basics of op amp

OPAMP...OPerational AMPlifier

Intro...● Op-amps are among the most widely used electronic devices

today.

● One of the basic building blocks of Analogue Electronic Circuits.

● A DC-coupled high-gain electronic voltage amplifier.

● Has a differential input and, usually, a single-ended output.

● High open loop gain, typically it is ~ 10^4-10^5.

● By using negative feedback, we throw most of that away!

● An ideal Operational Amplifier is basically a three-terminal device which consists of two high impedance inputs.

● One called the Inverting Input, marked with a negative or "minus" sign, ( - ) and the other one called the Non-inverting Input, marked with a positive or "plus" sign ( + ).

● The output signal is the amplification factor, known as the amplifiers gain ( A ) multiplied by the value of the input signal.

Vo=A(V+ - V-)

➔ A generalized form of a differential amplifier:● V1,V2: Inputs● The two identical transistors TR1

and TR2 are both biased at the same operating point with their emitters connected together and returned to the common rail, -Vee by way of resistor Re.

● The circuit operates from a dual supply+Vcc and -Vee which ensures a constant supply.

● Vout=V1-V2Equivalent Circuit for Ideal Operational Amplifiers:-

● V1 : Inverting input● V2: Inverting input● A: Gain● Vdiff: V1-V2● +Vsupply: +ve power supply● -Vsupply: -ve power supply

Properties of ideal op amp...● Infinite open-loop gain

● Infinite voltage range available at the output

● Infinite bandwidth with zero phase shift and infinite slew

rate

● Infinite input impedance. So zero input current and zero

input offset voltage

● Zero output impedance

● Zero noise

● Infinite Common-mode rejection ratio (CMRR)

● Infinite Power supply rejection ratio (PSRR).

Common-Mode Operation

● Same voltage source is applied at both terminals

● Ideally, two input are equally amplified● Output voltage is ideally zero due to differential voltage is

zero● Practically, a small output signal can still be measured● Note for differential circuits:

○ Opposite inputs : highly amplified○ Common inputs : slightly amplified

Common-Mode Rejection

Single-Ended Input

● + terminal : Ground● – terminal : Source● 180o phase change

● + terminal : Source● – terminal : Ground● 0o phase change

Ideal Practical

Open Loop gain A µ 105

Bandwidth BW µ 0-100Hz

Input Impedance

Zin

µ >1MW

Output Impedance

Zout

0 W 10-100 W

Output Voltage Vout Depends only on Vd =

(V+-V-)Differential mode signal

Depends slightly on

average input Vc =

(V++V-)/2 Common-Mode signal

CMRR µ 10-100dB

IDEAL

v/s

PRAC

TICAL

IC Product

DIP-741Dual op-amp 1458

device

741

Applications

•Non Inverting Amplifier•Inverting Amplifier•Adder (Summing amp)–(and Subtractor using an Inverter)•Differential Amplifier•Integrator•Differentiator

Noninverting amplifier Inverting amplifier

Voltage followerLess than unity gain

Vo=Vi

Vo = 1+(Rf/Ra)Vi Vo = 1+(Rf/Ra)(R1/(R1+R2)Vi

Differential amplifier