Colon
Member level 3
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hi guys,
So, I have an opamp circuit that has been having problems when I drive it with a large signal. I have stripped away a lot of the circuit to it's simplest state but the problem remains. What I now have is basically an ac coupled non-inverting stage (using a THS4031 opamp), powered from 24V. The non inverting input is biased with a low terminating resistor (300R in this case, AC coupled to ground with a 10uF cap) to mid rail (voltage produced by a resistive divider). I have removed my usual output load (which was a push pull stage) and replaced it with a 10k resistor to simplify things.
When I have a large input signal (peak voltage approx 20V) the output suddenly distorts massively, while the input bias current increases dramatically, pulling the input DC level down from mid rail by a few volts.
I suspicion was that I nearing the common mode input voltage limit. I have read that going near this limit results in increased input bias current and sudden phase inversion, but I'm at least 2V away from that. What's stranger still is that I attempted to limit the input voltage to 1V below the point at which I see this effect occur, via diodes connected to the appropriate voltages. The waveform clipped early as expected but the problem simply occurred at a lower voltage instead!
Upping the supply to 30V just moved the point at which the problem occurs up to around 24V, far from the rail.
I am really stumped now as to what is going on. Everyone else I work with, some with much more experience than me in opamps, are stuck too. Hoping someone on here has seen this before![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
Thanks
James
- - - Updated - - -
Is this maybe a case of large signal instability?
So, I have an opamp circuit that has been having problems when I drive it with a large signal. I have stripped away a lot of the circuit to it's simplest state but the problem remains. What I now have is basically an ac coupled non-inverting stage (using a THS4031 opamp), powered from 24V. The non inverting input is biased with a low terminating resistor (300R in this case, AC coupled to ground with a 10uF cap) to mid rail (voltage produced by a resistive divider). I have removed my usual output load (which was a push pull stage) and replaced it with a 10k resistor to simplify things.
When I have a large input signal (peak voltage approx 20V) the output suddenly distorts massively, while the input bias current increases dramatically, pulling the input DC level down from mid rail by a few volts.
I suspicion was that I nearing the common mode input voltage limit. I have read that going near this limit results in increased input bias current and sudden phase inversion, but I'm at least 2V away from that. What's stranger still is that I attempted to limit the input voltage to 1V below the point at which I see this effect occur, via diodes connected to the appropriate voltages. The waveform clipped early as expected but the problem simply occurred at a lower voltage instead!
Upping the supply to 30V just moved the point at which the problem occurs up to around 24V, far from the rail.
I am really stumped now as to what is going on. Everyone else I work with, some with much more experience than me in opamps, are stuck too. Hoping someone on here has seen this before
Thanks
James
- - - Updated - - -
Is this maybe a case of large signal instability?