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Operational Amplifier

The operational amplifier is an electronic circuit element designed to be used with other circuit elements to perform a specified signal processing operation. The power supplies, V+ and V-, are used to bias the opamp and cause certain conditions that are required for it to function properly.

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UEEA2413 Process Control and Instrumentation

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Op Amp Transfer Function

The output voltage, Vo, is plotted versus the difference between the two input voltages, V2 – V1. This input is called differential input voltage. When V2 is much larger than V1, the output is saturated at some negative voltage, -VSat. Conversely, when V1 is much larger than V2, the output is saturated at some positive voltage, +VSat. Other characteristics of the op amp are that the input impedances are very high, typically exceeding 1 MΩ, whereas the output impedance is very low, typically less than 100 Ω.

2

V2 – V1

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UEEA2413 Process Control and Instrumentation

Ideal Inverting Amplifier

R2 is used to feed back the output to the inverting input and R1 connects Vin to the summing point. The +ve terminal is grounded. The currents at the summing point –

I1 + I 2 = 0

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The summing point potential is assumed to be zero, by ohm’s law –

Vin Vout + =0 R1 R2 Vout R2 = − × Vin R1

The circuit is an inverting amplifier with gain R2/R1 that is shifted 180° in phase from the input. This device is also an attenuator by virtue of making R2 < R1.

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Example 4.12

Fix the values of R1 and R2 to obtain a gain = – 4.5. Max. op amp operating current is ≈ 20 mA.

Vout = −

R2 × Vin R1

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UEEA2413 Process Control and Instrumentation

5

Op Amp Circuits In Instrumentation

Typical op amp circuits and their basic characteristics,...