Almughtaribeen University
ALMUGHTARIBEEN UNIVERSITY
College of Engineering
Department of Electrical Engineering
Analog Electronic Circuits I
Lecture 5
BJT Modes of Operation
Almughtaribeen University – [ Abusabah I. A. Ahmed]
Lecture Outline
❑ Introduction
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❑ Operation Modes
❑ BJT in Active Mode
❑ BJT Analysis
❑ Practice Problem
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Introduction
❑ The basic of electronic system nowadays is semiconductor
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device.
❑ The famous and commonly use of this device is BJTs (Bipolar
Junction Transistors).
❑ It can be use as amplifier and logic switches.
❑ There are two types of BJT : pnp and npn.
❑ The term bipolar reflects the fact that holes and electrons
participate in the injection process into the oppositely polarized
material
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Operation Modes
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Operation Modes
❑Active:
❑Most importance mode, e.g. for amplifier operation.
❑The region where current curves are practically flat.
❑Saturation:
❑Barrier potential of the junctions cancel each other out
causing a virtual short. Ideal transistor behaves like a closed
switch.
❑Cutoff:
❑Current reduced to zero
❑Ideal transistor behave like an open switch.
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Operation Modes
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BJT in Active Mode
❑ Operation
✓ Forward bias of EBJ injects electrons from emitter into base (small
number of holes injected from base into emitter)
▪ Most electrons shoot through the base into the collector across the
reverse bias junction (think about band diagram)
▪ Some electrons recombine with majority carrier in (P-type) base region
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BJT in Active Mode
Beta () or amplification factor
❑ The ratio of dc collector current (𝐼𝐶 ) to the dc base current (𝐼𝐵 )
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is dc beta (dc ) which is dc current gain where 𝐼𝐶 and 𝐼𝐵 are
determined at a particular operating point, Q-point (quiescent
point).
❑ It’s define by the following equation:
30 < dc < 300 → 2N3904
❑ On data sheet, dc is derived from forward-current amplification
and common-emitter configuration respectively.
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BJT in Active Mode
Beta () or amplification factor
❑ For ac conditions an ac beta has been defined as the changes
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of collector current (IC) compared to the changes of base
current (IB) where IC and IB are determined at operating
point.
❑ It can defined by the following equation:
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BJT in Active Mode
Example 5-1
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From output characteristics of common emitter configuration,
find ac and dc with an Operating point at IB=25 A and VCE
=7.5V.
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BJT in Active Mode
Solution
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BJT Analysis
❑We would want to know the collector current (iC),
collector-emitter voltage (VCE), and the voltage
across 𝑅𝐶 .
❑To get this we need to fine the base current (iB) and
the base-emitter voltage (VBE).
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BJT Analysis
❑To start, let’s write Kirchoff’s voltage law (KVL)
around the base circuit.
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BJT Analysis
❑ Likewise, we can write KVL around the
collector circuit.
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BJT Analysis
❑Let’s assume that Vin(t) = 0.2 sin(ωt).
❑Then the voltage sources at the base vary from a
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maximum of 1.6 + 0.2 = 1.8 V to a minimum of 1.6
-0.2 = 1.4 V
❑We can then draw two “load lines” corresponding
the maximum and minimum values of the input
sources
❑The current intercepts then become for the:
❑Maximum value: 1.8 / 50k = 36 µA
❑Minimum value: 1.4 / 50k = 28 µA
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Practice Problem
In the circuit shown in the figure, the voltage at the
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emitter was measured and found to be –0.7 V. If
β = 50, find IE, IB, IC, and VC.
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Thank You
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