Greetings,
I am somewhere between newbie and semi-experienced, just in the are where I become dangerous 🙂
I am working on a (what I thought) simple gain stage for a guitar preamp and am experiencing some unexpected drop in gain as the signal approaches 20 khz. The circuit is a traditional non-inverting opamp with a 50 kOhm pot as feedback resistor, a 2.2 kOhm resistor followed by a 47 uF bipolar capacitor from negative input to ground, as in this schematic.
The idea is that one can vary the gain from 1x to about 22x.
At the maximum value for Rf I am measuring the expected gain, but when I sweep the input signal past approx. 18 kHz the gain starts dropping, all he way down to only 10x or so at 50 khz.
I am using a 2134 and have also tried the 4562 opamp with the same result, and I wouldn't expect slew rate or GBWP to be an issue with these devices.
Does anyone have any suggestions what might cause this? Am I missing something?
Thanks!
I am somewhere between newbie and semi-experienced, just in the are where I become dangerous 🙂
I am working on a (what I thought) simple gain stage for a guitar preamp and am experiencing some unexpected drop in gain as the signal approaches 20 khz. The circuit is a traditional non-inverting opamp with a 50 kOhm pot as feedback resistor, a 2.2 kOhm resistor followed by a 47 uF bipolar capacitor from negative input to ground, as in this schematic.
The idea is that one can vary the gain from 1x to about 22x.
At the maximum value for Rf I am measuring the expected gain, but when I sweep the input signal past approx. 18 kHz the gain starts dropping, all he way down to only 10x or so at 50 khz.
I am using a 2134 and have also tried the 4562 opamp with the same result, and I wouldn't expect slew rate or GBWP to be an issue with these devices.
Does anyone have any suggestions what might cause this? Am I missing something?
Thanks!
What is the meter frequency response? i.e. do a loop-through test without your amplifier.
Is there a capacitor across the pot? You don't show one, but for some folks it is just habit.
Does the top-cut vary with gain?
I could also note that a guitar amplifier rarely benefits from over-over provision of response range. 2Hz is liable to block on thumps; e-guitar "needs" a 5kHz top-cut for string overtone inharmonicity. But that's not what you are asking.
Is there a capacitor across the pot? You don't show one, but for some folks it is just habit.
Does the top-cut vary with gain?
I could also note that a guitar amplifier rarely benefits from over-over provision of response range. 2Hz is liable to block on thumps; e-guitar "needs" a 5kHz top-cut for string overtone inharmonicity. But that's not what you are asking.
Thanks PRR and Mark.
Right now there is no cap across the feedback pot. Initially I had one, but upon noticing the drop in gain I took it out to eliminate one potential issue. Once I figure out the bandwidth issue I'll put that one back in,
I am using a scope, not a meter to measure the output signal swing. The signal generator is flat up to its max 180 kHz, I eliminated that concern already.
Lower gain improves the high frequency behavior, at around 12x gain I don't notice it below 50 kHz or so.
The opamp is a 2134 with 8 Mhz GBP and 20 V/us slew rate. I also tried the 4562 with even higher GBP, same behavior so I don't think it is a device issue.
One thing I noticed: I am actually using two of these stages in series to get about 50db of gain. Each stage is identical and has its own local feedback. I have a 100 ohm resistor from the output of the first stage to the positive input of the second stage. Does the second stage expect something like a 20k resistor to ground at it's input? I was thinking that it already sees the feedback network of the first stage ?
Right now there is no cap across the feedback pot. Initially I had one, but upon noticing the drop in gain I took it out to eliminate one potential issue. Once I figure out the bandwidth issue I'll put that one back in,
I am using a scope, not a meter to measure the output signal swing. The signal generator is flat up to its max 180 kHz, I eliminated that concern already.
Lower gain improves the high frequency behavior, at around 12x gain I don't notice it below 50 kHz or so.
The opamp is a 2134 with 8 Mhz GBP and 20 V/us slew rate. I also tried the 4562 with even higher GBP, same behavior so I don't think it is a device issue.
One thing I noticed: I am actually using two of these stages in series to get about 50db of gain. Each stage is identical and has its own local feedback. I have a 100 ohm resistor from the output of the first stage to the positive input of the second stage. Does the second stage expect something like a 20k resistor to ground at it's input? I was thinking that it already sees the feedback network of the first stage ?