ECE 200 Lab 4 (072621)

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Electrical Engineering

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Dec 6, 2023

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1 Experiment 4 Audio Amplification and Operational Amplifiers In this experiment, we will construct an audio amplifier and measure its specifications. We will also analyze the transfer characteristics of an operational amplifier. You MUST show work for all calculations in the lab. Note: The use of the Op-Amp OP27 ( datasheet ) in an inverting configuration as an amplifier is only a single example of dozens of really interesting possible circuits. 4.1 Constructing the Amplifier We will begin the experiment by building and testing the inverting amplifier. Figure 4.1: (Left) The pinout of the OP27 operational amplifier. (Right) The inverting amplifier circuit. Preparation: • To assemble the circuit depicted in Figure 4.1: o Insert the op-amp (with 8-pins), spanning across the central breadboard divide. o Attach the V+ and V- wires of the Analog Discovery to the V+ and V- op-amp terminals (pins 7 and 4, respectively). Use header pins to connect the wires to the breadboard. o Connect the 20 kΩ feedback resistor from the op-amp output to the inverting input (pins 6 to 2). Remember to use the resistor color code chart or measure the resistance with your meter. o Connect the 10 kΩ input resistor from the inverting input (pin 2) to the waveform generator channel 1 "W1" (use an empty row in the breadboard away from the op-amp). Note this voltage source is grounded already inside the Analog Discovery, so no additional ground wire is needed.

2 o Connect any ground wire from the Analog Discovery to the non-inverting input (pin 3). Because this (pin 3) row on the breadboard will need at least three connections to wires from the Analog Discovery, you may find it helpful to insert a four-pin header into this row. o Connect the oscilloscope channel 1 across the output of the op-amp (via wire "1+" to pin 6 and wire "1-" to ground, i.e., pin 3). o Connect the oscilloscope channel 2 across the input of the amplifier (via wire "2+" to the same row as "W1" and "2-" to any ground, i.e., pin 3). Figure 4.2: The oscilloscope measurement window. o In the "Scope" Instrument, open the Measurements Panel by selecting the View/Measure menu option. This should open a new panel within the oscilloscope window. In this panel, select "Add/Defined Measurement" then "Ch1/Vertical/" to list the following measurements in this panel as shown in Figure 4.2. § Maximum § Minimum § DC RMS § Peak2Peak (Ch1) § Peak2Peak (Ch2) • Setup the power supplies: o Open the "Supplies" Instrument o Set them to +5 and -5 V o Click the "Positive Supply" and "Negative Supply" buttons until they say "Rdy" o Set the "Master Enable" to "On" • Set the input to 1 V and verify the output is -2 V. • Set the input to a 1 V sinusoid and verify the output is twice the amplitude with a 180° phase shift.

3 4.2 Measuring the Voltage Gain In this section, we will measure the voltage gain of the amplifier using a sinusoidal input signal from the waveform generator. Follow the instructions given below to prepare for the measurements. Preparation: • Power Supplies: ±5 V • Waveform Generator: Sinusoid, Offset = 0 V, Amplitude = 1 V, Frequency = 1 kHz • Oscilloscope (Both Channels): Time Scale = 500 µs/div, Trigger Mode = Auto o Ch1 (output): Voltage Scale = 500 mV/div o Ch2 (input): Voltage Scale = 500 mV/div • Workspace: make sure you can view the "Scope" and "Wavegen" Instruments at the same time, i.e., spanning across the top and bottom halves of your screen. This is done by clicking the small multi- window icon in the top right corner. • Verify that the oscilloscope shows the output signal in Channel 1 and the input signal in Channel 2. Both should be smooth and without distortion or clipping. Do not proceed unless your screen looks something like what is shown in Figure 4.3. Figure 4.3: The proper input and output signals for the operational amplifier. Measurements & Questions: 1. Save the oscilloscope display to include in your data summary. 2. Study the input and output signals. Is there a phase difference between them? 3. Measure the peak-to-peak voltages of the input and output signals. You may do this by simply using the measurements panel (add Ch2 if desired) or by observing the signal display scales with the cursor. Use A v = V pp (out) / V pp (in) to determine the voltage gain to one decimal place.

4 4.3 Frequency Response of an Audio Amplifier The frequency response of an Audio Amplifier is a graph that shows the power gain G as a function of input frequency. Most young people can hear audio frequencies in the range of 20 Hz to 20 kHz, but the upper limit degrades with age . Ideally, we expect an amplifier to treat all frequencies in this range equally, which is usually not true unless the amplifier is intended for high-fidelity applications. To obtain the frequency response of an amplifier we measure the voltage gain at different frequencies and then calculate the power gain using G = 20 log (A v ) . Preparation: • Verify the input signal level on the waveform generator is low enough to avoid clipping or any distortion of the sinusoidal output. If your input is 1 V, this should be true. • You will now vary the input frequency dramatically. Also vary the time scale to see a clear sinusoidal shape with between 1 and 10 periods displayed. Any more and the measurements may not be accurate. Measurements & Questions: 1. Measure and tabulate the peak-to-peak voltage of the output signal, voltage gain and power gain, G = 20 log(A v ), at the following frequencies: 1 Hz, 10 Hz, 100 Hz, 1 kHz, 10 kHz, 20 kHz, 50 kHz, 100 kHz, 200 kHz, 300 kHz, 400 kHz, 500 kHz, 600 kHz, 700 kHz NOTE: At 1 Hz use a time scale of approximately 200 ms/div. Reduce this manually as you increase the frequency. 2. Verify to yourself that regardless of the output frequency, the input peak-to-peak voltage is nearly always stable at 2 ± 0.1 V. Is this still true at 500 kHz? Write your observation below the table. 3. Plot the power gain as a function of frequency, use a logarithmic axis for the frequencies. Be sure to properly label your excel plot and apply a line of best fit to the data.

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