www.nortonkit.com 18 अक्तूबर 2013
Basic Summing: [Setting the Gain Coefficient] [Analog Addition] [Adding a Fixed Constant]
Variations in Feedback Circuits: [Integrators] [Differentiators] [Logarithmic Amplifiers] [Non-Inverting Amplifiers] [A Difference Amplifier] [Increasing the Output Current Capacity] [A Half-Wave Rectifier] [A Full-Wave Rectifier]
Mixing Analog and Digital Technologies: [Comparators] [Digital to Analog Conversion] [Analog to Digital Conversion]
Generating Waveforms: [A Square Wave Generator] [A Triangle Wave Generator] [A Sine Wave Generator]
Operational Amplifiers: [Characteristics of Operational Amplifiers] [Inside the 741]

Many equations require the inclusion of a fixed constant, which has no connection with any of the variables. For example, the general equation for a straight line is Y = mX + b, where m is the slope of the line and b is an offset, which defines the point at which this particular line will cross the Y axis on a graph. The next question to be answered, then, is how we can include such a fixed constant in our analog computer circuits.

To insert a constant into the equation, we need to introduce a fixed voltage. This is most easily done with a simple potentiometer, as shown to the left. The potentiometer is set to an appropriate voltage, and then connected to its own input resistor to the op amp. Thus, this input represents an "unvarying variable," which is a constant.

In an analog computer, a number of steps are taken to help ensure both accuracy and precision:

1. The ±10 volt source applied to the top of the potentiometer is carefully regulated. Either the positive or negative source can be connected to the potentiometer via a small jumper.
2. The potentiometer is designed to be highly linear, and has a knob that is designed to make ten full rotations in order to turn the potentiometer over its range (a ten-turn potentiometer). The knob also locks in place once the correct setting has been made and verified.
3. To allow for loading effects of the input resistor, the numbers on the dial are not assumed to be an accurate representation of the applied voltage. Instead, the actual voltage under load is measured and the potentiometer set to the desired voltage under loaded conditions.

If the required constant is in the range of ±10, it can be applied directly, with the gain of that input set to 1. If the constant is larger, it is scaled down and then amplified. In some cases, the constant might be very small. In that case, it can be scaled up and its input set to a fractional gain. This allows accurate setting of the potentiometer.