Basics of op amp
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Transcript of 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