When designing a summing amplifier, the use of operational amplifiers is extended nowadays, due to its huge gain, infinite in theory, and the simplicity of its design. With this configurations two or more signals can be added to a single signal in the output, depending the gain of each of the signals in its channel gain.
The purpose for using this circuit in the actual amplification, is to set the input AC voltage to only positive voltage and by the way amplify the signal to a reference voltage, which will ease the design of the second amplifier. The chosen voltage value is of 1V, in this way the amplification will be done in two steps, instead of amplifying the signal 100 times in one single amplification, avoiding problems of having much bigger amplifications, if internal resistance value is changed.
For starting to design the circuit the input AC voltage needs to be measured, for setting the DC voltage to the peak amplitude of the signal, not peak to peak. Then a resistance will be connected to each of those inputs, while the other side will be connected to an input of the operational amplifier. The input that has not been used will be grounded, this will produce the signal to be inverted, as currents will be flowing in opposite directions. The used R1 resistances have a value of 1kΩ, as the voltage is not big and because it is a standard value.
Once we have the input values the first resistances and we have set the output voltage the resistance that will produce the amplification will be estimated:
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