Experiment #8:
Design of Common Collector Amplifiers
Friday Group
Dr. Somnath
11-6-09
Ari Mahpour
Teddy Ariyatham
Jayson De La Cruz
Table of Contents
Objective ......................................................................................................................................... 3
Tools ............................................................................................................................................... 3
Theory ............................................................................................................................................. 4
Preliminary Calculations ................................................................................................................. 5
Part 1 ....................................................................................................................................... 5
Part 2 ....................................................................................................................................... 5
Part 3 ....................................................................................................................................... 5
Part 4 ....................................................................................................................................... 5
Discussion and Results ................................................................................................................... 6
Parts 2 and 3 ................................................................................................................................ 6
Part 4 ........................................................................................................................................... 8
Part 5 ........................................................................................................................................... 9
Part 6 ......................................................................................................................................... 11
Conclusion .................................................................................................................................... 13
Objective
The purpose of this laboratory experiment was to design a common collector amplifier
using a bipolar junction transistor. Prior to doing so, it was imperative to follow the collector
and base currents and beta values through the curve tracer. In previous experiments we had
exposure to the curve tracer so this did not become an obstacle in our experiment. Compared
to the sixth experiment, where there was a considerable amount of trouble with calculations,
the curve tracer portion seemed to be very straight forward. Using a series of methods such as
the ten points (provided by the professor) and notes from the corresponding lecture class, the
design was very straight forward, enabling the preliminary calculations portion to move
through quickly.
Tools
-
Oscilloscope
-
Functional generator
-
Power supply
-
Transistor: Q2N2222A
-
Capacitors: 10µF
-
Resistors: (Specific for each circuit design)
Theory
Using the curve tracer, the student is expected to find the corresponding beta and
current values that match up to the bipolar junction transistor that is used in the lab. In this
case, the students are instructed to use a 2N2222A transistor and must trace their own part.
Using another student’s values could potentially cause trouble since each and every transistor
was not created equally. All corresponding beta and current values can be different for each
transistor since the fabrication process is not always exact. When tracing ones own transistor,
experimental errors stay at a minimum (if none at all). Design calculations must be done by
hand and then verified using PSPICE. Both methods are necessary to ensure that the circuit will
function correctly.
Preliminary Calculations
Part 1
(Refer to calculations sheet)
Part 2
(Refer to calculations sheet)
Part 3
(Refer to calculations sheet)
Part 4
(Refer to calculations sheet)
V2
18
R1
365.4uA
40k
Rsig
50
0A
VOFF = 0.00000001 V3
VAMPL = 2.5
FREQ = 10kHz
0A
05.738mA
Q1
C1
Q2N2222A
5.373mA
C2
10uF
26.87uA
V
R2
338.5uA
10k
10uF
-5.400mA
Re
5.400mA
500
0
Figure 8.1: PSPICE Model
Rl V
5k
0A
Discussion and Results
Parts 2 and 3
IC (A)
RE (Ω)
VCE (V)
Theoretical Experimental Percent Error
0.00512
0.0054
5.47%
500
500
15.4
15.45
0.32%
V2
18
R1
365.4uA
40k
Rsig
50
0A
VOFF = 0.00000001 V3
VAMPL = 2.5
FREQ = 10kHz
0A
05.738mA
Q1
C1
Q2N2222A
5.373mA
C2
10uF
26.87uA
V
R2
338.5uA
10k
10uF
-5.400mA
Re
5.400mA
500
0
Figure 8.3a: PSPICE Model
Rl V
5k
0A
3.0V
2.0V
1.0V
-0.0V
-1.0V
-2.0V
-3.0V
0s
V(V3:+)
20us
V(Rl:2)
40us
60us
80us
100us
120us
140us
160us
Time
Figure 8.3b: PSPICE Model
Figure 8.3c: PSPICE Calculations
VIN (V)
VOUT (V)
AV
Theoretical Experimental Percent Error
2.499
0.108
2.462
0.106
0.9849
0.9815
0.35%
Since this amplifier is a common collector, the gain is 1. This circuit is also known as a
unity gain amp, or a buffer amp. The values we calculated result in a design that meets the
required specifications. The gain is 9.8, which is greater than the required 9.5. Output swing
across load is driven to 2.4 V which is greater than the 2 V required in the specifications.
180us
200us
Part 4
3.0V
2.0V
1.0V
-0.0V
-1.0V
-2.0V
-3.0V
0s
V(V3:+)
20us
V(Rl:2)
40us
60us
80us
100us
120us
140us
160us
180us
Time
Figure 8.4a: PSPICE Model
Clipping occurs around 2.5 V when the input is at 2.7 V as shown in Figure 8.4a.
We used PSPICE to measure the output voltage at which our V out begins to show
distortion, meaning the maximum voltage our amplifier is able to drive the output. In both our
experimental and PSPICE circuits, we were able to drive the load resistor to around 2.7V. This
surpasses the specified output swing of at least 2V across a 5kOhm load resistance.
200us
Part 5
V1 (V)
Resistance (Ω)
V2 (V)
Zi (Ω)
Theoretical Experimental
2
0.112
7500
6900
1
0.0536
7500
6900
V2
18
R1
40k
Rsig
0
Q1
C1
Q2N2222A
7.5k
10uF
C2
V
VOFF = 0.00000001 V3
VAMPL = 2
FREQ = 10kHz
10uF
V
R2
10k
Re
500
0
Figure 6.5a: PSPICE Model
Rl V
5k
2.0V
1.0V
0V
-1.0V
-2.0V
0s
V(V3:+)
20us
V(Rl:2)
40us
V(C1:1)
60us
80us
100us
120us
140us
160us
180us
Time
Figure 6.5b: PSPICE Simulation
Figure 8.3c: PSPICE Calculations
In experiment 6, we learned that we could essentially guess and check with various
input resistors to find out what the input impedance of our circuit was. This was done by
inserting an additional input resistor (Rin) with a value such that our Vn will be half the input
voltage (Vin) of the original circuit. We know this because whenever a voltage is passed through
2 equivalent resistors in series, the voltage is divided in half. In Figure 6.5b, we see that our
original PSPICE input voltage is 1.99V, and when a 7.5kΩ resistor is inserted, the voltage across
that resistor becomes 0.99V. Therefore we can conclude that our input impedance (Zin) is
approximately 7.5kΩ. This coincides with the generalization that common collector amplifiers
have high input impedance, which explains why they are commonly used as the last stage in
multi-stage amplifiers.
200us
Part 6
Theoretical Experimental
V1 (V)
0.098
0.11
Resistance (Ω)
10
47
V2 (V)
0.050
0.0536
Zo (Ω)
10
47
V2
18
R1
40k
Rsig
0
Q1
C1
Q2N2222A
500
VOFF = 0.00000001 V3
VAMPL = 100mV
FREQ = 10kHz
10uF
C2
10uF
V
V
R2
10k
Re
500
0
Figure 6.6a: PSPICE Model
Rl
10
100mV
50mV
0V
-50mV
-100mV
0s
V(V3:+)
20us
V(Rl:2)
40us
60us
80us
100us
120us
140us
160us
180us
Time
Figure 6.6b: PSPICE Simulation
As with finding input impedance, we use a similar guess and check method to find
output impedance. This is done by first measuring the output voltage with an open (infinite)
load resistance. In PSPICE, this is done by inserting a load resistance of < 1 MegaOhm, since
pspice doesn’t handle open circuits. Next, we increase the original load resistance such that the
output voltage (open load) is divided in half. Above in Figure 6.6b, we can see that inserting a
10Ω resistor changes our output voltage from 98mV to 50mV. Therefore our output impedance
(Zout) is approximately 10Ω.
200us
Conclusion
In this laboratory experiment we modeled a common collector amplifier
with little errors and ease of design. The most important factor to the beginning
of this laboratory experiment was to ensure that the proper curve tracer values
were found. In addition, the ten steps were very helpful in designing the circuit in
a fast manner. In real world applications people find common collectors to be an
integral part of the basic amplifier. They are in the middle range with respect to
the fastest and most commonly used amplifiers. In future experiments, the
laboratory manual will take the student through more complicated amplifiers that
require a bit more skill to design.