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OP-AMPS II
Physics 427
Lab # 6
1) Voltage follower
Construct the voltage follower shown below. Verify that Vout = Vin for several voltages.
The major application of a voltage follower is as an impedance buffer. The input
impedance of the follower is very large while its output impedance is very small. The
following experiment will convince you of this.
Charge a 1 F capacitor to 5 volts. (Use a high quality capacitor - i.e., one that doesn’t
have much leakage.) You might mount the capacitor in the breadboard and use
momentary contact with a wire from the +5 V supply to charge the capacitor. Measure
the voltage across the capacitor with a DMM. You should find that the voltage reading
of the DMM decreases with time. Determine approximately how long it takes for the
voltage to decrease to 37% of its initial value.
Now, as shown in the figure below, recharge the capacitor and connect it between V in of
the follower and ground and measure the voltage at Vout using the same DMM. What
happens? Explain how the voltage follower has improved the measurement. Can you
estimate the input impedance of the voltage follower?
2) Summing amplifier
Construct the summing amplifier shown below. Use the following nominal resistance
values RF = 100 K, R1 = 1 K, R2 = 10 K. (Be sure to measure the actual resistance
values and use them in all calculations.) The output voltage for this amplifier will be
given by:
Vout = – V1(RF/R1) – V2(RF/R2) = – (100V1 + 10V2)
Connect Frequency Generator # 1 (FG1) to V1 and Frequency Generator # 2 (FG2) to
V2. Set FG1 to produce a 200 Hz sine wave of amplitude 20 millivolts and FG2 to
produce a 2000 Hz sine wave of amplitude 20 mV. Sketch the output waveform. Is the
output waveform equal to – (100V1 + 10V2) ?
Observe the operation of the amplifier for a variety of input frequencies and amplitudes
from FG1 and FG2. Does it perform properly?
Bandwidth of the amplifier
The gain of any amplifier will begin to decrease when the frequency of the input voltage
is increased beyond a certain point. The frequency at which the gain is down to 0.707
of the DC gain (the gain for DC voltages) is called the bandwidth (BW) of the amplifier.
For signal frequencies above the BW, amplifier gain decreases rapidly. (Many
amplifiers also have limited response at low frequencies; but the op-amp in these
circuits does not have this problem, it really works down to zero frequency, which is
DC.)
Set FG1 to produce a 100 Hz sine wave of amplitude 80 mV and disconnect FG2.
Increase the frequency of FG1 until Vout decreases to 0.707 of its value at 100 Hz.
Record this frequency. This is the BW for a gain of 100.
Set FG2 to produce a 100 Hz sine wave of amplitude 800 mV and disconnect FG1.
Increase the frequency of FG2 until Vout decreases to 0.707 of its value at 100 Hz.
Record this frequency. This is the BW for a gain of 10.
Based on your observations, how does the bandwidth of an amplifier depend on its
gain? Can you make this relationship quantitative?