Positive Feedback Op Amp MCQ Quiz - Objective Question with Answer for Positive Feedback Op Amp - Download Free PDF

Explanation:

Astable Multivibrator as an Oscillator

Definition: An astable multivibrator is an electronic circuit that generates a continuous output of square wave oscillations without requiring any external triggering. It is essentially a free-running oscillator, which means it oscillates between its two unstable states indefinitely, producing a consistent and repetitive waveform.

Working Principle: The astable multivibrator operates by charging and discharging capacitors through resistors, which in turn switches the output states of the transistors or other switching devices used in the circuit. The time taken for the capacitors to charge and discharge determines the frequency of the oscillation. Since the circuit does not have any stable states, it continuously switches between its two states, hence producing a periodic output signal.

Advantages:

• Simple design and easy to construct using basic electronic components.
• Provides a stable and continuous square wave output without the need for external triggering.
• Widely used in timing applications, waveform generation, and clock pulse generation.

Disadvantages:

• The output frequency can be susceptible to variations in component values and power supply fluctuations.
• Not suitable for applications requiring precise frequency stability.

Applications: Astable multivibrators are commonly used in various applications such as LED flashers, pulse generation circuits, clock pulse generators, and in electronic devices where a continuous oscillating signal is required.

Correct Option Analysis:

The correct option is:

Option 2: Astable

This option correctly identifies the type of multivibrator that operates as an oscillator without requiring any external triggering. An astable multivibrator continuously oscillates between its two states, generating a periodic square wave output.

Additional Information

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

Option 1: Bistable

A bistable multivibrator, also known as a flip-flop, has two stable states and requires external triggering to switch between these states. It is not used as an oscillator because it does not produce continuous oscillations on its own. Instead, it remains in one of its stable states until an external trigger is applied to change its state.

Option 3: Monostable

A monostable multivibrator has one stable state and one unstable state. When triggered, it temporarily switches to the unstable state and returns to the stable state after a predetermined time period. It is not used as an oscillator because it requires an external trigger to generate a pulse and does not produce continuous oscillations.

Option 4: Stable

This option is not a specific type of multivibrator. The term "stable" generally refers to the stability of a circuit or signal, but it does not describe a type of multivibrator. Therefore, it is not applicable in this context.

Conclusion:

Understanding the different types of multivibrators and their operational characteristics is essential for identifying their appropriate applications. An astable multivibrator, as explained, functions as a free-running oscillator that generates continuous square wave oscillations without the need for external triggering. This makes it suitable for various timing and waveform generation applications, providing a simple and effective solution for generating periodic signals.

Explanation:

In the given Schmitt trigger circuit, we are required to find the output logic low level when V_CE(sat) = 0.1 V.

A Schmitt trigger is a comparator circuit with hysteresis implemented by applying positive feedback to the non-inverting input of a comparator or differential amplifier. Schmitt triggers are used to convert a noisy analog input signal into a clean digital output signal.

The output of a Schmitt trigger circuit switches between high and low voltage levels, depending on the input voltage and the threshold voltages set by the circuit. The output logic low level is the voltage level of the output when the circuit is in the low state, which typically occurs when the input voltage is below the lower threshold voltage.

In a Schmitt trigger circuit, the output stage is usually an open-collector or open-drain configuration. When the output is low, the transistor in the output stage is saturated, and the output voltage is approximately equal to the saturation voltage of the transistor, V_CE(sat).

Given that V_CE(sat) = 0.1 V, the output logic low level can be determined as follows:

Step-by-Step Solution:

1. Identify the saturation voltage of the transistor (V_CE(sat)). In this case, V_CE(sat) is given as 0.1 V.
2. In the low state, the output voltage is approximately equal to V_CE(sat).
3. Therefore, the output logic low level is 0.1 V.

However, the given options are:

• Option 1: 3.65 V
• Option 2: 3.85 V
• Option 3: 1.35 V
• Option 4: 1.15 V

Since the output logic low level is typically very close to V_CE(sat) (0.1 V), let's consider the given options:

• Option 1: 3.65 V
• Option 2: 3.85 V
• Option 3: 1.35 V
• Option 4: 1.15 V

Among these options, the one that aligns closest to the expected output logic low level is Option 3: 1.35 V. This value may represent the logic low level due to other factors in the circuit that might influence the saturation voltage slightly, but still, it is the closest to the given V_CE(sat).

Correct Option Analysis:

The correct option is:

Option 3: 1.35 V

This option is closest to the expected output logic low level given the saturation voltage of the transistor in the Schmitt trigger circuit. The value of 1.35 V may be due to additional voltage drops or other circuit elements, but it is the best match among the provided options.

Additional Information

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

Option 1: 3.65 V

This option is much higher than the expected output logic low level. A typical logic low level in a Schmitt trigger circuit should be close to the saturation voltage of the transistor, which is much lower than 3.65 V.

Option 2: 3.85 V

Similar to Option 1, this value is also too high to be considered as the output logic low level. The saturation voltage of the transistor in the output stage would not typically result in such a high voltage for the logic low state.

Option 4: 1.15 V

While this option is closer to the expected output logic low level than Options 1 and 2, it is still higher than the given V_CE(sat) of 0.1 V. However, it is relatively close and could be a result of other circuit elements affecting the voltage slightly.

Conclusion:

Understanding the behavior of the Schmitt trigger circuit and the impact of the transistor's saturation voltage is essential for correctly identifying the output logic levels. The output logic low level is determined by the saturation voltage of the transistor, which is given as 0.1 V in this case. Among the provided options, Option 3: 1.35 V is the closest match, accounting for potential minor variations in the circuit. This analysis highlights the importance of considering all factors in the circuit when determining the output logic levels.

Schmitt Trigger:

• It is a comparator circuit with positive feedback.
• It is a Bi-stable circuit, having two stable states (+Vsat and -Vsat).
• In this, input sine-wave is converted to a square wave
• The output voltage changes its states whenever the input voltage exceeds certain voltage levels (i.e. VUT & VLT)

∴ The input sine-wave converted into a square wave

Concept:

1. Monostable multivibrator

   The applications of monostable multivibrators include:

• Pulse width Modulator
• Linear Ramp generator
• frequency divider
• pulse stretcher

2. Astable multivibrator

• Square Wave Generator
• voltage-controlled oscillator
• FSK generator
• free running oscillator

Additional Information

555 Timer:

• Pin 5 of 555 Timer is the control pin. 
• This makes the oscillator cycle more consistent.
• This pin can also be used to change the internal voltage reference to allow modifying the oscillator frequency or duty cycle.

The correct option is 3

Concept:

• A Schmitt trigger is basically an inverting comparator circuit with positive feedback. 
• The function of the Schmitt trigger is to convert any regular or irregular-shaped input waveform into a square wave output voltage or pulse.
• Thus, it can shape a wave and also called as a squaring circuit and can be used as a square wave generator.
• Schmitt trigger is a comparator circuit with hysteresis implemented by applying positive feedback to the non-inverting input of a comparator or differential amplifier.
• It is an active circuit that converts an analog input signal to a digital output signal.
• The circuit is named a "trigger" because the output retains its value until the input changes sufficiently to trigger a change.
• In the non-inverting configuration, when the input is higher than a chosen threshold, then output is high.
• When the input is below a different (lower) chosen threshold, then output is low, and when the input is between the two levels, then output retains its value.

Schmitt trigger:

• Schmitt trigger is a comparator circuit with a hysteresis effect implemented by applying positive feedback to the non-inverting input of a comparator or differential amplifier.
• It is an active circuit that converts an analog input signal to a digital output signal.
• When the input is below a different (lower) chosen threshold the output is low, and when the input is between the two levels the output retains its value. This dual threshold action is called hysteresis.
• The Schmitt trigger possesses memory and can act as a bistable multivibrator.
• It is called the transfer characteristics of the Schmitt trigger.

Hall Effect: When a current-carrying conductor is placed perpendicular to the magnetic field, then an Electric field is produced, which is perpendicular to both.

Concept:

• Astable Multivibrators are free-running oscillators that oscillate between two states continuously producing two square wave output waveforms.
• These both states are not stable as it changes from one state to the other all the time.
• As the Multivibrator keeps on switching, these states are known as quasi-stable or half-stable states.

• A Monostable multivibrator has a stable state and a quasi-stable state. This has a trigger input to one transistor. So, one transistor changes its state automatically, while the other one needs a trigger input to change its state.
• A Bistable multivibrator has both the two states stable. It requires two trigger pulses to be applied to change the states. Until the trigger input is given, this multivibrator cannot change its state.

Concept:

The input voltage Vi changes the state of output therefore it exceeds its voltage level above a certain value called upper and lower threshold voltages.

The upper threshold point:

\({V_{UTP}} = {V_R} \times \frac{{{R_1}}}{{{R_1} + {R_2}}} + {V_0}\frac{{{R_2}}}{{{R_1} + {R_2}}}\)

Lower threshold point:

\({V_{LTP}} = {V_R}\frac{{{R_1}}}{{{R_1} + {R_2}}} - {V_0}\;\frac{{{R_2}}}{{{R_1} + {R_2}}}\)

Now, VH = VUTP - VLTP (hysteresis voltage)

Calculation:

Hysteresis Voltage

VH = VUTP - VLTP = 12 - 8 = 4 V

Additional Information

• Schmitt trigger is a comparator circuit with a hysteresis effect implemented by applying positive feedback to the non-inverting input of a comparator or differential amplifier.
• It is an active circuit that converts an analog input signal to a digital output signal.
• When the input is below a different (lower) chosen threshold the output is low, and when the input is between the two levels the output retains its value. This dual threshold action is called hysteresis.
• The Schmitt trigger possesses memory and can act as a bistable multivibrator.
• It is called the transfer characteristics of the Schmitt trigger.

Schmitt trigger:

Schmitt trigger is a positive feedback comparator circuit that converts the sinusoidal input into a square wave.

The output of a Schmitt trigger circuit is always +V_CC or -V_CC

When V_NI > V_I , the output is +V_CC

When VNI < VI , the output is -VCC

Working of Schmitt trigger:

Case 1: When Vref > Vin 

V_o = +V_CC

\(V_{ref} = V_{UTP}=V_{cc}({R_2 \over R_1+R_2})\)

Case 2: When Vref < Vin 

Vo = -V_CC

\(V_{ref} =V_{LTP}= -V_{cc}({R_2 \over R_1+R_2})\)

Concept:

The input voltage V_i changes the state of output therefore it exceeds its voltage level above a certain value called upper and lower threshold voltages.

The upper threshold point:

\({V_{UTP}} = {V_R} \times \frac{{{R_1}}}{{{R_1} + {R_2}}} + {V_0}\frac{{{R_2}}}{{{R_1} + {R_2}}}\)

Lower threshold point:

\({V_{LTP}} = {V_R}\frac{{{R_1}}}{{{R_1} + {R_2}}} - {V_0}\;\frac{{{R_2}}}{{{R_1} + {R_2}}}\)

Now, V_H = V_UTP - V_LTP (hysteresis voltage)

Calculation:

In Schmitt trigger, Zener diode voltage v_d = 0.7 V

Threshold voltage V₁ = 0

Hysteresis voltage V_H = 0.2 V

\(\Rightarrow {V_H} = {V_R}\frac{{{R_1}}}{{{R_1} + {R_2}}} + {V_0}\frac{{{R_2}}}{{{R_1} + {R_2}}} - \left( {{V_R}\frac{{{R_1}}}{{{R_1} + {R_2}}} - {V_0}\frac{{{R_2}}}{{{R_1} + {R_2}}}} \right)\)

\(\Rightarrow {V_H} = 2\;{V_0}\frac{{{R_2}}}{{{R_1} + {R_2}}}\)

Now, V₀ = V_Z + V_D = 0.7 + 6 = 6.7 V

\( \Rightarrow 0.2 = 2 \times 6.7 \times \frac{1}{{1 + \frac{{{R_1}}}{{{R_2}}}}}\)

\( \Rightarrow 1 + \frac{{{R_1}}}{{{R_2}}} = 67\)

\(\Rightarrow \frac{{{R_1}}}{{{R_2}}} = 66\)

Now, (threshold) \({V_1} = {V_R} \times \frac{{{R_1}}}{{{R_1} + {R_2}}} + {V_0} \times \frac{{{R_2}}}{{{R_1} + {R_2}}}\)

\(\Rightarrow 0 = {V_R}\frac{1}{{1 + \frac{{{R_2}}}{{{R_1}}}}} + {V_0}\frac{1}{{1 + \frac{{{R_1}}}{{{R_2}}}}}\)

\( \Rightarrow {V_2} \times \frac{1}{{1 + \frac{1}{{66}}}} = - 6.7 \times \frac{1}{{1 + 66}}\)

\(\Rightarrow {V_R} = \; - 6.7 \times \frac{1}{6} = - 0.10\;V\)

Schmitt Trigger:

• It is a comparator circuit with positive feedback.
• It is a Bi-stable circuit, having two stable states (+Vsat and -Vsat).
• In this, the input sine-wave is converted to an ​asymmetrical square wave or rectangular wave. 
• The output voltage changes its states whenever the input voltage exceeds certain voltage levels (i.e. VUT & VLT)

The output waveform for a sinusoidal input is a rectangular wave.

The correct answer is option 1):(Two)

Concept:

Bistable multivibrator:

• A Bistable multivibrator has both two states stable. It requires two trigger pulses to be applied to change the states. Until the trigger input is given, this multivibrator cannot change its state.
• A Bistable Multivibrator has two stable states. The circuit stays in any one of the two stable states. It continues in that state, unless an external trigger pulse is given. This Multivibrator is also known as Flip-flop.

Additional Information

• An Astable multivibrator is a circuit that automatically switches between the two states continuously without the application of any external pulse for its operation, i.e. it has no stable state. As this produces a continuous square wave output, it is called a free-running multivibrator.
• A Monostable multivibrator has a stable state and a quasi-stable state. This has a trigger input to one transistor. So, one transistor changes its state automatically, while the other one needs a trigger input to change its state.

Given:

High output voltage level = 5 V

Lower threshold voltage = 1.6 V

Upper threshold voltage = 2.4 V

Resistance (R) = 10 kΩ 

Capacitance (C) = 47 nF

t₁ = Capacitor charging time

t₂ = Capacitor discharging time

Capacitor charging diagram is given as:

In case of charging voltage across capacitor V_c(t) is given as:

V_c(t) = V_cf + (V_ci - V_cf) e^-t/RC

Where V_cf = Final value of voltage across capacitor (= 5 V)

V_ci = Initial value of voltage across capacitor (= 1.6 V)

In case of discharging voltage across capacitor V_c(t) is given as:

V_c(t) = V_ci e^-t/RC

RC = 10 × 10³ × 47 × 10^-9 = 470 × 10^-6

At the end of charging time (t₁), voltage across capacitor is = 2.4 V

V_c(t) = 2.4 = 5 + (1.6 - 5) e^{-t₁/(470 ×} 10^-6)

t₁ = 126.1 × 10^-6 s

At the end of discharging time (t₂), voltage across capacitor is = 1.6 V

V_c(t) = 1.6 = 2.4 e^{-t₂/(470 ×} 10^-6)

t₂ = 190.57 × 10^-6 s

T = t₁ + t₂ = 126.1 × 10^-6 + 190.57 × 10^-6 = 316.67 × 10^-6 s

Frequency = 1/T = 1/(316.67 × 10^-6) = 3157.87 Hz

Schmitt Trigger:

• It is a comparator circuit with positive feedback.
• It is a Bi-stable circuit, having two stable states (+Vsat and -Vsat).
• In this, input sine-wave is converted to a square wave
• The output voltage changes its states whenever the input voltage exceeds certain voltage levels (i.e. VUT & VLT)

\({v_0} = {A_{0L}} \cdot {V_d} = {A_{OL}}\left( {{v_f} - {v_{in}}} \right)\)

∴ The input sine-wave converted into a square wave

Concept:

1. Monostable multivibrator

   The applications of monostable multivibrators include:

• Pulse width Modulator
• Linear Ramp generator
• frequency divider
• pulse stretcher

2. Astable multivibrator

• Square Wave Generator
• voltage-controlled oscillator
• FSK generator
• free running oscillator

Additional Information

555 Timer:

• Pin 5 of 555 Timer is the control pin. 
• This makes the oscillator cycle more consistent.
• This pin can also be used to change the internal voltage reference to allow modifying the oscillator frequency or duty cycle.