Chapter 3:

Operational Amplifiers (Op Amps)

in Chemical Instrumentation

Look at Fig. 5-4, 7-29, 7-31

Operational Amplifiers

Functions

1. Precise measurement of voltage, current, and resistance

2. Mathematical operations

- addition or subtraction

- multiplication or division

- integration

- differentiation

3. Constant current and constant voltage sources

Operational Amplifiers

A: large open-loop gain (104 ~106)

Zi: high input impedance (very high: Zi ≥ 106 ~1013 Ω)

Zo: low output impedance (very small: Zo :1 ~ 10-3 Ω)

zero output voltage for zero input

Operational Amplifiers

General Characteristics

Operational Amplifiers

Inverting & noninverting input

- Either input may be connected to positive or negative signals

- Negative voltage to the inverting input positive output voltage

(positive voltage to the inverting input negative output voltage)

- AC signal to inverting input: 180o out of phase

8-pin commercial op amp.

Operational Amplifiers

General Characteristics

Operational Amplifiers: feedback circuits

Ri = Rf inverter

Rf > Ri Signal amplification

(inverting amplifier)

Inverter/scaler (inverting amplifier)

Operational Amplifiers Circuits

- output voltage = input voltage

- Supply large current as the output of the operational amplifier:

it can accept an input from a device that can not supply much current

(such as glass electrode) and offer the same voltage to a significant load (e.g. recorder)

Voltage Follower

Operational Amplifiers Circuits

Voltage (Potential) Measurement

Inverting amplifier

- offer a very high input impedance

- allows the measurement of a voltage without perturbing the voltage

significantly

-

Operational Amplifiers Circuits

Current-voltage converter (current follower)Current Measurement

Large Rf small current

can be measured.

Rf =100kΩ, i = 1 μA

Vo = 0.1 V

(easily detected)

Operational Amplifiers Circuits

Addition or Subtraction

-

Operational Amplifiers Circuits

Differentiation

Operational Amplifiers Circuits

Integration