Operational amplifiers

What is an Operation Amplifier ( Op-Amp)?

An Integrated circuit amplifier containing many transistors and resistors

What functions can Op-Amps do?

Amplification, Filtering, Multiplication, differentiation, Integration

Draw an Op-Amp

Do Op-Amps require power?

Yes from an external source (typically +15V to -15V and ground 0V)

What happens at a non-inverting input (Vp) pin?

When a signal is applied to this pin, it is amplified at the output with the same phase as the input

What happens at the interpreting input (Vn) pin?

When a signal is applied at this input pin, it is amplified at the output and inverted ( 180° out of phase)

How do you work out Vdiff for an Op-Amp?

Vp - Vn

How do you work out Voltage output of an op- amp?

gain x Vdiff

Draw a non-interpreting amplifier

Draw an inverting amplifier

Draw an amplifier with a common input

What does offset null do in real devices?

Since the two input channels are not perfectly balanced, an output might occur, the offset allows this to be ‘ tuned out’

Name some Op-Amp Charaterstics

• Usually >100,000 (the output is 100,000 times the input)

• Output voltage can only range from V- and V+ (output voltage ranges between ± 15V)

• To make good use of the high voltage gain, the differential voltage between the input terminals is usually very small to give a large output

• It is a difference amplifier, so if the inverting input has the higher potential, the Vout is more negative, vice versa

Draw the Open-Loop Op-Amp with a high open-loop gain of 10^5 and Vin of 10 microV

Draw an Open-Loop Op-Amp with high open loop gan 10^5 and Vin 10miliV

Draw a graph of Vout against Vdiff

In the linear region fo a Vout against Vdiff graph, how can you work out Vout?

AOL (high voltage gain) x Vdiff

For the linear operation what is true for Vp and Vn

Draw a Vout against Vdiff using different scaled for Vout and Vdiff

Draw a Vout against Vdiff graph using similar scaled for Vout and Vdiff

How can you amplify larger signals whilst avoiding saturation?

Using feedback to the negative terminal of the op-amp

Compare an open loop Vs. closed loop Op-Amp

A closed loop op-amp:

The voltage gain is low but can be controlled

Larger input signals can be amplified without saturation

Draw an inverting amplifier circuit

What is the equation for closed-loop gain in an inverting circuit?

Vout/Vin = -Rf/Ri

What is an application for an inverting amplifier circuit?

Summing amplifier

Draw a closed-loop Op-Amp non-inverting circuit

What is the equation for closed-loop gain for a non-inverting amplifier?

1 + Rf/Rs

What is an application for a non-inverting amplifier circuit?

Voltage comparator

What are some Ideal:typical Op-Amp characteristics?

Infinite open-loop voltage gain: high around 300,000+

Infinite input resistance: 100+ Mohms to 1Gohms

Zero output resistance: Low output resistance < 50 ohms

Derive the closed-loop gain ACL equation

Derive the closed-loop ACL of a non-inverting amplifier

Discuss an Open-loop Op-Amp as a comparator

Closed-loop feedback is not required as it benefits from output saturation.

Give an example of when an Open-loop Op-Amp acts as a comparator

(Where binary decisions are needed) voltage detection

Draw an open loop op amp comparator