11.3 and we're going to be looking at some of the popular operational amplifier applications. Op amps are used in a wide variety of applications in electronics. Some of the more common applications are: as a voltage follower, selective inversion circuit, a current-to-voltage converter, active rectifier, integrator, a whole wide variety of filters, and a voltage comparator. This is hardly a complete list; in fact, we have an entire quarter of curriculum at NSCC where we do nothing but study the various applications that are used with operational amplifiers and integrated circuits. The first thing we're going to look at is the voltage follower. Since the entire output is fed back, gain equals one. Here we have an input to the non-inverting input and notice the entire output is fed back so A is going to equal one in this case. This is akin to what we looked at with an emitter follower; we had the emitter follower look kind of like this. We had an output that came out right here and the input looked just like the output and they're basically the same size.
The value was so that it stepped up input imp … output impedance … excuse me, stepped it way down. That's what this is going … in fact these have … when you get into the formalised calculations through these, these have output impedance that is just tiny fractions of an Ohm; they're very, very small. If you use the formula, this is the formula for gain for a non-inverting amp, Rf over R1 divided by one and there's no Rf component so it would be zero and the R1 component, it would see it be the input impedance of the Op Amp which is infinity; so zero of infinity is zero … plus one that would give us the gain of one. Voltage followers have ultimately infinite input impedance, exceptionally low output impedance, no phase inversion, and unity voltage gain which means gain is one. Then we have a selective inversion circuit, since the Op Amp has an inverted and a non-inverted input, by selecting one pin or input or the other the user can choose the desired output. In this case, if you have the Op Amp and you had the positive or the negative, and you had a switcher over here and you could choose to switch on this one or this one. You could choose either to have your input go this way or have your input go this way. Your text has an actual circuit. Then a current-to-voltage converter … your text has a picture of a transducer that converts pressure to current. Here we have an actual use of one of these. This transducer measures applied pressure applied to a breaking system.
The output voltage will be proportional to the applied pressure. Here we have a transducer, remember what a transducer does; it converts one form of energy into another, so in this case, it's taking air pressure and it's converting it into a current. Now in this case, that current is being fed into this Op Amp; remember no current goes into the Op Amp, all the current goes through here. This will be converted into a proportional voltage and so that would be … typically in this type of situation that voltage would be sent to monitoring equipment. It would monitor the pressure that is being used in this system. Then an active rectifier, this is another use of Op Amps, this circuit is fairly straightforward; there's a lot of lines for most … OK, you come in here with our input, it's fed into the … this line here is not an input; this is the line that goes over the o-scope to see what we are looking at. The input comes in very much like a voltage follower; the input is fed back to the input. We have this diode right here, so here we have an input signal coming in … the same input signal goes out. It's just that the diode will only conduct on the positive alterations of that signal, on the negative it will shut off, and you'll see your output here. Here we see that on the positive … you have the signal on the input is cut off, and so we have rectification. This is a simulation … this from the … multi-sim simulation. Something I thought I might bring to your attention here, you'll notice that the output is … in fact the input and the output are about 169 millivolts, but you'll notice the input is 120 millivolts, and when you're using Electronics Workbench and you use some of these voltage sources, often times the voltage source … they're not in peak, they are in mass. You look at this and you look at your output and you think “what in the world's going on here?” So what you'll need to do is to make the conversion … and so what we would do … we say … 120 exponent minus three … and that is in millivolts. We would divide that by 0.707 and there would have 169 and so that coincides with the reading that we have on our o-scope. That is going to conclude this particular session. We've looked at just a few of the applications. We looked at the active rectifier, the current-to-voltage converter (commonly used with transducers), the selective inversion circuit, and the voltage follower, and we're going to look at more of these in the next sections.
Video Lectures created by Tim Fiegenbaum at North Seattle Community College.
by Robert Keim
by Kate Smith