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Collector to Base Bias Circuit Guide

The collector to base bias circuit is similar to the base bias circuit, except the base resistor RB is connected to the collector rather than the power supply. This connection provides negative feedback that improves stability by reducing changes in collector current IC. The required value of RB can be calculated using Kirchhoff's voltage law equations that relate IC, IB, RB, RL, and the supply voltage VCC. This circuit has better stability than a fixed bias circuit due to the feedback, but it reduces voltage gain as a tradeoff.

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David Talam
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2K views5 pages

Collector to Base Bias Circuit Guide

The collector to base bias circuit is similar to the base bias circuit, except the base resistor RB is connected to the collector rather than the power supply. This connection provides negative feedback that improves stability by reducing changes in collector current IC. The required value of RB can be calculated using Kirchhoff's voltage law equations that relate IC, IB, RB, RL, and the supply voltage VCC. This circuit has better stability than a fixed bias circuit due to the feedback, but it reduces voltage gain as a tradeoff.

Uploaded by

David Talam
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Collector to Base Bias

The collector to base bias circuit is same as base bias circuit except that the base resistor RB is
returned to collector, rather than to VCC supply as shown in the figure below.

This circuit helps in improving the stability considerably. If the value of IC increases, the
voltage across RL increases and hence the VCE also increases. This in turn reduces the base
current IB. This action somewhat compensates the original increase.
The required value of RB needed to give the zero signal collector current IC can be calculated as
follows.
Voltage drop across RL will be

VRL=(IC+IB)RL≅ICRL
From the figure,
ICRL+IBRB+VBE=VCC
Or
IBRB=VCC−VBE−ICRL
Therefore

Applying KVL we have


(IB+IC)RL+IBRB+VBE=VCC
Or
IB(RL+RB)+ICRL+VBE=VCCIB
Therefore

Since VBE is almost independent of collector current, we get


This value is smaller than (1+β) which is obtained for fixed bias circuit. Thus there is an
improvement in the stability.
This circuit provides a negative feedback which reduces the gain of the amplifier. So the
increased stability of the collector to base bias circuit is obtained at the cost of AC voltage gain.

Biasing with Collector Feedback resistor

In this method, the base resistor RB has its one end connected to base and the other to the
collector as its name implies. In this circuit, the zero signal base current is determined by
VCB but not by VCC.
It is clear that VCB forward biases the base-emitter junction and hence base current IB flows
through RB. This causes the zero signal collector current to flow in the circuit. The below figure
shows the biasing with collector feedback resistor circuit.
The required value of RB needed to give the zero signal current IC can be determined as follows.
VCC=ICRC+IBRB+VBE

Since IC=βIB
Alternatively,
VCE=VBE+VCB
Or
VCB=VCE−VBE
Since
Mathematically,
Stability factor, S<(β+1)
Therefore, this method provides better thermal stability than the fixed bias.
The Q-point values for the circuit are shown as

VCE=VCC−ICRC

Advantages

 The circuit is simple as it needs only one resistor.


 This circuit provides some stabilization, for lesser changes.

Disadvantages

 The circuit doesn’t provide good stabilization.


 The circuit provides negative feedback.

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