Amplifier used in DC motor control loop - uses diode-switched feedback ladder to produce output proportional to inverse of square root of input

摘要

This amplifier is used in a control loop that controls the position of an object moved by a d.c. motor. The control loop can be used with a film cassette in an X-ray machine. The characteristic of the loop is such that the motor slows when the object is nearing its required position, thus limiting vibration or shock. A voltage proportional to the difference between the required and actual positions of the object is fed to the amplifier input. The amplifier output is inversely proportional to the square root of the difference signal. The input is fed via a resistor (Ri) to the inverting input of an operational amplifier (11). The feedback impedance is a fixed resistor (R1) in parallel with a ladder of diode-switched resistances. Each resistance in the ladder is in series with a number of back-to-back diodes or reversed pairs of Zener diodes, which will only conduct if the voltage between the output and input amplifier exceeds a level dependent on the number of diodes or on the Zener voltage of those diodes.