■ WITH TJ BYERS
In this column, I answer questions about all
aspects of electronics, including computer
hardware, software, circuits, electronic theory,
troubleshooting, and anything else of interest
to the hobbyist.
Feel free to participate with your questions,
comments, or suggestions.
You can reach me at: TJBYERS@aol.com
● Build a Lambda diode, play with it.
✓Automatic cigarette lighter switch.
●✓Burned-out trailer taillight monitor.
✓Simple auxiliary battery charger.
FUN WITH NEGATIVE
QA while back in my Circuits
101 class for Electrical Engineering, the professor mentioned something about negative resistance. This was a totally new
■ FIGURE 1
■ FIGURE 2
concept to me — and he made almost
no effort to explain it. It seems very
strange, almost like having a negative
weight or length. Can you please explain this phenomenon in more detail?
AIn electronics, this is common. In fact, it’s desirable.
Without going to the math of
imaginary numbers and
the like, there is really no such thing as
■ FIGURE 3
negative resistance — as in there’s no
such thing as negative gravity. In both,
it’s all in how you define negative.
Ohm’s Law states that as the voltage across a resistance increases, so
does the current. It’s a linear function.
With negative resistance, on the other
hand, the current decreases as the
voltage increases, as shown in Figure
1. Several devices exhibit negative
resistance, including neon lamps, tunnel diodes, and Lambda diodes, which
you can make yourself from a pair of
Junction field-effect transistors (JFETs).
There are two types of field-effect
transistors: those that operate in the
enhancement mode and those that
operate in the depletion mode.
Enhancement mode means that the
FET is forward biased (turned on), like
a bipolar transistor. Depletion mode
means that the FET is reverse biased
(turned off), like a vacuum tube. JFETs
are depletion-mode devices.
When configured as shown in
Figure 2, the two transistors interact
with each other and produce a
negative-resistance Lambda diode. In
fact, the curve in Figure 1 are actual
measurements taken from this combination. Up to point A, the current
increases as the voltage is increased.
At point A, the diode current peaks
and begins a decline from point A to
point C, while the voltage continues
to increase — negative resistance!
Want to play with it? For most
applications, the diode is biased in the