Weird output for Single Supply Amplifier.

[jose_peeterson]

Dear all,
For the first stage I removed a 0.3V connected to the positive supply of first stage. This resulted in the output like this. Can I please know why this happens and how to fix it?

 

[BradtheRad]

I can think of no other reason for your waveform to move upward gradually except that a capacitor is charging. Eventually the waveform should stop rising.

In some cases it will rise all the way to the supply rail, depending on interaction between the capacitor and the feedback.

To speed up the process, you can try reducing a capacitor value or two.

 

[jose_peeterson]

thanks for ur reply brad.

The problem was with not using (3.3v)/2 = 1.6v at the 0v of non op-amp supply. I don't know why it has to be done? Do you know why?.
I mean, instead of ground at the feedback resistors it is connected to 3.3/2 = 1.6v. This is done in stage 1 and stage 3. After this was done the waveform was flat and normal AC sine wave. I read that it was because of single power supply. BUT DON'T UNDERSTAND A THING?

thanks.

 

[BradtheRad]

Yes, when you use a single (positive) supply, the op amp needs to operate in the positive region. This means the waveform has to occupy the region between 0V and supply V.

Therefore your AC waveform needs to have a DC component added at the input.

You would have an easier time if you were to start off powering the op amp from a bipolar supply. That would provide both positive and negative regions for the waveform to operate in.

It would help if you were to make everything operate properly with a bipolar supply, then decrease the negative supply bit by bit, while you adjust component values to maintain the correct waveform in the positive region.