Justified Amplifier

So I came across a problem the other day when I was working on a project of mine where the output of a micro-controller is only between 0 and 5 volts. This is a problem for me since I would like to have a AC signal passed that is centered on 0 volts. I first thought what if I off set the power to the micro by 2.5 volts. while in theory supplying a micro with -2.5V and 2.5V will work for a floating circuit, If something grounded is introduced there can be damaging back current pushed into the micro controller’s output pins. Realizing this I came to the conclusion that I needed to use an out side circuit. That is when I began to think about how to subtract a voltage from a AC signal. Normally to get rid of a DC offset you could just place a capacitor on the output and the signal is coupled, but not in this case. This is because the output of the micro-controller does not have a DC offset. The signal will look more rectified than actually coupled. After thinking a bit I came up this circuit.

The First part of the circuit that is in green, is the main part of the circuit. This part is a differential amplifier where the inverting terminal of the op amp is feed with a voltage reference of 2.5 volts. This in turn subtracts 2.5 volts from any voltage on the input. I used 2.5 volts because the micro-controller I am using outputs waves that go from 0 to 5 volts. After this first stage the wave is changed to vary from -2.5 to 2.5 volts. The second stage the blue part, is a regular non inverting amplifier. It is simply there to add gain to the signal. Finally the transistors are put in place to add power to the output. If I use Transistors with a Higher current rating than the op amp, the circuit will be able to output more current without fear of overloading the op amps. 

When I got home that day I decided to test out the circuit in LTSpice and what do you know … it worked first try. After seeing the circuit work in simulation i decided to breadboard it up and try it for real. The circuit worked perfectly. I still have yet to give it much testing but the circuit works for this application non the less.

6 months ago  /  0 notes