Positive and Negative Feedback 2/13

  On Monday we talked about positive and negative feedback. Our discussion and a little googling made what was going on in the circuit we built the last week a little clearer. Here is a quick recap of op-amps; an op-amp is a thing with a positive input and a negative input, and one output. The equation that goes along with the op-amp is:
                                                                  Vo = A (V+ - V-)
If V+>V-, then Vo is positive. If  V+<V-, then Vo is negative.
    Last week when we built our circuits,  Vo  fed back into V+. Therefore our circuit had positive feedback. Because it had positive feedback it was an op-amp with hysteresis. Then we discussed what the output of an op-amp with negative feedback would look like, and we made it on a computer program called LT Spice.
Here is the diagram of a circuit with negative feedback...

As you can see, the output of the op-amp connects back into the negative input, which is why it's negative feed back! And here's a diagram pf a circuit with positive feedback, where the output of the op-amp feeds back into the positive input:

Below are graphs of what the output looks like for op-amps with positive and negative feedback. An op-amp with positive feedback has hysteresis, so the output depends on what it was before. As you can see on the graph, the output only changes from -12 to +12 when the input is -6 or 6.

  The second graph shows the output of an op-amp with negative feedback.  Unlike a circuit with positive feedback, a circuit with negative feedback can have an output that is inbetween -12V and 12V. I think Cailey's post explains it very well.
   I find LTspice  difficult to use, but I am sure it will get it easier with practice. My main objection to LTspice is that it only runs on Windows, so we have to run it in Virtual Box on our Macs. Running Windows on a Mac just seems sacrilegious to me.