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OPA552: is my power amplifier oscillating? and if so, how to avoid it?

Ma Tianfu
Prodigy 100 points
Part Number: OPA552
Other Parts Discussed in Thread: OPA551

Tool/software:

I am building a simple power amplifier using OPA552. I copied a schematic from this document (https://www.ti.com/lit/an/sboa224a/sboa224a.pdf) and thought it is simple enough (I rarely design an analog circuit), but actually it seems not.

Here are screenshot of my schematic and pcb file. The design is to amplifiy a 200kHz sinewave from a signal generator, with a Vpp around 1-5V, to a Vpp of around 10V. The output is used to drive some home-made, capacitive electrode, similar to ultrasonic transducer, but I don't know the exact details.

During testing, I applied a voltage of ~12.2V to the opamp. The voltage between R2 and R3 is 6.1~6.2V, biased as expected. But when no input signal is applied, the output with no load, or a 680ohm resistor load, is a wave with a frequency of around 1.6MHz.

If I generate a 3V square wave from a microcontroller and feed it into the input, I get this result on oscilloscope, where channel 1 (yellow) is the input and channel 2 (blue) is the output. It seems that the input signal is properly amplified but the high frequency noise/oscillation persists and imposed ont the output signal.

Hopefully you guys can figure out how it happens and give me some advice on circuit design (or pcb layout, if my layout put to much parasite capacitance into feedback network).

  • 0
    TI__Genius 9835 points

    Hi Ma,

    Simulating the AC portion of your schematic shows some extreme gain peaking. It is not surprising that you are seeing such oscillations.

    The transient step response confirms that oscillations are expected.

    The problem is the OPA552 is a decompensated amplifier that is only stable for gains of 5 V/V or greater.

    I recommend using the OPA551 which is the unity gain stable version of the device. See below simulated step response for your AC gain of 2 V/V.

    OPA551_AC_HPF.TSC

    Depending on the capacitance of your load, you may require some additional compensation to completely remove the oscillations. However, switching to a device that is unity gain stable is the first step. If you continue to see oscillations using OPA551 feel free to reply to this thread and we can provide some additional stabilization techniques.

    Regards,

    Zach

  • 0 in reply to Zach Olson
    Prodigy 100 points

    Thank you Zach for your impressive explanation.