The precise combination of common, inexpensive components can yield a scaled regulated voltage with a very low output impedance and high current output capability. It's especially useful when the output is scaled to one-half of the input value to create a mid-rail reference in a single-polarity circuit. The transfer function of the circuit (Fig. 1) is simple:
The negative feedback via the inverting terminal of U2 ensures the accuracy of this transfer function. Accuracy depends on the tolerance values of R1 and R2, as well as on the voltage-offset specification of U2. The RDS of Q1 is controlled in such a way to ensure that:
satisfies the following design equation for proper operation of LM317, or of any other adjustable regulator used in the circuit:
where IADJ is an error term equal to approximately 100 mA and VREF represents the reference voltage between the output and the adjustment terminal. Figure 2 shows a PSPICE simulation of the circuit in Figure 1, with the input swept from 4 to 18 V. With R2 = R1, the output will measure half of the input. Figure 2 demonstrates that the output voltage actually measures half of the input over the input-voltage range of 8 to 18 V.