Please
'Boom' Responsibly As
most of you have noticed, the noise ordinances have
become much tougher lately. Most of this is due to
idiots, yes IDIOTS, who drive through residential areas
with their windows down while their system is playing at
full power. To make things worse, the music they listen
to has all sorts of foul language that's not suitable for
small children, (who may be playing outside). There are
even a few people, who are even beyond idiot status, that
play their systems at full power through residential
areas after 10:00 PM (when many people go to bed). I
don't believe that this type of behavior is good for the
industry. If the fines get too stiff, people will stop
buying large systems. If this happens, more people will
get out of car audio (who wants a mediocre system).
People get interested in things because they're exciting.
A deck and four 6.5" speakers are not going to
interest many of the younger car audio enthusiasts. If
car audio enthusiasts keep annoying more and more people,
the fines will keep getting tougher. All of this will
only reduce interest in the equipment that fuels the
industry. If you want to listen to your system at full
volume, get out on the highway where there's little
chance of bothering anyone. When you get to a red light,
turn it down. If the only thing attractive about you is
your 'system', you have some work to do. Bottom line...
Think about what you're doing. Think about other people.
It's not the end of the world if you have to turn the
volume down for a little while.
Field-Effect Transistor:
The function of Field Effect
Transistors is similar to bipolar transistors
(especially the type we will discuss here) but
there are a few differences. They have 3
terminals as shown below. Two general types of
FETs are the 'N' channel and the 'P' channel
MOSFETs. Here we will only discuss the N channel.
Actually, in this section, we'll only be
discussing the most commonly used enhancement
mode N channel MOSFET (Metal Oxide Semiconductor
Field Effect Transistor). Its schematic symbol is
below. The arrows show how the LEGS of the actual
transistor correspond to the schematic symbol.
Current Control:
The control terminal is called
the gate. Remember that the base terminal of a
bipolar transistor passes a small amount of
current. The gate on the FET passes virtually no
current when driven with D.C. When driving the
gate with high frequency pulsed D.C. or A.C.
there may be a small amount of current flow. The
transistor's "turn on" (a.k.a.
threshold) voltage varies from one FET to another
but is approximately 3.3 volts with respect to
the source.
When FETs are used in the audio
output section of an amplifier, the Vgs (voltage
from gate to source) is rarely higher than 3.5
volts. When FETs are used in switching power
supplies, the Vgs is usually much higher (10 to
15 volts). When the gate voltage is above
approximately 5 volts, it becomes more efficient
(which means less voltage drop across the FET and
therefore less power dissipation).
MOSFETS are commonly used
because they are easier to drive in high current
applications (such as the switching power
supplies found in car audio amplifiers). If a
bipolar transistor is used, a fraction of the
collector/emitter current must flow through the
base junction. In high current situations where
there is significant collector/emitter current,
the base current may be significant. FETs can be
driven by very little current (compared to the
bipolar transistors). The only current that flows
from the drive circuit is the current that flows
due to the capacitance. As you already know, when
DC is applied to a capacitor, there is an initial
surge then the current flow stops. When the gate
of an FET is driven with a high frequency signal,
the drive circuit essentially sees only a small
value capacitor. For low to intermediate
frequencies, the drive circuit has to deliver
little current. At very high frequencies or when
many FETs are being driven, the drive circuit
must be able to deliver more current.
Note:
The gate of a MOSFET has some
capacitance which means that it will hold a
charge (retain voltage). If the gate voltage is
not discharged, the FET will continue to conduct
current. This doesn't mean you can charge it and
expect the FET to continue to conduct
indefinitely but it will continue to conduct
until the voltage on the gate is below the
threshold voltage. You can make sure it turns off
if you connect a pulldown resistor between the
gate and source.
High Current Terminals:
The 'controlled' terminals are
called the source and the drain. These are the
terminals responsible for conducting the current
through the transistor.
Transistor Packages:
The MOSFETs use the same
'packages' as bipolar transistors. The most
common in car stereo amplifiers is currently the
TO-220 package (shown above).
Transistor In Circuit:
This diagram shows the voltages
across the resistor and the FET with 3 different
gate voltages. You should see that there is no
voltage across the resistor when the gate voltage
is around 2.5 volts. This means that there is no
current flowing because the transistor is not
turned on. When the transistor is partially
turned on, there is a voltage drop (voltage)
across both components. When the transistor is
fully turned on (gate voltage approx. 4.5 volts),
the full supply voltage is across the resistor
and there is virtually no voltage drop across the
transistor. This means that both terminals
(source and drain) of the transistor have
essentially the same voltage. When the transistor
is fully turned on, the lower lead of the
resistor is effectively connected to ground.
Voltage
applied to gate
Voltage
across resistor
Voltage
across transistor
2.5
volts
no
voltage
approximately
12 volts
3.5
volts
less
than 12 volts
less
than 12 volts
4.5
volts
approximately
12 volts
virtually
no voltage
In the following demo, you can
see that there is an FET connected to a lamp.
When the voltage is below about 3 volts, the lamp
is completely off. There is no current flowing
through the lamp or the FET. When you push the
button, you can see that the capacitor starts to
charge (indicated by the rising yellow line and
by the point where the capacitor's charging curve
intersects with the white line sweeping from left
to right. When the FET starts to turn on, the
voltage on the drain starts to fall (indicated by
the falling green line and the point where the
green curve intersects with the white line). When
the voltage on the gate starts to drop, the
voltage across the lamp starts to increase. The
more it increases, the brighter the lamp becomes.
After the voltage on the gate reaches about 4
volts, you can see that the bulb is fully on (it
has the full 12 volts across its terminals).
There is virtually no voltage across the FET. You
should notice that the FET is fully off below 3
volts and fully on after 4 volts. Any gate
voltage below 3 volts has virtually no effect on
the FET. Above 4 volts, there is little effect.
Design
Parameters
GATE VOLTAGE
As you already know, the FET is
controlled by its gate voltage. For this type of
MOSFET the maximum safe gate voltage is ±20
volts. If more than 20 volts is applied to the
gate (referenced to the source) it will destroy
the transistor. The transistor will be damaged
because the voltage will arc through the
insulator that separates the gate from the
drain/source part of the FET.
CURRENT
As with bipolar transistors,
each FET is designed to safely pass a specified
amount of current. If the temperature of the FET
is above 25c (approx. 77 degrees farenheit), the
transistor's "safe" current carrying
capabilities will be reduced. The safe operating
area (S.O.A) continues to be diminished as the
temperature rises. As the temperature approaches
the maximum safe operating temperature, the
transistor's current rating approaches zero.
VOLTAGE
FETs will be damaged if its
specified maximum drain-source voltage is
exceeded. You can obtain a data sheet from the
manufacturer. The data sheet will give you all of
the information you need to use it.
POWER DISSIPATION
FETs are similar to bipolar
transistors as far as packages and power
dissipation go, and you can follow this link back to the bipolar page for more
information. Hit you're "back" button
to return.
If you
find a problem with this page or feel that some part of
it needs clarification, E-mail
me.