Inverting Amplifier

The following figure shows an inverting amplifier. The input signal is amplified and inverted. This is the most widely used constant-gain amplifier circuit.

V_o = -R_f.V_in /R₁

Voltage gain A = (-R_f /R₁)

Non-Inverting Amplifier

The following figure shows an op-amp circuit that works as a non-inverting amplifier or constant-gain multiplier and it has better frequency stability.

The input signal is amplified but it is not inverted.

Output V_o = [(R₁ + R_f) / R₁] V₁

Voltage gain A = (R₁ + R_f) / R₁

Inverting Summing Amplifier

The following figure shows an inverting summing amplifier. It is the most used circuit of the op-amp. The circuit shows a three-input summing amplifier, which provides a means of algebraically summing three voltages, each multiplied by a constant-gain factor. The output voltage is expressed as,

V_o = [(-R₄ / R₁) V₁][(-R₄ / R₂) V₂][(-R₄ / R₃) V₃]

V_o = -R₄(V₁ / R₁ + V₂ / R₂ + V₃ / R₃)

If, R₁ = R₂ = R₃ = R₄ = R & R_s = R/3

V_o = -(V₁ + V₂ + V₃)