fundamentals of op-amp operation modes golden rules...

Operational Amplifiers


Fundamentals of op-amp Operation modes

Golden rules of op-amp Op-amp circuits

Inverting & non-inverting amplifier

Unity follower, integrator & differentiator


IntroductionAn operational amplifier, or op-amp, is a very high gain differential amplifier with high input impedance and low output impedance.

Shown in the figure is a basic op-amp with two inputs and one output.

Each input results an output either in phase or out of phase with respect to the input.


Figure: Basic op-amp


Operation Modes Single-ended input

Only one input is applied with input signal while the other is connected to ground.The input applied to the plus input results in an output having the same polarity as input.

The input applied to the minus input results in an output being opposite in phase to the applied signal.


Figure: Single-ended input


Double-ended input

When signal is applied between both inputs, it is referred to as double-ended input.

The amplified output is in phase with the difference between the two inputs.

This is the reason that this mode is calleddifferential input.


Figure: Double-ended input


Double-ended output

The op-amp can also be operated with opposite outputs.

The figure shows a single-ended input with a double-ended output.

The signal applied to the plus input results in two amplified output of opposite polarity.


Figure: Double-ended output with single-ended input


Common-modeWhen the same input signals are applied to both inputs, common-mode operation results.

Ideally, the output is zero due to the two opposite output components.

This means that signals common to both inputs will be suppressed, referred to as common-mode rejection.


Figure: Common-mode operation


Basic Op-amp Connection

The basic circuit connection using an op-amp is shown in the figure.An input signal v1 is applied through resistor R1 to the inverting input terminal.The output is connected back to the inverting input terminal through resistor Rf .The noninverting input terminal is connected to the ground.


With the assumption that:

Input impedance is infinite.

The output impedance is zero.

We get

The voltage gain is infinite.

11 RR

vv fo

which means that the relationship is only determined by the two resistors.


Golden rulesIf there is negative feedback and if the output is not saturated, it is true that:

both inputs are at the same voltage;

no current flows in or out of either input.

This leads to the virtual short-circuit between the two inputs.And the inverting terminal is at virtual ground.


Figure: Basic op-amp connection


Common Connections

Inverting Amplifier

As discussed previously, the inverting amplifier is shown in the figure.The output is obtained by multiplying the input by a fixed gain, set by R1 and Rf .

11

vRR

v fo


Figure: Inverting amplifier


Non-inverting Amplifier

The non-inverting amplifier is shown in the figure.

With the concepts of virtual short-circuit, the output is obtained

11

)1( vRR

v fo

The input signal is fed through non-inverting input terminal.


Figure: Non-inverting amplifier


Unity Follower

The unity-follower circuit is shown in the figure.

With the concepts of virtual short-circuit, the output is obvious.

1vvo

It provides a gain of unity with no polarity or phase reversal.


Figure: Unity follower


11. Op-amp Applications Voltage Summing

Based on the inverting amplifier, the voltage summing is shown in the figure.The output is the sum of the three inputs, each multiplied by a independent gain.

)( 33

22

11

vRR

vRR

vRR

v fffo


Figure: Summing amplifier


IntegratorBased on the inverting amplifier and with the feedback resistor replaced by a capacitor, it is integrator as shown in the figure.For a capacitor, we know that

dttdvCti C

C)()(

Also from the circuit, we get

RvtiC

1)(


And that vo = -vC(t) .

So, we obtain that

dttdvC

Rv C )(1

That isRCv

dtdvo 1

dtdvC o

Then,

dttvRC

tvo )(1)( 1


Figure: Integrator


Differentiator

After the investigation of integrator, exchanging the position of the resistor and capacitor, it is differentiator circuit as shown in the figure.

We obtain that

dttdvRCtvo)()( 1


Figure: Differentiator


Summary Operation modes

Golden rules of op-amp

Op-amp circuitsInverting & non-inverting amplifier

Unity follower, integrator & differentiator

Single-ended input, double-ended input Common-mode

fundamentals of op-amp operation modes golden rules...

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