# Introduction to BJT Amplifier - Portal by hcj

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• pg 1
```									Introduction to BJT
Amplifier

BJT (Review)

The emitter current (iE) is the sum of the collector current (iC) and the base
current (iB)

iB << iE and iC            OTHER PRAMETERS &
EQUATIONS?
BJT
n Basic structure and schematic symbol

approximate
equivalents

transistor
symbols

npn type                     pnp type
Refresh..
n Common-emitter
current gain, β
n   Range: 50 < β < 300
n Common-base current
gain, α
n   Range: always slightly
less than 1
n The current relationship
between these 2
parameters are as
follows:
Refresh..
n BJT as amplifying device
n B-E junction is forward-biased
n B-C junction is reverse-biased
BIASING OF BJT
n Remember…! for normal operation

Ä   emitter-base junction is always forward-
biased
AND

Ä   collector-base junction is always reverse-
biased
FORWARD BIASING E/B JUNCTION
REVERSE BIASING C/B JUNCTION
BIASING NPN TRANSISTOR
Common-Emitter Circuit

(a) with an npn transistor
(b) with a pnp transistor
(c) with a pnp transistor biased with a positive voltage source
DC Analysis
- Common-Emitter Circuit
Transistor current-
voltage characteristics
of the common-emitter
circuit
DC Analysis
- Common-Emitter Circuit

Common-emitter circuit with an   Common-emitter dc equivalent
npn transistor                   circuit, with piecewise linear
parameters
DC Analysis
- Common-Emitter Circuit

Usually VBE(on) = 0.7 V

Common-emitter dc equivalent circuit

Look for calculation examples in Neamen (Chapter 3), Example 3.3 & 3.4
DC Analysis
- Load Line & Modes of Operation

Figure A

Base on Figure A, using KVL
around B-E loop:
Base-emitter junction characteristics
DC Analysis
- Load Line & Modes of Operation
Base on Figure A, 2 end
points of the load line are
found by setting IC = 0

So, VCE = VCC = 10 V
When VCE = 0,
IC = VCC/RC = 5 mA

IBQ is the value from the
previous slide = 15 µA
So, ICQ = βIBQ
If β = 200,
ICQ = 3000 µA = 3 mA

Common- emitter transistor characteristics   So, VEQ = 4 V
BJT as an Amplifier
• Amplification of a small ac
voltage by placing the ac
signal source in the base
circuit
• Vin is superimposed on the
DC bias voltage VBB by
connecting them in series
with base resistor RB:

• Small changes in the base
current circuit causes large
changes in collector current
circuit
E ND

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