The Circuit Designer's Companion - diagramas.diagram...

Active components 113may be adequate for your application. Alternatively, use dual transistors to ensureidentical junction temperatures, with a more complex circuit arrangement to achievevery accurate compensation. This latter is the basis for many op-amp temperaturecompensation schemes, since extra transistors are essentially free and being on thesame chip, are as close as it is possible to get to the required temperature.4.1.2 Reverse biasSo far we have only considered the forward characteristic, that is for positive appliedvoltage. An ideal diode would block all current flow in the reverse direction. A practicaldiode doesn’t. There are two main reverse characteristics, reverse leakage current I Rand reverse breakdown voltage V BR . The diode equation (4.1) holds good in the reversedirection until V BR is approached; in the low-voltage region I R is almost equal to I S .BreakdownV BR is that voltage at which the reverse-biased junction can no longer withstand theapplied electric field. At this point, avalanche breakdown occurs and a current limitedmainly by the external source impedance will flow. If the device maximum powerdissipation is exceeded the junction will be destroyed. Diodes operated conventionally,as opposed to Zener diodes to which we will return shortly, are always run at reversevoltages lower than V BR .A common over-voltage excursion is the inductive turn-off transient (see section3.4.4) where a diode is used, intentionally or not, to block the transient. It can bedifficult to predict the maximum voltage of the transient and, since the energydissipated by a breakdown may be much less than needed to destroy the diode, suchbreakdowns may go unnoticed during the evaluation of the design. Diodes are availablewhich are characterised for the amount of avalanche breakdown energy they canwithstand, and should be used if a circuit is expected to deliver predictable transientsabove the normal breakdown voltage.4.1.3 LeakageSince reverse leakage current is of fundamental importance to circuit operation, alldiode data sheets quote a specification for maximum leakage. Unfortunately, this hidesas much as it reveals.Leakage current I R is relatively constant with voltage until V BR is approached, atwhich point it starts to increase rapidly. It is not, however, constant with temperature,but roughly doubles for every 10˚C rise of junction temperature. This characteristic,like the forward voltage temperature coefficient, is common to all reverse-biased p-njunctions and we will meet it again later. Most diode leakage currents are specified bothat 25˚C and at a higher temperature, and the 25˚C figure is highly misleading if youapply it over a typical temperature range. A leakage of 100nA at 25˚C translates to2.2µA at 70˚C, for instance. This is a common factor in the poor high-temperatureperformance of high impedance circuits, or those which employ very low currentlevels.Leakage variabilityTo make matters more complicated, leakage is susceptible to process variations. It canvary by up to an order of magnitude from batch to batch under otherwise identicalconditions. Therefore, manufacturers will put an artificially high maximum value ofleakage in their specifications compared to the actual performance of the majority of