OPA835: OPA836 stability simulation

Javier Castillo

Prodigy 30 points

Part Number: OPA835

Hello,

I am founding lot of problems calculating the phase margin of a circuit.

To measure the phase margin I am using this other circuit, dividing fb(s)/inm(s) at the output.

This is what the output look like, with the variable resistor at 20%. Do you have any idea of why it is not working?

Thank you

over 5 years ago

0 Hasan Babiker31 over 5 years ago

TI__Genius 15555 points

Hello Javier,

Before trying to do a stability analysis, I have a few questions:

1) Could you clarify the function of the circuit? You state the output is correct, what should you be achieving?

2) Is the poteniometer setting supposed to be fixed or are you trying to vary it using VG1? What setting % do you have it set at?

3) Can you attach your TINA files?

Best,

Hasan Babiker

0 Javier Castillo over 5 years ago in reply to Hasan Babiker31

Prodigy 30 points

Hello Hasan,

Thank you for your answer.

1) The circuit is a simple amplifier, with variable gain controlled with the potentiometer and a DC offset.

2) The potentiometer changes the gain of the amplifier

3) I have attached the circuit

Regards

6355.amplifier.TSC

0 Hasan Babiker31 over 5 years ago in reply to Javier Castillo

TI__Genius 15555 points

Hello Javier,

Thank you for the clarification. Because you are using the part in a single supply configuration, you will want to reference your circuit to midsupply (2.5V). This way the amplifier is not railing at DC and the amplifier can utilize its full output range. I've modified your stability circuit below:

JavierStability.TSC

Where you seem to be getting a phase margin of 83.65 degrees. I would recommend simulating stability as shown in this modified circuit:

JavierStability_Modified.TSC

Where I am getting a phase margin of 75.64 degrees of phase margin. Note the added resistor and cap (200k and 1.2pF) is the input impedance of the amplifier.

Best,

Hasan Babiker