78 August 2015
READER - TO - READER TECHFORUM
>>> QUESTIONS
Headphone Hookup
My flat panel TV doesn’t have
a headphone jack. It has dual RCA
jacks labeled “R/L line out” and a
“TOSLINK” connector. What would
be the simplest way to get my
headphones hooked up to this TV?
#8151 Chad Kessler
Marion, IN
Dim Bulb
What’s the difference circuit-wise
between a “dimmable” and a “
non-dimmable” 110V home LED light
bulb?
Jeremy Tores
#8152 Garden City, NY
Servo Cycler
I would like to make a servo
tester that would continuously run
a servo from 0 to 180 degrees and
back. I have found a number of 555
servo tester designs, but none that
actually cycle the servo. Does anyone
have a schematic for such a thing?
#8153 Owen Corlis
Plymouth, MI
Voltage Mod
I am trying to build a small micro
ampere meter project I found on the
Internet (Figure 1). The design calls
for a +9V and -9V supply. Is it possible
to modify this circuit to use a single
9V battery instead of two?
#8154 Stephan Barth
Grand Bay, AL
>>> ANSWERS
[#6153 - June 2015]
Bipolar vs. MOSFET
I’ve read that bipolar transistors
are current devices and MOSFET
transistors — like old-fashioned vacuum
tubes — are voltage-operated devices.
What does that mean from a
practical perspective? For example,
does this mean that bipolar are best
for high power applications and
MOSFETs are best for low voltage
applications?
MOSFETs come in many flavors:
there are N-type and P-type of
course, but there is enhancement
mode and depletion mode.
Enhancement mode is off at zero gate
voltage; you have to apply a positive
gate voltage for N-type or a negative
gate voltage for P-type to turn it on.
N-type MOSFETs are available with
drain voltage ratings from 30V to
800V or more. Most MOSFETs are
designed for switching; an 800V, 10
amp device would quickly burn up
unless it could turn on and saturate
even quicker.
MOSFETs are characterized by
their saturation resistance which can
be very low (like .01 ohms). Bipolar
transistors on the other hand, are
characterized by their saturation
voltage which can’t get as low power
as MOSFETs. The threshold voltage
(the point where the transistor just
turns on) is not well controlled,
so you can’t really know what the
drain current will be at a particular
voltage. That makes it difficult to
design a linear circuit. I avoid that
problem by using bipolar transistors
in linear circuits, or using pulse width
modulation in a switching circuit
which can be filtered to produce an
analog signal.
Depletion mode MOSFETs are on
at zero gate voltage and you have to
apply a negative gate voltage to an
N-type to turn it off. Junction FETs are
also depletion mode devices and the
zero gate voltage drain current is not
well controlled, so they are usually
binned and labeled so you have some
idea of what you are designing with.
Russell Kincaid
Milford, NH
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Always use common sense and
good judgment!
n FIGURE 1
