Bipolar Transistors:
Bipolar transistors are made by placing a p-type layer between two n-type layers
(NPN transistor) or by placing an n-type layer between two p-type layers (PNP
transistor). In essence transistor is equivalent to placing two P-N junctions backto-back. A transistor has three terminals: (1) collector, (2) base, and (3) emitter.
In an NPN transistor, both the collector and emitter are of n-type material and the
base is of p-type material. In a PNP transistor, however, both the collector and
emitter are of p-type material and the base is of n-type material.
Emitter is heavy doped (to have plenty of free electron or holes, depending on
the type of the transistor) and is a medium-sized layer. The base is made up of a
very thin layer and is medium doped. The collector is lightly doped and is made
of a thick layer (to allow collection of electrons from the base). In normal
operation, the collector-to-base junction is kept reverse-biased and the base-toemitter junction is kept forward-biased. Figure 1 shows and NPN transistor with
proper bias conditions.
Figure 1: An NPN transistor.
With base-to-emitter junction forward biased (VBE > 0.6 V) a large amount of
electrons from the heavily doped emitter gain sufficient energy to cross the baseemitter depletion region and enter into the base. Note that the base-to-collector
junction is reverse biased and, therefore, no electron can enter to the base from
the collector. Since the base is made very thin, the electron entering the base
from the emitter over-crowd the base region. These electrons have two possible
paths to move forward: (1) return to the positive terminal of the source voltage at
the base (VB), or (2) dash ahead to collector by crossing the base-to-collector
junction. Note that in case (2) electrons go from base to collector. In reality, the
electrons take both of these paths. However, a majority of electrons sweeps
through to the collector and a small fraction returns to the base terminal. The
base current (IB) and the collector current (IC) are related by the following
equation:
I C = β X IB
where β is called the current gain of the transistor. Note that a small variation in
IB (say due to a variation in the input voltage at the base) causes a large variation
in IC (β times the variation in IB). In this way, the transistor act like an amplifier for
the input signal. Figure 2 shows a simple single-stage amplifier.
Figure 2: A single-stage amplifier.
Example 1:
For the circuit shown in Fig. 2, RB = 66 KΩ, RC = 3.3 KΩ, VCC = 10 V, VBB = 1.6 V.
Find iB and iC (assume β = 100, vin(t) = 0, and VBE = 0.6 V).
Solution:
Applying Kirchoff’s voltage law on base-emitter loop gives us
iB RB + VBE - VBB = 0
or
iB = 15.15 µA
and iC = β X iB = 1.5 mA
Collector characteristics of a Common-Emitter amplifier are a set of iC versus VCE
curves obtained for different values of iB (see Figure 3).
Figure 3: Typical collector characteristics.
Assignment 1:
Build the circuit shown in Fig. 2 using Electronics Workbench. For each value of
RB shown in Table 1 measure iC and VCE by changing VCC from 0.0V to 10.0 volts
in 0.5 volts steps (see table 2: you would need to take a total of 21
measurements of iC and VCE for each value of RB). Then plot the collector
characteristics curve and submit with your report.
Table 1.
RB in KΩ
200
100
66
50
40
33
Table 2.
VEC (in volts)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.0
10.0
IC (in mA)
VCE (in volts)
Assignment 2:
Build the circuits shown in Fig. 4 and 5 using Electronics Workbench. Use RB =
5.1KΩ, RC = 2 KΩ, and VCC = 5 volts. Use transistor # 2N222A.
For Figure 4: Use input as 0 volts and 5 volts and compute the corresponding
output. Record your observations. What kind of a circuit is this?
For Figure 5: Apply the voltages shown in Table 3 and measure the output.
Record your observations in the table. What kind of a circuit is this?
Figure 4.
Figure 5.
Table 3.
Input (in volts)
Output
A
B
C
0
0
0
0
0
5
0
5
0
0
5
5
5
0
0
5
0
5
5
5
0
5
5
5
(in volts)