Download 4 TRANSISTOR CHARACTERISTICS

Electronic Circuits I Laboratory
4 TRANSISTOR CHARACTERISTICS
4.1 Objectives
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Understanding the characteristics and symbols of transistors.
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Calculation of parameters of a transistor by practical measurements.
4.2 Basic Description
4.2.a
Structures of the transistors
As you know, transistors are classified into two major types, PNP and NPN type.
Their structure diagrams are respectively shown in Fig. 4.1 (a) and (b). E (Emitter), B
(Base) and C (Collector) respectively represents the three terminals of the transistor.
Fig. 4.1
As shown in Fig. 4.2 (a), if a forward bias (P and N are respectively connected to
positive and negative polarity) is applied across the terminals of E-B so that VBE will
reach the cut-in voltage (0.6V for silicon and 0.2V for germanium), a forward current IB
will be generated between E-B. As shown in Fig. 4.2 b), if a reverse bias (P and N are
respectively connected to negative and positive polarity) is applied across the terminals
of E-B, no current will flow through B-C (the reverse leakage current is so small that it
can be neglected) and the current IC that flows through C terminal is zero. If Fig. 4.2 (a)
and (b) are combinatively connected as Fig. 4.2 (c) or (d), inspite of the originally
reverse bias between B-C (as shown in Fig. 4.2 (d), VCB=VCC-VBE, VCC >> VBE ), a
significant current Ic will be generated due to the forward conduction of VBE. The
equation Ic = β.IB ( where β is the current amplification factor) describes the relation
between Ic and IB. Why is IB smaller than IC? This is beacuse the base of transistor is
very narrow and is very lowly doped with impurity. VBE will drive the electrons in E (NPN
type) to massively enter B. B is so narrow that only small portion of electrons will
combine with holes and will move toward B whereas most electrons will move toward
the B-C junction. This phenomenon along with the higher voltage (VBC or VCC) applied in
C will generate the significant amount of IC. As shown
in Fig. 4.2 (c) and (d), we
can see that IE = IB + IC . Similarly, if the bias shown in Fig. 4.3 is applied to PNP
transistor, this transistor will function the same as NPN transistor.
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Fig. 4.2
Fig. 4.3
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4.2.b Symbols and Casing
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Symbols
Fig. 4.4
Fig. 4.4 shows the symbols of the transistors that are used to differentiate
between NPN and PNP wherein the outward pointed arrow represents NPN type and
the inward pointed arrow represents PNP type. Arrow is used to indicate the direction of
the current.
•
Casing
Transistors come in many different packages. The two main categories are
through-hole (or leaded), and surface-mount, also known as surface mount device
(SMD). The ball grid array (BGA) is the latest surface mount package (currently only for
large transistor arrays). Because they are smaller and have shorter interconnections,
SMDs have better high frequency characteristics but lower power rating.
Fig. 4.5 Internal Construction of Fairchild Transistor in a TO-92 Package
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Transistor packages are made of glass, metal, ceramic or plastic. The package
often dictates the power rating and frequency characteristics. Power transistors have
large packages that can be clamped to heat sinks for enhanced cooling. Additionally,
most power transistors have the collector or drain physically connected to the metal
can/metal plate.
Fig. 4.6 – Various packaging types of general-purpose or switching transistors: (a) low
power; (b) medium power; (c) medium to high power
Often a given transistor type is available in different packages. Transistor
packages are mainly standardized, but the assignment of a transistor's functions to the
terminals is not: different transistor types can assign different functions to the package's
terminals. Even for the same transistor type the terminal assignment can vary (normally
indicated by a suffix letter to the part number- i.e. BC212L and BC212K).
4.3 Experiment Equipments
1. KL- 200 Linear Circuit Lab. Device
2. Experiment Instruments: Oscilloscope, Multimeter
3. Connection cables
4. Resistors 47K, Potentiometers 1K, 10K
5. Transistor, 2N 3904
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4.4 Procedure
Derivation of IC – VCE – IB characteristic curve
B
------- Step 1 -----(1) Setup the circuit shown in “Step 1”.
a. Use main module pots for VR1(1K) and VR2(10K).
b. Connect analog ameter for base current (IB)
measurement.
c. Connect digital voltmeter for Collector-Emitter voltage (VCE) measurement.
(2) Now only adjusting VR2 (10K), adjust base current (IB) to 10uA.
(3) Then relocate analog ameter so that you are able to measure collector current
(IC) as shown in “Step 2”.
---- Step 2 ----
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(4) Now, by using VR1(1K), read and adjust VCE to the desired values in Table 4.1.
For each VCE value, measure the resulting collector current (IC).
(5) After all values of VCEs are accomplished, return to step 1 and repeat the same
measurements for IB=30 uA and IB=50 uA.
STEP 2
VCE
STEP 1
IB=10 uA
IC
IB=30 uA
IC
IB=50 uA
IC
B
B
B
0.1
0.2
0.3
0.5
0.7
1.0
3.0
5.0
7.0
Table 4.1
(6) By using the values of Ic and IB, calculate the average value of β by using the
equation
β ave =
1
N
∑
Δ IC
Δ IB
VCE =constant
where N is the number of measurements that you use for calculation.
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