ENGR225 Circuit Analysis Name_________________________
Lab #4 - Op-Amp Circuits Date__________________________
· To gain experience with op-amp circuits in the lab and with Multisim.
· To get a feel for how precisely the gain is determined by the external resistor values.
· To see the different standard op-amp circuits that are often used.
1. Calculate the gain for the inverting amplifier shown as circuit a) on the attached sheet. In other words get an expression for the output voltage divided by the input voltage. Gain A = Vout/Vin. Hint: Write equations taking advantage of the two assumptions that we spoke about in class last time - namely, the currents into the inputs are zero and the two inputs are at the same potential.
2. Build circuit a) on the breadboard and in Multisim. We will use the 741 op-amp with supply voltages of +15V. In Multisim you already know where to find the resistors, power supplies, and ground symbol, and you will find the 741 op-amp in the ANA (analog) part of the menu under the upper left corner button. First, use a 1-V dc supply for the input and measure the output voltage with a dc DMM. The ratio of the output voltage to the input will be the gain. On the breadboard you can use the Wavetek function generator as your input. Press the button to take it out of the Continuous mode and adjust the Offset control to get an output of 1 Vdc as read on the DMM. Then measure your output voltage with the DMM to get the gain.
3. Change the input source from a 1-Vdc source to a 1-Vrms sinusoidal source with a 1-kHz frequency. On the Wavetek press the button to select the Continuous mode, snap the Offset control to the Off position, and adjust the amplitude to give you 1 Vac measured on the DMM. Again measure the gain of the amplifier using the DMM in the ac voltage range. Note that the ac voltage measured on the DMM is Vrms.
4. With this same ac input signal, use the oscilloscope to observe the input and output waveforms. Record the waveforms and determine the gain from them as well. Note whether the output waveform is in phase or 180 degrees out of phase with the input. Are the waveforms consistent with what you would expect from this amplifier?
5. Change the feedback resistor to increase the gain by a factor of 5. Select a standard resistor value close to the one you calculated. Again use the oscilloscope to record the input and output waveforms and explain what you see.
6. Summarize in a table your gain results from your breadboard circuit measurements and from your simulations for the dc, ac, and waveform measurements. Discuss their agreement with the calculated gain from part 1.
7. Repeat procedures 1 through 6 for the non-inverting amplifier shown as circuit b) on the attached sheet.
a) Inverting amplifier with dc source. The ac source ready to connect for part 3. The oscilloscope is shown as it will be connected for parts 4 and 5.
b) Non-inverting amplifier.