# Types of BJT Biasing Circuits

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Types of BJT Biasing Circuits
By G.Kavitha ECE Dept Paavai Engineering College

Biasing Circuits of the BJT
Normally, three types of circuits are used to establish dc biasing in a BJT, these are; (a) Fixed bias circuit (b) Collector-to-Base Bias Circuit (c) Voltage-Divider with Self-Bias Circuit (or) Self-Bias (or) Emitter Bias (or) Emitter-Stabilized Bias Circuit While designing the all four types of biasing circuits of BJT it is aimed to obtain Q-point parameters (IBQ, ICQ, VCEQ)

Fixed Bias

The Base-Emitter Loop From Kirchhoff’s voltage law:
+VCC – IBRB – VBE = 0 Solving for base current:

Collector-Emitter Loop
Collector current: From Kirchhoff’s voltage law:

Saturation When the transistor is operating in saturation, current through the transistor is at its maximum possible value.

The end points of the load line are: ICsat
Load Line Analysis The end points of the load line are: ICsat IC = VCC / RC VCE = 0 V VCEcutoff VCE = VCC IC = 0 mA The Q-point is the operating point: where the value of RB sets the value of IB that sets the values of VCE and IC

Collector to Base Bias

Base-Emitter Loop From Kirchhoff’s voltage law: Where IB << IC:
Knowing IC = IB and IE  IC, the loop equation becomes: Solving for IB:

Collector-Emitter Loop
From Kirchhoff’s voltage law: Since IC  IC and IC = IB: Solving for VCE:

Voltage Divider Bias (or) Self Bias

Approximate Analysis Where IB << I1 and I1  I2 :
Where bRE > 10R2: From Kirchhoff’s voltage law:

Voltage Divider Bias Analysis
Transistor Saturation Level Load Line Analysis Cutoff: Saturation:

Emitter-Stabilized Bias Circuit

Base-Emitter Loop From Kirchhoff’s voltage law: Since IE = (b + 1)IB:
Solving for IB:

Collector-Emitter Loop
From Kirchhoff’s voltage law: Since IE  IC: Also:

Improved Biased Stability
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 () values. Adding RE to the emitter improves the stability of a transistor.

Saturation Level The endpoints can be determined from the load line.
VCEcutoff: ICsat:

Emitter-Stabilized Bias Circuit

Base-Emitter Loop From Kirchhoff’s voltage law: Where IB << IC:
Knowing IC = IB and IE  IC, the loop equation becomes: Solving for IB:

Collector-Emitter Loop
From Kirchhoff’s voltage law: Since IC  IC and IC = IB: Solving for VCE:

Base-Emitter Bias Analysis
Transistor Saturation Level Load Line Analysis Cutoff: Saturation:

THANK YOU

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