Linear Regulator - Simple Series Regulator

Simple Series Regulator

Adding an emitter follower stage to the simple Zener regulator forms a simple series voltage regulator and substantially improves the regulation of the circuit. Here, the load current I_R2 is supplied by the transistor whose base is now connected to the Zener diode. Thus the transistor's base current (I_B) forms the load current for the Zener diode and is much smaller than the current through R2. This regulator is classified as "series" because the regulating element, viz., the transistor, appears in series with the load. R1 sets the Zener current (I_Z) and is determined as -

where, V_Z is the Zener voltage, I_B is the transistor's base current and K = 1.2 to 2 (to ensure that R1 is low enough for adequate I_B).

where, I_R2 is the required load current and is also the transistor's emitter current (assumed to be equal to the collector current) and h_FE(min) is the minimum acceptable DC current gain for the transistor.

This circuit has much better regulation than the simple Zener regulator, since the base current of the transistor forms a very light load on the Zener, thereby minimising variation in Zener voltage due to variation in the load. Note that the output voltage will always be about 0.65V less than the Zener due to the transistor's V_BE drop. Although this circuit has good regulation, it is still sensitive to the load and supply variation. This can be resolved by incorporating negative feedback circuitry into it. This regulator is often used as a "pre-regulator" in more advanced series voltage regulator circuits.

The circuit is readily made adjustable by adding a pot across the Zener, moving the transistor base connection from the top of the Zener to the pot wiper. It may be made step adjustable by switching in different Zeners. Finally it is occasionally made microadjustable by adding a low value pot in series with the Zener; this allows a little voltage adjustment, but degrades regulation.