With limited components, how do I emulate a reverse-biased zener diode?
The 'amplified diode' circuit below is used often to bias the output stage of class B amplifiers. It can be tuned by resistor selection to any voltage, and you can use a potentiometer to make it adjustable in circuit. With the values shown, the circuit below is a nominally 4.2v 'zener', 6x 0.7v.

Beware that its knee is only as relatively sharp as a diode, it has a diode's tempco, and relatively high leakage at low voltages. Use with a darlington transistor for lower leakage.

If you have a low power zener diode to hand, and for some reason need a higher power one, then replacing R2 with a zener results in a circuit which behaves like the zener with another 0.7v drop, but the dissipation capability of Q1.







simulate this circuit &ndash; Schematic created using CircuitLab

It depends how complete you want the emulation to be. D1 is only there to emulate the forward conduction. Most applications of zeners in voltage references will not need that.

Note that the voltage is not constant above the conduction voltage, but then neither is that of a zener, both have an effective series resistance.