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ELectronic Circuits I

Biasing is the process of applying external voltages to transistors to ensure proper operation. It involves forward or reverse biasing transistor junctions to place the device in specific regions like active, saturation, or cutoff. Common biasing circuits include fixed bias using a base resistor, collector-to-base bias which provides feedback, and emitter bias where a resistor is added to stabilize operating point. Proper biasing establishes a quiescent point and load line on the transistor characteristics curve for linear amplification.

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BIASING  Biasing means to obtain an output  ie) voltage or current from various point in order to establishing a proper operating system.  A biasing is required to activate the transistor and prevents it to either to saturation mode or cut-off mode.  Biasing is the process of applying external voltages to the transistor. 2
BIASING BJT  A BJT is capable of operating in four different regions, depending on the biasing. The regions of operation are :  I. Cutoff region  II. Saturation region  III. Active region  IV. Inverse active region  To operate the transistor in these regions the two junctions of a transistor should be forward or reverse biased according to the application. 3
TRANSISTOR BIASING  Biasing is the process of providing DC voltage which helps in the functioning of the circuit.  A transistor is based in order to make the emitter base junction forward biased and collector base junction reverse biased.  so that it maintains in active region, to work as an amplifier.  a transistor acts as a good amplifier, if both the input and output sections are biased. 4
NEED FOR DC BIASING  If a signal of very small voltage is given to the input of BJT, it cannot be amplified. Because, for a BJT, to amplify a signal, two conditions have to be met.  The input voltage should exceed cut-in voltage for the transistor to be ON.  The BJT should be in the active region, to be operated as an amplifier. 5
A TRANSISTOR AMPLIFIER 6
OPERATING POINT  The operating point of a device, also known as a bias point, quiescent point or Q-point,  It is the steady-state DC voltage or current at a specified terminal of the transistor with no input signal applied.  Q-point is an abbreviation of quiescent point, it’s the points where the transistor is currently operating.  Q-point is an abbreviation of quiescent point, it’s the points where the transistor is currently operating 7
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DC LOAD LINE  When the transistor is given the bias and no signal is applied at its input, the load line drawn at such condition, can be understood as DC condition. Here there will be no amplification as the signal is absent.  Let VCC of a bipolar junction transistor is the DC voltage that is supplied to the collector of the transistor. 9
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OBTAINING END POINTS A& B  To obtain the load line, the two end points of the straight line are to be determined. Let those two points be A and B.  To obtain A  When collector emitter voltage VCE = 0, the collector current is maximum and is equal to VCC/RC. This gives the maximum value of VCE. This is shown as VCE=VCC−ICRC 0=VCC−ICRC IC=VCCRC  This gives the point A (OA = VCC/RC) on collector current axis, shown in the above figure. 12
OBTAINING END POINTS A& B CONTD… To obtain B  When the collector current IC = 0, then collector emitter voltage is maximum and will be equal to the VCC. This gives the maximum value of IC.  VCE=VCC−ICRC  =VCC  (As IC = 0)  This gives the point B, which means (OB = VCC) on the collector emitter voltage axis shown in the above figure. 13
DC ANALYSIS 14
BIASING METHODS The common biasing circuits used in the bipolar transistor amplifiers are  Fixed bias  Collector-to-base bias  Fixed bias with emitter resistor Voltage divider bias  Emitter bias 15
Fixed Bias (Base Resistor Bias) 16
 In the base circuit, Apply KVL, we get  Therefore,  VCC = IBRB + VBE 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. In the Collector circuit Apply KVL, we get Therefore, VCC = ICRC + VCE VCE = VCC - ICRC 17
 The common-emitter current gain of a transistor is an important parameter in circuit design, and is specified on the data sheet for a particular transistor. It is denoted as β.  IC = βIB  In this circuit VE =0 18
Collector to Base Bias 19
COLLECTOR TO BASE BIAS  Figure shows the dc bias with voltage feedback. It is also called as collector to base bias circuit. It is an improvement over fixed bias method. In this, biasing resistor is connected between collector and base of the transistor to provide feedback path.  Circuit analysis:  Base circuit:  Consider the base circuit and applying voltage law then we get, 20
COLLECTOR TO BASE BIAS 21
 Only the difference between the equation for IB and that obtained for fixed bias configuration is βRC, so the feedback path results in a reflection of the resistance RC to the input circuit.  Collector circuit:  Applying KVL to the collector circuit, VCC – (IC + IB) RC – VCE = 0 VCE = VCC – (IC + IB) RC 22
Modified collector to base bias circuit: 23
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