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

Figure 1 shows an ideal op amp in an open-loop configuration having open-loop gain A_OL = ∞. V_o is the op amp output. +V_sat and −V_sat 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 +V_sat.

Vo = + V_sat   (when V₊ > V₋)

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

Vo = − V_sat   (when V₋ > V₊)

Let us understand by taking values.

(a) V₊ = 0.2 V and V₋ = 0.1 V

then V_o = A_OL[0.2-0.1] = A_OL × 0.1 = ∞ × 0.1 = +∞

but the voltage can’t exceed the op amp supply voltage. Therefore, V_o = +V_sat

(b) V₊ = 0.3 V and V₋ = 0.1 V

then V_o = A_OL[0.3-0.1] = A_OL × 0.2 = ∞ × 0.2 = +∞

but the voltage can’t exceed the op amp supply voltage. Therefore, V_o = +V_sat

(c) V₊ = 1 V and V₋ = 0.1 V

then V_o = A_OL[1-0.1] = A_OL × 0.9 = ∞ × 0.9 = +∞

but the voltage can’t exceed the op amp supply voltage. Therefore, V_o = +V_sat

(d) V₊ = 0.1 V and V₋ = 0.2 V

then V_o = A_OL[0.1-0.2] = A_OL × (−0.1) = ∞ × (−0.1) = −∞

but the voltage can’t exceed the op amp supply voltage. Therefore, V_o = −V_sat

(e) V₊ = 0.1 V and V₋ = 0.3 V

then V_o = A_OL[0.1-0.3] = A_OL × (−0.2) = ∞ × (−0.2) = −∞

but the voltage can’t exceed the op amp supply voltage. Therefore, V_o = −V_sat

(f) V₊ = 0.1 V and V₋ = 1 V

then V_o = A_OL[0.1-1] = A_OL × (−0.9) = ∞ × (−0.9) = −∞

but the voltage can’t exceed the op amp supply voltage. Therefore, V_o = −V_sat

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