In this article, We are providing the 2nd part of the study notes on Circuits Analysis and Applications of Diodes, BJT, and MOSFET for GATE and other Electrical Engineering exams. The study notes cover the topic such as Analysis of the operating mode of BJT, Operating Point, DC and AC load line, Instability in collector current and BJT Biasing.
➤ ANALYSIS OF OPERATING MODE OF BJT
Method-1
To determine the mode of operation of a BJT, the following steps are followed.
Method-2
The mode of operation of the Transistor is found by follwing the given steps.
Method-3
To determine the mode of operation of a BJT, the following steps are followed.
➤ OPERATING POINT
➤ THE DC AND AC LOAD LINES
The capacitance considered in the circuit of figure 3(a) is large enough so that they act as open circuits under DC conditions but short circuits at the lowest frequency of operation of the circuit. Since the capacitor are open-circuited under DC conditions so the circuit can be redrawn as shown below in figure 3(c)
.....(i)
➤ INSTABILITY IN COLLECTOR CURRENT
From the above discussion, it is found that to obtain faithful reproduction of the input signal it should necessary to maintain a stable Q-point.
The collection current can be calculated by the following equation.
Ic = β IB + (1+β) Ico …………(ii)
From equation (ii), It can be concluded that Ic is unstable due to the following reasons:
Hence from the above discussion, it can be concluded that Ic is unstable due to variation in Ico, β and VBE i.e
Ic = f (ICO, VBE, β) …………(iii)
From the above equation, It is clear that Ic is a function of Ico, β and VBE.
By Applying partial differentiation on equation (iii),
ΔIc = s.ΔIco + s’ΔVBE + s”Δβ ………….(iv)
Where s, s’ and s” are stability factors.
Procedure to find stability factor
Step 1: Find expression for IB by applying KVL in a proper loop in the given circuit.
Step 2: Find ∂IB/∂IC by differentiating of IB with respect to IC.
Step 3: Substitute ∂IB/∂IC in S =
➤ BJT BIASING
Biasing is a technique to make the Q-point (Quiescent point or operating point) stable with respect to temperature variation.
Commonly used biasing circuits are:
(i) fixed bias circuit
(ii) collector to base bias circuit
(iii) self-bias circuit
3 videos|33 docs|64 tests
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1. What is the purpose of diodes in circuit analysis? |
2. How do BJT (Bipolar Junction Transistors) differ from FET (Field Effect Transistors)? |
3. What are the applications of MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors)? |
4. How does a diode in forward bias operate? |
5. What is the difference between a diode and a zener diode? |
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