op-amp as comparator.

A circuit which compares an input voltage with an external reference voltage is known as comparator. In inverting comparator, a reference voltage is applied to non inverting terminal and input is applied to inverting terminal.

When input < $V_{ref}$, o/p is at +ve saturation means output changes whenever $V_{ref}$ level is crossed by $V_{in}$, hence comparator is also known as voltage level detector.

Diode $D_1$ and $D_2$ protects op-amp from excessive input voltage.

Output voltages, $V_o = gain \ (V_{1d}$)

$\because$ op-amp is operated in open loop

Ideally gain = $\infty$

$v_2 = \infty(V_{cef} - V_{in})$

When $V_{ref} \gt V_{in} , V_2 = + \infty$

$V_{ref} \lt V_{in} , V_0 = - \infty$

Practically, Gain = $2 \times 10^5$ (High)

When $V_{ref} \gt V_{in}$

$V_o = + V_{sat}$ (Highest possible positive voltage)

When $V_{ref} \lt V_{in}$

$V_o = - V_{sat}$ (Highest possible negative voltage)

$V_{ref}$ can be obtained from op-amp power supply as

Above arrangement can provide positive as well as negative references.