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Summing Amplifier: Inversion Properties and Op-Amp Varieties

Key Takeaways

  • Learn what is a summing amplifier.

  • Find out how a summing amplifier works.

  • Explore how summing amplifiers are used in practical applications.

Blackboard sketch of an amplifier circuit

The summing amplifier is configured from an op-amp. 

What is 5,592 + 9,861? And that’s how my mind goes blank. Mental arithmetic was never my forte, despite being in the engineering profession. Instead, I relied on one of man’s greatest inventions, the calculator, as I struggled through one assignment to another during my university days. 

In electronics, the summing amplifier proves to be as useful as the calculator or more, in adding up the value of currents from various inputs. It proves to be a useful component in mixing up analog signals. You ought to understand the in’s and out’s of the summing amplifier before using one in design. 

What is a Summing Amplifier

A summing amplifier is, in fact, an operational amplifier (op-amp) configured in a specific manner. An ideal op-amp has characteristics like infinite input impedance, infinite gain, and zero output impedance. Based on its unique properties, an op-amp can be configured for various functions, including a summing amplifier.

Also known as the adder, the summing amplifier produces a voltage output that is equivalent to an amplified sum of two or more input voltages. Usually, a summing amplifier is in fact an inverting amplifier, which means the output voltage has a negative value when measured to ground. 

How Does A Summing Amplifier Work

If you’re familiar with an inverting amplifier, you’ll find that the summing amplifier shares a similar structure. The summing amplifier merely has more input resistors connected in parallel to the inverting terminal of the op-amp. 

It is important to remember two basic rules of op-amp when understanding how a summing amplifier functions. First, the no current flows into the input of an op-amp, and second, the voltage difference between both input terminals is zero. 

Circuit of a summing amplifier

Based on the above golden rules, the inverting terminal becomes a virtual earth summing point as the non-inverting terminal is connected to 0V earth ground. Therefore, the relationship between the current flowing through the resistors can be formed with:

If = I1 + I2 + … + In

This can be interpreted into 

-Vout/Rf = V1/R1 + V2/R2 + … + Vn/Rn

Vout = -Rf  (V1/R1 + V2/R2 + … + Vn/Rn)

Assuming that each of the input resistors has the same value, the equation can be simplified into.

Vout = -Rf/R (V1 + V2 + …. +Vn)

The value of the feedback resistor, Rf, and input resistors determine the gain of the summing amplifier. When the values of both types of resistors are equal, the gain is equal to 1 and the equation becomes.

Vout = -(V1 + V2 +  …. +Vn)

While summing amplifiers are usually derived from inverting amplifiers, it can also be configured from a non-inverting amplifier. The principle is the same as the amplifier sums the total voltage inputs from the resistors connected to the non-inverting terminal of the circuit. 

Using PSpice, you can easily determine voltage gains through proper circuit layout and amplifier parameters, as well as utilize the massive, over 34,000 component model library to find the accurate components for your design necessities. What’s more is you can perform vital analysis  options like determining inversion, plotting waveforms, performing stress tests, and determining necessary component tolerances. 

Applications Of Summing Amplifier

Summing amplifiers are commonly used to process analog signals. You’ll find summing amplifiers in audio mixers. It allows audio experts to combine signals from various channels and reproduce them into a single track. Every single audio input can be configured independently without affecting the output. 

Another common application of the summing amplifier is the digital to analog converter. For example, an 8 bit DAC will have 8 inputs, where the voltage is set to logic 1 or 0 based on the binary representation of the value. The input resistors of each digit will have a different value, usually in a 2x increment. When used as a DAC, the output will be an analog voltage that represents the binary value set at the input resistors. 

Summing amplifiers are used in audio mixers. 

Theoretically, a summing amplifier can take on unlimited numbers of input. However, an op-amp in the real world is far from ideal. The output voltage range of an ideal op-amp is unlimited, but for a real opamp, it is limited by the DC supply connected to it. If the summing amplifier is powered by +15V and -15V, the output will be clamped before it reaches the limit. 

You’ll also need to take into consideration the feedback resistor used in the summing amplifier. A resistor heats up when current passes through it. Even if you’re using a resistor with a large power rating, it is still impossible to connect an infinite number of input resistors. Eventually, the feedback resistor will be overheated.

To prevent running into these real-life limitations, it is important that you simulate your design with a suite of PCB design and analysis tools like those offered by Cadence. By using PSpice Simulator, you’re able to get accurate simulations on various nodes of the summing amplifier. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts