Technical University of Gdañsk
Department of Medical and Ecological Electronics
Laboratory of Basic Electronics
Exercise 4
prepared by:
Piotr Jasiñski
Gdañsk 1997
Exercise 4
Active elements - basic applications of transistors and operational
amplifiers
1. Exercise programme
1.
2.
3.
4.
5.
Transistor common emitter amplifier
Transistor common collector amplifier (emitter follower)
Operational amplifier as inverting amplifier
Operational amplifier as low-pass filter
Operational amplifier as high-pass filter
2. List of devices used in the exercise
1.
2.
3.
4.
5.
6.
7.
8.
Laboratory circuit
Power supply unit
Two multimeters
Oscilloscope
Generator
Tuneable power supply
Selective nanovoltmeter
Decade resistor
3. Exercise
The main aim of this exercise is to present general applications of transistor and operational
amplifiers. Figure 1 shows the laboratory circuit. Proper connection allows to study transistor
common-emitter amplifier, transistor common-collector amplifier, op-amp inverting
amplifier, op-amp low-pass filter, op-amp high-pass filter. The role will be dependent on
electrical connections on board of the circuit.
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Figure A. Laboratory circuit
Switch on all the instruments which will be used in exercise. Set +15V and -15V supply
voltage in the power supply unit. Apply supply volatge to the laboratory circuit with correct
polarity.
3.1 Transistor common emitter amplifier
Make connections in the laboratory stand to obtain the circuit as shown on Figure 8. Check
values of elements in the measurement protocol and apply them to the laboratory circuit.
Switch multimeter into DC voltage measurement mode. Switch the multimeter’s range into
10V. Measure voltage drop on Rc resistor by connecting Hi input of multimeter to the upper
connector of resistor and Lo input of multimeter to lower connector of resistor. Write the
measured value of voltage into Table 1.a.
Make connections in the laboratory stand to obtain the circuit as shown on Figure 9. At first
do not apply Ce capacitor and Ro load. Switch multimeters into alternating voltage
measurement mode. Switch multimeters range into 10V. Set frequency and amplidute of
generator as indicated in table 1.b. If multimeters range will be not sensitive enough, change
it.
UOUT
UIN
generator
1kHz
multimeter
UAC
oscilloscope
multimeter
UAC
Figure B. Transistor common-emitter amplifier circuit.
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Apply to the circuit Ce capacitor and fill the Table 2. Then apply to the circuit Ro resistor
(decade resistor) and fill in Table 3.
3.2 Transistor common collector amplifier (emitter follower)
Change connections in the laboratory stand to obtain the circuit as shown on block diagram
Figure 3 (or connection diagram Figure 10).
generator
1kHz
UIN
UOUT
multimetr
UAC
multimeter
UAC
RO
oscilloscope
Figure C. Transistor common-collector amplifier circuit.
Check values of elements in the measurement protocol and apply them to laboratory circuit.
Fill in Tables 4, 5 with measurements results.
3.3 Operational amplifier as inverting amplifier
3.3.1 DC measurements
Switch multimeters into direct voltage measurement mode. Switch multimeters range into
10V. Set tunable power supply output into 0V. Make connections in the laboratory stand to
obtain the circuit as shown on block diagram Figure 4 (or connections diagram Figure 12). At
first do not apply decade resistor Ro. Check values of elements in the measurement protocol
and apply them to laboratory circuit. Fill in Table 6.
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UIN
UOUT
tunable
power supply
multimeter
UDC
multimeter
UDC
RO
oscilloscope
Figure D. Inverting amplifier DC circuit.
Make connections in the laboratory stand to obtain the circuit as shown on connections
diagram Figure 11. Fill in Table 7 with measurement results.
Make connections in the laboratory stand to obtain the circuit as shown on connections
diagram Figure 12. Fill in Table 8 with measurement results.
3.3.2 AC measurements
Switch multimeters into alternating voltage measurements. Switch multimeters range into
10V.
Make connections in the laboratory stand to receive the circuit as shown on Figure 5 (or
connection diagram Figure 13).
UIN
UOUT
generator
multimeter
UAC
multimetr
UAC
oscilloscope
Figure E. Inverting amplifier AC circuit.
Check values of elements in the measurement protocol and apply them to the circuit. Fill in
Table 9 with measurement results. If multimeter is not sensitive enough or overloaded change
the range of multimeter.
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3.4 Operational amplifier as low-pass filter
Change connections in the laboratory stand to obtain the circuit as shown on Figure 6 (or
connection diagram Figure 14).
UIN
UOUT
generator
multimeter
UAC
multimeter
UAC
oscilloscope
Figure F. Low-pass filter circuit.
Check values of elements in the measurement protocol and apply them to the circuit. Fill in
Table 10 with measurement results. Change multimeter range if readout is overloaded or out
of range.
3.5 Operational amplifier as high-pass filter
Change connections in the laboratory stand to obtain the circuit as shown on Figure 7 (or
connection diagram Figure 15).
UIN
UOUT
generator
multimeter
UAC
multimeter
UAC
oscilloscope
Figure G. High-pass filter circuit.
Check values of elements in the measurement protocol and apply them to the circuit. Fill in
Table 11 with measurement results. Change multimeter range if readout is overloaded or out
of range.
4. Measurement data evaluation
For measurement data evaluation check protocol.
6
Figure H. Connection diagram to fill table 1.a. (dashed lines - necessary cable connections)
Figure 9. Connection diagram to fill tables 1.b., 2. and 3. (dashed lines - necessary cable connections)
8
Figure 10. Connection diagram to fill table 4. and 5. (dashed lines - necessary cable connections)
9
Figure 11. Connection diagram to fill table 6. and 7. (dashed lines - necessary cable connections)
10
Figure 12. Connection diagram to fill table 8. (dashed lines - necessary cable connections)
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Figure 13. Connection diagram to fill table 9. (dashed lines - necessary cable connections)
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Figure 14. Connection diagram to fill table 10. (dashed lines - necessary cable connections)
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Figure 15. Connection diagram to fill table 11. (dashed lines - necessary cable connections)
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5. Literature
P.Horowitz, W.Hill, Sztuka elektroniki, pp.72-121, 187-247.
6. Bipolar Junction Transistors and their applications as amplifier
The bipolar junction transistor is constructed with three doped semiconductor regions
separated by two PN junctions. The three regions are called emitter, base and collector. There
are two types of bipolar transistors: PNP (two P regions separated by N region) and NPN (two
N regions separated by P region). The PN junction joining the base region and the emitter
region is called the base-emitter junction. The junction joining the base region and the
collector region is called the base-collector junction. Wire leads connected to each of three
regions are named E, B and C for emitter, base and collector respectively.
Figure shows the schematic symbols for the NPN and PNP bipolar transistors.
Figure 16. Transistor symbols
The term bipolar refers to the use of both holes and electrons as carriers in the transistor
structure.
6.1 Common - emitter amplifier
Figure 16 shows a typical common - emitter amplifier. C1 and C2 are coupling capacitors
used to pass the signal into and out of the amplifier so that the source or load will not affect
the DC bias voltage. CE capacitor is a bypass capacitor that shorts the AC emitter signal
voltage to ground without disturbing the DC emitter voltage. Because of the bypass capacitor,
the emitter is at signal ground (but not DC ground), thus making the circuit a common emitter amplifier. The purpose of the bypass capacitor is to increase the signal voltage gain.
The voltage gain of the CE amplifier without the bypass capacitor shorting RE is:
Av =
U out
I C RC
RC
25mV
=
≅
where re ≅
IE
U in
I E ( re + RE ) re + RE
The voltage gain of the CE amplifier with the bypass capacitor shorting RE is:
Av ≅
RC
re
where
re ≅
25mV
IE
Resistance re is a very important transistor parameter because it determines the voltage gain of
CE amplifier in conjunction with RC.
RO
Figure 17. Typical common-emitter amplifier.
All presented equations assumed that output load resistance (RO) does not exist or is very big
(which practically means that RO is much bigger than RC). In the case when this assumption is
not true there is the following equation which evaluates voltage gain.
Av ≅
RC RO
re
=
1  RC * RO 


re  RC + RO 
where
re ≅
25mV
IE
It is important to notice that the voltage gain depends a lot on load resistance value.
6.2 Common - Collector amplifiers (emitter follower)
The common - collector (CC) amplifier is the second most important basic amplifier
configuration. It is commonly referred to as an emitter-follower. Figure shows an emitter follower.
RO
Figure 18. Typical emitter - follower (common-collector) amplifier.
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The voltage gain of the CC amplifier is:
U out
I E RE
RE
=
=
≅1
Av =
U in
I E ( re + R E ) re + R E
It is important to notice that the gain is always lower than 1. If re is much lower than RE
(which is usually true), then a good approximation for the value of gain is 1.
Again all presented equations assumed that output load resistance (RO) does not exist or is
very big. In the case when this assumption is not true there is the following equation which
evaluates voltage gain.
Av ≅
R E RO
( R E RO ) + re
where
re ≅
25mV
IE
It is important to notice that the voltage gain even with very small load resistance does not
practically change the value. This is a very important feature of emitter follower and indicates
that output resistance is very small. Input resistance of emiter follower is dependent on load
resistance. This feature is called transformation of resistance.
7. Operational Amplifiers (OP) and their applications
The operational amplifier is a linear integrated circuit which typically consists of two or more
differential amplifier stages. As we will not go into details how OP is build it is enough to say
that OP has two input terminals (called the inverting input and the noninverting input) and one
output terminal. The typical OP operates with two DC supply voltages, one positive and the
other negative. The standard OP symbol is shown in Figure 19.
Figure 19. Operational Amplifier symbol
The main characteristic of OP can be illustrated by equation:
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VOUTPUT=A*(VINPUT+ - VINPUT-)
where A is open-loop voltage gain (usually bigger than 10000). Also input impedance is very
big (close to infinite) and output impedance is very small (close to zero).
Application of OP as amplifier usually using a negative feedback design. This feedback
stabilizes the gain and increases frequency response. Negative feedback takes a portion of the
output and applies it back out of the phase with the input, creating an effective reduction in
gain. This gain is called closed-loop gain and is independent of open-loop gain.
7.1 OP as inverting amplifier
The circuit shown in the Figure represents OP connected as an inverting amplifier.
Figure 20. Inverting amplifier
To understand how this amplifier is working and to calculate close-loop gain, let’s assume
that an input impedance of OP has infinite value (what is good approximation). The infinite
input impedance implies zero current to the inverting input. If there is zero current flow
through the input impedance, then there must be no voltage drop between the inverting and
noniverting inputs. That means, the voltage at the inverting input is zero because the other
input is grounded. The zero voltage at the inverting input terminal is referred to as virtual
ground. On this basis we can calculate closed-loop voltage gain.
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IR 2
−U out
=
R2
I R1
U IN
=
R1
I R1 = I R 2
U out
R2
A=
=−
U IN
R1
This clearly shows, that the closed-loop gain is independent of the OP internal open-loop gain.
Thus, the negative feedback stabilizes the voltage gain. The negative sign indicates inversion.
7.2 OP as low-pass filter
Figure shows a basic RC low-pass filter circuit.
Figure 21. Single-pole RC low-pass filter
The equation shows amplitude gain of this filter.
A=
R2
R1 1 + ( 2πfR2 C) 2
Amplitude is strongly dependent on frequency (f). Figure shows the filter response curve.
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fc
0.01
0
0.1
1
10
100
-5
-10
-15
-20
-25
-20dB/decade
-30
-35
-40
Figure 22. Response curve of single pole low-pass filter (fc=1Hz, R2/R1=1)
A filter has flat characteristic till fc (critical frequency) and from this point -20dB/decade rolloff begins. The term „-20dB/decade” means that the voltage gain decreases by ten times (20dB) when the frequency increases by ten times (decade). A filter with 20dB/decade slope is
called a first-order filter or a single pole.
Critical frequency fc can be calculated from the formula fc=1/(2πR2C). The graphical
representation of this frequency can be described: critical frequency is the frequency where
amplitude gain drop -3dB.
Note, that amplitude gain in flat area depends only on R2/R1 rate and can be bigger than 1.
7.3 OP as high-pass filter
Figure shows a single pole high-pass filter circuit.
Figure 23. Single pole high-pass filter circuit
The equation shows amplitude gain of this filter.
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A=
2πfR2 C
1 + ( 2πfR1C ) 2
Amplitude is strongly dependent on frequency (f). Figure shows the filter response curve.
fc
0.01
0
-5
0.1
1
10
100
-10
-15
-20
-25
-30
-35
20dB/decade
-40
-45
Figure 24. Response of single-pole high-pass filter (fc=1Hz, R2/R1=1).
In this case a filter has flat characteristic over critical frequency and till this point
20dB/decade roll-on run. This time, term „20dB/decade” means that the voltage gain increases
by ten times (20dB) when the frequency increases by ten times (decade). Again, filter with
20dB/decade slope is called a first-order filter or a single pole.
Critical frequency fc can be calculated from the same formula as low-pass filter:
fc=1/(2πR2C). The graphical representation of this frequency can be described: critical
frequency is the frequency where amplitude gain is 3dB lower than maximum gain. Gain in
the flat area is again dependent on R2/R1 rate.
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