Electronic Design

Digitally Adjust Regulator's Output Using A JFET Interface

Often, electronic systems are guaranteed to operate with any supply voltage, within a given VCC range. Testing these circuits requires a regulated voltage supply that can be automatically incremented, from a minimum to a certain maximum. This makes taking parametric measurements for the circuit-under-test (CUT) easier.

Such a circuit uses the ubiquitous 317 voltage regulator (see the figure). Output of the regulator is calculated by VOUT = 1.25 × (RADJ/240+1). The variable RADJ is constructed from a linearized p-channel JFET, which has its resistance controlled by the op amp's output.

The amplifier increases the output of the R-2R digital-to-analog converter (DAC). For the given circuit values, VOUT spans 10 to 15 V, with increments of about 0.3 V. Changing the gain of the noninverting op amp will adjust that range. Also, increasing the DAC's number of bits will grant finer resolution for VOUT.

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