What will be the output voltage of an Op-amp inverting adder.

What will be the output voltage of an Op-amp inverting adder for the input voltages V1=-10V, V2=+10V, V3= +5V and resistances R1= 600K Ω , R2= 3000K Ω & R3= 2M Ω if the feedback resistance is considered to be 2M Ω ?

a. -38.3V
b. 38.3V
c. 40V
d. -40 V
Correct Answer : a. -38.3V

Explanation :

Given data:
V1= -10V, R1= 600k
V2= 10V, R2= 300K
V3= 5V, R3= Rf=2M

The output voltage of an inverting adder or summer circuit is evaluated by,

V_o= -(K₁V₁ + K₂V₂ + K₃V₃)

where, 'K' represents the constant gain factor.
K₁= R_f/ R₁ = 2M/ 600K= 2000K/ 600K = 3.33

K₂ = Rf / R2 = 2M/300K = 2000K/ 300K = 6.66
K₃ = R_f / R₃ = 2M/ 2M = 1

Therefore, V_o = -(K₁V₁ + K₂V₂ + K₃V₃)
= -[ 3.33 x (-10) + 6.66 x (10) + 1 x 5]
= - [ -33.3 + 66.6+ 5]
= -38.3 V

Hence, the final output value of voltage of an inverting amplifier is nothing but summation of all input voltages estimated to be in terms of negative voltage of about -38.3 V.