Biasing (electronics)

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"Biasing" redirects here. For other uses, see Biasing (disambiguation).

Biasing in electronics is the method of applying a predetermined voltage and current to the junction of a transistor to set the appropriate quiescent point.

Amplification is the process of raising the strength of a weak signal without any change in its general shape. This is achieved using amplifiers. Hence small ac signal is applied to the input and larger output signal is obtained. Before applying ac signal, proper biasing of the transistor is necessary.

Contents 1 Need for Biasing 1.1 Reasons for Shift of Q-pt 2 Requirements of Biasing Circuit 3 Types of Biasing Circuits 4 Configurational Bias 4.1 Comparison of three configs

[edit] Need for Biasing

Before applying ac input signal, we have to setup the Q-point (quiscent point) of operation, typically near the middle of the dc load line. The process of obtaining certain dc collector current at a certain dc collector voltage by setting up operating point is called biasing.

After establishing the operating point, when input signal is applied, the Q-pt should not move either to saturation or cut-off region. But this unwanted shift might occur due to various reasons.

[edit] Reasons for Shift of Q-pt

The shifting of operating point is due to two reasons -

1. Parameters of transistor depend on temperature. As it increases, leakage current due to minority charge carriers (I_CBO) increases. As I_CBO increases, I_CEO also increases, causing increase in collector current I_C. This produces heat at the collector junction. This process may get repeated, and finally Q-pt may shift into saturation region. Sometimes the excess heat produced at the junction may even burn the transistor. This is known as thermal runaway.

2. When a transistor is replaced by another of the same type, the Q-pt may shift, due to change in parameters of transistor such as current gain (β) which changes from unit to unit.

Hence, to avoid shift of Q-pt, bias-stabilization is necessary. Various biasing circuits can be used for this purpose.

[edit] Requirements of Biasing Circuit

1. Q-pt is established in centre of active region of transistor characteristic. It should not shift to saturation region or cut-off region, when input is applied.
2. Q-pt should be independent of transistor parameters ie. should not shift if transistor is replaced by another of the same type.
3. Collector current should be stabilized against changes in temperature.
4. It must be practical in its implementation, and cost-effective.

[edit] Types of Biasing Circuits

There are five main types of biasing circuits.

1. Fixed bias
2. Collector-to-base bias
3. Fixed bias with emitter resistor
4. Voltage divider bias
5. Emitter bias

[edit] Configurational Bias

There are three configurations of a transistor. Configuration is the method of connecting any one terminal of transistor common to both input and output circuits. The three types are listed below.
1. Common base (Grounded base)

Here base is common to both input and output. Emitter-base jn is fwd-biased and collector-base jn is rev-biased.
See full article: Common base

2. Common emitter

Here emitter terminal is common to both input and output. Emitter-base jn is fwd-biased and collector-base jn is reverse biased.
See full article: Common emitter

3. Common collector (Emitter follower)

Here collector terminal is common to both input and output. Load is connected to emitter.
See full article: Common collector

[edit] Comparison of three configs

Parameter	CB	CE	CC
Phase shift between input and output	zero	180	zero
Current gain	less than 1	high	high
Voltage gain	high	high	less than 1
Power gain	moderate	high	low to moderate
Input resistance	low	moderate	high
Output resistance	high	moderate	low