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Biasing in electronics is the method of establishing predetermined voltages and/or currents at various points of a circuit to set the appropriate operating point.

Requirement

In electronics, a bias point, also known an operating point, quiescent point or Q-point, is a dc voltage which, when applied to a device, causes it to operate in a certain desired fashion. The term is normally used in connection with devices such as transistors and diodes which are used in amplification or rectification.

Linear circuits involving transistors typically require a specific P-n junction voltage to operate correctly, which can be achieved using a biasing circuit. The method of keeping a device to operate in the active region is also referred to as biasing of the circuit. In amplifiers, a small input signal gives larger output signal without any change in its general shape. Before applying an AC signal, proper biasing of the transistor is necessary.

For example, for Bipolar Junction Transistors the bias point would keep the transistor operating in the active mode, drawing a DC current. A small signal is then applied on top of this bias voltage, thereby either modulating or switching the current, depending on the design of the circuit. The input dc voltage is chosen to satisfy the required large signal parameters.

The quiescent point of operation is 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.

Quiescent current is the current that flows in an electrical circuit when no load is present. This term is commonly used in circuit analysis of electronic amplifier and voltage regulator circuits. It is basically the current which flows through a component/circuit without actually contributing in any way to the load and usually of the order of milliamperes to microamperes.

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

Reasons for Shift of Q-pt

The shifting of operating point is due to two major 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 repeat, 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 (${\displaystyle \beta }$) which changes from unit to unit.

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

BJT Transistor biasing

Requirements of Biasing Circuit

1. Q-pt is established in center 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. The circuit must be practical in its implementation, and cost-effective.

Types

There are five main types of biasing circuits used with Bipolar transistors.

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

Fixed Bias

This form of biasing is also called Base Bias. In the example image on the right, the single power source (ie. battery) is used for both collector and base of transistor, although seperate batteries can also be used.

In the given circuit,

V_CC = I_BR_B + V_be

Therefore,

I_B = (V_CC - V_be)/R_B

For a given transistor, V_be does not vary significantly during use. As V_CC is of fixed value, on selection of R_B, the base current I_B is fixed. Therefore this type is called fixed bias type of circuit.

Also for given circuit,

V_CC = I_CR_C + V_ce

Therefore,

V_ce = V_CC - I_CR_C

From this equation we can obtain V_ce. Since I_C = βI_B, we can obtain I_C as well. In this manner, operating point given as (V_CE,I_C) can be set for given transistor.

Sources

• Sedra, Adel; Smith, Kenneth (2004). Microelectronic Circuits. Oxford University Press. ISBN 0-19-514251-9.{{cite book}}: CS1 maint: multiple names: authors list (link)

See also

• Small signal model
• Bipolar Junction Transistor
• MOSFET

• Electronics
• Electronic engineering