Operational Amplifiers (Op Amps) in Chemical Instrumentation Chapter 3: Operational Amplifiers (Op Amps) in Chemical Instrumentation Look at Fig. 5-4

Operational Amplifiers

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

General Characteristics Operational Amplifiers General Characteristics 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

Inverting & noninverting input Operational Amplifiers 8-pin commercial op amp. 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

General Characteristics Operational Amplifiers General Characteristics

Inverter/scaler (inverting amplifier) Operational Amplifiers: feedback circuits Inverter/scaler (inverting amplifier) Ri = Rf  inverter Rf > Ri  Signal amplification (inverting amplifier)

Operational Amplifiers Circuits Voltage Follower - 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 (Potential) Measurement 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 Measurement Current-voltage converter (current follower) Large Rf small current can be measured. Rf =100kΩ, i = 1 μA Vo = 0.1 V (easily detected)

Addition or Subtraction Operational Amplifiers Circuits Addition or Subtraction -

Operational Amplifiers Circuits Differentiation

Operational Amplifiers Circuits Integration