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Revision as of 15:19, 5 July 2007 by Rogerbrent (talk | contribs) (rv)(diff) ← Previous revision | Latest revision (diff) | Newer revision → (diff) "Biasing" redirects here. For other uses, see Biasing (disambiguation).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 (ICBO) increases. As ICBO increases, ICEO also increases, causing increase in collector current IC. 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 () 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
- 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.
- Q-pt should be independent of transistor parameters ie. should not shift if transistor is replaced by another of the same type.
- Collector current should be stabilized against changes in temperature.
- The circuit must be practical in its implementation, and cost-effective.
Types
There are five main types of biasing circuits used with Bipolar transistors.
- Fixed bias
- Collector-to-base bias
- Fixed bias with emitter resistor
- Voltage divider bias
- 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 separate batteries can also be used.
In the given circuit,
VCC = IBRB + Vbe
Therefore,
IB = (VCC - Vbe)/RB
For a given transistor, Vbe does not vary significantly during use. As VCC is of fixed value, on selection of RB, the base current IB is fixed. Therefore this type is called fixed bias type of circuit.
Also for given circuit,
VCC = ICRC + Vce
Therefore,
Vce = VCC - ICRC
From this equation we can obtain Vce. Since IC = βIB, we can obtain IC as well. In this manner, operating point given as (VCE,IC) can be set for given transistor.
Merits:
- It is simple to shift the operating point anywhere in the active region by merely changing the base resistor (RB).
- Very few number of components are required.
Demerits:
- The collector current does not remain constant with variation in temperature. Therefore the operating point is unstable.
- When the transistor is replaced with another one, slight (if not large) change in the value of β can be expected. Due to this the operating point will shift.
Usage:
Due to the above inherent drawbacks, fixed bias is rarely used in linear circuits, ie. those circuits which use the transistor as a current source. Instead it is often used in circuits where transistor is used as a switch.
Collector-to-Base bias
In this form of biasing, the base resistor RB is connected to the collector instead of connecting it to the battery VCC.
Similar to above,
Vce = VCC - ICRC (Since IB << IC)
In case of increase in temperature, collector current tends to increase, causing the voltage drop across resistor RC to increase. Hence Vce decreases. Therefore base current reduces, thereby compensating for the increase in collector current.
It can be noted that for the given circuit,
IB = (VCC)/(RB+βRC)
Merits:
- Circuit has a tendency to stabilize the operating point against variations in temperature and β (ie. replacement of transistor)
Demerits:
- The resistor RB causes an ac feedback, reducing the voltage gain of the amplifier. This is a mostly undesirable effect.
Usage:
Due to the major drawback of feedback, this biasing form is rarely used.
Fixed bias with emitter resistor
The fixed bias circuit is modified by attaching an external resistor to the emitter. Since Vbe is very small, we get
Ib = (VCC - IERE)/RB
When the temperature increases, the leakage current increases. Therefore there is increase in IC and IE. This increases the emitter voltage, which reduces the voltage across the base resistor. This reduces the base current which results in less collector current. Thus collector current is not allowed to increase, and operating point is kept stable.
Similarly, if the transistor is replaced by another, there may be a change in IC corresponding to change in β-value. By similar process as above, the change is negated and operating point kept stable.
Vce = VCC - (RC+RE)IC (since IC roughly equals IE as IB is very small)
Merits:
The circuit has the tendency to stabilize operating point against changes in temperature and β-value.
Demerits:
In this circuit, for proper functioning, the following condition must be met:
RE >> RB/β
As β-value is fixed for a given transistor, this relation can be satisfied either by keeping RE very large, or making RB very low.
- If RE is of large value, high VCC is necessary. This increases cost as well as precautions necessary while handling.
- If RB is low, a separate low voltage supply should be used in the base circuit. Using two supplies of different voltages is impractical.
In addition to the above, RE causes ac feedback which reduces the voltage gain of the amplifier.
Usage:
Due to the above disadvantages, this type of biasing circuit is generally not used.
Voltage divider bias
The voltage divider is formed using external resistors R1 and R2. The voltage across R2 forward biases the emitter junction. By proper selection of resistors R1 and R2, the operating point of the transistor can be made independent of β.
In this circuit we get,
VB = V across R2 = (R2*VCC)/(R1+R2)
Also VB = Vbe + IERE
When temperature increases, IC increases. As IC makes up the majority of IE, IE also increases. When IE increases, Vbe decreases. Therefore IC decreases and the operating point remains stable.
Also, VC = VCC - ICRC
Since IC is roughly equal to IE,
Vce = VC - (RC+RE)IC
We note that β is absent from all the above equations. Therefore, if the transistor is replaced by another having a different value of β, the operating point is largely unaffected.
Merits:
- Unlike above circuit, only one dc supply is neccessary.
- Operating point is almost independent of β variation.
- Operating point stabilized against shift in temperature.
Demerits:
- Ac feedback is caused by RE, which reduces the voltage gain of the amplifier. A method to avoid this is discussed below.
Usage:
The circuit's stability and merits as above make it the most widely for linear circuits.
Voltage divider with capacitor
The standard voltage divider circuit discussed above faces one critical drawback - ac feedback caused by resistor RE. This can be avoided using a capacitor (CE) in parallel with RE, as shown in circuit diagram.
The impedence of the capacitor(XC) is given by the equation,
XC = 1/(2*π*F*C)
where-
- F is the frequency of input signal
- C the value of capacitance.
- π is pi
Therefore, the capacitor offers low impedence to ac input as ac signal has high frequency. Thus the emitter is placed at ground potential for ac input. Only dc feedback is provided for stabilization of operating point.
Emitter bias
When a split supply (dual power supply) is available, this biasing circuit is the most effective. The negative supply VEE is used to forward-bias the emitter junction through RE. The positive supply VCC is used to reverse-bias the collector junction. Only three resistors are neccessary.
We know that,
VB - VE = Vbe
If RB is small enough, base voltage will be approximately zero. Therefore emitter current is,
IE = (VEE - Vbe)/RE
The operating point is independent of β if RE >> RB/β
Merit:
Good stability of operating point similar to voltage divider bias.
Demerit:
This type can only be used when split (dual) power supply is available.
Sources
- Sedra, Adel; Smith, Kenneth (2004). Microelectronic Circuits. Oxford University Press. ISBN 0-19-514251-9.
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: CS1 maint: multiple names: authors list (link) - P.K. Patil;M.M. Chitnis (2005). Basic Electricity and Semiconductor Devices. Phadke Prakashan.
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: CS1 maint: multiple names: authors list (link)