Next: Transistor Circuits
Up: Diode Circuits
Previous: Diode Protection

 Make a sketch showing the current through an ideal diode as a
function of the applied voltage. Also sketch the current
through a real signal diode as a function of voltage.
 Make a sketch showing the current drawn through a Zener diode as
a function of the applied voltage. Show how to determine the
forward resistance ( ), the reverse resistance ( ) and
the Zener resistance ( ) from your sketch. Label the
voltages and .

Sketch the expected output waveforms when
 a 100 Hz sine wave with a peak voltage of 5 V, and
 a 100 Hz square wave with a peaktopeak voltage of 10 V
are applied separately to each of the circuits below.

The Zener diode in the following circuit is characterized by
V and V. Terminal B is at ground and there
is no external load resistor.
 What is the voltage at terminal A in the above circuit?
 What is the current through the Zener? (What reasonable
approximation makes this straightforward?).
 If the Zener is dissipating 1.0 W (in heat), what can we
conclude to be the effective impedance of the diode for this
situation?

The effects of a Zener diode in a circuit can be treated analytically
by using an equivalent circuit model of the reversebiased condition.
In the low voltage region, before breakdown, the Zener diode can be
treated like any other reversebiased diode.
However, the Zener diode is normally operated in the breakdown region.
 Write down an equivalent linearcircuit model in the breakdown
voltage region.
 Replace the Zener diode in the following voltage reference
circuit by your equivalent circuit model.
 Determine the contribution of the of Zener diode to this voltage
reference circuit by calculating the elements of a Thevenin
equivalent circuit representation.
 Show that for small Zener diode effective resistance the
Thevenin equivalent voltage is close to the Zener breakdown
voltage and thus is insensitive to changes in the source
voltage.
 Show that for small Zener diode effective resistance, the
Thevenin equivalent resistor gives the voltage source a
reasonably small output impedance.
 The combined results show a voltage reference that is
insensitive to voltage changes in the original EMF and to
changes in the load current.
State the assumptions under which this result is valid.
Next: Transistor Circuits
Up: Diode Circuits
Previous: Diode Protection
Doug Gingrich
Tue Jul 13 16:55:15 EDT 1999