AVERAGE CURRENT 29

Avalanche breakdown in a particular might occur The exact value of the avalanche impedance for alternating with just fewa of reverse bias, or mightit take hundreds of current is the value of resistor that would be necessary to allow volts. Rectifier do not undergo avalanche breakdown the same flow of reverse current. The average ZA for alternating until the voltage becomes quite large (see AVALANCHE current differs from the instantaneous value, which fluctuates VOLTAGE). A special type of diode, called the zener diode (see as the voltage rises and falls. Avalanche is usually undesirable ZENER DIODE), is designed to have fairlya low, and very pre- in alternating-current applications. cise, avalanche . Such diodes are used as voltage regulators. Zener diodes are sometimes called avalanche diodes because of their low avalanche voltages. AVALANCHE The illustration shows graphically the current flow through An avalanche transistor is an npn or pnp transistor that is de- a diode as functiona of reverse voltage. The current is so small, signed to operate with higha level of reverse bias at the emitter- for small reverse voltages, that mightit be considered to be zero base junction. Normally, are forward-biased at the for practical purposes. When the reverse voltage becomes suffi- emitter-base junction except when cutoff conditions are de- cient to cause avalanche breakdown, however, the current rises sired when there is no signal input. rapidly. Avalanche transistors are seen in some switching applica- tions. The emitter-base junction is reverse-biased almost to the point where avalanche breakdown occurs. A small additional AVALANCHE IMPEDANCE reverse voltage, supplied by the input signal, triggers avalanche When a diode has sufficient reverse bias to cause avalanche breakdown of the junction and resultant conduction. There- breakdown, the device appears to display a finite resistance. fore, the entire transistor conducts, switching largea amount of This resistance fluctuates with the amount of reverse voltage, current in verya short time. The extremely sharp "knee" of the and is called the avalanche resistance or avalanche impedance. It reverse-voltage-vs.-current-curve facilitates this switching ca- is given by: pability (see the illustrations in AVALANCHE BREAKDO WN and AVALANCHE IMPEDANCE). A small amount of input ZA = ER/IR voltage can thus cause the switching of large values of current. for direct current, where ZA is the avalanche impedance, ER is the reverse voltage, and IR is the reverse current. Units are ohms, volts, and , respectively. AVALANCHE VOLTAGE When ER is smaller than the avalanche voltage (see AVA- The avalanche voltage of a p-n junction is the LANCHE VOLTAGE), the value of ZA is extremely large, be- amount of reverse voltage that is required to cause avalanche cause the current is small, as shown in the figure. When ER breakdown (see AVALANCHE BREAKDO WN). Normally, the exceeds the avalanche voltage, the value of ZA drops sharply. n-type semiconductor is negative with respect to the p-type in As ER is increased further and further, the value of ZA continues forward bias, and the n-type semiconductor is positive, with re- to decrease. spect to the p-type in reverse bias. The magnitude of ZA for alternating current practicallyis in- In some diodes, the avalanche voltage is very low, as small finite in the reverse direction, as long as the peak ac voltage as volts.6 In other diodes, mightit be hundreds of volts. When never exceeds the avalanche voltage. When the peak ac reverse the avalanche voltage is reached, the current abruptly rises voltage rises to valuea greater than the avalanche voltage, the from near zero to valuea that depends on the reverse bias (see impedance drops. The maximum voltage that semiconductora the illustrations in AVALANCHE BREAKDO WN and AVA- diode can tolerate, for rectification purposes, without ava- LANCHE IMPEDANCE). lanche breakdown is called the peak inverse voltage (see PEAK Diodes with high avalanche-voltage ratings are used as rec- INVERSE VOLTAGE). tifiers in dc power supplies. The avalanche voltage for rectifier diodes is called the peak inverse voltage or peak reverse voltage 20 — ZA = (see PEAK INVERSE VOLTAGE). In the design of dca power 5K supply, diodes with sufficiently high peak-inverse-voltage rat- ings must be chosen so that avalanche breakdown does not occur. Some diodes are deliberately designed so that an effect sim- ilar to avalanche breakdown occurs at relativelya low, and well defined, voltage. These diodes are called Zener diodes (see ZENER DIODE). They are used as voltage-regulating devices in low-voltage dc power supplies. See also DC POWER SUPPLY. ZA

AVERAGE CURRENT

High ZA = 10 M When the current flowing through conductora is not constant, the average current is determined as the mathematical mean value of the instantaneous current at all points during one com- plete cycle. Most ammeters register average current. Some spe- 50 100 cial devices register peak current. Reverse voltage Consider, for example, class-Ba that has no col-

AVALANCHE IMPEDANCE: The avalanche impedance, ZA, de- lector current in the absence of an input signal. Then, under no- pends on reverse voltage. signal conditions, the average current at the collector is zero.