• BME 210
• Lab 4: Operational Amplifiers

1. Objectives

By the end of this laboratory session students will be able to:

• Design, build, and test circuits containing op-amps

2. Background

The operational amplifier (op-amp) is an integrated circuit consisting of many transistors, resistors, and capacitors and is used to amplify signals. It is drawn as a triangle with + (non-inverting) and – (inverting) terminals on it. Op-amps are useful because they have gain—they can take an input and amplify it by some amount controlled by the person building the circuit. We use them in biomedical instrumentation to measure small signals from the body (on the order of microvolts to millivolts) and amplify them to the 1–2 V range, so that they can be more easily observed. Filters built with op-amps are called active filters.

Op-amps amplify the DIFFERENCE between the two inputs. To simplify calculations, the op-amp is often modeled as an ideal op-amp. Some of the assumptions in the ideal op-amp model:

• The open-loop gain of the amplifier is infinite
• Input impedance is infinite, so no current flows into or out of the input terminals
• The output impedance of the amplifier is zero, so it can supply infinite current
• The bandwidth of the amplifier is infinite, with zero phase shift

In practice, amplifiers employ negative feedback to control gain. Two assumptions are used to simplify feedback calculations:

1. No current will flow into the inputs
2. The input voltages are equal

One practical limit on op-amps is that the output voltage can never exceed the supply voltage(s). When the amplifier’s output approaches one of the supply voltages, the amplifier is said to “hit a rail”. The inverting op-amp amplifies an input voltage and inverts the output. The gain for an inverting op-amp is

$$v_\textrm{out}=-v_\textrm{in}\frac{R_\textrm{f}}{R_\textrm{i}}$$

The schematic for the inverting op-amp is shown below.

The non-inverting op-amp amplifies an input voltage but does not invert the output. The gain for a non-inverting op-amp is

$$v_\textrm{out}=v_\textrm{in}\left(1 + \frac{R_\textrm{f}}{R_\textrm{i}}\right)$$

The schematic for the non-inverting op-amp is shown below:

3. Laboratory Equipment

1. Resistors
2. DC Power supply
3. Op-amps
4. Oscilloscope
5. Function generator

4. Procedure

4.1. Inverting amplifier

1. Op-amps are very sensitive to static electricity. Be sure to ground yourself before touching one! Construct the circuit shown below. Show your TA the circuit before you turn on the DC power supply connected to your op-amp.

2. Calculate the gain for this circuit.

3. Attach the function generator to $$v_\textrm{in}$$. Adjust the function generator so that it outputs a 1 V peak-to-peak ($$\mathrm{V_{pp}}$$) sine wave signal at 10 Hz. Press UtilityOutput SetupHigh ZDone to make sure the proper signal is sent from the function generator. This makes the function generator assume it is delivering a signal into an infinite resistance load.

4. Attach Channel 1 from the oscilloscope to $$v_\textrm{in}$$ on your circuit. Attach Channel 2 to $$v_\textrm{out}$$. Display both at the same time. You may want to push the Auto-Scale button to display both signals and then manually adjust the vertical (Volts/Div) resolution of each. You can adjust the vertical position of each waveform by using the knobs just above where the grey probes plug into the oscilloscope.

5. Sketch the waveforms of both channels and record the peak-to-peak voltage of each. Press MeasureVoltageVpp for each channel to get the voltage amplitudes. Be sure to pay attention to the voltage scale (Volts/Div) of each signal when comparing them on the oscilloscope.

6. Record the peak-to-peak amplitude of vout for the frequencies 10, 500, 1000, 5000, 10000 , 500000, and 1 MHz.

7. Turn off the DC power supply and replace the 30 k$$\Omega$$ feedback resistor with a 100 k$$\Omega$$ resistor. Turn the DC power supply back on.

8. Set the function generator to 100 Hz and the amplitude to 5 $$\mathrm{V_{pp}}$$. Adjust the vertical resolution and position of Channel 2 ($$v_\textrm{out}$$) so that both channels are on the screen at the same time. Sketch both waveforms and the amplitude of each.

9. Decrease the function generator amplitude to 1 $$\mathrm{V_{pp}}$$.

4.2. Buffer

1. Construct the circuit shown below.

2. Set the function generator to 2 $$\mathrm{V_{pp}}$$ at a frequency of 100 Hz. Measure $$v_\textrm{out}$$ with the oscilloscope. Record the peak-to-peak amplitude of $$v_\textrm{out}$$.

3. Add an op-amp buffer to the circuit, as shown below.

4. Measure $$v_\textrm{out}$$ with the oscilloscope. Record the peak-to-peak amplitude of $$v_\textrm{out}$$.

4.3. Comparator

1. Design and build a comparator that gives an output of +12 V when the input is > 3V and -15 V when the input is < 3V. Use a sine wave as input.
2. Make sure the oscilloscope probe connected to your output is set to DC coupling. (If your probe is on Channel 2, press 2 then press the grey button next to Coupling until it says DC. If you are using Channel 1, press 1 to change the coupling). Now you should see your output flipping between +12 V and -15 V.
3. Once you have your circuit built and working, show your TA.
4. Make a detailed sketch of your circuit, including component values, for your lab report.
5. Simulate your comparator circuit in MultiSim. You may do this during lab or later on your own.

5. Results

1. What gain did your inverting amplifier have in steps 2 and 7 from Section 4.1.?
2. Include your sketches from step 5, Section 4.1..
3. Tabulate your results from step 6, Section 4.1..
4. Include your sketches from step 8 in Section 4.1.. What $$\mathrm{V_{pp}}$$ did each waveform have? Why?
5. Include your results from steps 2 and 4 in Section 4.2.. Why is the voltage different? Theoretically what voltage should you have measured in each circuit? Hint: sketch out both circuits, including the input resistance of the oscilloscope (10 M$$\Omega$$).
6. Sketch the circuit you built in Section 4.3., including component values.
7. Simulate your comparator circuit in Multisim (see tutorial in your textbook pp. 200-203) using the same sine wave input (AC_POWER in Multisim) you used in lab. Does it give a similar result to what you measured experimentally?
8. Create screen shots of your Multisim schematic and simulation.

Last updated:
January 7, 2018