We can configure an NPN or PNP transistor in three basic circuits. We can make
each of them in a Common Emitter, Common Collector or Common Base. Each has its
advantages and disadvantages.
This is the most popular config
uration for NPN and
PNP transistors. The Emitter is tied to ground and is common
to the Input circuit (Emitter
Base) and the Output circuit
The PNP transistor can also be configured in a
Common Emitter configuration. The basic des
holds true. The input circuit is the Emitter
Base and the
Output is the Emitter
In both cases the Emitter is what is connected to the
power line, be it VCC or Ground.
For NPN devices configured this way applying a
voltage at the input causes Base current to flow,
turning the transistor on.
For PNP devices moving the input towards ground
causes current to flow and turns on the transistor.
Not too often seen in use, but still it is a working
cuit, we have the Common Collector. The Collector lead is
connected to the power line. The input is still the Base and
the Emitter is the Output.
We can also do this with a PNP.
For NPN devices we have to raise the input up to a
positive voltage above th
e emitter to turn the transistor on.
For PNP devices we have to move the input to a
voltage below that of the Emitter to turn the transistor on.
We can also have the Base as the common lead and have the
Emitter as the input and t
he Collector as the output. This configuration
is less popular but we can find a few common examples.
With an NPN in a Common Base configuration the Base goes
towards the +V line. We will see this later when we look at what is
inside TTL logic devices. T
his is the typical input structure for TTL
We can also use this with the Base going towards ground in
some Voltage Regulator circuits with the input being the Collector
and the output being the Emitter.
For a PNP transistor in a Common Base
circuit the Base lead
goes towards the
V or Ground line. We will see this in practical
application in Voltage Regulators.
these circuits in
The most common configuration used is the common emitter whether it is an
NPN with the emitter connected to ground or a PNP with the emitter connected to +V.
Now that we can identify the basic type of c
ircuit it is the next is figuring out what the
or Missing Pulse Detector
Seen often in one configuration or
another is the Pulse Stretcher
. This circuit
is noted by a relatively large capacitor
connected at the Colle
ctor of a transistor.
Q1 is off
and C is charged up. Q2 is on and the
A positive going pulse
turns Q1 on for a short time, discharging
C, turning off Q2 making the output high.
e input pulse returns low C
charging. The larger C is the longer it will
take to charge up. Event
ually the charge
gets over about 4 Volts (3.3 V of the
Zener plus about 0.6 V for the Base of Q2), and Q2 turns on again making the output
created a long
with a time
equal to the input pulse plus the
RC time constant.
To qualify as a Pulse Stretcher the time set by R x C must be notably
longer than the input pulse, otherwise we might call this circuit a
If R x C is
about the same
length as the input pulse we have replicated the input pulse as a cleaner
pulse appearing at the output.
We may also use this circuit as a Missing Pulse Detector. As long as we keep
getting input pulses at a rate quicker than R x C the capacitor C never g
ets a chance to
and the output stays high
. If we miss a puls
C charges up and the output goes
low indicating that we missed a pulse.
This requires that the design of R x C is precisely
set to just over the timing of the input pulse.
g the timing of R x C and the timing of the Input pulse we can
identify what function the circuit is designed to accomplish.
Is this a Common Base or a Common Collector
Drawn as it is it looks like it is a Common Base
We can argue that this is really a Common
Collector circuit drawn deceptively. The Collector goes to
some +V source
and is common to both the input circuit
and the output circuit
. The input is a regulated voltage at
the Base and the output is taken off of
the Emitter as a regulated voltage
about 0.6 Volts
We see these circuits where we need a regulated voltage at a current lev
than we can get from a Zener alone.
Typically these current levels are more than 10 mA
ut less than a few hundred milli
If we need higher current levels
we would likely
use an IC voltage regulator.
The only other advantage of this circuit over an IC regulator I can see is that it lets
us tailor the output to a precise voltage. By chan
ging the current through the Zener we
can set the voltage it regulate
s at to a very precise voltage.
Common Emitter, Common
Collector or Common Base? In an
all we have is Emitters and a Base. The
Cathode is almost always connected to
nd the Gate (Base) is referenced
to that voltage. Other configurations may
be possible, but I can
t think of a circuit
that has ever done so.
circuit we see using an SCR is called a Crowbar circuit. We see this on
as a safety
Over Voltage of the output if the Regulator
If the output should exceed 5.1 Volts (with the Zener shown) the SCR
triggers and shorts the output of the power supply to ground
, blowing the fuse
troubleshoot these circuits
connect the power supply to a safe load, remove the SCR and
apply power. S
ee what the initial problem
actually is. After the circuit is working and
regulating again check the SCR to not be shorted
and put it back into the circuit, then
recheck the power s
upply for proper operation.
If we want to control a
lamp or AC
motor, such as a hopper, from a DC signal we
need to use an optoisolator or a relay. The
Relay option we seldom see in gaming any
more. The optoisolator has taken its pla
an LED and a photo
. When we turn on the LED
the light hits the phototransistor and turns it
on. We need no electrical connection between
the LED side and the phototra
nsistor side. Our
phototransistor can be any type of circuit at all. Made more specifically for driving AC
loads we have a more specialized version of an optoisolator called a Solid State Relay. It
operates as a relay. We have an input voltage that control
s an AC voltage at the output. It
is specifically these that we will find most often in gaming to control AC motors and
Okay, Common Emitter, Common Base or
The NPN is connected as a
. The Emitter is grounded. The o
is taken off of the Collector. How about the Triac?
we look at how a Triac is built
on the inside, simplified
as transistors, we have to see that it can
’t be anything
but a Common Emitter since both of the main terminals
are Emitters of transistor
s. Main Terminal 1 (MT1) is
almost always connected to the ground, or common,
and the output is
taken off of MT2
. The Gate goes to
Bases of transistors that must have a voltage referenced
’t be impossible to construct it otherwise, but I
’t think I have ever
seen it as such.
that must go actively high and low might
use a circuit like the ones shown here. For an analog output that
may be any voltage between two rails (+V and
or +V and
will likely have a Push
Pull output as shown with the
Emitters tied together
at the output
Common Base, Common
Emitter or Common Col
For the analog output voltage we
have the Common Collector. The Collectors of both transistors go to voltage sour
output is taken from the Emitters.
Less often seen is the Common Emitter version of this
e is a very good reason here. One of the output
transistors is only off when the input voltage is very close to one
of the rail voltages. When t
he input is close to +V Q1 is off.
When the input is close to
(or ground if we use +V and
Ground) Q2 is off. Any time the voltage is in
between these two
both Q1 and Q2 are on to some degree and we have a
tance path between the tw
o rail voltages. This
circuit would only be used with current limiting resistors between the emitters and the rail
or for some reason we only want to drive str
ongly to the rails and don
the signal to spend much time in
We will en
counter this when we get into
Common, Emitter or
The Emitter is
connected to the +V rail,
even if it does go through
a resistor. The output is
taken off of the
Collector. I would say
R9 and C11 are
just a noise filter to keep
the noise this circuit
generates off of the +V
line. If this is the only
circuit we could
eliminate R9 and C11.
R13 sets our
output voltage level. The
output is a sine wave
with a frequency we can c
changing the R
its of C8
4, and C10
5. I didn
’t notice this until the picture was already
included here, but I would think R11 should be 56K, R12, R14 and R15 should be 10K
get 800 Hz
The output signal I would not exp
ect to go all the way from ground to +V. I
would likely expect it to be about 2 V Peak to Peak running from 5 V to 6V of +V. R13
would adjust this a bit, but not much.