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26. Explain forward resistance, static resistance and dynamic resistance of a
pn junction diode.
• Forward Resistance: Resistance offered in a diode circuit, when it is forward
biased, is called forward-resistance.
• DC or Static Resistance: DC resistance can be explained as the ratio of the
dc-voltage across the diode to the direct current flowing through it.
• AC or Dynamic Resistance: It can be defined as the reciprocal of the slope of
the forward characteristic of the diode. It is the resistance offered by a
diode to the changing forward current.
27. How does Zener phenomenon differ from Avalanche breakdown?
The phenomenon when the depletion region expands and the potential barrier
increases leading to a very high electric field across the junction, due to
which suddenly the reverse current increases under a very high reverse voltage
is called Zener effect. Zener-breakdown or Avalanche breakdown may occur
independently or both of these may occur simultaneously. Diode junctions that
breakdown below 5v are caused by Zener Effect. Junctions that experience
breakdown above 5v are caused by avalanche-effect. The Zener-breakdown occurs
in heavily doped junctions, which produce narrow depletion layers. The
avalanche breakdown occurs in lightly doped junctions, which produce wide
depletion layers.
28. Compare JFET’s and MOSFET’s.
Comparison of JFET’s and MOSFET’s:
• JFET’s can only be operated in the depletion mode whereas MOSFET’s can be
operated in either depletion or in enhancement mode. In a JFET, if the gate is
forward-biased, excess-carrier injunction occurs and the gate-current is
substantial.
• MOSFET’s have input impedance much higher than that of JFET’s. Thus is due to
negligible small leakage current.
• JFET’s have characteristic curves more flat than that of MOSFET is indicating
a higher drain resistance.
• When JFET is operated with a reverse-bias on the junction, the gate-current
IG is larger than it would be in a comparable MOSFET.
30. Explain thin film resistors and wire-wound resistors
a. Thin film resistors- It is constructed as a thin film of resistive material
is deposited on an insulating substrate. Desired results are obtained by either
trimming the layer thickness or by cutting helical grooves of suitable pitch
along its length. During this process, the value of the resistance is monitored
closely and cutting of grooves is stopped as soon as the desired value of
resistance is obtained.
b. Wire wound resistors – length of wire wound around an insulating cylindrical
core are known as wire wound resistors. These wires are made of materials such
as Constantan and Manganin because of their high resistivity, and low
temperature coefficients. The complete wire wound resistor is coated with an
insulating material such as baked enamel
31. What is a differential amplifier? Also, explain CMRR.
Differential Amplifier: The amplifier, which is used to amplify the voltage
difference between two input-lines neither of which is grounded, is called
differential amplifier. This reduces the amount of noise injected into the
amplifier, because any noise appearing simultaneously on both the
input-terminals as the amplifying circuitry rejects it being a common mode
signal.
CMRR: It can be defined as the ratio of differential voltage-gain to common
made voltage gain. If a differential amplifier is perfect, CMRR would be
infinite because in that case common mode voltage gain would be zero.
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