ABE425 Engineering Measurement Systems Operational Amplifiers (OpAmps) Dr. Tony E. Grift Dept. of Agricultural & Biological Engineering University of Illinois

Agenda Packaging OpAmp as a near-ideal Voltage Amplifier Non-inverting OpAmp Virtual ground derivation Inverting OpAmp Virtual ground derivation Differentiating OpAmp Integrating OpAmp Voltage follower (buffer)

Dual-in-line (DIP) and Surface Mount Device (SMD) packaging

Typical OpAmp pin assignment (741)

OpAmp as a near-ideal Voltage amplifier Open loop amplification A very large Input impedance very high Output impedance very low OpAmp has a Voltage source output

Non-inverting OpAmp

Non-inverting OpAmp : Virtual ‘ground’ principle Since no current is flowing into the OpAmp: What is the input resistance for the source ?

Inverting OpAmp

Inverting OpAmp: Virtual ground principle Since no current is flowing into the OpAmp: What is the input resistance for the source ?

Capacitor equation Charge across Capacitor (Coulomb) Capacitance value (Farad) Voltage across Capacitor (Volt) Current through Capacitor (Ampere)

Integrating Inverting OpAmp : Virtual ground

Differentiating Inverting OpAmp: Virtual ground

Integrating Inverting OpAmp : Virtual ground

Differentiating Inverting OpAmp: Virtual ground

Unity gain OpAmp (Voltage follower, buffer) This circuit has no amplification, but provides a very high impedance input and low impedance output Use it after any sensor configuration to prevent loading errors You will need a symmetric power supply

Comparator function

OpAmp as comparator I What could this circuit be used for if Vref can be varied? Slope is termed ‘slew rate’

Voltage range detector When does the LED come on? Why do we need the diodes? How would you limit the current through the LED? When Vin > Vref_high OR Vin < Vref, low

Comparator with hysteresis (Schmitt Trigger)

Schmitt trigger as debouncer in noisy signals Regular comparatorSchmitt trigger with hysteresis

Subtracting OpAmp Use superposition!! Use virtual ground principle!! First consider

Subtracting OpAmp Use superposition!! Use virtual ground principle!! Next consider

Subtracting OpAmp Add influences of both sources

Current to voltage converter

Application current to voltage converter

Active filters combine amplification and filtering in one circuit! What is the input impedance the source ‘sees’?

Active Low-Pass filter analysis (1 st order) Without CIn general with impedances

Active High-Pass filter with OpAmp and Capacitor / Resistor pair (1 st order) What is the input impedance the source ‘sees’?

Active High-Pass filter analysis Without C In general with impedances

High-pass and low-pass section separated by OpAmp

ABE425 Engineering Measurement Systems Operational Amplifiers (OpAmps) The End Dept. of Agricultural & Biological Engineering University of Illinois