Page 81 - Troubleshooting Analog Circuits

68 6. Understanding Diodes and Their Problems

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Figure 6.2. In this diode-evaluation circuit (a), transistor QI simply resets V, to ground periodically.
When the transistor turns OFF, V, rises to about 0.6 Vat which point the diode starts con-
ducting. In (b), when dV,/dt is 8 V/ps, this I N4 I48 overshoots as much as I40 mV at input
frequencies below IO kHz before it turns ON. At higher frequencies- I20,240,480,960, and
I920 kHz-as the repetition rate increases, the overshoot shrinks and disappears. Maximum
overshoot occurs when fin < 7 kHz. In (c), when dV,/dt increases to 20 V/ks, this same
I N4 148 overshoots as much as 450 mV at 7 kHz but only 90 mV at 480 kHz and negligible
amounts at frequencies above 2 MHz. In (d), various diode types have different turn-on char-
acteristics. The superimposed, 120-kHz waveforms are all invariant with frequency except for
the bad I N4 148.

sions to “improve” the product. One man’s “improvement” is another man’s poison.
Thus, you must always be alert for production changes that may cause problems.
When manufacturers change the diffusions or the process or the masks, they may
think that the changes are minor, but these changes could have a major effect on your
circuit.
Many circuits, obviously, require a diode that can turn ON and catch, or clamp, a
voltage moving much faster than 20 V/ps. Therefore, if you want any consistency in
a circuit with fast pulse detectors (for example), you’ll need to qualify and approve
only manufacturers whose diodes turn ON consistently. So, as with any other unspec-
ified characteristic, be sure to protect yourself against “bad” parts by first evaluating
and testing and then specifying the performance you need. Also if you want to see