Summing and Averaging Amplifiers – using Op Amps
Summing Amplifier
The diagram below illustrates an N Input Summing Amplifier using an Op Amp in the Inverting amplifier
configuration.
If the resistors are selected such that R1 = R2 = R3 = … = RN = Rf the voltage gain for each input
is AV,N = - Rf/RN = - 1
then
An audio mixer is a good application of a Summing Amplifier.
Non-Inverting Summing Amplifier
The diagram below illustrates a 3 Input Summing Amplifier using an Op Amp in the Non-Inverting
amplifier configuration.
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The value of R1 must equal R and RF = (N-1) x R where N is the number of inputs.
VOUT = V1 + V2 + V3
Example
Let R = R1 = 10 kΩ, so the value of RF = (3 – 1) x 10 kΩ = 20 kΩ
V1 = 4V
V2 = 2V
V3 = - 1 V
VOUT = 4 + 2 - 1 = 5V
Averaging Amplifier
An Averaging Amplifier is a variation of a Summing Amplifier. The input resistors must all be the same –
in this case they are all set to 100 kΩ. The feedback resistor is selected so that its value of the value of
RF divided by the number of inputs. So RF = 100 kΩ/4 = 25 kΩ
VOUT = - 25 kΩ/100 kΩ x (VIN,1 + VIN,2 + VIN,3 + VIN,4) = - (-1 V + 2V – 3V + 4V)/4 = - 1/4 x 2V = - 0.5 V
Application Note
Written by David Lloyd
Computer Engineering Program
Humber College
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