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open loop stability

Kailyn Chen
Genius 9560 points


Hello，

One customer built the voltage to current circuit with OP, and it involves the positive feedback, the input signal is voltage, and the output signal is current, how to judge the open loop stability? 

I am not sure if the below three methods about the loop stability could be used? Please provide your opinion:

1.Using the open loop spice simulation to obtain the closing  rate and phase margin, but it need to cut the loop circuit and added the external signal source.

2. Test the overshoot of the output signal about the step response or square wave response.

3. Test the peak value  of the gain

Best regards

kailyn 

  • 0
    Guru 48395 points

    Can you provide the TINA file, one easy approach is to sweep the output spot noise wideband and look for spikes. 

  • +1
    TI__Guru* 77541 points

    Hi Kailyn,

    I think that you want to find a closed loop stability of the V-to-I converter.  

    There are two feedback loops within the circuit. The inner loop is negative feedback and outer loop is positive voltage feedback. I think that you have to solve this circuit with superposition method. 

    2. Test the overshoot of the output signal about the step response or square wave response.

    The feedback loop is stable. If you want to check the loop stability, 2nd option will be the easier, where you perform a small signal step function and look for overshoot in responses signal in square wave/step function. 

    OPA376 V-I pos feedback 09232021.TSC

    If you want to obtain BW, phase margin and other closed loop performance parameters about the circuit, please let me know. 

    Best,

    Raymond

  • +1 in reply to Raymond Zhang1
    Guru 48395 points

    I don't see any spikes in the broadband output noise either - pretty encourgaging

  • 0 in reply to Raymond Zhang1
    TI__Guru* 77541 points

    Hi Kailyn,

    I made some improvement in the above circuit. The initial V-to-I converter has an inadequate gain margin (-3.8dB). I reduced the feedback capacitor from 100pf to 2pf and obtained the following performance parameters. 

    BW of the circuit: 387kHz, phase margin: 82.4 degree and gain margin: -27dB

    If you have additional questions, please let me know. 

    Best,

    Raymond