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and Circuit Theory, 10/e. Robert L. Boylestad and Louis Nashelsky. 3 ...
Chapter 3: Bipolar Junction Transistors
Transistor Construction There are two types of transistors: • pnp • npn
pnp
The terminals are labeled: • E - Emitter • B - Base • C - Collector npn
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Transistor Operation With the external sources, VEE and VCC, connected as shown: • The emitter-base junction is forward biased • The base-collector junction is reverse biased
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Currents in a Transistor Emitter current is the sum of the collector and base currents: IE = IC + IB
The collector current is comprised of two currents: IC = IC + I CO majority minority
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Common--Base Configuration Common
The base is common to both input (emitter–base) and output (collector–base) of the transistor.
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Common--Base Amplifier Common
Input Characteristics This curve shows the relationship between of input current (IE) to input voltage (VBE) for three output voltage (VCB) levels.
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Common--Base Amplifier Common
Output Characteristics This graph demonstrates the output current (IC) to an output voltage (VCB) for various levels of input current (IE).
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Operating Regions •
Active – Operating range of the amplifier.
•
Cutoff – The amplifier is basically off. There is voltage, but little current.
•
Saturation – The amplifier is full on. There is current, but little voltage.
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Approximations Emitter and collector currents: I
C
≅ I
E
Base-emitter voltage: VBE = 0.7 V (for Silicon)
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Alpha (α (α ) Alpha (α) is the ratio of IC to IE : αdc =
IC IE
Ideally: α = 1 In reality: α is between 0.9 and 0.998 Alpha (α) in the AC mode: mode αac =
Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky
∆I C ∆I E
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Transistor Amplification
Currents and Voltages:
Voltage Gain:
V 200mV I E = Ii = i = = 10mA Ri 20Ω I
C
I V
≅I
Av =
VL Vi
=
50V
= 250
200mV
E
L
≅ I = 10 mA i
L
= I R = (10 ma )(5 kΩ) = 50 V L
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Common––Emitter Configuration Common
The emitter is common to both input (base-emitter) and output (collectoremitter). The input is on the base and the output is on the collector.
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Common--Emitter Characteristics Common
Collector Characteristics
Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky
Base Characteristics
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Common--Emitter Amplifier Currents Common Ideal Currents IC = α IE
IE = IC + IB
Actual Currents IC = α IE + ICBO
where ICBO = minority collector current ICBO is usually so small that it can be ignored, except in high power transistors and in high temperature environments.
When IB = 0 µA the transistor is in cutoff, but there is some minority current flowing called ICEO.
ICEO =
Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky
ICBO 1− α
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I B =0 µA
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Beta (β) β represents the amplification factor of a transistor. (β is sometimes referred to as hfe, a term used in transistor modeling calculations) In DC mode:
β dc
IC = IB
In AC mode:
β ac =
Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky
∆ IC ∆ IB
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VCE = constant
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Beta (β (β ) Determining β from a Graph
(3.2 mA − 2.2 mA) (30 µA − 20 µA) 1 mA = V = 7.5 10 µA CE = 100
β AC =
2.7 mA V CE = 7.5 25 µ A = 108
β DC =
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Beta (β (β ) Relationship between amplification factors β and α
α=
β β+1
β=
α α −1
Relationship Between Currents
I C = βIB
Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky
I E = (β + 1)I B
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Common– Common –Collector Configuration
The input is on the base and the output is on the emitter.
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Common– Common –Collector Configuration
The characteristics are similar to those of the common-emitter configuration, except the vertical axis is IE.
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Operating Limits for Each Configuration VCE is at maximum and IC is at minimum (ICmax= ICEO) in the cutoff region. IC is at maximum and VCE is at minimum (VCE max = VCEsat = VCEO) in the saturation region. The transistor operates in the active region between saturation and cutoff.
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Power Dissipation Common-base:
PCmax = VCB I C Common-emitter:
PCmax = VCE I C
Common-collector:
PCmax = VCE I E
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Transistor Specification Sheet
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Transistor Specification Sheet
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Transistor Testing •
Curve Tracer Provides a graph of the characteristic curves.
•
DMM Some DMMs measure βDC or hFE.
•
Ohmmeter
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Transistor Terminal Identification
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