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OPA548: OPA548 - Distortion when I add capacitor at the output

P SUBRAHMANYA ADIGA5
Prodigy 80 points
Part Number: OPA548

Hello,

1.I am trying to design an power amplifier using OPA548 and 2N3055/MJ2955 transistors. But when I add capacitor of 100nF and 10 ohm resistor  (in series) at the output, am getting distortion in the output signal. This experiment was carried out without any load. The images are in the attached file. I am unable to find the reason.

 2. Upon connecting a induction motor at the load, If I suddenly change the amplitude of the signal (from around 7V to 30V), the PNP transistor is failing. I am unable to explain this phenomenon.capacitor.pdf

  • 0
    TI__Guru* 93105 points

    Hello P SUBRAHMANYA,

    If you are driving the bases of the two output transistors with the OPA548, then adding the series RC network may be introducing a loop stability issue. When the output transistors are not driving a load their junction capacitances are likely different than when they are driving a high current load. Is there a reason you are adding the power transistors after the OPA548?

    Please provide information about your OPA548 applications circuit. We need to see the circuit schematic, power supply information, input signal characteristics, detailed output load information and output signal expectations. 

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0
    Guru 140200 points
    Hi Subrahmany,

    without knowing your circuit it's difficult to tell anything useful. Are the additional transistors sitting inside the feedback loop of OPA548? This would not be a good idea, because the transistors add phase shift and erode the phase margin of OPA548. Usually, it's no good idea to add a booster stage to an operational amplifier and include it in the feedback loop.

    If you need more load current why not paralleling two OPA548, as discussed in the datasheet?

    Regarding the failing of PNP: Have you omitted the free wheeling diodes (figure 38 of datasheet)?

    Kai