Hello,
I'm trying to use an LM675 power op-amp IC as an output buffer for a signal generator. For stability, I've configured this IC as a non-inverting amplifier with a gain of 11 as per the circuit diagram below. As also recommended in the datasheet, I routed separate ground returns for the signal input and power output. The circuit is powered from a dual PSU at +/-10V. The output is connected to a 10 Ohm power resistor returned to ground (using 4cm long wires).
However, there are oscillations on all positive output voltages, especially when Vcc exceeds the maximum amplified input signal by around 2V (a little below which there would be clipping). By way of illustration, I have captured a scope trace of the output below for a 200mHz 200mV sine-wave input. This low frequency gave me enough time to increase Vcc between each positive cycle as labeled in the trace. I omitted and interchanged various values for C4 (in cct above) with minimal effect, except to slightly decrease the oscillations near the crossover point. I also added large electrolytic capacitors across the supply rails beside the chip (and C2/C3) to no avail. The oscillations are in the MHz range and are sinusoidal in nature. The negative-going output swings remain clean at all frequencies (up to around 300kHz). There are no oscillations without the load.
For a 500mV sine-wave input, the picture is as below.
This behavior persisted when I used batteries instead of the PSU. I'd be grateful for any pointers/suggestions please.
Thanks,
John.
I'm trying to use an LM675 power op-amp IC as an output buffer for a signal generator. For stability, I've configured this IC as a non-inverting amplifier with a gain of 11 as per the circuit diagram below. As also recommended in the datasheet, I routed separate ground returns for the signal input and power output. The circuit is powered from a dual PSU at +/-10V. The output is connected to a 10 Ohm power resistor returned to ground (using 4cm long wires).
However, there are oscillations on all positive output voltages, especially when Vcc exceeds the maximum amplified input signal by around 2V (a little below which there would be clipping). By way of illustration, I have captured a scope trace of the output below for a 200mHz 200mV sine-wave input. This low frequency gave me enough time to increase Vcc between each positive cycle as labeled in the trace. I omitted and interchanged various values for C4 (in cct above) with minimal effect, except to slightly decrease the oscillations near the crossover point. I also added large electrolytic capacitors across the supply rails beside the chip (and C2/C3) to no avail. The oscillations are in the MHz range and are sinusoidal in nature. The negative-going output swings remain clean at all frequencies (up to around 300kHz). There are no oscillations without the load.
For a 500mV sine-wave input, the picture is as below.
This behavior persisted when I used batteries instead of the PSU. I'd be grateful for any pointers/suggestions please.
Thanks,
John.
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