[time-nuts] Lars GPSDO on EEVblog

Achim Gratz Stromeko at nexgo.de
Sun Sep 9 04:13:54 EDT 2018


Charles Steinmetz writes:
>> The diode-connected 2N3904 has less leakage at a small forward
>> voltage than any small-signal diodes I tested
>
> Generally, one uses the B-C junction rather than the B-E junction for
> this, because (i) being a smaller junction (in most small-signal
> BJTs), it has less leakage than the B-E junction,

The smaller junction in a discrete BJT is always the emitter.  The
breakdown and reverse leakage characteristics are different due to the
different doping profiles.

> and (ii) it has a much higher reverse breakdown voltage than the ~5v
> breakdown of the B-E junction (NB: the B-E junction is degraded with
> even very brief operation in the breakdown region).

I think the circuit depends on the forward characteristics of the diode
from the brief look I've had.  In this case, a diode connected npn as
shown (shorting the base and collector) produces a much better diode
characteristic than any "true" diode would.


Regards,
Achim.
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