An amplifier is an analogue circuit. This page is about a summing amplifier based on an inverting amplifier. The output voltage (Vout) of the circuit depends on the input voltage of two separate inputs (VA and VB) and the Gain of each input.
For all the circuits shown below, the amplifier is assumed to a have a positive and a negative power supply, usually ±15V, so that the output voltage can be both positive and negative.
The circuit is based on an inverting amplifier
The output voltage (Vout) depends on each of the two input voltages (VA and VB) and the ratio of the input resistors (RA and RB) and the feedback resistor (Rf)
The voltage gain of each input can be calculated separately as the two inputs are completely independent and do not affect each other in any way
The output voltage is given by:
If all the resistor values are equal:
Note: RA, RB and Rf should all be >1kΩ and <10MΩ
The summing amplifier multiplies each input voltage by the gain of that input and then adds them together to give the output voltage. Input voltages can be both positive and negative and the summing amplifier takes account of this. Input voltages can be fixed or changing with time, analogue or digital.
The timing diagram shows two input voltages to a summing amplifier and the output voltage assuming the gain of each input is −1
VA is a constant voltage of +2V
VB is a square wave that oscillates between +1V and −1V
Vout is the inverted sum of the two inputs. Initially VA = +2V and VB = +1V and therefore Vout = −3V
After some time VA = +2V and VB = −1V and therefore Vout = −1V
The equation to calculate the output voltage can be rearranged to give:
Consider the following example. What is the output voltage?
For input A, the gain is 470k/220k = ×2.14
For input B, the gain is 470k/10k = ×47
The output voltage is therefore −(+3 × 2.14 + −0.2 × 47) = −3V
Notice how each input has a gain defined by the input resistor and the feedback resistor. The gains for each input do not have to be the same.
A Summing Amplifier can be used to mix two signals, such as in the music industry. The circuit shown has an input for a line source such as a CD player or guitar and a separate input for a microphone (Mic).
The input capacitors (nominally 1µF) block any D.C. bias
The 47kΩ Logarithmic potentiometers act as volume controls
The gain of the Mic input is ×10 so an inout of 100mV will give an output of 1V
The gain of the Line input is ×1 so an line level input of 1V will give an output of 1V
The summing amplifier mixes the two signals so, using the numerical values given, the maximum output would be 2V
The output capacitor and resistor block any D.C. bias being presented to the next subsystem
Just like the Inverting Amplifier, the Summing Amplifier uses negative feedback to keep the inverting input at almost zero volts. The inverting input is a virtual earth. As both inputs are connected, through their respective input resistors, to this virtual earth they do not affect each other - the voltage at input A does not affect the voltage at input B and vice versa.
The Gain Equation
Combining the above equations gives:
© Paul Nicholls
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