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LAB 7
Analog Applications of the Operational Amplifier
Objective: This project will demonstrate some of the analog applications of an operational amplifier through
a summing circuit and a bandpass filter circuit.
Components: 741 op-amp
Introduction:
Figure 2-1 shows a weighted summer circuit in the inverting configuration. This circuit can be used to sum
individual input signals with a variable gain for each signal. The virtual ground at the inverting input
terminal of the op-amp keeps the input signals isolated from each other. This isolation makes it possible for
each input to be summed with a different gain.
The bandpass filter shown in Figure 2-2 uses an op-amp in combination with resistors and capacitors. Since
the op-amp can increase the gain of the filter, the filter is classified as an active filter. This bandpass filter
circuit is extremely useful because the center frequency can be changed by varying a resistor instead of
changing the values of the capacitors. The center frequency is given by:
The center frequency can be changed by varying the variable resistor R3. Increasing R3 decreases the center
frequency while decreasing R3 increases the center frequency. The bandwidth is given by:
Notice that the bandwidth is independent of the variable resistor R3 so the center frequency may be varied
without changing the value of the bandwidth. The gain at the center frequency of the bandpass filter is given
by:
Design:
1. Find the relationship between the output and inputs for the weighted summer circuit of Figure 2-1.
2. Design a bandpass filter with a center frequency of 2.0 kHz and a bandwidth of 200 Hz. Let the voltage
gain at the center frequency be 20. Check your design with PSPICE®. Use ± 15 V supplies for the op-amp.
Use RL
Figure 2 - 1: Weighted Summer
Figure 2 - 2: Bandpass Filter
Lab Procedure:
1. Construct the summing amplifier of Figure 2-1. Design for the transfer function to
be VO = -2 VIN1 - VIN2. Use ± 15 V supplies for the op-amp. Use RL
2. Let VIN1 be a 1 V peak sine wave at 1 kHz and VIN2 equal to 5 V DC. Verify the amplifier's operation
by monitoring the output waveform on the oscilloscope.
3. Construct the bandpass filter of Figure 2-2. Use the designed values for the resistors and capacitors. Use
±15 V supplies for the op-amp. Use RL
4. Record and plot the frequency response (you may want to use computer control for the sweep and data
collection). Find the center frequency, corner frequencies, bandwidth, and center frequency voltage gain to
verify that the specifications have been met.
5. Change R3 to lower the center frequency from 2.0 kHz to 1.0 kHz. Repeat part 4 for the new frequency
response. Verify that the new center frequency is 1.0 kHz. What is the new bandwidth? What is the new
center frequency voltage gain? Compare with the measurements of Procedure 4.
Questions:
1. Could the summer circuit be used with the inputs connected to the noninverting terminal and produce the
same affect without the inversion? Explain.
2. What is/are the benefit(s) of using an op-amp circuit to produce a bandpass filter over using an RLC
circuit with a noninverting op-amp at the output of the RLC circuit?