Download Preliminary Work

ECEN 313
The Electronic Amplifier
Objectives:
1. This lab is a step-by-step lab to show the student how to build and test an electronic
amplifier using an operational amplifier (op amp).
2. Review the use of test equipment and test methods for performing basic amplifier
measurements.
3. Examine the frequency response of an op-amp based inverting amplifier.
Equipment:
1.
2.
3.
4.
5.
Oscilloscope
Signal generator
Two power supplies
LF347 IC operational amplifier
Breadboard and miscellaneous components
Preliminary Work
Please complete the preliminary work before coming to lab. This will greatly reduce the
amount of time spent in the lab and will allow you to get more meaningful help from the TA.
1. Calculate the transfer function, A f  
Vout
for the circuit shown in Fig. 1 .
Vin
2. Determine the numeric value for the lower 3-dB frequency, fL, of the amplifier.
3. Obtain an approximate value for the gain-bandwidth product (GBW) of the LF347 op amp
from a product datasheet. You should get a datasheet off of the internet.
4. Using the GBW from the datasheet calculate the upper 3-dB frequency, fH.
Figure 1
Important note: Every time you use an op amp chip it must have the power supply pins
bypassed with a capacitor to ground. This is standard practice in industry and an op amp
should never be used without bypassing. Put capacitors located as close as possible to the IC
from the positive supply pin to the ground pin and from the negative supply pin to the ground
pin. Typical values of capacitance are 0.01 F to 0.1 F---non-electrolytic.
Procedure
1. Simulate the circuit shown in Fig. 1.
a. Determine the midband gain, upper and lower 3dB frequencies.
b. Compare the simulated results to the values obtained during the preliminary work.
If there are large discrepancies please correct your preliminary calculation.
c. Determine the input voltage amplitude that will produce a 20Vpp output. 20 Vpp is
the total voltage from the maximum voltage to the minimum voltage. A 20 Vpp
would be vt   10 sin 2 f t  .
2. Connect the circuit as shown in Fig.1. The pin out of the LF347 chip should be in the
datasheet that you downloaded off of the internet.
a. Vary the frequency of the input signal to observe the low and high frequency roll-off
and then record the upper and lower 3 dB frequencies. Make sure that the input
signal is small enough so that the output signal has a peak value less that the voltage
supplies (12 V).
b. Take sufficient data to (1) permit the accurate construction of the frequency
response near the corner frequencies, and (2) determine the slope of the low and
high frequency roll-off.
c. Make sure that you include this plot in your lab write up. The plot should be in dB
scale.
d. You need to compare this measured plot to the Bode plot calculated as part of your
preliminary work.
e. What is the measured gain-bandwidth product (GBW)? The GBW should be
calculated using the upper frequency corner fH.
f. Apply a sinusoidal signal to the input that has a frequency between low and high
log10  f H  log10  f L 


2
.
frequency corners  f  10



g. Set the input to produce an output level of 20 Vpp. You may need to use a voltage
divided in order to get a small enough input voltage.
h. Display both the input and output signals simultaneously on the oscilloscope.
i. Measure the midband gain.
j. Compared the measured midband gain to the simulated value. Explain any small
discrepancies. Fix the design if the discrepancy is large.
3. Plot the gain as a function of input voltage.
a. Determine the maximum undistorted output signal.
b. Plot the input and output signals on the oscilloscope. Describe the distortion in the
output signal.
4. Simulate the circuit shown in Fig. 1 with a square wave input. Set the frequency of the
square wave to a point between the high and low frequency corners (see part 3). (Use the
source in Pspice called VPULSE. The only parameters that you need to change are V1, V2,
PW, and PER.)
a. Show what happens if the frequency of the square wave is increased. Find the
frequency at which the output voltage amplitude decreases by 10%.
b. With the frequency of the square wave set to a midband frequency, calculate the rise
time (10%-90%) of the circuit.
c. With the frequency of the square wave set to a midband frequency, calculate the fall
time (90%-10%) of the circuit.
d. Calculate the time constant of the circuit (=RC). Hint: use the equation 𝑣(𝑡) =
𝑡
𝑣𝑓 − (𝑣𝑓 − 𝑣𝑖 )𝑒 −𝜏 . The square wave provides in the initial and final voltages.
e. Compare the time constant to the upper 3dB frequency calculated in Part 1. Derive
an equation to estimate the upper 3dB frequency using the rise time (10% - 90%) of
the circuit.
5. Change the input signal applied to your circuit to be a square wave and set the amplitude
such that the output is not driven to it maximum swing.
a. Increase the frequency until the amplitude of the output voltage decreases to 90%.
(This is called the 10% sag.) Demonstrate the waveform with 10% sag to a TA and
obtain a completion stamp.
b. Decrease the frequency until there is no sag. Measure the rise time (10%-90%) of
the circuit.
c. Compute the expected bandwidth of the amplifier based on the rise-time
measurement.
d. Compare the computed bandwidth with Part 2.