The breakdown voltage VZ varies with temperature and this is undesirable especially in precision voltage-reference applications. A popular way to compensate for this variation is to use an avalanche...

The breakdown voltage VZ varies with temperature and this is undesirable especially in precision voltage-reference applications. A popular way to compensate for this variation is to use an avalanche diode, which has TC . 0, in series with an ordinary forward-biased diode, which has TC , 0, as depicted in Fig. P1.76. As we know, the latter has TC 5 22 mV/8C, so if we use an avalanche diode with TC 5 12 mV/8C, the opposing TCs will cancel each other out, resulting in a very stable combined voltage drop. Avalanche diodes with TC 5 12 mV/8C fall in the neighborhood of 6.2 V, so the voltage drop of the series combination is about 6.2 1 0.7 5 6.9 V. (a) Assuming VO > 6.9 V, specify R for an avalanche-diode current of 3 mA at VI 5 12 V and IL 5 2 mA. (b) Assuming suffi ciently small line and load variations to justify the small-signal approximation for D1, fi nd the line and load regulation (in mV/V and mV/mA) if rz 5 8 V and nVT 5 26 mV.