01 February 2008
As usual, I've moved the January 2008 page to archives,
reachable by clicking here, or via the navigation
table at the top of this page.
In electronics, as in life, it's often not what you don't
know that gets you in trouble, but rather the things you think you know but in
fact are wrong. I ran into that yesterday when I add a 1N5711 Schottky diode to
clamp the reverse base voltage across a 2SC1945 transistor RF power amplifier.
When I measured the reverse voltage clamping level,
I expected it to be around 0.5 volts or less. After all, a Schottky diode has a
drop around 0.4 to 0.5 volts, compared with a standard silicon diode's 0.7
volts, right? When I measured around 1.2 volts across the 1N5711, I first
thought I installed a 1N4148 silicon diode instead. The diode was clearly (well,
clearly after using a magnifying glass) marked 1N5711.
That lead me to look at data sheets for several diodes and
to measure the forward current versus forward voltage of four diode types.
Because this subject is likely to be of continuing interest, I've added a new
page with my data and analysis. You may view it by clicking
here, or via the link at the left side of this
If you want the one-sentence summary: A 1N5711
Schottky signal diode does not always have 0.4 V forward voltage and in fact the
forward voltage drop across a 1N5711 Schottky diode and a 1N4148 Silicon diode
may be quite similar as the forward current exceeds 10-20 mA.