Voltage Regulators MCQ Quiz - Objective Question with Answer for Voltage Regulators - Download Free PDF

Explanation:

Phase-Locked Loop (PLL)

Definition: A Phase-Locked Loop (PLL) is an electronic circuit that synchronizes an output signal's phase and frequency with a reference signal. PLLs are widely used in communication systems, signal processing, and control systems to generate stable and precise frequency signals.

• Time Domain: In the time domain, a PLL ensures that the timing of the output signal matches the timing of the reference signal. By adjusting the VCO, the PLL can correct any timing errors, ensuring that the output signal remains synchronized with the reference signal.
• Frequency Domain: In the frequency domain, a PLL locks the frequency of the VCO to the frequency of the reference signal. This is crucial for applications requiring stable and precise frequency control, such as in communication systems where accurate carrier frequencies are necessary.
• Phase Domain: In the phase domain, a PLL maintains a constant phase relationship between the output signal and the reference signal. This phase alignment is essential for coherent signal processing and minimizing phase noise, which can degrade system performance.

Therefore, a PLL operates in all three domains, making Option 4 the correct choice

Explanation:

In FM Demodulation Using PLL

Definition: Frequency Modulation (FM) demodulation using a Phase-Locked Loop (PLL) is a method where the frequency of the received signal is tracked and converted back to the original information signal. The PLL is an electronic circuit that consists of a phase detector, a low-pass filter, and a voltage-controlled oscillator (VCO).

Working Principle: In FM demodulation using PLL, the incoming FM signal is first fed into the phase detector of the PLL. The phase detector compares the phase of the incoming signal with the phase of the signal generated by the VCO. The difference in phase generates a voltage signal that is proportional to the frequency deviation of the FM signal. This voltage signal is then passed through a low-pass filter to remove high-frequency components and noise. The filtered signal is used to adjust the VCO, which in turn changes its frequency to match the frequency of the incoming FM signal. The output of the VCO is the demodulated signal, which is the original information signal.

Components of PLL:

• Phase Detector: Compares the phase of the incoming FM signal with the VCO signal and generates a voltage proportional to the phase difference.
• Low-Pass Filter: Filters the output of the phase detector to remove high-frequency noise and provides a smooth control voltage to the VCO.
• Voltage-Controlled Oscillator (VCO): Generates a signal whose frequency is controlled by the input voltage. The VCO adjusts its frequency to match the frequency of the incoming FM signal.

Explanation:

Voltage-Controlled Oscillator (VCO)

Definition: A Voltage-Controlled Oscillator (VCO) is an electronic oscillator whose oscillation frequency is controlled by a voltage input. The frequency of the output signal varies with the applied input voltage, making VCOs crucial components in various electronic devices such as phase-locked loops (PLLs), frequency synthesizers, and communication systems.

Correct Option Analysis:

The correct option is:

Option 4: External R and C

This option correctly identifies the components that the center frequency of a VCO depends on. In most VCO designs, external resistors (R) and capacitors (C) are used to set the oscillation frequency. The value of these components determines the resonant frequency of the circuit, which is modulated by the input control voltage to produce the desired oscillation frequency.

The resonant frequency f of an LC circuit (inductor-capacitor circuit) or an RC circuit (resistor-capacitor circuit) is given by:

For an LC Circuit:

f = 1 / (2π√(LC))

Where:

• L is the inductance
• C is the capacitance

For an RC Circuit:

f = 1 / (2πRC)

Where:

• R is the resistance
• C is the capacitance

In both cases, the external R and C components play a crucial role in determining the center frequency of the VCO. By adjusting these components, designers can set the desired frequency range for the oscillator

Explanation:

Phase Detector Output

Definition: A phase detector is an essential component in various electronic systems, particularly in phase-locked loops (PLLs), which are used to synchronize an output signal's phase with a reference signal. The phase detector compares the phase of two input signals and generates an output that represents the phase difference between these signals.

Working Principle: The primary function of a phase detector is to generate a signal that is proportional to the phase difference between two input signals. This output signal can be used to adjust the frequency of a voltage-controlled oscillator (VCO) to maintain phase synchronization in a PLL system. Phase detectors can be implemented using different circuit designs, including mixers, digital logic gates, or specialized integrated circuits.

Correct Option Analysis:

The correct option is:

Option 2: DC voltage

This option is correct because the output of a phase detector in a phase-locked loop is typically a DC voltage. The phase detector produces a voltage that is proportional to the phase difference between the input signals. This DC voltage is then used to control the frequency of the VCO, ensuring that the output signal remains in phase with the reference signal. The output voltage is zero when the input signals are in perfect phase alignment, and it increases or decreases as the phase difference changes.

Additional Information:

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

Option 1: Sine wave

This option is incorrect. A sine wave is a continuous wave that oscillates smoothly, representing a periodic oscillation. While phase detectors work with sine waves as input signals, their output is not typically a sine wave. The output of a phase detector is intended to represent the phase difference, which is more effectively done with a DC voltage or a pulse train.

Option 3: Pulse train

This option is partially correct but not typically the final output used in PLL systems. A pulse train can be produced by certain types of phase detectors, especially digital ones, where the width or frequency of the pulses represents the phase difference. However, in most practical applications, this pulse train is further processed to produce a DC voltage that can be used to control the VCO. Hence, the pulse train is more of an intermediate step rather than the final output.

Option 4: Square wave

This option is incorrect. A square wave is a type of periodic waveform that alternates between two levels at a constant frequency. While a square wave can be one of the input signals to a phase detector, the output of the phase detector is not a square wave. The phase detector's output needs to indicate the phase difference, which a square wave does not effectively do.

Conclusion:

Understanding the function and output of a phase detector is crucial for its application in phase-locked loops and other synchronization systems. The correct output of a phase detector is typically a DC voltage, which reflects the phase difference between the input signals. This DC voltage is then used to control the VCO, maintaining phase alignment. While pulse trains can be intermediate outputs in some phase detector designs, the final output used in PLL systems is a DC voltage. Options like sine waves and square waves are not appropriate as they do not represent the phase difference in a usable form for controlling the VCO.

Explanation:

Phase-Locked Loop (PLL) and Voltage-Controlled Oscillator (VCO)

• In a PLL, the VCO generates a frequency that is compared to the reference signal. The phase detector compares the phase of the VCO output with the reference signal.
• If there is a difference in phase or frequency, the phase detector generates an error signal. This error signal is filtered and then fed back to the VCO, adjusting its frequency to match the reference signal.
• When the PLL is locked, the output frequency of the VCO is synchronized with the frequency of the input reference signal.

There are some characteristics of 3-pin voltage regulators ICs which are given below,

• It has built-in all protection circuits
• It does not require any feedback connections
• It does not require any external component for normal operation
• It can provide fixed and adjustable positive and negative operations
• It can deliver output current more than 1 A
• It requires minimum heat sinking
• It can provide dual tracking voltages
• The design of a regulated DC power supply is very easy
• It is very easy to use
• It is conveniently used for local regulation

In Voltage ICs, Zener diode and transistor are used inside it during IC fabrication because these are a semiconductor device and its fabrication is easier as compared to the capacitor, resistor, and transformer IC.

The circuit diagram for a voltage regulator IC is as shown:

In this, the transistor is called the series pass transistor because the collector and emitter terminals

are in series with the input and output voltages.

 Brown-Boveri voltage regulator:

• In this type of regulator, exciter field rheostat is varied continuously or in small steps instead of being first completely cut in and then completely cut out as in Tirril regulator.
• For this purpose, a regulating resistance is connected in series with the field circuit of the exciter.
• Fluctuations in the alternator voltage are detected by a control device that actuates a motor.
• The motor drives the regulating rheostat and cuts out or cuts in some resistance from the rheostat, thus changing the exciter and hence the alternator voltage.
• The figure below shows the schematic diagram of a Brown Boveri voltage regulator.
• It also works on the “overshooting the mark principle” 

• The control system is built on the principle of induction motor.
• It consists of two windings A and B on an annular core of laminated sheet steel.
• The winding A is excited from two of the generator terminals through resistances U and U’ while a resistance R is inserted in the circuit of winding B.
• The ratio of resistance to the reactance of the two windings is suitably adjusted so as to create a phase difference of currents in the two windings.
• Due to the phase difference of currents in the two windings, the rotating magnetic field is set up.
• This produces electromagnetic torque on the thin aluminum drum C carried by steel spindle; the latter being supported at both ends by jewel bearings
• . The torque on drum C varies with the terminal voltage of the alternator. The variable resistance U’ can also vary the torque on the drum.
• If the resistance is increased, the torque is decreased and vice versa.
• Therefore, the variable resistance U’ provides a means by which the regulator may be set to operate at the desired voltage.

In the given question, Supply is varying. Hence the load current is constant.

The maximum Zener current (I_Z(max)) is defined as:

\(I_{Z(max)}=\frac{power~rating}{Zener ~voltage}\)

Given power rating = 2 W. 

\(I_{Z(max)}=\frac{2}{10}=0.2~A\)

The current across the load I_L will be constant to the value:

\(I_L=\frac{V_Z}{R_L}=\frac{10}{500}\)

I_L = 0.02 A

Now, the maximum current across the resistor R_s will be:

I_S(max) = I_L + I_Z(max)

IS(max) = 0.02 + 0.2 = 0.22 A

The maximum voltage across R_s will be:

V_S(max) = V_i(max) - V_Z = 20 - 10 = 10 V

VS(max) = 10 V

∴ The maximum power that can be dissipated across the R_s will be:

P = V_max I_s(max) = (20 - 10) × 0.22 W

= 2.2 W = 2200 mW

Concept:

A Zener diode is a silicon semiconductor device that permits current to flow in either a forward or reverse direction. The diode consists of a special, heavily doped p-n junction, designed to conduct in the reverse direction when a certain specified voltage is reached.

Voltage Vz: The Zener voltage refers to the reverse breakdown voltage.

Current Iz (max.): Maximum current at the rated Zener voltage Vz.

Current Iz (min.): Minimum current required for the diode to break down.

Calculation:

Open circuit the zener diode to calculate the voltage appearing across the diode.

V_x = V_i × 1.2 K/( 1.2 K + 0.22 K)

To operate in zener breakdown region:  

Vi(min) × 1.2 K/( 1.2 K + 0.22 K) > 20 V

Vi(min) > [20( 1.2 K + 0.22 K)]/1.2 K 

Vi(min) > 23.66 V

Current through the zener diode will be maximum if maximum possible input voltage is applied.

⇒ I_R = I_Zmax + I_L

⇒ IR = 60mA + (V_L/R_L) [∵ V_L = V_Z = 20 V]

⇒ IR = 60mA + (20/1.2 K)

⇒ IR = 76.66 mA

(V_i(max) - V_Z)/ 0.22K = I_R

⇒ (Vi(max) - 20)/ 0.22K = 76.66

⇒ Vi(max) = 36.87 V

Given:

V_BE = 0.7 V, β = 100, V_z = 4.7 V, and V_D = 9V,

The given circuit is a voltage Regulator circuit. ∴ The Zener diode will be working in the breakdown region.

The circuit is redrawn as:

V_z = V⁺ = 4.7 V

Due to virtual ground concept : V⁺ = V^-

Since no current enters the input of the Op-Amp, the current flowing through the 1 kΩ Resistance will be:

\(I = \frac{{{V_0} - {V^ - }}}{{1\;k{\rm{\Omega }}}} = \frac{{9 - 4.7}}{{1\;k{\rm{\Omega }}}}\)

\(I = 4.3\;mA\)

Current (I) will flow through the Resistance R, i.e.

\({V^ - } = I \cdot R\)

\(R = \frac{{{V^ - }}}{I} = \frac{{4.7\;V}}{{4.3\;mA}}\)

R = 1093.02 Ω 

​Fixed-Negative Voltage Regulator:

• The series 79XX regulators are the three-terminal IC regulators that provide a fixed negative output voltage.
• This series has the same features and characteristics as the series 78XX regulators except for the pin numbers which are different.

​Negative-Voltage regulators in the 79XX series are as shown:

Fixed-Positive Voltage Regulator:

The series 78XX regulators are the three-terminal devices that provide a fixed positive output voltage.

IC 7805:

7805 is a voltage regulator IC which is used to provide a constant output voltage of 5V.

The last two terms (78  05) denote the output voltage of the Regulator.

Specifications of 7805 IC:

• Drop out voltage = 2 V
• Input voltage range = 7V- 35V
• Current rating Ic = 1A
• Output voltage range VMax = 5.2V, VMin = 4.8V

IC 723:

It is used as an op-amp voltage regulator.

Specifications of 723 IC:

• Output voltage range: 2 V to 37 V
• Output current: up to 150 mA
• Max supply voltage: 40 V
• Line regulation: 0.01%

IC 741:

The Pin diagram of 741 IC Op-amp is shown below:

• 8 Pin IC
• Pin 1 & 5 offset null pins
• Pin 2 is inverting Input (V-)
• Pin 3 is non-inverting Input (V+)
• Pin 4 is - VEE supply
• Pin 6 is for Output
• Pin 7 is + Vcc supply

Pin 8 is not connected and not used

Voltage Regulator :

• The circuit which is used to maintain a constant output voltage.
• The usage of filter circuit in any application is to dampen the fluctuations or the variations in the concerned signal.
• The filter circuits are used in order to bypass the voltage fluctuations and thereby to improve the accuracy of the regulator circuit.

​

Choke Filter :

The inductor is connected to rectifier output.

In choke filter best voltage regulation is obtained.

Hence correct option is "1"

There are some characteristics of 3-pin voltage regulators ICs which are given below,

• It has built-in all protection circuits
• It does not require any feedback connections
• It does not require any external component for normal operation
• It can provide fixed and adjustable positive and negative operations
• It can deliver output current more than 1 A
• It requires minimum heat sinking
• It can provide dual tracking voltages
• The design of a regulated DC power supply is very easy
• It is very easy to use
• It is conveniently used for local regulation

In Voltage ICs, Zener diode and transistor are used inside it during IC fabrication because these are a semiconductor device and its fabrication is easier as compared to the capacitor, resistor, and transformer IC.

The circuit diagram for a voltage regulator IC is as shown:

In this, the transistor is called the series pass transistor because the collector and emitter terminals

are in series with the input and output voltages.