To get an AC voltage from a DC voltage supply we use an inverter with a filter to get a pure sine wave.
And to make the filtering easier we use pulse width modulation in which we need a sine wave with the fundamental frequency of the desired output and compare it with a triangular wave.
Instead of this, why we don't amplify this sine wave using a power transistor and get the desired output?

 

You do know transistors (mosfet, IGBT, BJT) are DC devises don't you?

 

You could use an audio power amplifier and feed it with a sine wave input to get a more powerful AC sine wave output.
Why do you think that method isn't used?

 

To get an AC voltage from a DC voltage supply we use an inverter with a filter to get a pure sine wave.
And to make the filtering easier we use pulse width modulation in which we need a sine wave with the fundamental frequency of the desired output and compare it with a triangular wave.
Instead of this, why we don't amplify this sine wave using a power transistor and get the desired output?

You can do it with one of these:

 

You do know transistors (mosfet, IGBT, BJT) are DC devises don't you?

Are you sure!!
In electronics lab we used to amplify sine wave using BJTs and MOSFETs.

 

You could use an audio power amplifier and feed it with a sine wave input to get a more powerful AC sine wave output.
Why do you think that method isn't used?

I don't know if it's used or not.
But if we use it instead of PWM circuit, we can save the power losses due to switches and reduce the cost.

 

But if we use it instead of PWM circuit, we can save the power losses due to switches and reduce the cost.

What makes you think an audio amplifier is 100% efficient?
The efficiency of a linear audio amplifier (around 65%) is much poorer than a PWM switching circuit inverter.
If you use a Class D (switching) audio amp then the efficiencies would be more comparable.

 

Instead of this, why we don't amplify this sine wave using a power transistor and get the desired output?

"Pure" is a problem The Bi-polar amp will work. or AC mains simulations, there are devices that do just that. PWM is more efficient.
The simple inverters use a stepped waveform where the RMS value is say 120 VAC. The so-called sine wave inverters essentially use class D techniques.

 

Are you sure!!
In electronics lab we used to amplify sine wave using BJTs and MOSFETs.

If using your logic, then all devices are AC. The truth is, any single transistor only works with one direction of current flow. It is the more or less of that current flow which looks like AC but the transistor always uses DC for its power.

 

Just to be clear, which is better to be used
1.

2.

 

Circuit #1 is not suitable. That is a design of an audio signal amplifier.

Circuit #2 is not suitable. That is a design of an H-bridge.

What are you really trying to do?