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Lab 2 for Amplifier Frequency Response - Engineer Electronics II | ECE 3110, Lab Reports of Electrical and Electronics Engineering

University of Utah (The U)Electrical and Electronics Engineering
Prof. R. R. Harrison

Material Type: Lab; Professor: Harrison; Class: Engineer Electronics II; Subject: Electrical & Computer Engg; University: University of Utah; Term: Unknown 1989;

Typology: Lab Reports

Pre 2010

Uploaded on 08/31/2009

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UNIVERSITY OF UTAH
ELECTRICAL AND COMPUTER ENGINEERING DEPARTMENT
ECE 3110 LABORATORY EXPERIMENT NO. 2
AMPLIFIER FREQUENCY RESPONSE
Objectives
This experiment will demonstrate the frequency and time domain response of a single-
stage common emitter BJT amplifier. The measured data will be compared to SPICE
simulations from SPICE assignment #1. To save a lot of time and possible frustration,
read the section you are working on entirely before performing any measurements. There
are often important hints or subtleties in following paragraphs.
Experiment
Build the amplifier shown in Fig. 1. You may use standard value components that are
within 10% of the specified values, but be sure to measure and record the actual
values.
During this experiment, you will be making measurements at frequencies in the 10 MHz
range. At these higher frequencies, the parasitic capacitance of your breadboard, wires,
and terminals of your discrete components can cause additional poles to appear in your
circuit’s measured transfer function. To minimize this effect, use the shortest possible
wires and clip the terminal wires of your components to be as short as possible. Also, be
sure the polarized electrolytic capacitors are connected with the proper polarity. NOTE:
A common mistake in wiring this circuit is to get the emitter and collector reversed, so
make sure you look at the data sheet.
1
pf3
pf4
pf5

Partial preview of the text

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UNIVERSITY OF UTAH

ELECTRICAL AND COMPUTER ENGINEERING DEPARTMENT

ECE 3110 LABORATORY EXPERIMENT NO. 2

AMPLIFIER FREQUENCY RESPONSE

Objectives

This experiment will demonstrate the frequency and time domain response of a single- stage common emitter BJT amplifier. The measured data will be compared to SPICE simulations from SPICE assignment #1. To save a lot of time and possible frustration, read the section you are working on entirely before performing any measurements. There are often important hints or subtleties in following paragraphs.

Experiment

Build the amplifier shown in Fig. 1. You may use standard value components that are within 10% of the specified values, but be sure to measure and record the actual values.

During this experiment, you will be making measurements at frequencies in the 10 MHz range. At these higher frequencies, the parasitic capacitance of your breadboard, wires, and terminals of your discrete components can cause additional poles to appear in your circuit’s measured transfer function. To minimize this effect, use the shortest possible wires and clip the terminal wires of your components to be as short as possible. Also, be sure the polarized electrolytic capacitors are connected with the proper polarity. NOTE: A common mistake in wiring this circuit is to get the emitter and collector reversed, so make sure you look at the data sheet.

R = 45 KΩ

R = 85 KΩ

R = 4.7 KΩ

R = 3.0 KΩ

R = 4.7 KΩ

C = 1 μF C = 1 μF C = 10 μF Q = 2N

+–

IN C

+V

C OUT

v

R

R C

R

Q

R

v

vcc

C

E E

1

2 B

i L o

RC

CC

1 2 C E L B C E

Fig. 1. Single stage bipolar voltage amplifier.

  1. Calculate the expected DC voltages at all nodes of the circuit assuming a +10V power supply (ignore the base current also). With no AC signal applied to the circuit, set the DC supply to +10V. Measure and record the DC voltage at all nodes. Compare these measurements with calculations. Before proceeding, be sure that calculations and measurements are in reasonable agreement (to make sure the transistor is inserted properly).
  2. Apply a small (less than 0.01-volt peak-to-peak) sinusoidal voltage to the input at a frequency of about 10 kHz. Use the bench-mounted signal generator with a voltage divider using a 1 kΩ and a 51Ω resistor to attenuate the signal generator output by a factor of approximately 20. Use a 10X scope probe to avoid unnecessary loading of the amplifier output. Observe the amplifier output and, if necessary, reduce the magnitude of the input until the output shows no distortion. Measure and record this input signal amplitude. Don’t forget to take this voltage divider into account when calculating the gain.

To get an idea of the overall transfer function, do a quick frequency sweep to locate approximately both the upper and lower corner frequencies of the amplifier gain (where the midband gain changes by 3 dB). Note the approximate corner frequencies.

in the output (you will probably need the attenuator again). Carefully measure and record V 1 and V 2 and the value of R, but make sure to use two probes for this. One probe will measure V 1 relative to ground, and the second will measure V 2 relative to ground. If you connect one probe directly across the resistor you short out the scope and may blow a fuse and possibly destroy your circuit as well. This is because the negative input of the scope probe is connected to earth ground. You can also use a multimeter in AC mode to measure the RMS voltage, but only do this if the other measurement is too noisy. From the measurements, calculate the value of Rin. Measurements of V 1 and V 2 must be as accurate as possible because both values will be only a few millivolts.

+–

C

V

C

R

R C

R

R

v

C

E E

1

2 B

i L

RC

CC

Signal Generator

V V

R = 1 KΩ

(^1 )

Rin

Fig. 2. Input impedance measurement.

  1. Measure the response of this amplifier to a square-wave input. Because the amplifier gain is dependent on frequency, an input square wave will not result in a perfect square wave at the output. For a discussion of the shape of the output pulse, see Appendix D, pp. 12-15. First use a low-amplitude, high frequency square wave on the input. The amplitude of the input should be about 0.01 volt. Measure the rise time (see Fig. D-13 in the book) and fall time (which should be very close to the rise time). Now decrease the frequency of the square wave by several orders of magnitude and measure the “percentage sag” (see Fig. D-14 in the book). The cursors feature of the scope make these measurements easier.

Report

  1. Calculate the mid-band gain of the amplifier AM , the input impedance Rin , and the lowest corner frequency (use equations developed in the textbook). Using the simulated results from SPICE assignment #1, make a table that compares the mid- band gain, input resistance, and lowest corner frequency. How do they compare? Why are they different?
  2. Compare, in table form, your simulated DC node voltages, measured DC node voltages, and the values calculated by hand. How do they compare? Why are they different?
  3. Using your spice simulation data from SPICE assignment #1, measured data with the 10x probe, and measured data with the coax probe, prepare a Bode plot of magnitude and phase for all three on the same set of axes using MATLAB.

Make sure the phase starts at zero for all three sets of data, and use unwrap on the phase if there are any 180-degree jumps in the measured data. Also, don’t plot the measurements as a continuous line, use a * and ^ to plot the measured points (i.e. plot(x,y,’*’) ).

How do the plots compare? What causes the simulated to vary from the measured? Which type of probe is better for measurements? Is the SPICE model accurate?

  1. Compute the upper and lower corner frequencies, i.e. the 3-dB frequencies, of your amplifier from your low-amplitude square-wave measurements. Compare these values with those obtained from your transfer function measurements. Refer to Appendix D in the text for the relationship between corner frequencies and time constants.