Laboratory 2.
Exercise 5.
Exercise 5
Testing basic transistor amplifiers
Introduction
In the field of analogue electronics, the basic transistor amplifiers represent a typical
example of such electrical engineering knowledge where both the theory and the practical
experiences are important.
Object of the measurement exercise
The students get acquainted with the work connected to the basic transistor amplifiers
(calculation of the operation point data, the parameters of the equivalent circuit, calibrating
measurements etc.).
Special literature, references 0
[1]
Dr. Hainzmann János – Dr. Varga Sándor – Dr. Zoltai József: Elektronikus áramkörök
(„Electronic circuits”), Műegyetemi Kiadó, 44570, Bp., 1992
1.1 Aszimmetrikus (hárompólusú) erősítők („Asymmetrical /three-pole/
amplifiers”), pp. 18-22.
2.4 Aszimmetrikus erősítők visszacsatolásának négy alapesete („The four basic
kinds of feedback with asymmetrical amplifiers”), pp. 72-80.
7. Aszimmetrikus tranzisztoros erősítő alapkapcsolások („Asymmetrical
transistor amplifiers”), pp. 203-219.
8.3 Fázishasító kapcsolás („Phase splitter”), pp. 224-225.
[2]
Dr. Székely Vladimír: Elektronika I („Electronics I.”)., Műegyetemi Kiadó, 55054,
Bp., 2003.
Kisjelű helyettesítőképek („Small signal equivalent circuits”), (Chapter 5, pp. 4256. )
Tasks for preparation 0.
Do the listed actions below at home as preliminary preparation.
1. Read thoroughly the book chapters listed under Special literature, references.
2. Read and think over the Measurement tasks.
3. Answer the Check questions (at the end of this description).
The preparation may be checked by the leader of the measurement by oral questions.
1
Laboratory 2.
Exercise 5.
Applied instruments
Oscilloscope
Agilent 54622A
Supply unit
Agilent E3630
Function generator
Agilent 33220A
Digital multi-meter (6½ digit)
Agilent 33401A
Digital multi-meter (3½ digit)
Metex ME-22T
Test board
The device under test is built on a test board, the top view of which can be seen on Figure 51.
Figure 5-1. The test board
2
Laboratory 2.
Exercise 5.
On the test board, using jumpers, we can execute measurements on:
•
bipolar transistor amplifiers:
ƒ
common emitter,
ƒ
common collector,
ƒ
phase splitter,
•
JFET common source amplifiers,
•
transistor current generator.
Measurement tasks0.
1. Common emitter amplifier
Inserting the appropriate jumpers configure the following connection arrangement
(Figure 5-2.):
Figure 5-2. Common emitter amplifier
(Be = IN; Ki = OUT; U = V)
Connect the supply unit and set the prescribed supply voltage (VS ): because of the diode
D, which protects against reverse polarity supplying, on the supply unit a battery voltage
VB should be set which is by 0.6 V greater than the VS. Measure the operation point
voltages.
B
1.1. From the measured data calculate the values of IC, IB and B.
B
1.2. Give on the input a sinusoidal signal of f = 1 kHz. Check with an oscilloscope
the operation of the stage. Set the amplitude of the input signal so, that the
output signal remains undistorted. Using the following equivalent circuit (Figure
5-3.) measure:
ƒ
the value of h11e,
⎛
⎞
⎜ Δi = vBE1 − vBE , h = ΔvBE
⎟
11e
⎜ BE1
ΔiBE1 U ⎟
RB
KI 1=0 ⎠
⎝
3
Laboratory 2.
ƒ
the value of h21 = ß,
ƒ
the gain of the stage,
ƒ
the output resistance inserting the load by the jumper JP15.
Exercise 5.
Measure the AC voltages with the Agilent multi-meter. Compare the measured
results with the preliminary calculated values.
Figure 5-3. Equivalent circuit of the transistor to the measurement of the h parameters
1.3. Testing the maximum output voltage of the unloaded amplifier.
1.3.1. Observe the distortion of the output signal in case of voltage generator
(JP1 = ON), i.e. current generator (JP1 = OFF) driving. Explain what you
experience.
1.3.2. Determine for both cases the maximum allowable input, i.e. output
voltages.
2. Common emitter amplifier with emitter degeneration (or emitter feedback)
Create the connection below (Figure 5-4.):
Figure 5-4. Common emitter amplifier with emitter degeneration
4
Laboratory 2.
Exercise 5.
2.1. Measure the voltages on points B, E and C, compare them with the preliminary
calculated values.
2.2. Calculate the actual values of IE and IC.
2.3. Measure the voltage gains in case of unloaded amplifier and with a load of 1
kΩ-mal, compare the measured and the preliminary calculated values.
2.4. Determine the output resistance of the amplifier.
2.5. Measuring the frequency response in terms of the voltage gain.
In this measurement we take the Bode amplitude diagrams, and we analyse the
effect of the different components on the frequency response (JP7: 4.7 nF; JP14:
470 nF; JP15: 2.2 μF, 1 k). In the case of such amplifiers („correctors”) the signal
level should be chosen carefully.
Two aspects should be considered:
o
the signal level should be as large as possible to get a good signal-to-noise
ratio,
o
the amplifier should not be overdriven at the frequency of the maximum
peaking.
On the base of the above, think over which one is the jumper to be inserted so that
the frequency response will be modified in such a way that the output signal will
become maximal. Make a quick check and determine the optimal input signal
belonging to this state.
Set the frequency of the generator to 1 kHz and measure this signal level. Choose
from among the Math functions of the Agilent multi-meter the dB function, match
the 0 dB level to this 1 kHz signal level.
In the range 20 Hz – 200 kHz take the frequency response of the output at three
points in each decade (2; 5; 10) in the following variations:
2.5.1. Unloaded amplifier (JP7, JP14 and JP15 OFF)
2.5.2. Unloaded amplifier with emitter capacitance (JP7 and JP15 OFF, JP14
ON)
2.5.3. Loaded amplifier with collector (JP14 OFF, JP7 and JP15 ON)
2.5.4. Inserting all jumpers.
Evaluate the diagrams, compare the break-point frequencies with the preliminary
calculated values.
2.6. Measuring the square-wave transfer of the amplifier
Apply a square-wave signal of 1 kHz on the input at the jumper positions: JP7,
JP14, and JP15 OFF.
2.6.1. Take the time function of the output signal and measure the voltage
droop and the rise time. (Which time-constant is responsible for the
voltage droop?)
2.6.2. Insert the jumper JP7, measure again the rise time.
2.6.3. Insert also the jumper JP14, measure the overshot of the signal. (Be sure
that the amplifier does not limit even at the overshot.)
Explain the results comparing them with the measurements in the frequency range.
5
Laboratory 2.
Exercise 5.
3. Testing a phase splitter
Configure the connection arrangement bellow (Figure 5-5.):
Figure 5-5. Phase splitter arrangement
3.1. Measure the operation point voltages at the points B, E, C.
Apply at the input a sinusoidal signal of 1 kHz and 1 Vpp. Connecting the Y1 and
Y2 channels of the oscilloscope to the point C and E respectively using the divider
probes. Observe the operation (phase relations, signal levels). Using the Quick
Measure Peak-Peak function of the oscilloscope, measure the signal levels on the
collector and the emitter:
3.1.1. without load,
3.1.2. with an emitter load of 1 kΩ (JP13, JP15 ON),
3.1.3. with a collector load of 1 kΩ (JP12, JP15 ON).
Explain the experiences.
4. Testing a common collector (emitter follower) amplifier
Inserting the jumper JP8, we can make an emitter follower from the previous phase
splitter. Prove this.
Connect the two channels of the oscilloscope to the emitter and the base points by DC
coupling. (The zero lines and the sensitivities should be identical.) Apply to the input a
sinusoidal signal of 1 kHz. Analyse and explain the operation. Increase the amplitude of
the input signal till the signal on the emitter is about to be distorted.
4.1. Measure the magnitude of the maximum undistorted signal.
4.2. Load the emitter follower by 1 kΩ. What do you experience? Determine again
the magnitude of the available maximum signal.
6
Laboratory 2.
Exercise 5.
4.3. Prove by calculation the result measured under 4.2.
4.4. Measure the output resistance of the emitter follower at f = 10 kHz in case of
the following generator resistances: Rg = 0 and Rg = ∞.
We have seen before that measuring the output resistance of the emitter follower using
the usual loading methods can be problematic because of the small output resistance and
the available maximum output signal of the stage. Therefore the measurement is retraced
to measuring the output voltage on the output resistance caused by a measuring current
forced upon the output.
Design the measuring arrangement. The voltage on E, which is proportional to the output
resistance, should be measured with the Agilent multi-meter used as an AC voltmeter. At
the measurement frequency, the 2.2 μF capacitance can already be regarded a shortcircuit.
4.4.1. Determine the output resistance with short-circuited input (Rg = 0) by
calculation and by measurement.
4.4.2. Determine the output resistance by calculation and by measurement with
open-circuited input as well.
Additional measurement tasks
5. Testing a JFET common source amplifier
5.1. Measure the operation point voltages of the amplifier on the test board (at the
points G, S and D), and using the measured values determine the operation
point currents ID and IS.
5.2. Measure the gain of the amplifier without feedback (JP16 ON) at f = 1 kHz.
Determine from the measured value the g21 parameter of the FET (also called
„trans-conductance”). (With different notation: gm = g21.)
5.3. Measure also the gain of the amplifier with feedback. All measurement result
should be compared with the preliminary calculated ones.
5.4. Testing the maximum output voltage swing of the FET amplifier. Increase the
amplitude of the input signal until the output wave-form begins to be visibly
distorted.
5.4.1. Measure the maximum input signal of the amplifier without feedback.
5.4.2. Measure also the maximum input signal of the amplifier with feedback.
Give explanation for the kind of distortion in both cases.
6. Testing a transistor current generator
Increase the supply voltage (VS) to 20 V at this measurement. Calculate the current of the
generator to be expected supposing an output voltage of V2 = 10 V.
6.1. Check the calculation by measurement.
6.2. Take the current-voltage characteristic of the current generator in the range
4 V < VS < 20 V.
6.3. Determine the output resistance of the current generator.
7
Laboratory 2.
Exercise 5.
7. Measuring the maximum output voltage swing of an amplifier
Configure the common emitter amplifier of Figure 5-4. with emitter degeneration.
The quantitative characterisation of the maximum output voltage swing of amplifiers can
be made with input and output voltages having given distortion factor. The harmonic
distortion can be determined with the following formula:
n
k=
∑v
i =2
2
i
v1
where v1 denotes the effective (r.m.s.) value of the fundamental harmonic, and v2......vn
denote that of the over-harmonics.
7.1. Determine at a frequency of f = 1 kHz the value of the output voltage belonging
to a distortion factor k = 2%.
The unloaded amplifier is tested (JP7, JP14 és JP15 OFF).
Using the FFT function of the oscilloscope, measure the dependence of the
distortion factor k on the effective value of the input signal at 4 different levels.
Choosing the 4 signal levels:
Increase the input signal until a just visible distortion can be seen on the output
signal. Regard this as a 100 % level. Increase now the level so far that you see a
signal of broken line wave-form (this is an overdriven state). The remaining two
input signal levels should correspond to the percentages of 50 % and 75 % .
It is enough to make the analysis up to the 9-th over-harmonic component (fSR =
20 kS/s). Ensure that the signal on the oscilloscope has always a proper magnitude,
but the Y channel should not be overdriven.
Check questions
1. Define the matching quantities (the input and the output impedances), and the transfer
quantities (voltage and current gains, transfer impedance and admittance) of an
asymmetrical amplifier.
2. How to calculate the input and the output time-constants of a common emitter amplifier
stage?
3. How do we define the rise time and the fall time?
4. What is the cause of the voltage droop?
5. What is the relation between the rise time and the high limit frequency?
6. What is the relation between the low limit frequency and the voltage droop?
7. How can be determined the operation point of the transistor in a common emitter /
collector / base configuration in case of a single supply or a double supply?
8. How to calculate the conduction (g) and hybrid (h) parameters from the operation point
data of the transistors?
9. How to calculate the voltage gain, the input resistance and the output resistance of the
basic transistor amplifiers if the g or h parameters of the transistor are known?
8
Laboratory 2.
Exercise 5.
10. Draw the high frequency hybrid π equivalent circuit of the bipolar transistors, and
knowing the operation point data calculate the value of the components in it.
11. What does it mean „physical parameter”?
12. What is the effect of the emitter resistance on the high frequency time constants?
13. Draw the Bode plot of a common emitter stage at low frequency.
14. What is the effect of the emitter capacitance on the Bode plot of the common emitter
amplifier?
9