1. An operational amplifier (Op-Amp) is a:
Correct Answer: (B) Voltage-controlled device
Explanation: An operational amplifier is a voltage-controlled device because its output voltage is determined by the voltage difference between its input terminals. It has a very high input impedance, meaning it draws negligible current from the input signal.
2. The ideal characteristics of an Op-Amp include:
Correct Answer: (D) All of the above
Explanation: The ideal characteristics of an operational amplifier include: - Infinite input impedance (draws no current from the input) - Infinite open-loop gain (AOL = ∞) - Zero output impedance (can drive any load without voltage drop) - Infinite bandwidth (no frequency limitations) - Zero offset voltage (output is zero when inputs are equal)
3. The gain-bandwidth product of an Op-Amp is:
Correct Answer: (A) Constant
Explanation: The gain-bandwidth product (GBWP) is a constant for a given operational amplifier. It represents the product of the amplifier's bandwidth and the gain at which it is measured. This means as the closed-loop gain increases, the bandwidth decreases proportionally, and vice versa, keeping their product constant.
4. An ideal Op-Amp has:
Correct Answer: (A) Infinite input impedance and zero output impedance
Explanation: An ideal operational amplifier has: - Infinite input impedance (no current flows into the input terminals) - Zero output impedance (can drive any load without voltage drop) - Infinite open-loop gain (AOL = ∞) - Infinite bandwidth (no frequency limitations) These characteristics allow the Op-Amp to be used in various configurations with predictable behavior.
5. The open-loop gain of an ideal Op-Amp is:
Correct Answer: (C) Infinity
Explanation: The open-loop gain (AOL) of an ideal Op-Amp is infinite. In practical Op-Amps, the open-loop gain is very high (typically 105 to 106) but not infinite. This high gain is what allows the Op-Amp to be used in negative feedback configurations where the closed-loop gain is determined by external components rather than the Op-Amp's internal characteristics.
6. Which power supply configuration is commonly used for Op-Amps?
Correct Answer: (B) Dual supply (+V and -V)
Explanation: Operational amplifiers are most commonly used with dual power supplies (positive and negative voltages relative to ground). This configuration allows the output to swing both above and below ground potential, enabling the amplification of AC signals without DC offset. However, many modern Op-Amps can also operate with single supply configurations for specific applications.
7. Virtual ground in an Op-Amp means:
Correct Answer: (B) The inverting terminal is at the same potential as ground but not physically connected
Explanation: The virtual ground concept arises in negative feedback configurations where: 1. The non-inverting input is connected to actual ground (0V) 2. The Op-Amp tries to make the inverting input equal to the non-inverting input (due to high gain) 3. Thus the inverting input appears to be at ground potential, but no current flows into the actual ground This virtual ground is a powerful concept for analyzing inverting amplifier circuits.
8. The virtual ground concept is applicable in:
Correct Answer: (B) Inverting Op-Amp configurations
Explanation: The virtual ground concept specifically applies to inverting amplifier configurations where: - The non-inverting input is grounded - Negative feedback is applied from output to inverting input - The high gain of the Op-Amp forces the inverting input to remain at virtual ground In non-inverting configurations, both inputs follow the input signal, and in open-loop or comparator circuits, the virtual ground concept doesn't apply.
9. The virtual ground concept is used to:
Correct Answer: (A) Maintain equal voltage at both input terminals
Explanation: The virtual ground is a direct consequence of the Op-Amp's high gain and negative feedback: 1. The Op-Amp tries to make V+ = V- (golden rules) 2. In inverting configuration with V+ grounded, V- becomes virtual ground 3. This simplifies circuit analysis as the inverting input can be considered at 0V 4. All input current flows through the feedback resistor (Iin = If)
10. In an inverting Op-Amp configuration, the input is applied to:
Correct Answer: (B) Inverting terminal
Explanation: In the inverting amplifier configuration: - The input signal is applied to the inverting terminal (-) through an input resistor (Rin) - The non-inverting terminal (+) is grounded - Negative feedback is provided by a feedback resistor (Rf) from output to inverting input - This configuration produces an output that is inverted (180° phase shift) relative to the input
11. The gain of an inverting Op-Amp is given by:
Correct Answer: (C) Av = -Rf/Rin
Explanation: The voltage gain (Av) of an inverting amplifier is: Av = Vout/Vin = -Rf/Rin Where: - Rf is the feedback resistor - Rin is the input resistor - The negative sign indicates phase inversion This formula is derived using the virtual ground concept and Ohm's Law applied to the input and feedback currents.
12. The output of an inverting Op-Amp is:
Correct Answer: (C) Out of phase by 180°
Explanation: The inverting amplifier configuration produces an output that is: - Amplified by the factor Rf/Rin - Phase shifted by 180° relative to the input This phase inversion occurs because the input signal is applied to the inverting terminal. When the input voltage increases, the output voltage decreases proportionally, and vice versa.
13. In an ideal inverting amplifier, the input impedance is:
Correct Answer: (A) Rin
Explanation: In an inverting amplifier configuration: - The input impedance is approximately equal to Rin - This is because the inverting input is at virtual ground (0V) - The input current is therefore Vin/Rin - The input impedance is Zin = Vin/Iin = Rin Note that this is different from the non-inverting configuration where the input impedance is much higher.
14. In a non-inverting Op-Amp configuration, the input is applied to:
Correct Answer: (A) Non-inverting terminal
Explanation: In the non-inverting amplifier configuration: - The input signal is applied directly to the non-inverting terminal (+) - The inverting terminal (-) is connected to ground through a resistor and receives feedback - This configuration provides high input impedance and no phase inversion
15. The gain of a non-inverting Op-Amp is given by:
Correct Answer: (C) Av = 1 + Rf/Rin
Explanation: The voltage gain of a non-inverting amplifier is: Av = 1 + (Rf/Rin) Where: - Rf is the feedback resistor - Rin is the resistor to ground from the inverting input - The gain is always greater than or equal to 1 - There is no negative sign (no phase inversion)
16. The output of a non-inverting Op-Amp is:
Correct Answer: (A) In phase with input
Explanation: The non-inverting amplifier configuration produces an output that is: - Amplified by the factor (1 + Rf/Rin) - In phase with the input signal (0° phase shift) This is in contrast to the inverting amplifier which produces a 180° phase shift.
17. The input impedance of a non-inverting Op-Amp is:
Correct Answer: (A) Infinite
Explanation: The non-inverting configuration provides extremely high input impedance because: - The input signal is applied directly to the non-inverting terminal - The Op-Amp's input impedance is typically in the megaohm or gigaohm range - Practically no current flows into the input terminal This makes it ideal for applications where loading of the source must be minimized
18. Which of the following is not an application of an Op-Amp?
Correct Answer: (B) Rectifier
Explanation: While Op-Amps are used in many applications including: - Voltage followers (unity gain buffers) - Comparators (open-loop configuration) - Integrators (with capacitors in feedback) They are not typically used as standalone rectifiers. Specialized rectifier circuits use diodes, though precision rectifiers may incorporate Op-Amps with diodes.
19. A unity gain buffer (voltage follower) has a gain of:
Correct Answer: (B) 1
Explanation: A voltage follower (unity gain buffer) has: - Gain of exactly 1 (Vout = Vin) - Very high input impedance (minimal loading of source) - Very low output impedance (can drive heavy loads) - No phase inversion It's commonly used for impedance matching between circuits
20. An Op-Amp differentiator produces:
Correct Answer: (B) The derivative of the input
Explanation: An Op-Amp differentiator circuit: - Uses a capacitor in series with the input - Produces an output proportional to the rate of change of input voltage - Output voltage Vout = -RC(dVin/dt) - Useful for wave shaping and frequency detection - Can be unstable at high frequencies due to gain increase
21. An Op-Amp integrator produces:
Correct Answer: (A) The integral of the input
Explanation: An Op-Amp integrator circuit: - Uses a capacitor in the feedback path - Produces an output proportional to the time integral of input voltage - Output voltage Vout = -(1/RC)∫Vindt - Useful for waveform generation (ramps, triangles) - Requires a reset mechanism to prevent DC drift
22. A Schmitt trigger is used to:
Correct Answer: (B) Eliminate noise
Explanation: A Schmitt trigger is a comparator with hysteresis that: - Has two threshold voltages (upper and lower) - Provides noise immunity by requiring the input to change significantly before the output switches - Converts noisy signals into clean digital outputs - Prevents chatter when input signal is near the threshold - Commonly used in switch debouncing circuits
23. An Op-Amp comparator is used to:
Correct Answer: (A) Compare two voltages and produce a digital output
Explanation: An Op-Amp comparator: - Operates in open-loop mode (no negative feedback) - Compares an input voltage with a reference voltage - Produces a digital output (either high or low saturation voltage) - Output indicates which input is at higher potential - Used in analog-to-digital conversion, zero-crossing detectors, and threshold detection