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Analog Electronics Part-9 - Note

DC biasing is essential for turning on BJTs to amplify AC signals, establishing a quiescent point (Q-point) for operation. The document outlines the three states of operation (linear, cutoff, saturation), various biasing circuits (fixed-bias, emitter-stabilized, voltage divider), and the importance of stability in maintaining consistent performance. Load-line analysis and shifting Q-points are discussed to illustrate how changes in biasing affect transistor operation.

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8 views26 pages

Analog Electronics Part-9 - Note

DC biasing is essential for turning on BJTs to amplify AC signals, establishing a quiescent point (Q-point) for operation. The document outlines the three states of operation (linear, cutoff, saturation), various biasing circuits (fixed-bias, emitter-stabilized, voltage divider), and the importance of stability in maintaining consistent performance. Load-line analysis and shifting Q-points are discussed to illustrate how changes in biasing affect transistor operation.

Uploaded by

sureshpv8793
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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DC Biasing—BJTs

Biasing: The DC voltages applied to a transistor in order to turn it on so that it


can amplify the AC signal.
DC Biasing—Operating Point
The DC input establishes an operating or quiescent point called the quiescent
Point or Q-point.

By definition, quiescent means quiet, still, inactive.


The Three States of Operation

Linear-region operation:
Base–emitter junction forward-biased
Base–collector junction reverse-biased

Cutoff-region operation:
Base–emitter junction reverse-biased
Base–collector junction reverse-biased

Saturation-region operation:
Base–emitter junction forward-biased
Base–collector junction forward-biased
DC Biasing Circuits

• Fixed-bias circuit
• Emitter-stabilized bias circuit
• Collector-emitter loop
• Voltage divider bias circuit
FIXED-BIAS CONFIGURATION

DC equivalent
FIXED-BIAS CONFIGURATION

The Base-Emitter Loop

From Kirchhoff’s voltage law:

Solving for base current:


FIXED-BIAS CONFIGURATION

Collector-Emitter Loop

From Kirchhoff s voltage law:


Transistor Saturation

When the transistor is operating in saturation, current


through the transistor is at its maximum possible value.
Load-Line Analysis
Shifting of Q-points
If the level of IB is changed by varying
the value of RB , the Q -point moves up
or down

For increasing values of IB


Shifting of Q-points
If VCC is held fixed and RC increased,
the load line will shift.
Shifting of Q-points
If RC is fixed and VCC decreased, the
load line shifts.
Emitter-Stabilized Bias Circuit

Adding a resistor (RE) to the emitter circuit


stabilizes the bias circuit.

DC equivalent
Emitter-Stabilized Bias Circuit- Base–Emitter Loop
Impedence Reflection Rule
Load-Line Analysis
Stability refers to a circuit condition in which the currents and
voltages will remain fairly constant over a wide range of
temperatures and transistor Beta (β) values.

Adding RE to the emitter improves the stability of a transistor.


VOLTAGE-DIVIDER BIAS CONFIGURATION (VDB)

This is a very stable bias circuit. The currents


and voltages are nearly independent of any
variations in β.

DC components
VDB- Exact Analysis
VDB- Approximate Analysis
COLLECTOR FEEDBACK CONFIGURATION

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