Inverting Summing Amplifier

An inverting amplifier can be constructed as a summing amplifier. The output voltage is a weighted sum of the inverting input signals V1, V2, ……, Vn. Note that each summing coefficient may be independently adjusted by adjusting the corresponding “feed-in” resistor (R1 to Rn). Figure below is an application of inverting configuration and the Vout to Vin transfer function.

summing_amp.jpg
summing_amp_transfer_function.jpg

Example:
An inverting summing amplifier below has three inputs using 100kΩ resistors and a feedback resistor of 50kΩ.
What is VOUT in terms of V1 and V2? If V1 = 2V and V2 = 2V, V3 = 3V what is VOUT ?

summing_example1.jpg

Answer:
VOUT = -50 (V1/100 + V2/100 + V3/100)

VOUT = -50 ((2V+ 2V + 3V )/100 = -3.50V

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