Amplifying an AC signal

hadroneater

I have a sinusoidal AC voltage with roughly Vrms = 50mV. I wanted to amplify with a non-inverting amplifier circuit with the LM358 op-amp. I used resistors of 100ohm and 10kohm for a theoretical gain of 100x. However, my output signal is rectified so that only the positive half of the signal shows. I reckon this is because the LM358 is a low power op-amp and only uses a single, positive power supply. What can I do to ensure that a full AC signal is amplified with the LM358? I thought of giving the input signal an offset using diodes but that seem to complicated.

chroot

Depending upon the frequency response you need, you might be able to capacitively couple your signal of interest into the middle of a resistor divider. That will effectively add a constant offset voltage, putting the op-amp input signal within its supply range.

- Warren

chroot

Alternatively, you could simply use two power supplies, one below and one above the ground level of your signal.

- Warren

vk6kro

You can do this:



The supply shown as 2.5 volts at left can be a pair of resistors (maybe 3.3 K) in series across the power supply with an electrolytic capacitor across the bottom one. Maybe 22 uF.
Take the output from the junction of the two resistors.

The 100 K brings this voltage to the non inverting input and the signal is coupled to this input via a 0.1 µF capacitor. V1 is the input signal.

The gain is about 100.