Electronics - AS Module 1 & 2

Operational Amplifiers

An operational amplifier is a very high gain amplifier. Op amps can be used in many different ways. Two of the most common uses are

a) as comparators

b) as amplifiers (either NON-INVERTING or INVERTING amplifiers)

An op amp has TWO inputs; one non-inverting (called the "+" input) and one inverting (called the "-" input). The voltage at the output (Vo) depends on the DIFFERENCE between the voltage at the non-inverting input (V+) and the voltage at the inverting input (V-). Expressed mathematically,

Vo = A.(V+ - V-)

where A is the gain or voltage amplification of the op amp (typically 100000)

The diagram below shows an op amp's circuit symbol

For an ideal op-amp

Vout = Gain(V+ - V- )

V+ is the voltage input to the non-inverting input.

V- is the voltage input to the inverting input

Gain is known as the Open Loop Gain

For an ideal op-amp Gain has a very large value.

Typically Gain = 2 x 10^6

Input Resistance

The ideal op-amp has a very large input resistance.

Current flowing into the inputs is negligible.

"Differential Amplifier"

The op-amp is a differential amplifier since it amplifies the difference between two voltages

Output Saturation

The output voltage cannot exceed in magnitude the value of the voltages applied to the supply rails.

That is

-Vs < Vout < + Vs

In practice Vout can only get to within 1 Volt of Vs .

This process is known as "Output Saturation".

Transfer Characteristic

The Inverting Amplifier

Using the second approximation for the Op-Amp,

V+ = V- = 0 Volts

The potential difference across R1 is Vin

The current through R1 is given by

I = Vin/ R1

Using the first approximation for the Op-Amp, no current flows through the Op-Amp.

The current that flows through R2 is the same as the current that flows through R1 .

The potential difference across R2 is given by

V = I R2

V = VinR2/ R1

Since the output MUST be at a lower potential than the point X, Vout must be negative.

Vout = -VinR2/ R1

Gain = Vout/Vin = -R2/ R1

This is known as an inverting amplifier, because a positive voltage IN gives a negative voltage OUT.

A negative voltage IN gives a positive voltage OUT.

The Non-Inverting Amplifier

Gains of greater than 1 can be achieved by feeding back only a fraction of the output. This is done by using a "Potential Divider" between the output and the OV rail.

V+ = Vin

V-= Vout R1/ R1 + R2

Using the second approximation for the Op-Amp

V+ = V-

And so

Vin = R1 Vout/ R1 + R2

or

Vout/Vin = R1 + R2/ R1

And so

Gain = 1 +(R2/ R1)

NOTE. The amplification (gain) depends only upon the value of the resistors. It does NOT depend upon the Op-Amp as long as the approximations are valid.

The Comparator

If V+ > V- then Vout is high and the LED is on.

This happens when the LDR is in the dark.

If V+ < V- then Vout is low ( 0 volts) and the LED is off.

This happens when the LDR is in the light.