Adjustable-voltage pass regulators normally allow the output infinite adjustment between some maximum V_OUT and one bandgap voltage away from ground. The internal bandgap reference for the error amplifier limits the minimum output voltage. Some regulators have well-behaved, constant ground currents so that you can "float" the pass regulator on top of an adjustable resistor. This scheme requires a relatively high quiescent current to swamp out the pass regulator's ground current. Other regulators have a dedicated high-input-impedance adjust input and, therefore, a lower quiescent current. Fig 1 shows both of these standard options. These designs produce quick and easily adjustable regulators, but their outputs can never be less than 1.25V, which is the zero-temperature-coefficient point of most semiconductor bandgap references found inside ICs.

Fig 2's alternative scheme shows a simple and practical single-supply pass regulator for variable output laboratory supplies and for fixed-output regulator applications requiring an output of less than 1V. The circuit uses a second bandgap reference (available in separate components in very small packages) to fool a regulator, and a feedback divider to make an adjustable supply that can supply a V_OUT of 0 to 20V. The circuit uses the second bandgap reference to translate the output of an adjustable regulator up to a "virtual V_OUT" and then uses that virtual V_OUT as the top of the feedback divider.

When R₁ goes to 0ê, the output is 0V; the virtual V_OUT is one bandgap voltage above ground; and, therefore, the adjust input is also one bandgap voltage above ground. The regulator's error-amplifier circuitry is satisfied that both of its inputs are at one bandgap voltage and, therefore, it keeps the output voltage constant at 0V. The circuit fools the regulator into providing a lower than one bandgap output. The virtual V_OUT tracks any increases in R₁, albeit at a fixed one bandgap voltage above the actual V_OUT, and the pass regulator's output rises from ground. The maximum possible V_OUT equals the regulator's maximum input voltage minus the approximately 2V housekeeping voltage required by the current-source FET and the external bandgap reference.

The current source, composed of the 2N3687 JFET and R₃, provides approximately 77 µA-70 µA for the bandgap and 7 µA for the resistor string (approximately 100 times the nominal 60-nA input current of the pass regulator's adjust input). The optional R₂ bleeds off the 70 µA from the external bandgap reference if no load is present and the circuit is set for 0V out. (DI #1687)