Important Questions EDC

1) Explain the effect of coupling and bypass capacitors on amplifier

1. Coupling Capacitors

Coupling capacitors are used to "couple" or transfer an AC signal from one stage of the amplifier
to the next while blocking any DC component. This is essential for isolating different stages of
the circuit electrically while allowing the signal to pass through.

Effects on Amplifier:

● Signal Integrity: By blocking DC, coupling capacitors prevent unwanted DC shifts that
could affect subsequent stages or components in the amplifier.
● Frequency Response: Coupling capacitors can impact the amplifier's low-frequency
response. Their impedance decreases with increasing frequency, so they allow higher
frequencies to pass more readily. If the capacitor value is too small, it may attenuate low-
frequency signals, reducing bass response in audio amplifiers.

Bypass Capacitors

Bypass capacitors are used in amplifiers to stabilize the voltage at certain points in the circuit,
often across emitter or source resistors in transistor or FET amplifiers. They help to reduce the
impact of unwanted AC signals or fluctuations, essentially “bypassing” AC around a particular
component.

Effects on Amplifier:

● Gain Enhancement: By bypassing the AC signal around an emitter or source resistor,

the bypass capacitor increases the amplifier's gain. Without the bypass capacitor, part of
the AC signal would drop across the emitter resistor, reducing the overall gain.
● Frequency Stability and Response: The size of the bypass capacitor affects the
amplifier's low-frequency response

2) Explain crossover distortion in power amplifiers

Crossover distortion is a type of distortion that occurs in push-pull class B or class AB
power amplifiers.
It manifests as a "kink" or "notch" in the output waveform, particularly around the zero-
crossing points. This distortion is caused by the brief period when both transistors in the
output stage are off, leading to a momentary absence of output signal.
Several techniques are employed to reduce or eliminate crossover distortion:
1. Class AB Biasing: By biasing the transistors slightly above the cutoff point, a small
current flows through them even when there is no input signal. This ensures that at least
one transistor is always partially conducting, reducing the dead zone and minimizing
distortion.

3) Define parameters of differential amplifiers

Differential Gain (Ad)

● This is the gain with respect to the difference between the two input signals. If the inputs
are V1 &V2 the differential output Vout is proportional to V1−V2 by the differential
gain:
●
● Vout=Ad⋅ (V1−V2)

Common-Mode Gain (Ac)

● Common-mode gain is the amplifier's response to signals that are identical on both
inputs. This is generally undesirable since the differential amplifier should ideally ignore
common signals.
●

t is defined by:

Vout=Ac⋅ (V1+V2/2)

Common-Mode Rejection Ratio (CMRR)

● CMRR is the ratio of differential gain to common-mode gain and indicates how
effectively the amplifier rejects common-mode signals. It is given by:

●
Input Resistance (Rin)

● This parameter defines the impedance seen by the source connected to the amplifier
inputs. High input resistance is desirable for minimal loading on the signal source.

Output Resistance (Rout)

● This parameter represents the impedance at the amplifier's output. Low output resistance
is ideal for driving loads without significant signal loss.

4) Explain E-MOSFET as differential amplifier with diagram

An E-MOSFET (Enhancement-Mode MOSFET) is a type of transistor that can be used to
construct a differential amplifier. It's a versatile device that can operate in both analog
and digital circuits.

How it Works:
1. Bias Current: A constant current source, I, is applied to the drain terminals of the two
MOSFETs, M1 and M2. This current source ensures that the total current flowing
through the two transistors remains constant.
2. Input Signals: Two input signals, V1 and V2, are applied to the gate terminals of M1
and M2, respectively.
3. Output Signals: The output signals, Vout1 and Vout2, are taken from the drain terminals
of M1 and M2, respectively.

Operation:
● Common-Mode Input: If both V1 and V2 increase or decrease by the same amount, the
current through M1 and M2 remains unchanged. This is known as a common-mode input.
In an ideal differential amplifier, the output voltage remains unaffected by common-mode
inputs.

● Differential-Mode Input: If V1 increases and V2 decreases (or vice versa), the current
through M1 increases and the current through M2 decreases. This results in a difference
 in the output voltages, Vout1 and Vout2. The differential amplifier amplifies this
difference.

5) Explain high frequency model of BJT

At increasing frequencies, the reactance xc will decrease in magnitude resulting in a shorting

effect across the output and a decrease in gain.
For high frequency response, various parasite capacitances (cbe, cbc, cce) of the transistors are
included along with the wiring capacitors ( cwi, cwo) for analysis. For high frequency response,
cs, cc, ce are assumed to be in short circuit state. Input capacitance ci includes wiring
capacitance cwi, the transistor capacitance. Cbe and miller capacitance cmi. The o/p capacitance
co includes wiring cce and miller capacitance cmo

6) Draw and explain two transistor or constant current source using MOSFET

Two-Transistor MOSFET Constant Current Source

A two-transistor MOSFET current source is a widely used circuit configuration that
provides a relatively stable current output, independent of variations in supply voltage
and temperature.

Bias Transistor (M1):

● The gate and drain of Q1 are connected together.

● This configuration forces Q1 to operate in the saturation region.
● The gate-source voltage (VGS1) of Q1 is fixed by the voltage divider formed by R1 and
R2.
● This fixed VGS1 establishes a specific drain current (ID1) through M1.

Current Mirror Transistor (M2):

● Q2 has the same gate-source voltage as Q1 due to their identical gate-source connections.
● This ensures that Q2 also operates in the saturation region.
● Since both transistors have the same VGS and are in the saturation region, they will have
approximately the same drain current (ID2 ≈ ID1).

Key Points:

● Current Stability: The current source provides a relatively stable current output because
the gate-source voltage of M2 is fixed by the gate-source voltage of M1.
 ● Output Resistance: The output resistance of the current source is high, making it less
sensitive to variations in the load resistance.
● Temperature Compensation: The two transistors are closely matched, so they tend to
track each other's temperature variations, improving the overall temperature stability of
the current source.

Applications:

● Biasing transistors in amplifier circuits

● Providing a reference current for other circuits
● Implementing current mirrors for various applications
●