This lab focuses on several op-amp circuits, including summing amplifier, differentiator, integrator, and bandpass filter configurations.
Items needed:
breadboard, +5V and +-12V
power supply, wiring kit, resistors, capacitors
function generator, 2-channel
scope
4741 quad op-amp
I. Simple Summing Amplifier (described in Ch. 29)
Construct a 741 unity-gain, inverting, summing amplifier circuit with
2 inputs (G = -1.0 for each input).
Connect R1 input resistor
to +5V and record the output voltage.
Connect input resistor R2
also to +5V and record the new output voltage.
Draw an equivalent circuit
with only one input resistor.
II. Differentiator
Construct an op-amp differentiator using R ~ 10 kohm and C ~ 0.1 ufd.
Set the function generator to 100 Hz and connect the 50 ohm output to the
circuit input. Connect Chnl-1 of the scope to the function generator, and
connect Chnl-2 to the circuit output. Set DC coupling on both channels.
1. For a sine wave, compare the input and output waveforms
(phase and amplitudes).
Compare the measured
gain to that computed from the measured component values.
Q: What happens to the output
when the input frequency is increased? Explain.
2. Switch to square wave input. Compare (plot)
the input and output waveforms.
Q: What happens to the output
when the input amplitude is increased/decreased? Explain.
Q: What happens to the output
when the input frequency is increased? Expain.
3. Switch to sawtooth wave. Compare (plot)
the input and output waveforms.
Q: What happens to the output
when the input amplitude is increased? Expain.
Q: What happens to the output
when the input frequency is increased? Expain.
III. Integrator
Use the same setup as the differentiator (2), except the resistor and
capacitor are interchanged. Connect another large capacitor (>=1 ufd) in
the input (between the function generator and the input resistor) to block
any DC offset from the frequency generator. (The output waveform will drift
due to the nonzero input bias voltage. You should have two wires sticking
up from either side of the capacitor so that it can be discharged periodically.)
1. For a 2V peak-peak square wave input, compare (show) the input and output waveforms? Expain?
Q: What happens to the output
when the input amplitude is increased? Expain.
Q: What happens to the output
when the input frequency is increased? Expain.
Optional - do for extra credit
Active Bandpass Filter
Consider the "Multiloop" bandpass filter in the diagram below. Choose
appropriate size capacitors (C=C1=C2). For a gain of K=100, "Q"=10,
and center sine wave frequency of
fo
= wo/2
p
= 5 kHz, compute R1, R2 (=R1), and R3.
R1 = Q / ( |K| C wo
)
R2 = Q / [( 2Q2 -|K| ) C wo
] |K|<2Q2
R3 = 2Q / (C wo
)
If the resistors are less than a few hundred ohms, then to avoid loading
down the function generator, choose a smaller C.
Construct the multiloop bandpass filter using components which
are within 10-20% of the computed values.
Measure the components used in the circuit and recompute
K,
fo, Df
and
Q from these values.
|K| = (R3/R1) [ C1 / ( C1+C2 ) ]
wo = 2 pfo
= sqrt [ ( 1/R1 + 1/R2 ) / (R3 C1 C2 ) ]
Q = sqrt [ (R3/R1) ( 1 + R2/R1) ] / [ sqrt(C1/C2)
+ sqrt(C2/C1) ]
Dw(3-db) = ( 1/C1 + 1/C2
) / R3
Connect the sinewave function generator and circuit output to the two scope inputs. Use the measure function on the scope to display the channel voltages (peak-to-peak or rms) and the frequency. Vary the frequency near fo and tabulate the frequency and voltages. Be sure to get 8-12 voltages values on each side of the maximum between the maximum and 1/10 of the maximum. Record data in a table and plot roughly as you take data.
Plot gain=Vout/Vin versus f. Curvefit the data to a Lorentzian function.
Q: What is the maximum
gain (K)?
Q: At what center frequency
(fo) is the gain maximum?
Q: What is the frequency
width (Df = Dw/2p)
between the 3-db points?
Q: What is the "Q"
of the circuit?
In a table, compare the measured and computed values of K, Q, fo, and Df . Discuss the results.