To design and simulate Summing, Scaling and Averaging Amplifier circuits using 741.

An operational amplifier is a direct coupled high gain amplifier consisting of one or more differential amplifiers and usually followed by a level translator and an output stage which is usually a push-pull or push-pull complementary symmetry pair.

An operational amplifier can be used to amplify ac as well as dc input signals and was originally designed for computing such mathematical functions as addition, subtraction, multiplication and integration.

Figure shows the inverting configuration of 741 to implement the summing, scaling and averaging amplifiers. Depending on the relationship between the feedback resistor, R_F and the input resistors R_A, R_B and R_C the circuit can be used as a summing amplifier, a scaling amplifier or an averaging amplifier. The offset minimizing resistor R_OM is used to minimize the effect of input bias currents on the output-offset voltage The circuit functionality can be explained using the equation for output voltage obtained by applying Kirchoff’s current law at node A. Referring to the figure

since R_i and A of the op-amp are ideally infinity,

If the resistor values in the above circuit are selected such that

Then equation 3 can be rewritten as

i.e., the output voltage is equal to the negative sum of all the input voltage times the gain of the circuit R_F/ R; hence the circuit is called a summing amplifier. When the gain of the circuit is 1,i.e, , the output voltage is equal to the negative sum of all the input voltages. Thus,

The circuit for summing amplifier can be converted to a scaling amplifier if each input voltage is amplified by a different factor i.e, weighted differently at the output. This condition can be accomplished if R_A, R_B and R_C are different in value. Thus the output voltage of the scaling amplifier is

The above circuit can be used as an averaging circuit in which the output of the circuit is equal to the average of all the input voltages. This is accomplished by using all resistors of equal value, . In addition, the gain by which each input is amplified must be equal to 1 over the number of inputs; that is,

where n is the number of inputs.

Thus for a circuit with 3 inputs, . Consequently from equation 3

EDWin 2000 -> Schematic Editor: The circuit diagram is drawn by loading components from the library. Wiring and proper net assignment has been made.

The values are assigned for relevant components.

EDWin 2000 -> Mixed Mode Simulator: The circuit is preprocessed. Waveform markers are placed in input and output of the circuit. GND net is set as reference net. The Transient Analysis parameters have been set. The Transient Analysis is executed and output waveform is observed in the Waveform Viewer.

The output waveform may be observed in the waveform viewer. The output for Summing, Scaling and Averaging Amplifiers are shown below.