Operational Amplifier Concepts

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Questions and Answers

What does the slew rate (SR) measure?

The average response time of an op amp

The maximum frequency of an op amp

The change in output voltage over change in time (correct)

The output voltage at peak signal

In the equation $SR = \frac{\Delta v_{out}}{\Delta t}$, what does the Greek letter delta (Δ) indicate?

A fixed time period

A change in value (correct)

A measurement of voltage

A constant rate

What will happen if the output sine wave signal is very large and the frequency is high?

The output will become a square wave

Slew rate will have no effect

Slew rate will distort the output signal (correct)

The output will remain unchanged

For a 741C op amp, what is the slew rate given?

0.5 V/μs Signup and view all the answers

How can calculus be used regarding slew rate and sine waves?

To derive the equation $SS = 2\pi f V_P$ Signup and view all the answers

What does it mean when $SS \leq SR$ in terms of output distortion?

There is no distortion in the output signal Signup and view all the answers

If the initial slope of an output waveform is represented by which variable?

$SS$ Signup and view all the answers

Why is the slew rate crucial in the data sheet of an op amp?

It limits the response to large signals Signup and view all the answers

What does a voltage follower circuit produce at its output?

A voltage exactly equal to the input voltage Signup and view all the answers

What is the closed-loop input impedance of a voltage follower circuit like?

Much higher than the open-loop input impedance Signup and view all the answers

In a voltage follower, what happens to the closed-loop output impedance?

It is much lower than the open-loop output impedance Signup and view all the answers

What is the power supply rejection ratio (PSRR) used to measure?

The ability to reject variations in the supply voltage Signup and view all the answers

Why is a voltage follower ideal for interfacing a high-impedance source with a low-impedance load?

It allows all input voltage to appear across the load Signup and view all the answers

What does a closed-loop bandwidth equal to in a voltage follower?

The unity gain frequency Signup and view all the answers

What is a key advantage of using a voltage follower regarding output offset error?

The input errors are not amplified Signup and view all the answers

Which linear ICs are commonly used in conjunction with op amps?

Various linear circuits including audio and video amplifiers, and voltage regulators Signup and view all the answers

What is the output voltage equation for a differential amplifier?

$v_{out} = A_v (v_1 - v_2)$ Signup and view all the answers

What is the phase relationship of the output voltage to the noninverting input voltage?

In phase Signup and view all the answers

In applications where only one input is used, which input is typically active while the other is grounded?

The inverting input is grounded Signup and view all the answers

What happens to the output voltage when the two input voltages are equal?

The output voltage is zero Signup and view all the answers

How does a single-ended output differ from a differential output in terms of voltage gain?

It has half the voltage gain Signup and view all the answers

Which component is removed in the single-ended output configuration of a differential amplifier?

Collector resistor on the left Signup and view all the answers

What is the total input referred to when both noninverting and inverting input voltages are present?

Differential input Signup and view all the answers

What characterizes the output polarity when $v_1$ is greater than $v_2$?

The output voltage has the same polarity as $v_1$ Signup and view all the answers

In a summing circuit, what is the significance of the virtual ground at the inverting input?

It directs all input currents through the feedback resistor. Signup and view all the answers

When the resistances in a summing circuit are equal, what is the resulting closed-loop voltage gain?

Equal to unity Signup and view all the answers

What defines the output voltage in a summing amplifier with multiple inputs?

Output is the negation of the total input voltages. Signup and view all the answers

What happens to the voltage gain of a voltage follower circuit?

It equals one due to maximum negative feedback. Signup and view all the answers

What is the role of Thevenin resistance in a summing circuit?

To determine compensation resistance at the noninverting input Signup and view all the answers

In the formula for output voltage of a summing circuit, which term shows the contribution from each input voltage?

The sum of all input voltages multiplied by their gain factors. Signup and view all the answers

How is the effective resistance looking from the inverting input in a summing amplifier calculated?

As the parallel equivalent of all resistances connected to the virtual ground. Signup and view all the answers

If you wanted to reduce the output voltage in a summing circuit, which modification would help?

Increase the feedback resistor. Signup and view all the answers

What is the output voltage equation when only the noninverting input is active?

$v_{out} = A_v (v_1)$ Signup and view all the answers

When both inputs are grounded in the dc analysis of a diff amp, what assumption is made regarding component characteristics?

All transistors are identical and all resistors are equal. Signup and view all the answers

In the inverting input configuration, how does the phase relationship between the input and output change?

The output is out of phase with the input. Signup and view all the answers

What distinguishes the differential single-ended output configuration from the differential differential output configuration?

The output is taken from one side only. Signup and view all the answers

What is the overall behavior of the voltage gain $A_v$ in a single-ended output configuration compared to a differential output configuration?

It is half. Signup and view all the answers

Which statement reflects a characteristic of the general case of a differential amplifier?

It has two differential inputs and a differential output. Signup and view all the answers

In the given configurations, which input does the equation $v_{out} = -A_v (v_2)$ specifically pertain to?

Inverting input with grounded $v_1$. Signup and view all the answers

What is meant by the term 'tail current' in the context of a differential amplifier?

The current through the common resistor shared by the transistors. Signup and view all the answers

Study Notes

Slew Rate

The slew rate is the fastest rate of change of the output voltage of an operational amplifier (op amp).
It is calculated as the change in output voltage divided by the change in time.
The slew rate is specified in the datasheet for an op amp and represents the maximum speed at which the output voltage can change.
A larger slew rate indicates a faster response time.
Slew-rate distortion can occur when the input signal is large and the frequency is high, limiting the output signal's fidelity.

Inverting Summing Amplifier

An inverting summing amplifier combines multiple input signals by multiplying each input by a corresponding gain factor and summing the results.
If the feedback resistor (Rf) and all input resistors (R1, R2, etc.) have equal values, each input channel has a unity gain (1).
This allows for combining multiple input signals while preserving their relative magnitudes.

Voltage Follower

A voltage follower is an op-amp circuit that employs maximum negative feedback, producing a closed-loop voltage gain of unity.
Since the output voltage directly feeds back to the inverting input, the output voltage is essentially equal to the input voltage.
Voltage followers offer a low output impedance and high input impedance which makes it effective for interfacing between a high-impedance source and a low-impedance load.
The closed-loop bandwidth of a voltage follower is maximized, as it is equal to the unity-gain frequency (fUNITY).
Output offset errors are minimal because input errors are not amplified.

Differential Amplifier

A differential amplifier (diff amp) amplifies the difference between two input signals.
The noninverting input (v1) produces an output in phase with the input signal, while the inverting input (v2) produces an output that is 180° out of phase.
The equation for the output voltage is: vout = Av (v1 - v2), where Av is the voltage gain.
A diff amp can be configured with different input and output modes:
- Differential Input and Differential Output: The output is the difference of the two collector currents (vc2 - vc1)
- Differential Input and Single-Ended Output: The output is taken from one collector resulting in half the voltage gain.
- Single-Ended Input and Differential Output: One input is grounded and the other is the active input, providing a differential output and providing the same voltage gain as differential input.
- Single-Ended Input and Single-Ended Output: Similar to single-ended input and differential output with the same voltage gain but the output is taken from one collector only.
Diff amps are used in circuits like operational amplifiers and in applications where the difference between two signals needs to be amplified.

DC Analysis of a Diff Amp

The diff amp is sometimes called a long-tail pair, and the current flowing through the common emitter resistor (RE) is called the tail current.
Ideal analysis assumes identical transistors with equal collector resistors and assumes both bases are grounded.
The tail current provides a reference point for controlling the operating point of the transistors.
Changes in input signals cause changes in the base currents of the transistors, which affects the collector currents and ultimately the output voltage.

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Description

This quiz covers key concepts related to operational amplifiers, including slew rate, inverting summing amplifiers, and voltage followers. Test your understanding of how these circuits operate, their characteristics, and their applications in electronic design.