Oscillators and Feedback Amplifier MCQ Quiz - Objective Question with Answer for Oscillators and 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.

Explanation:

Feedback Amplifiers

Definition: A feedback amplifier is an electronic amplifier that uses feedback to control its gain, bandwidth, and other performance characteristics. Feedback can be positive or negative, and it involves returning a portion of the amplifier's output signal to its input through a feedback network.

Working Principle: In a feedback amplifier, a fraction of the output signal is fed back to the input. Depending on the type of feedback, this signal can either reinforce (positive feedback) or oppose (negative feedback) the input signal. Negative feedback is commonly used to stabilize the amplifier, improve linearity, increase bandwidth, and modify input/output impedance.

Correct Option Analysis:

The correct option is:

Option 3: Positive feedback increases gain, but bandwidth remains almost unchanged.

This statement is incorrect. Positive feedback indeed increases the gain of the amplifier, but it does not leave the bandwidth unchanged. Instead, positive feedback tends to reduce the bandwidth of the amplifier. This is because the amplified signal, when fed back in phase with the input signal, increases the overall loop gain, which can lead to instability and can amplify noise or undesired frequencies. Therefore, the bandwidth is adversely affected when positive feedback is applied.

Additional Information

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

Option 1: If the feedback signal is returned to the input in series opposition with the applied signal, the input impedance increases.

This statement is correct. In series feedback (negative feedback), the feedback signal is added in opposition to the input signal, and this configuration increases the input impedance of the amplifier. The opposition of the feedback signal reduces the effective input current, which in turn increases the input impedance.

Option 2: If the feedback signal is returned to the input in shunt with the applied signal, the input impedance decreases.

This statement is correct. In shunt feedback (negative feedback), the feedback signal is applied in parallel with the input signal, and this configuration decreases the input impedance of the amplifier. The parallel connection effectively reduces the overall resistance seen at the input.

Option 4: If the feedback signal is subtracted from the applied signal, the amplifier's bandwidth increases.

This statement is correct. Negative feedback, where the feedback signal is subtracted from the input signal, increases the amplifier's bandwidth. This is because negative feedback reduces the gain at low frequencies, which allows the amplifier to maintain a more consistent gain across a wider range of frequencies.

Conclusion:

Understanding the effects of feedback on amplifier characteristics such as gain, input/output impedance, and bandwidth is crucial. Negative feedback is widely used in amplifiers to improve stability, linearity, and bandwidth, while positive feedback is generally used in oscillators or specific applications where gain enhancement is desired but at the cost of reduced bandwidth and potential instability. The incorrect statement in this case is option 3, as positive feedback not only increases gain but also reduces the bandwidth of the amplifier.

The primary function of a crystal oscillator in a frequency synthesizer is to generate a stable output frequency.

Explanation

• A frequency synthesizer creates various output frequencies from a single, fixed reference frequency.

• A crystal oscillator is used to generate this highly stable and accurate reference frequency. The stability of the crystal oscillator directly impacts the accuracy and stability of all the frequencies generated by the synthesizer.

• While the synthesizer then uses phase-locked loops (PLLs), dividers, and multipliers to derive other frequencies, the crystal oscillator provides the fundamental, precise timing standard.

• 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

 

Power Amplifier	Conduction Angle	Maximum Efficiency	Figure of Merit
Class A	360°	50%	2
Class B	180°	78.5%	0.4
Class AB	180° - 360°	50 – 78.5%	0.4 – 2
Class C		≥ 90°

Concept of Virtual Ground:

• The differential input voltage Vid between the noninverting and inverting input terminals is essentially zero.
• This is because even if the output voltage is few volts, due to a large open-loop gain of the op-amp, the difference voltage Vid at the input terminals is almost zero.

Where Vid is differential voltage, Vin1 is noninverting voltage, Vin2 is inverting voltage.

If the output voltage is 10 V and A i.e., the open-loop gain is 104 then, 

V out = A Vid

Vid = V out / A

= 10 / 104

= 1 mV.

Hence Vid is very small, for analyzing the circuit assumed to be zero.

Vid = Vin1 - Vin2

(Vin1 - Vin2) = V out / A

= V out / ∞ = 0

Calculation:

Circuit diagram:

Two terminals of Op-Amp i.e.; Inverting Terminal and Non-Inverting Terminal are at equipotential.

Apply KCL at node 1V_pp,

Closed-loop gain is given by the ratio of output to the input.

so the Closed-loop voltage gain is given by,

Barkhausen stability criteria

The Barkhausen stability criteria is a mathematical requirement used in electronics to predict whether a linear electronic circuit may oscillate.

It is commonly utilized in the design of electronic oscillators as well as general negative feedback circuits such as op-amps to keep them from oscillating.

Barkhausen's criteria is a necessary but not sufficient condition for oscillation:

Barkhausen Conditions For Oscillation:

It states that if 'A' is the gain of the amplifying element in the circuit and β(s) is the feedback path transfer function, so βA is the loop gain around the circuit's feedback loop, the circuit will maintain steady-state oscillations only at frequencies for which:

1. The loop gain is equal to one in absolute magnitude, which means that |βA| = 1
2. The phase shift through the loop is either zero or an integer multiple of 2π or 360⁰.

Important Points

RC phase shift oscillator:

• It consists of three pairs of RC combinations, each providing a 60° phase shift, thus a total of 180° phase shift.
• RC oscillators are used to generate low or audio-frequency signals. Hence they are also known as audio-frequency oscillators.

The frequency of oscillation is given by:

Calculation:

Given, f = 1000 kHz

R = 500 kΩ 

Phase shift oscillator:

• The phase shift oscillator is a linear electronic circuit that produces a sine wave output.
• It consists of an inverting amplifier element such as a transistor or op-amp with its output feedback to its input through a phase shift network consisting of resistors and capacitors in a ladder network.
• The feedback network shifts the phase of the amplifier output by 1800 at the oscillation frequency to give positive feedback. 

Wein bridge oscillator :

• The Wein bridge oscillator uses two RC networks connected together to produce a sinusoidal oscillator.
• The Wein bridge oscillator uses a feedback circuit consisting of a series RC circuit connected with a parallel RC of the same component values producing a phase delay or phase advance depending upon the circuit frequency
• At the resonant frequency, the phase shift is 00.

Clapp oscillator:

• The Clapp oscillator is an LC oscillator that uses a particular combination of an inductor and three capacitors to set the oscillator frequency.
• The Clapp is often drawn as a Colpitts oscillator that has an additional capacitor placed in series with the inductor.
• This comes under linear or harmonic oscillators, which produces a sine wave output.
• Clapp oscillator is also one kind of phase shift oscillator containing L, C elements, and a transistor or op-amp along with feedback, so it provides phase shift.

​Relaxation oscillator:

• A relaxation oscillator is a nonlinear electronic oscillator circuit that produces nonsinusoidal repetitive output signals such as a triangle wave or square wave.
• Relaxation oscillators are generally used to produce low-frequency signals.
• These oscillators will not provide any phase shift in their output
• Examples of Relaxation oscillators are Astable multivibrator, flyback or sweep oscillator, etc.

The feedback amplification factor is given by 

where A is open-loop gain and βA is loop gain.

As feedback increases the gain decreases thereby bandwidth increases.

The negative feedback in amplifiers causes

1. reduced the voltage gain and increases the stability in gain

2. increases the bandwidth by the factor (1+Aβ) to maintain constant gain-bandwidth product

3. Reduces the distortion and noise in the amplifier

4. but the signal to noise ratio is not affected

Colpitts oscillator:

• The Colpitts oscillator consists of one inductor and one split capacitor in the tank circuit.
• A capacitor with a center tap is used in the feedback system of the Colpitts oscillator
• It is used for the generation of sinusoidal output signals with very high frequencies

RC phase shift oscillator:

The circuit diagram of the RC phase shift oscillator is shown below:

The frequency produced by the above phase shift oscillator is given by:

Wein bridge oscillator:

The circuit diagram of the Wein bridge oscillator is shown below:

The frequency of oscillation is given by:

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.

Concept:

Negative feedback circuit:

The feedback amplification factor is given by:

 A_f = 

Where,

A_o is open-loop gain

A_oβ is the loop gain.

Explanation:

The negative feedback in amplifiers causes:

• Reduced the gain and increases the stability in G.
• 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.
• The voltage gain (A_v) of an amplifier is defined as the ratio of output voltage to the input voltage.

Av = V_o/V_in

Here, V_o is the output voltage of an amplifier and V_in is the input voltage of an amplifier.

• In a negative feedback amplifier, closed-loop voltage gain is given 

            A_v = V_o / V_in = 1/(1+A_oβ)

Here, β = feedback factor,

A_o = open-loop gain of the amplifier.

• This expression clearly shows that closed-loop voltage gain has reduced by introducing negative feedback. 
• We know that a product of gain and bandwidth is inversely proportional so here bandwidth of amplifier will increase ;

            (gain × bandwidth = 0.35)

• A negative feedback amplifier decreases the current gain.

• One of the most important features of the crystal oscillator is its frequency stability as it has the ability to provide a constant frequency output under varying load conditions.
• The stability of the crystal oscillator is closely related to its quality factor or Q.
• High-Q crystal oscillator will oscillate at constant frequency because it produces oscillation only when it is nearer to its resonance frequency.
• A typical Q for a crystal oscillator ranges from 104 to 106.

Important Points:

• The crystal of crystal oscillator is usually made of the quartz material and provides a high degree of frequency stability and accuracy.
• It uses a piezoelectric crystal and when an ac voltage is applied across a crystal it starts vibrating at the frequency of supply voltage this effect is known as piezoelectric effect and the crystal which exhibits this effect is known as piezoelectric crystals.
• Conversely, when these crystals are placed under mechanical strain to vibrate, they produce an ac voltage