Discrete Transistor Power Supplies

Capacitance Multiplier

A transistor can act as a variable series resistance to control the supply voltage to a circuit. The first consideration is a NPN voltage follower. A pair of resistors provide forward biasing to the base-emitter junction. The capacitor is filtering again, but this time we also get capacitance multiplication due the base current's control over the collector current. Our capacitor's Farad value will be multiplied roughly by the transistor's beta value, so the capacitor can be significantly smaller than in the passive circuit. The resistors must be low enough to provide stable biasing. This creates a serious limitation on this primitive circuit. Low value resistors provide stable biasing at the expense of the wasted current passing through them.

Primitive Regulator with a transistor and Zener Diode

Instead of biasing the base with two resistors, we can replace the bottom resistor with a Zener diode. Whatever Zener value we choose, the emitter voltage will be about 0.6V lower. For this reason, a lot of Zener diodes are available in "X.6V" values so that the final voltage is X. Also see the basic Zener regulator page.

Two Transistor Regulator

Adding a second transistor introduces the concept of the "error amplifier" and negative feedback. Our new transistor will sample the output voltage via the resistor divider network at the base, and this sample will have a direct and opposite effect on the base of the pass transistor. If the sample voltage increases, the pass transistor's output decreases and vice versa. The resistor divider network can be made adjustable over a small range by placing a potentiometer at the base of the new transistor.

The "error amplifier" here takes the base current as one input, the emitter voltage as the other input, and the collector current changes according to the difference, thus regulating the pass transistor's emitter voltage based on the selected Zener diode and output sampling resistors.

This type of transistor regulator can be expanded upon for further refinement and complexity, but this is rarely seen because refined and complex circuits are readily available inside of cheap voltage regulator integrated circuits.