Jun 30, 2016

Transistor Biasing



Introduction of Transistor Biasing

  • Transistors are used in a large variety if applications in many different ways. Transistors can be used as a switch, a current source, linear circuit and also used in radio, TV and communication circuits.
  • The basic function of a transistor is to amplify a weak ac signal when transistors is used for voltage or current amplification or as control (ON or OFF); it is necessary first to bias the device in a proper way.
  • The usual reason for this transistor biasing is to turn the device ON, and in particular, make it operate in the linear region of its characteristics.

DC Transistor Biasing

  • DC transistor biasing is a static operation. It deals with setting a fixed level of the current which should flow through the transistor with a desired fixed voltage drop across the transistor junctions. Usually, the current IC, IB and the voltage VBE and VCE are required to be set the biasing circuit. The proper values of these currents and voltages allow a transistor allow to amplify the weak signals faithfully.
  • The proper flow of zero signal collector current and the maintenance of proper collector emitter voltage during the passage of signal is known as transistor biasing.

Why a bias a transistor?

DC biasing is a static operation. It deals with setting a fixed level of current with a desired voltage drop across the device. the purpose of DC transistor biasing is to obtain certain DC collector current at a certain DC collector voltage. These values if current and voltage define the point, at which the transistor operates. This point is known as operating point or quiescent point.

Operating point

After knowing that transistor functions most linearly when it is constrained to operate in its active region. Consider a common-emitter circuit as shown in the fig below. The transistor is biased with two dc supplies namely VBB and VCC to obtain the fixed values of IB, IC, IE and VCE.
common emitter configuration
The output (collector) characteristic curves for this particular VBB forward biases the emitter-base junction whereas the dc supply VCC reverse biases the collector-base junction. the resistor RB is a current limiting resistors, used to limit the base current within certain specified levels. Applying Kirchhoff’s voltage law to the collector circuit,

we get VCC + ICRC + VCE
where ICRC is the voltage across RC.

ICRC + VCC – VCE

IC = (-1/RC)VCE + (VCC/RC)

The above equation is in the form of y=mx+c which is the equation of the straight line. Here m=(-1/RC) is the slope of the line and c=Vcc/RC is the intercept on Y-axis. As Vcc and RC constants, it is a first degree line which is commonly known as the load line equation on the output characteristics. The load line determines the locus of VCE and IC for any given value of RC.
common emitter circuit - output characteristic curves
The two points needed to draw the dc load line are located a s follows:
  1. When VCE = VCC i.e., the voltage drop across RC must be zero. This corresponds to the cut off point B (OB = VCE max) on the VCE (X – axis) axis.
  2. When VCE = 0,    IC = VCC/RC
This corresponds to the saturation point A(OA – ICmax) on the IC (Y-axis) axis.
  • The straight line AB drawn through the points A and B is the DC load line, the inverse of the slope of which is equal to the resistance of the load.
  • The line on the output characteristic s of a transistor which gives the value of IC and VCE corresponding to zero signal is called the dc load line. Then the region lying in between the saturation and cut-off points of the load line is called active region or linear region of the transistor’s operation.
  • The region along the load line including all points between saturation and cut-off is known as the linear region of the transistor’s operation. Applying Kirchhoff’s voltage law to the base circuit in the above figure;
we get VBB = IBRB = VBE
where IB = VBB/RB

Re-arranging the above equations IB = VBB – (VBE / RB)

since VBE<<VBB,      IB = VBB/RB
If we draw a characteristic curve for this IB; then intersection of this curve and dc load line gives the operating point Q. This point is the fixed point on the output characteristics. So it is called the quiescent point or simply Q-Point. Once an operating point is established, time-varying excursions of the input signal should cause an output signal of the same waveform.

If the output signal is not a faithful reproduction of the input signal, the operation point is unsatisfactory and should be relocated on the collector characteristics. As long as the transistor is operated in the active region, the output voltage is a linear reproduction of the input. Note that for normal operation, the Q-point of a transistor is selected midway between the saturation and cut-off i.e., where the collector to emitter voltage VCE is equal to half of the VCC supply.


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