any potential sources of interference.

What techniques can be used to improve the stability of an amplifier?

Some techniques that can be used to improve the stability of an amplifier include adding compensation networks, using bypass capacitors, increasing the value of feedback resistors, and improving the grounding and shielding of the circuit.

Can unstable amplifier performance impact the overall performance of a system?

Yes, unstable amplifier performance can greatly impact the overall performance of a system. It can cause distortion, noise, and signal loss, which can affect the accuracy and reliability of the system's output.

How can I prevent unstable amplifier performance in future designs?

To prevent unstable amplifier performance in future designs, it is important to carefully select high-quality components, ensure proper circuit design and layout, and perform thorough testing and troubleshooting before finalizing the design. It is also essential to consider factors such as thermal management and electromagnetic compatibility during the design process.

Hot Threads

Recent Insights

Back
Top