● Op-amps are among the most widely used electronic devices
● One of the basic building blocks of Analogue Electronic
● 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
● The circuit operates from a dual
supply+Vcc and -Vee which
ensures a constant supply.
Equivalent 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
● Infinite input impedance. So zero input current and zero
input offset voltage
● Zero output impedance
● Zero noise
● Infinite Common-mode rejection ratio (CMRR)
● Same voltage source is applied at
● Ideally, two input are equally amplified
● Output voltage is ideally zero due to differential voltage is
● Practically, a small output signal can still be measured
● Note for differential circuits:
○ Opposite inputs : highly amplified
○ Common inputs : slightly amplified
Open Loop gain A µ
Bandwidth BW µ 0-100Hz
0 W 10-100 W
Depends only on V
Differential mode signal
Depends slightly on
average input V
CMRR µ 10-100dB