COMSATS Institute of Information Technology
Virtual campus
Islamabad
Dr. Nasim Zafar
Electronics 1
EEE 231 – BS Electrical Engineering
Fall Semester – 2012
DC Analysis of Transistor CircuitsI
Lecture No:
16
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NPN Transistor
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Operation of an NPN Transistor:
Large current
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Modes of Operation of a BJT Transistor:
Mode:
BE Junction:
BC Junction:
cutoff
reverse biased
reverse biased
Active(linear)
saturation
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Forward Biased
forward biased
Reverse Biased
forward biased
5
Transistor Characteristics:
IC
IB
Output
Circuit
Input
Circui
t
IE
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DC Current and Voltage Analysis:
NPN
IC
IB
base
collector
Large current
+
VBE emitter
IE
Small Current
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DC Current and Voltage Analysis:
PNP
IC
IB
base
collector
Large current
+
VBE emitter
IE
Small Current
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DC Analysis of Transistor Circuits.
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DC Analysis of Transistor Circuits:
Ø
The DC Analysis of the transistor circuits involves solving for all
(or most of) the currents and voltages in the circuit.
Ø
Ø
Ø
The most important DC parameters to solve are IC and VCE.
The “Qpoint” of a transistor, gives the values of IC and VCE that are
present in the given transistor circuit.
The first step in the DC analysis, of any transistor circuit, is to solve for
one of the unknown currents, IB, IC, or IE.
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Ø
Ø
DC Analysis of Transistor Circuits:
We need a set of equations – a model of the transistor – to be used in
transistor circuit theory.
In a basic transistor circuit, we have three terminals collector, base and
the emitter.
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DC Analysis of Transistor Circuits:
Ø
Ø
Ø
In a basic transistor circuit, we have three terminals collector, base and
emitter; and correspondingly we have:
Three possible DC voltages: VBE, VCB, VCE but only two are
independent due to KVL; and
Three transistor DC currents: IB, IC, IE but only two are independent
due to KCL.
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DC Analysis of Transistor Circuits:
Ø
Ø
Ø
If we solve for one of these unknowns, the other two can be found by using
the current gain equations and the given value of .
The recommended way of solving for one of the currents is to write a
Kirchhoff's Voltage Law (KVL) loop.
The KVL states the sum of all voltages around a closed loop equals zero.
Va = V1 + V2
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BJTCurrent and Voltage Analysis:
Ø
When the baseemitter junction, in an NPN transistor is forward biased,
it is like a forward biased diode and has a forwardvoltage drop of:
VBE = 0.7 V
Ø
Since the emitter is grounded, by Kirchhoff’s voltage law, the voltages in
the input circuit are:
NPN
VBB = VRS + VBE
VRS = VBB VBE
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BJTCurrent and Voltage Analysis:
Using Ohm’s law: VRS = IB RS
NPN
IB RS = VBB VBE
The drop across RL is:
VRL = IC RL
The collector voltage is:
VCE = VCC IC RL
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DC Analysis of Transistor Circuits
Examples and Exercises:
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Summary of equations for a BJT:
IE IC
IC = β IB
β is the current gain of the transistor 100200
VBE = 0.7V(NPN)
VBE = 0.7V(PNP)
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Example 41: CommonEmitter Configuration:
Given: IB = 50 A , IC = 3.65 mA
Determine: IE , dc and dc
Solution:
IE= IB+ IC= 50 A + 3.65 mA= 3.7
mA
dc = IC / IB = 3.65 mA / 0.05 mA = 73
dc = IC / IE = 3.65 mA/ 3.7 mA =
0.986
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Exercise 41
A certain transistor has a dc of 200. When the base current IB = 50 A.
Determine the collector current. Also calculate dc .
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Example 42
A given NPN transistor has dc = 150. Determine IB, IC , IE , VCB ,
VCB and VBE in the circuit shown below (ignore ac signal):
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BJTOutput Characteristics:
Common Emitter Configuration.
Graphical construction for determining the dc collector current IC
and the collectortoemitter voltage VCE.
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BJTOutput Characteristics:
Common Emitter Configuration
Ø
Ø
We must operate the transistor in the linear region.
A transistor’s operating point (Qpoint) is defined by IC, VCE, and IB.
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Example 4.3
From the output characteristics of the common emitter
configuration shown below, find ac and dc with an Operating point
at IB=25 A and VCE =7.5V.
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Summary of DC Analysis:
Ø
Bias the transistor so that it operates in the linear region
Ø
BE junction forward biased, CE junction reversed biased.
Ø
Use VBE = 0.7 (NPN), IC IE, IC = βIB
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Write BE, and CE voltage loops.
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For DC analysis, solve for IC, and VCE.
Ø
For design, solve for the resistor values (I C and VCE specified).
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