A summing amplifier, also known as an inverting summing amplifier or virtual ground summer, is an electronic circuit that combines multiple input signals and produces an output voltage proportional to the sum of those input voltages. It is an example of an operational amplifier (op-amp) configuration.
The summing amplifier is typically based on the inverting amplifier configuration, which means that the output voltage is the inverted and amplified version of the input voltage. The basic configuration of a summing amplifier looks like this:
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R1 R2 R3 Rn
+--->--/\/\--+--/\/\--+-- ... --/\/\--+--/\/\--->--+
V1 | | | Vn
+--| +--| +--|
| | | | | |
Rf | Rf | Rf |
| | | | | |
+--| +--| +--|
| | |
GND GND GND
(Virtual Ground)
Here's how the summing amplifier works:
Each input voltage (V1, V2, V3, ..., Vn) is connected through a resistor (R1, R2, R3, ..., Rn) to the inverting input (-) of the operational amplifier.
All the resistors are connected together at a single node, which is connected to the inverting input of the op-amp. This node is known as the "virtual ground." The virtual ground concept assumes that the voltage at the inverting input is virtually equal to the voltage at the non-inverting input (+), which is typically connected to the actual ground (GND).
The non-inverting input (+) of the op-amp is connected directly to the actual ground.
The output of the op-amp is connected to the junction of a feedback resistor (Rf) and the inverting input.
Due to the virtual ground concept, the op-amp's inverting input is held at a voltage close to zero volts (ground potential). This means that the current flowing through each input resistor is proportional to the input voltage at that node, as determined by Ohm's law (V = I * R).
Since the inverting input is at virtual ground, the current sum flowing into the junction point (the inverting input) is given by:
I_sum = (V1 / R1) + (V2 / R2) + (V3 / R3) + ... + (Vn / Rn)
The op-amp amplifies this current sum by its gain (which is determined by the feedback resistor Rf) and produces an output voltage:
V_out = -Rf * I_sum
Therefore, the output voltage is proportional to the negative sum of the input voltages. The negative sign indicates that it is an inverting amplifier, meaning the output is an inverted version of the sum of the input voltages.