# Can I use a power amplifier for power stage of a pure sine wave inverter?

As the title reads, I wish to use a power amplifier (say OPA541) in the power stage of a 50W pure sine wave inverter? How to find the efficiency of this approach? Since the amplifier itself will be sourcing all the current...efficiency may be low? is that true? how low will it be?

I need a variable frequency variable voltage AC source where I can move the frequency from 40Hz to 80Hz. It should be easy to create a variable frequency sinewave, either analog or mcu based. This can be sent to the input of a power amplifier to get the required signal which can then be stepped-up using a suitable transformer. The amplitude can be changed by changing the feedback resistance on the amplifier. My power requirements are 120VAC, 3A for the output. So I was thinking OPA541 (80VDC, 10A) should be a good option.

Is this a good design for a lab source?

what I really need is a 3 phase inverter !!. Once single phase is done, I was planning to use a cd4027 to get 120degrees phase shifted signals and send them to independent power stages.

Thanks and best regards, Vishal

• You really want to use something more akin to a class D amplifier, otherwise your efficiency is going to be absolutely horrible, far less than 50%, which would require some very significant cooling. Besides, 120V AC requires 340V peak-to-peak, so the OPA541 will not work at all unless you step the output voltage up with a transformer. Sep 22, 2015 at 8:37

The OPA541 data sheet says it can operate on power supplies as high as 80 volts and produce a peak current of 10 amps.

Now, if these are translated to a sinewave, the RMS voltage would be 80/2.828 = 28.3 volts and the current would be 10A/2.828 = 3.53 amps.

This would lead to a power output of 100 watts.

As you require 120V AC at 3A (power = 360 watts) you are well off-the-mark with this device and I have given it the most favourable chances in my analysis such as assuming it is a rail-to-rail output device.

For instance, if you dug deeper into the data sheet you would read that the output voltage swing on a 5A load and +/-35 volt power supplies is about +/-30V and this ratio won't be any better at +/-40V rails so, I'd expect the maximum output voltage to be only 70Vp-p or about 25 volts, leading to a power output of only 87 watts.

You would do better buying one of these: -

They cost about £20 from Maplin (UK source) and are rated at 200 watts into 4 ohms with a frequency response from 3Hz to 200kHz.

• Thanks Andy for the insight. Initially I had thought that parallel operation of two opamps would work..and looks like you have suggested something on the same lines. Please let me know what this "board" is called. Sep 22, 2015 at 14:05
• Also, I am thinking that using DDS to synthesize a sine may be the best option for me. Earlier I was thinking of creating square waves separated by 120degrees using a shift register (4035), adding a negative voltage shift to get +/- square wave and then low pass filtering them to get fundamental sine. However, now it dawned on me that when I change the frequency of the fundamental, the filter phase shift and amplitude changes, which will change the power amplifier input, which inturn changes the output voltage. Sep 22, 2015 at 14:08
• Since this is a lab source we need independent control on frequency and voltage. So, I must synthesize the sine itself. Welcome to MCU-DAC world :)) Sep 22, 2015 at 14:09
• rapidonline.com/design-technology/… Sep 22, 2015 at 14:19