Hi guys,
I bought an op-amp yesterday thinking it was unity gain stable, turns out that it's only stable at gains above 5 (the ADA4637):
http://www.analog.com/static/imported-files/data_sheets/ADA4627-1_4637-1.pdf
So I've spent today trying to stabilize this op-amp for gains below 5, with varied success. I'll explain what I think I understand about uncompensated op-amps first.
Definitions:
Open loop gain - Very high, the intrinsic gain of the op-amp
Closed loop gain - Vin*[1+(Rf/Rg)] for a non-inverting amp
Loop gain - open loop gain * feedback network gain (i.e. Rf/Rg)
At low frequencies, there's no roll-off and no phase shift at the output of the op-amp. As frequency increases, roll-off starts to occur due to a filter within the op-amp (first pole). This causes a 90 degree phase shift from the input to output (is this correct?) and 6dB/octave. As frequencies continue to rise, a second pole within the op-amp kicks in (for want of a better phrase). This creates a 180 degree phase shift between the input and output. If this second pole occurs at a frequency where the gain is less than 1 (i.e. it's being attenuated indefinitely) then there's no problem. However - if this pole is located at a frequency where the gain is greater than 1, you get oscillation (180 degree phase shift, positively amplified).
Now you can compensate the intrinsic instability by using it at a gain of 5 or more, the reason that works is excellently explained here:
https://www.youtube.com/watch?v=Db16d88ZziE
So with our chip, an inverting amplifier of gain 2 would be unstable (Rf = 2k, Rin = 1k say). If I wanted to make that chip stable and keep the low frequency gain below 5, how would I do that? Also, if I wanted to configure this amp as a unity gain non-inverting buffer, how could I possibly compensate that circuit? Does all of this have to do with the loop gain?
Sorry about the vague questions, I'll probably be more specific tomorrow after some sleep. Any help/teaching/guidance is greatly appreciated.
Dan
I bought an op-amp yesterday thinking it was unity gain stable, turns out that it's only stable at gains above 5 (the ADA4637):
http://www.analog.com/static/imported-files/data_sheets/ADA4627-1_4637-1.pdf
So I've spent today trying to stabilize this op-amp for gains below 5, with varied success. I'll explain what I think I understand about uncompensated op-amps first.
Definitions:
Open loop gain - Very high, the intrinsic gain of the op-amp
Closed loop gain - Vin*[1+(Rf/Rg)] for a non-inverting amp
Loop gain - open loop gain * feedback network gain (i.e. Rf/Rg)
At low frequencies, there's no roll-off and no phase shift at the output of the op-amp. As frequency increases, roll-off starts to occur due to a filter within the op-amp (first pole). This causes a 90 degree phase shift from the input to output (is this correct?) and 6dB/octave. As frequencies continue to rise, a second pole within the op-amp kicks in (for want of a better phrase). This creates a 180 degree phase shift between the input and output. If this second pole occurs at a frequency where the gain is less than 1 (i.e. it's being attenuated indefinitely) then there's no problem. However - if this pole is located at a frequency where the gain is greater than 1, you get oscillation (180 degree phase shift, positively amplified).
Now you can compensate the intrinsic instability by using it at a gain of 5 or more, the reason that works is excellently explained here:
https://www.youtube.com/watch?v=Db16d88ZziE
So with our chip, an inverting amplifier of gain 2 would be unstable (Rf = 2k, Rin = 1k say). If I wanted to make that chip stable and keep the low frequency gain below 5, how would I do that? Also, if I wanted to configure this amp as a unity gain non-inverting buffer, how could I possibly compensate that circuit? Does all of this have to do with the loop gain?
Sorry about the vague questions, I'll probably be more specific tomorrow after some sleep. Any help/teaching/guidance is greatly appreciated.
Dan