Hello everyone 
I have a question about opamp dc servo with power amp.
Here is the circuit I usually use.
I have tried several opamp models.
This works well with opa604 and opa132.
But not with opa1641 and ada4522-1. (High DC at output)
[I cannot remember if I tried opa134 too. If I did, it was not working too.]
The blue words are the voltages when it is not working using ADA4522-1. [under +-23V supply]
I have read some post in the forum and understand that this circuit needs a JFET input opamp.
while opa604 and opa132 are JFET opamp and ada4522-1 may not be so, opa1641 is JFET opamp but still not work properly with this circuit.
Are there any more criteria besides JFET input?
Do I need to change anything if I use 10K in all R1 R2 R3 R5? (as preamp)
Thank you.
I have a question about opamp dc servo with power amp.
Here is the circuit I usually use.
I have tried several opamp models.
This works well with opa604 and opa132.
But not with opa1641 and ada4522-1. (High DC at output)
[I cannot remember if I tried opa134 too. If I did, it was not working too.]
The blue words are the voltages when it is not working using ADA4522-1. [under +-23V supply]
I have read some post in the forum and understand that this circuit needs a JFET input opamp.
while opa604 and opa132 are JFET opamp and ada4522-1 may not be so, opa1641 is JFET opamp but still not work properly with this circuit.
Are there any more criteria besides JFET input?
Do I need to change anything if I use 10K in all R1 R2 R3 R5? (as preamp)
Thank you.
Attachments
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First remove the connection between R3 and C1.
The lower end of C1 is not grounded.
Thank you for you reply!
Ada4522-1 input resistance for differential mode is just 30KR so it doesn’t work.
What should be the cap value if I use 1mohm?
And what are the differences with different values?
I have checked opa1641 input impedance is 10^13ohm.
Should be high enough?
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I suggest reading the lme49600 datasheet. There is a circuit included that has an lme49710, lme49600, and lme49720. The lme49720 functions as a DC servo, and the datasheet explains the capacitor values and resistor choices. It is most important to understand why values were chosen and not to just choose random values.
If halving resistance to 1meg then capacitor value should be doubled.
10^13 is excellent.
But I cannot understand why some opamp work, but some cannot.
If the input impedance is too low they wont work.
Maybe a different pin out ?
Some op amps do odd things if the inputs go near the rails, some op amps invert the output.
You need to put the op amp in and check voltages with a meter and a scope in high impedance mode (x100) to see what is going on.
Most opamps are rated +/-18V MAX.
A servo certainly does not need super voltages.
Maybe you are all barking up the wrong tree.
The annotated input and output voltages of the main amplifier in post #1 make no sense at all: positive input at 0 V DC, negative input at -9.5 V and output at -14 V. Maybe the main amplifier suffers from polarity inversion when the negative input goes too far negative? Either that or some oscillation messes up the bias point.
The annotated input and output voltages of the main amplifier in post #1 make no sense at all: positive input at 0 V DC, negative input at -9.5 V and output at -14 V. Maybe the main amplifier suffers from polarity inversion when the negative input goes too far negative? Either that or some oscillation messes up the bias point.
Select your servo op amp with care and read the data sheets
- very low input bias current and offset voltage (could be fet input but not necessarily) so the capacitor accurately monitors the error voltage
- rail to rail input voltage range so that you know the correction voltage can remain in operating control range
- rail to rail output voltage swing for the same reason
This should help you understand why some op amps do not work as expected.
- very low input bias current and offset voltage (could be fet input but not necessarily) so the capacitor accurately monitors the error voltage
- rail to rail input voltage range so that you know the correction voltage can remain in operating control range
- rail to rail output voltage swing for the same reason
This should help you understand why some op amps do not work as expected.
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