1. SMALL SIGNAL ANALYSIS OF CE AMPLIFIER
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EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE

2. EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE
INTRODUCTION
One of the primary uses of a transistor is to amplify ac signals.
This could be an audio signal or perhaps some high frequency radio signal.
It has to be able to do this without distorting the original input.
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EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE

3. Definition of Small Signal
ac input signal voltages and currents are in the order ±10% of Q-point voltages and currents. eg
If dc current is 10mA, the ac current (peak to peak) is less than 0.1 mA.
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EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE

4. EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE
HOW BJT AMPLIFIES?
When a weak ac signal is given to the base, a small ac base current start flowing.
Due to BJT, a much larger ac current flow through RC
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EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE

5. EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE
R1, R2, RE – form the biasing circuit & stabilization.
C1(Coupling capacitor) – couple the signal to the base.
C2(Bypass capacitor) - use in parallel with RE to provide low reactance path to amplified ac signal
C3(Coupling capacitor)- couple the signal to the next stage
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EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE

6. EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE
Circuit Diagram
Common-Emitter Biasing(CE)
input = VBE & IB
output = VCE & IC + _ IC
VCE IB
VCC
= Common-emitter current gain =
= Common-base current gain =
The relationships between the two parameters are:
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EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE

7. Collector-Current Curves
Common-Emitter
Collector-Current Curves IC
Active Region IB
Saturation Region
Cutoff Region
IB = 0
Region
of Operation
Description
Active
Small base current controls a large collector current
Saturation
VCE(sat) ~ 0.2V, VCE increases with IC
Cutoff
Achieved by reducing IB to 0, Ideally, IC will also equal 0.
VCE
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EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE

8. DC Analysis of Transistor Circuits Common-Emitter Circuit
Common-emitter circuit with an npn transistor
Assume B-E junction: forward biased  V drop is the cut-in / turn-on V [VBE (on)]
IC represented as a dependent I source (function of IB)
Neglect reverse-biased junction leakage current & Early effect
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EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE

9. EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE
Common- Emitter Transistor Characteristics And The Collector-emitter Load Line
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EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE

10. EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE
Amplifier Operation
The boundary between cutoff and saturation is called the linear region.
A transistor which operates in the linear region is called a linear amplifier.
Only the ac component reaches the load because of the capacitive coupling and that the output is 180º out of input phase.
Fig 6-2 amplifier circuit
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EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE

11. dc equivalent circuit of transistor amplifier
Only dc condition are to be considered
No signal is applied (All ac source reduce to zero)
dc cannot flow through a capacitor (open circuit)
To calculate the dc currents & voltages
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EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE

12. DC Equivalent for the BJT Amplifier
DC Equivalent Circuit
All capacitors in the original amplifier circuit are replaced by open circuits, disconnecting vI, RI, and R3 from the circuit and leaving RE intact. The the transistor Q will be replaced by its DC model.
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EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE

13. ac equivalent circuit of transistor amplifier
Only ac condition are to be considered
dc voltage not important, considered zero or ground. (VCC=0)
the capacitors used to couple or bypass the ac signal. (short circuit)
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EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE

14. AC Equivalent for the BJT Amplifier
The coupling and bypass capacitors are replaced by short circuits.
The DC voltage supplies are replaced with short circuits, which in this case connect to ground.
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EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE

15. Small-signal hybrid- equivalent circuit
Using common-emitter current gain (β) parameter
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EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE

16. Small-signal equivalent circuit
Small-signal hybrid- equivalent circuit
Small-signal equivalent circuit r Vs RB RC Vo Ic Ib
gmVbe
Vbe
Vce + -
Output signal voltage
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EC-1/UNIT 2 SMALL SIGNAL ANALYSIS OF CE AMPLIFIE S.PREMA/AP/ECE
Input signal voltage