Feedback Amplifier MCQ Quiz - Objective Question with Answer for Feedback Amplifier - Download Free PDF

The requirement of a good instrumentation amplifier is that it should provide a finite gain.

Explanation:

1. Low CMRR (Common Mode Rejection Ratio): This is incorrect. A good instrumentation amplifier requires a high CMRR. A high CMRR allows the amplifier to effectively reject common-mode noise and interference while amplifying only the desired differential signal.

2. Low slew rate: This is generally incorrect. A higher slew rate is desirable for an amplifier to accurately reproduce rapidly changing input signals without distortion. A low slew rate would limit the amplifier's frequency response and ability to handle fast transients.

3. High power consumption: This is incorrect. In most electronic designs, especially for precision amplifiers, low power consumption is a desirable characteristic, as it reduces heat dissipation and improves efficiency.

4. Finite gain: This is correct. While instrumentation amplifiers aim for high gain for the differential signal, that gain must be finite and precisely controllable. An amplifier with truly infinite gain would be impractical and unstable. The ability to set a specific, stable, and finite gain (often adjustable) is a key feature that allows instrumentation amplifiers to scale signals accurately.

A voltage-shunt feedback amplifier, also known as a transresistance amplifier, is a type of feedback amplifier where the feedback network samples the output voltage and feeds back a current to the input. This type of feedback amplifier is widely used in various applications because it stabilizes the gain, reduces distortion, and improves the bandwidth of the circuit.

Key Features of Voltage-Shunt Feedback Amplifiers:

• The feedback network is connected in parallel with both the input and output of the amplifier.
• The input impedance of the amplifier is reduced due to the parallel connection of the feedback network.
• The output impedance is also reduced, making it suitable for driving low-impedance loads.
• These amplifiers are commonly used in applications requiring low output impedance and high stability, such as current-to-voltage converters and transimpedance amplifiers.

Correct Option Analysis:

The correct option is:

Option 3: A non-inverting op-amp is an example of a voltage-shunt feedback amplifier.

This option is incorrect because a non-inverting op-amp does not use a voltage-shunt feedback mechanism. In a non-inverting op-amp configuration, the feedback is voltage-series, meaning that the output voltage is fed back to the input in series with the input signal. This type of feedback increases the input impedance of the amplifier and stabilizes the voltage gain but does not involve the shunt feedback mechanism characteristic of voltage-shunt feedback amplifiers.

Important Information

To further understand the analysis, let’s evaluate the other options:

Option 1: An inverting operational amplifier uses a voltage-shunt feedback mechanism.

This option is correct. In an inverting op-amp configuration, the feedback network connects the output to the inverting input in parallel, creating a shunt feedback mechanism. The output voltage is sampled, and the feedback current flows back to the input, stabilizing the circuit's gain and improving its performance.

Option 2: A collector-base bias circuit is an example of a voltage-shunt feedback amplifier.

This option is correct. A collector-base bias circuit in a transistor amplifier uses a voltage-shunt feedback mechanism. The feedback is applied by connecting the collector to the base through a resistor. This configuration stabilizes the operating point of the transistor by providing negative feedback.

Option 4: It is also known as a transresistance amplifier.

This option is correct. A voltage-shunt feedback amplifier is indeed referred to as a transresistance amplifier because its output is a voltage, and its input is a current. The gain of such an amplifier is expressed in terms of resistance (volts per ampere).

Feedback Amplifiers

Definition: A feedback amplifier is an electronic amplifier that uses feedback to control its gain, stability, and other characteristics. Negative feedback is commonly employed to improve performance by feeding a portion of the output signal back to the input in a manner that opposes the input signal.

Correct Option Analysis:

The correct option is:

Option 2: In a feedback topology, sampling the output current reduces the output impedance.

This statement is incorrect. Sampling the output current, in a feedback topology, does not inherently reduce the output impedance. Instead, when current sensing (sampling) is used in a feedback system, it affects the system's ability to control or modify the output based on the type of feedback network implemented. Output impedance reduction is typically associated with voltage sampling (not current sampling) in series-shunt feedback configurations. Therefore, the given statement does not accurately describe the behavior of feedback amplifiers.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: In a feedback topology, sampling the output voltage reduces the output impedance.

This statement is correct. In a voltage-sampling (shunt) feedback configuration, the output impedance of the amplifier is reduced. This reduction occurs because the feedback network effectively modifies the output characteristics of the system, making it more stable and less sensitive to changes in load conditions.

Option 3: The gain-bandwidth product remains constant for a negative feedback amplifier.

This statement is also correct. Negative feedback amplifiers are designed to maintain a constant gain-bandwidth product. When the gain is reduced due to feedback, the bandwidth increases proportionally to maintain the product constant. This is a fundamental characteristic of feedback amplifiers and is one of the reasons why negative feedback is widely employed in amplifier design.

Option 4: Negative feedback minimizes nonlinear distortion in the amplifier's output.

This statement is correct. Negative feedback reduces nonlinear distortion by linearizing the amplifier's operation. By feeding back a portion of the output to the input, the system compensates for deviations from linearity, thereby improving the fidelity of the output signal.

Feedback Amplifiers

Definition: Feedback amplifiers are electronic circuits in which a portion of the output signal is fed back to the input. The feedback can be either positive (regenerative) or negative (degenerative). The choice between positive and negative feedback determines the amplifier's behavior, affecting parameters like gain, bandwidth, linearity, and stability.

Correct Option Analysis:

The correct option is:

Option 1: Positive feedback increases both gain and bandwidth.

This statement is FALSE. Positive feedback indeed increases the gain of an amplifier but does not increase the bandwidth. In fact, positive feedback typically leads to a decrease in bandwidth. Additionally, excessive positive feedback can result in instability and oscillation, making it unsuitable for most amplifier applications where stability and fidelity are required. The primary purpose of positive feedback is in circuits like oscillators, where sustained oscillations are desired, rather than amplifiers intended for linear operation.

Three-Stage Cascade Amplifier and Overall Noise Figure Calculation

Problem Statement: In a three-stage cascade amplifier, each stage has a gain of 10 dB and a noise figure of 10 dB. We are tasked with calculating the overall noise figure of this cascade system.

Solution:

To calculate the overall noise figure (F_overall) of a cascade amplifier, we use Friis's formula for noise figure in cascaded systems:

Friis's Formula:

F_overall = F₁ + ^{(F₂ - 1)}/_G1 + ^{(F₃ - 1)}/_{G1 × G2} + ...

Where:

• F_n = Noise figure of the nth stage (in linear scale).
• G_n = Gain of the nth stage (in linear scale).

Step 1: Convert Gains and Noise Figures from dB to Linear Scale

The given gain (G) and noise figure (F) for each stage are 10 dB:

• G = 10 dB → G (linear) = 10^G(dB)/10 = 10^10/10 = 10.
• F = 10 dB → F (linear) = 10^F(dB)/10 = 10^10/10 = 10.

Step 2: Apply Friis's Formula

Since there are three stages, Friis's formula for the overall noise figure becomes:

F_overall = F₁ + ^{(F₂ - 1)}/_G1 + ^{(F₃ - 1)}/_{G1 × G2}

Substitute the values for F₁, F₂, F₃, G₁, and G₂:

• F₁ = 10 (linear).
• F₂ = 10 (linear).
• F₃ = 10 (linear).
• G₁ = G₂ = 10 (linear).

Thus:

F_overall = 10 + ^{(10 - 1)}/₁₀ + ^{(10 - 1)}/_{10 × 10}

F_overall = 10 + 0.9 + 0.09

F_overall = 10.99

Final Answer: The overall noise figure of the three-stage cascade amplifier is 10.99 (linear scale).

• The transistor amplifier in which collector current flows for the entire cycle of input AC signal is called class A amplifier.
• The transistor amplifier in which collector current flows for the half-cycle of an AC signal is called a class B amplifier.
• The transistor amplifier in which collector current flows for less than half the cycle of an AC signal is called a class C amplifier

The correct option is 4

Concept:

A. Hartley oscillator - II. LC tuned circuit

Explanation: The Hartley oscillator is an electronic oscillator circuit in which the oscillation frequency is determined by an LC (inductor-capacitor) tank circuit. The frequency can be adjusted based on the values of the inductors and capacitors used.

B. Crystal oscillator - III. Greater stability

Explanation: A Crystal oscillator uses a quartz crystal for frequency control and offers excellent frequency stability due to the quartz crystal's high Q-factor. This makes a crystal oscillator more stable compared to the other oscillator circuits.

C. Wien bridge oscillator - I. Two-stage RC coupled amplifier

Explanation: The Wien Bridge Oscillator employs a feedback circuit with an RC (resistor-capacitor) network to produce sinusoidal oscillations. Its design can involve a two-stage RC coupled amplifier and it's often used for generating audio frequencies.

Explanation:

• An amplifier can be converted into an oscillator by doing some changes in the amplifier circuit as:
• Connect the output of the amplifier to the input by a positive feedback circuit.
• The phase-shifted output by 180° and feed this phase-shift output to the input via a feedback circuit.
• An arrangement of the RC-tuned circuit is connected as a load to the amplifier.
• Oscillator circuit block diagram.

Negative feedback amplifier:

A negative-feedback amplifier is an electronic amplifier that subtracts a fraction of its output from its input.

The transfer function of a negative-feedback amplifier is given by:

\({V_o(s)\over V_i(s)} = {A_{ol}\over 1+\beta A_{ol}}\)

Effects of negative feedback:

1. Increases input resistance
2. Reduces gain
3. Increases bandwidth
4. Stability increases hence providing a more linear operation
5. Reduces noise and distortion

The four basic feedback topologies are as shown:

i) Voltage-series feedback with voltage amplifier

ii) Current-series feedback with a transconductance amplifier

iii) Current-shunt feedback with current amplifier

iv) Voltage-shunt feedback with trans resistance amplifier

The four basic feedback topologies are as shown:

i) Voltage-Series feedback with voltage amplifier:

Input Resistance:

\({R_{if}} = {R_i}\left( {1 + A\beta } \right)\) (Increases)

Output Resistance:

\({R_{of}} = \frac{{{R_o}}}{{1 + A\beta }}\) (Decreases)

ii) Current-Series feedback with a transconductance amplifier:

Input Resistance:

\({R_{if}} = {R_i}\left( {1 + A\beta } \right)\)  (Increases)

Output Resistance:

\({R_{0f}} = {R_0}\left( {1 + A\beta } \right)\) (Increases)

iii) Current-Shunt feedback with a current amplifier:

Input Resistance:

\({R_{if}} = \frac{{{R_i}}}{{1 + A\beta }}\)  (Decreases)

Output Resistance:

\({R_{of}} = {R_o}\left( {1 + A\beta } \right)\) (Increases)

iv) Voltage-Shunt feedback with trans resistance amplifier:

Input Resistance:

\({R_{if}} = \frac{{{R_i}}}{{1 + A\beta }}\) (Decreases)

Output Resistance:

\({R_{of}} = \frac{{{R_o}}}{{1 + A\beta }}\) (Decreases)

The net input resistance for the current shunt is given by:

\({{\rm{R}}_{{\rm{iF}}}} = \frac{{{{\rm{R}}_{\rm{i}}}}}{{1 + {\rm{A\beta }}}}\)

And the output resistance is:

ROF = Ro (1 + Aβ)

Conclusion: The effect of current-shunt Feedback in an amplifier is to decrease the Input resistance and increase the output resistance.

Negative feedback circuit:

The feedback amplification factor is given by:

 \({A_f} = \frac{A}{{1 + A\beta }}\),

where,

A is open-loop gain

Aβ is the loop gain.

The negative feedback in amplifiers causes:

• Reduced the gain and increases the stability in gain
• Increases the bandwidth to maintain constant gain-bandwidth product
• Reduces the distortion and noise in the amplifier
• The signal to noise ratio is not affected.

Concept-

Identification of Feedback topology-

1. Identify the feedback network/element.

2. If at the output side, feedback is connected to the output of the circuit directly, name it as 'voltage sampling', or else 'current sampling'

3. If at the input side, feedback is connected to the input given to the circuit directly, name it as 'shunt mixing ' or else 'series mixing'

Analysis:

Step-1: feedback element is both R₁­ Resistor

Step – 2: The feedback element is directly connected to output so voltage sampling.

Step – 3: The feedback element is directly connected to input so shunt mixing.

Hence, Voltage-shunt is the right answer. 

Transistor as an Amplifier

A transistor is a 3-terminal device i.e. base, collector, and emitter 

When the base-emitter junction is forward-biased and the collector-emitter is reversed-biased, the transistor acts as an amplifier.

The working of the amplifier is based on negative feedback.

In a negative feedback circuit, some part of the output is fed back to the input.

Explanation

The closed loop gain for negative feedback is given by:

\(A_f={A\over 1+AM}\)

where, Af = Closed loop gain

A = Open loop gain

M = Feedback factor

So, when negative feedback amplifier, the gain reduces by the factor (1+AM)