Op amp as a Comparator | Working | Electricalvoice

Op amp as a Comparator | Working

In an open-loop configuration, an ideal operational amplifier (op amp) behave as a comparator. The behaviour of op amp as a comparator is due to infinite open-loop gain. When it is working as a comparator, there are only two outputs possible at the output of op amp.

Op amp as a Comparator

op amp as a comparator
Fig. 1 Op amp open-loop configuration

Figure 1 shows an ideal op amp in an open-loop configuration having open-loop gain AOL = ∞. Vo is the op amp output. +Vsat and −Vsat are the op-amp power supply voltage. There are only two outputs possible at the output of op amp based upon inverting terminal voltage (V) and non-inverting terminal voltage (V+). As we know that

V_o=A_{OL}[V_+-V_-]

When non-inverting terminal voltage is greater than inverting terminal voltage then op amp output is +Vsat.

Vo = + Vsat   (when V+ > V)

When non-inverting terminal voltage is greater than inverting terminal voltage then op amp output is −Vsat.

Vo = − Vsat   (when V > V+)

Let us understand by taking values.

(a) V+ = 0.2 V and V = 0.1 V

then Vo = AOL[0.2-0.1] = AOL × 0.1 = ∞ × 0.1 = +∞

but the voltage can’t exceed the op amp supply voltage. Therefore, Vo = +Vsat

(b) V+ = 0.3 V and V = 0.1 V

then Vo = AOL[0.3-0.1] = AOL × 0.2 = ∞ × 0.2 = +∞

but the voltage can’t exceed the op amp supply voltage. Therefore, Vo = +Vsat

(c) V+ = 1 V and V = 0.1 V

then Vo = AOL[1-0.1] = AOL × 0.9 = ∞ × 0.9 = +∞

but the voltage can’t exceed the op amp supply voltage. Therefore, Vo = +Vsat

(d) V+ = 0.1 V and V = 0.2 V

then Vo = AOL[0.1-0.2] = AOL × (−0.1) = ∞ × (−0.1) = −∞

but the voltage can’t exceed the op amp supply voltage. Therefore, Vo = −Vsat

(e) V+ = 0.1 V and V = 0.3 V

then Vo = AOL[0.1-0.3] = AOL × (−0.2) = ∞ × (−0.2) = −∞

but the voltage can’t exceed the op amp supply voltage. Therefore, Vo = −Vsat

(f) V+ = 0.1 V and V = 1 V

then Vo = AOL[0.1-1] = AOL × (−0.9) = ∞ × (−0.9) = −∞

but the voltage can’t exceed the op amp supply voltage. Therefore, Vo = −Vsat

Hence, an open-loop operational amplifier amplifies a small applied differential input voltage to a huge value, but this significant value at the output cannot go beyond the supply voltage of the operational amplifier. Hence it does not violate the law of conservation of energy.

Op-amp as a Comparator is used in the following circuits

  • Schmitt triggers
  • Oscillators
  • Voltage level detectors
  • Discriminators
  • Digital interfacing

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