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Design Ideas |
You can use the
venerable 723 voltage regulator in new and unusual ways. For example, you can
modify its topology as in Figure 1 to yield a low-component-count,
dual-polarity tracking voltage reference. The 723, a long-time standard in
power-supply design, is a flexible IC that includes a temperature-compensated
voltage reference, an error amplifier, and an output pass transistor with a
voltage-shifting zener diode in its emitter circuit. The error amplifier
resembles a simple operational amplifier with a single output transistor. The
output transistor's emitter and collector are accessible to the user. Also, the
emitter connects to the cathode of a 6.2V zener diode, both of whose terminals
are accessible.
In common applications, the 723's reference, VREF, serves as one input to the error amplifier. The amplifier's output usually controls a large power transistor connected as a pass element, which supplies current to the load. A portion of the output-load voltage returns to the amplifier's other input as an error signal. The circuit in Figure 1 uses this basic arrangement, but without the power transistor, to produce both positive and negative reference voltages. The 723 derives its power from the unregulated output of a rectifier-filter. The positive reference (for a positive regulated supply) comes directly from the VREF output of the 723. This positive reference, nominally 7.15V, also connects to the noninverting input of the error amplifier via the 2-to-1 voltage divider R1-R2.
The negative voltage-reference circuit uses a shunt-regulator configuration. The error amplifier's VZ output connects to a load comprising the 8.2V zener diode (D1) and resistor R5. The amplifier supplies approximately 1.5 mA through D1 and R5, which returns to the unregulated negative input of the rectifier-filter. Zener-diode D1 and the 723's output zener diode provide a voltage drop of approximately 14.4V. A portion of the output voltage developed across current sink R5 feeds back to the error amplifier's inverting input via R4. Capacitor C1 provides frequency compensation for the error amplifier.
With the component values shown, the voltage at the noninverting input is approximately 3.6V. R3, RV1, and R4 set the inverting input at 3.6V. Potentiometer RV1 trims the negative output voltage to the exact desired value. Although the circuit shown is designed to provide equal positive and negative output voltages, it can accommodate other negative output voltages. Zener diode D1 should have a voltage equal to or slightly greater than the desired negative output voltage. You can determine other component values from the following relationships:
The circuit uses just six components: the 723, the zener diode, a potentiometer, two capacitors, and a 14-pin DIP resistor pack. For a dual voltage reference, this is about as simple as it gets. (DI #1943)
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