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LAB 12: Op-amp invertor and follower
Names___________________________
This lab introduces the op amp in two of its simplest (and most common) modes: the inverting
amplifier and the voltage follower. We’ll be using the LF411, which is an FET version of the
famous bi-polar 741. The pin-out is as follows:
Pin 7 is always connected to the
positive supply voltage +VS!
Pin 4 is always connected to the
negative supply voltage -VS!
PART A: Open-loop gain.
Hook up the following circuit:
1) Use the 10 k pot from one of our previous labs, or solder up a new one. Set the
supply voltages to ±10 V. Be sure you always remember to connect pins 7 and 4 to
their proper supply voltages! These connects are seldom indicated on circuit
diagrams.
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2) Experiment with the pot, changing the input voltage from + VS to – VS. You’ll see that
for all input voltages the output will be saturated at a large negative or positive voltage
respectively. What are these ‘saturation’ limits? How close are they to the supply
voltages?
positive Saturation
negative Saturation
V
V
PART B: DC closed-loop gain.
Now we’ll include negative feedback by including two resistors: a feedback resistance RF and an
input resistance RI. Hook up the following circuit:
1) As usual, measure carefully the resistor values you use! As we discussed in lecture, this
circuit acts as an inverting amplifier whose gain is given by
𝐺=
RI
RF
Gain G
𝑉𝑂
𝑅𝐹
=−
𝑉𝐼
𝑅𝐼
()
()
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2) In steps of about 0.5 V, vary the input voltage VI (using the pot) from -10 V up to +10 V.
For each of these values of VI measure the output voltage VO. Note that the input and
output voltages are measured relative to ground.
3) In Excel, enter your data and plot a graph of output voltage verses input voltage. You
will see that when the output voltage approaches either of the supply voltages the
transfer function becomes non-linear, i.e. saturates.
4) In another column, calculate the ratio VO / VI. From the portion of your data in the
linear region, calculate the average of this ratio.
Compare your measured gain with what you calculate from the measured resistances.
G (theory)
G (measured)
SHOW ME YOUR WORK ……..
APPROVAL?
PART C: AC Distortion and frequency response.
Replace the voltage-divider input with a sine-wave generator, and now measure VI and
VO (peak-peak) with your oscilloscope ( Ch 1 = VI and Ch 2 = VO ):
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1) Set the frequency of the input sine wave to about 1 kHz. Note that the input and output
are 180⁰ out of phase with the input. This simply reflects the inverting character of the
amplifier. If you haven’t done so yet, adjust the input voltage to be about 1 V peak-topeak. Now gradually increase the input voltage. You’ll notice that as the output
increases to large amplitudes (like the supply voltages) the amplification is no longer
linear and the output signal becomes “clipped”.
THE LESSON: always work in the linear region!
2) Now, set your input wave to have an amplitude of about 1 V (peak-to-peak). Let’s look
at the frequency response of our amplifier on a logarithmic scale! Measure VI and VO as
a function of frequency in the following steps: 1 kHz, 5KHz, 10 kHz, 50 kHz, and 100 kHz.
Between 100 kHz and 1 MHz you’ll see the gain VO / VI start to drop, so measure at
more frequencies in this region, say ... 200 kHz, 400 kHz, 600 kHz, 800 kHz, and as close
to 1 MHz as possible.
Plot the gain as a function of log (frequency). You should also notice that in this region
the output becomes distorted and shifted in phase.
Show me your graphs, if your results meet my standards I will check the following BOX:
PART D: The voltage follower.
Another op-amp configuration using feedback is the voltage follower. Despite its simplicity,
it’s one of the most useful op-amp applications. The follower provides a buffer between
one circuit stage and the next. It has a large input resistance (that of an FET input) and so
does not load the input circuit; a vast simplification in design. Alternately, it has a low
output resistance and can drive reasonable loads.
Hook it up:
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1) For various input voltages, measure the output. Compare the accuracy. How close
to the plus or minus supplies can the output swing before the output differs by 10%
from the input?
VI
VO
% error = 100 ∗ (𝑉𝑂 − 𝑉𝐼 )/𝑉𝐼
2) Measure the output resistance RO of your LF411: Set the input to some fixed
voltage VREF, say about 5 V. Measure the output voltage when you load the output
with a small load resistor 𝑅𝐿 ≈ 100 . For a small enough load resistance, VO will
deviate from VREF. You can then figure the output resistance from the Thevenin
equivalent circuit:
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RL
VREF
VO
()
(V)
(V)
𝑅𝑂 = 𝑅𝐿
RO
𝑉𝑅𝐸𝐹 − 𝑉𝑂
𝑉𝑂
()
SHOW ME YOUR WORK ……..
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