EUT 1040 Lecture 4 Operational Amplifiers Differential Amplifier

EUT 1040 Lecture 4: Operational Amplifiers

Differential Amplifier • Not Practical Prior to IC Fabrication • 2 Inputs, Output is Av*(V 1 - V 2)

Common Mode Rejection • Differential Amps won’t Amplify Signals that are Common to both Inputs

Operational Amplifier • Three Stages: – Differential Amplifier (Input Stage) – Voltage Amplifier (High Gain) – Output Stage (low output impedance)

Op Amp Characteristics • • High Input Impedance ( 1 M-Ohm) Low Output Impedance (100 Ohms) High Voltage Gain (A = 100, 000) 2 Differential Inputs – Inverting Input (V-) – Non-Inverting Input (V+)

Equivalent Circuit Model • Differential Input Voltage determines Output Voltage

Ideal Op Amp • Input Impedance is Infinite (No input Current, I- = I+ = 0) • Voltage Gain is Infinite (A = Infinity) • Inverting and Non-inverting inputs at equal potential (V+ = V- )

Feedback • Open Loop Voltage Gain is so High that External Feedback is always used • Output Signal is Fed Back to an Input Terminal

Analysis of Ideal Op Amp Circuits • Set V- = V+ and I- = I+ = 0 • Write Nodal Equation(s)

Basic Op Amp Circuits • Voltage Follower: Impedance Matching or Isolation • Unity Gain: Vout = Vin

Inverting Amplifier • Vout = -(R 2/R 1)*Vin

Non-Inverting Amplifier • Vout = Vin*(R 1+R 2)/R 1

Summing Amplifier • Vout = -(V 1*Rf/R 1 + V 2*Rf/R 2 + V 3*Rf/R 3)

Difference Amplifier • Vout = (V 2 -V 1)*R 2/R 1

Analysis of the Voltage Follower • Provides “Current Isolation” – No Power Drawn from Input Circuit

References • Heathkit, Electronic Circuits, EB-6104 A, 2002 • Alexander, Fundamentals of Circuit Analysis – 2 nd Edition, Mc. Graw-Hill, 2004