Problem with estimating crossover frequency and corresponding isolation resistor value

Hello All,

I need some help with estimating few parameters with respect to the operational amplifier. Please refer the screenshot that i attached for the circuit diagram and the thing I want to achieve which are highlighted in yellow. As per the screenshot, I was trying to crossover frequency analysis of the RC circuit (attached simulink file). It was mentioned that they selected the isolation resistor value Rs based on the crossover analysis. They mentioned they selected 1 Mhz as crossover frequency such that their phase margin is greater 60 degree at their operating frequency of 20 kHz. Actually, I want to increase my operating frequency to the full range of the amplifier which is at 140 kHz. So if I increase the frequency, i would like like to know how the phase margin is getting affected (whether it is stable or not) with the provided Rs value of 7.5 ohms and capacitance of 82nF. I built the entire circuit on simulink and was trying to see the bode plot which gave me constant magnitude of 40 dB (my gain is 100) and a phase of +180 degrees (file attached). Can anyone tell me the following please?

1. I wanna know the phase margin of my circuit at 140 kHz so that i can choose a resistor value for Rs that will keep my system stable.
2. I wanna know the current across the resistor Rs so that i can choose a proper watt rating. Currently when i measure the current or voltage across Rs, it is showing 0 in the scope.

Please shed some lights. Your help is much appreciated. Thanks.

Answer 1

[Debadipto]

Hello @Yaser,

I understand that you have queries in estimating parameters for the operational amplifier circuit, namely the phase margin of the circuit and the current across the resistor.

To answer the first question, the following MATLAB documentation describes the method to calculate the phase margin of a Simulink model:

https://www.mathworks.com/help/robust/ug/stability-margin-of-a-simulink-model.html

As for the second question, I did not find the current to be zero across the resistors. Most likely, the graph is not scaled to your view which is why you might be seeing a constant line at 0. Try clicking the “Scale X & Y Axes Limits” button on the Scope window to fit the graph to your view.