Analog Integrated Circuits & Lab – Week 6 Video Lab Transcript

Oscillator Circuit

In this video we're going to investigate the beam bridge oscillator. Simulating oscillator circuits is inherently difficult because you have to more or less establish an imperfect situation in order to have oscillation. Oscillation starts with the imbalances in certain circuit components such as resistors and capacitors and the nonlinearities that are there create the oscillation and we attempt to control that by the external circuitry. But, if you look at the circuit that we have here, you'll notice that there is no input power supply voltage supply for a waveform. We just have the power supply that's up for the op-amp itself. So what we're actually doing is we're taking advantage of these nonlinearities, and then creating an output waveform, and trying to control the frequency of that oscillation and this is just an example of one of the circuits you could find on the internet, and what we'd like to do in this lab is have you build this circuit and then investigate some of the properties that control oscillation.

So we want to build a circuit, and then we want to be able to answer questions such as, you know, what is the frequency of oscillation of this circuit, and then how can we modify this circuit to allow for variable frequency on the output, and then we'd like to understand the significance of the ratio of RF to RG where RF is going to be the feedback resistor and then we have RG on the input. So we want to understand the significance of the ratio of those two in order to result in oscillation, and then for this particular circuit you will actually have some clipping on the output, and we'd like to understand what we need to do to modify the circuit to avoid clipping on the output.

Notice here that I've got RF set as a potentiometer, a variable resistor, and that's so that you could vary the value of this feedback resistor to achieve oscillation. Oscillation will not occur at any value, but only at specific points, and then beyond that point when the ratio is established appropriately, so that's the reason for the variable resistor here.

Also, I've chosen to use the agilent oscilloscope and that's just due to its better selectivity. The oscillations that we're talking about here are very small oscillations, so this is a better oscilloscope, more realistic to what you might use in a real lab scenario.

Also, you'll notice that we've got capacitors and resistors here, certain values, and there's some rules of thumb associated with the selection of these and that's something else that I'd like for you to investigate as you're working with this circuit in adjusting these and seeing how they affect the overall output frequency. So I'm just going to go ahead and run the circuit here, give you an idea of what the output looks like, at least for the case in which I'm set up for, and you see that we do have some clipping on the output, but we have an output waveform where we did not have any input, you know, waveform, so we've actually generated, so to speak, an oscillating output.

So what we'd like to do is take a screenshot of this and answer the questions in a lab section, and include those in the word document for submission, and there's a lot that you can investigate on the internet related to oscillator circuits, and I highly recommend that you read the theory on that just to get an idea of, like I said, the rules of thumb that are typically associated with the selection of these external component values.