6.070
Transistors
Dr. James Bales
Transistors:
Transistors are at the heart of the electronics revolution of the past seven decades. Transistors are
three-terminal devices that are typically used either as electronic switches or as amplifiers. As a
switch, a transistor allows an electrical signal at one of its three terminals to either permit or
prohibit the flow of current into a second terminal and out of the third. As an amplifier, the signal
at one terminal is presented in an amplified form at another terminal.
There are two major classes of transistors—bipolar junction transistors (BJT’s or bipolar) and
field effect transistors (or FETs). We will consider only bipolar transistors in this subject1. There
are two types of bipolar transistor, they are called NPN and PNP. The letters refer to how the
devices are constructed. We will consider both types. You can use an NPN and a PNP together to
form a complementary pair2.
A bipolar transistor is a three-terminal device, that is, it has three connections to a circuit, rather
than the two of resistors or diodes. The three terminals are called the emitter, collector, and base.
Figure 1 shows the schematic symbol for the NPN and PNP transistors, with the leads labeled.
Fig 1 Schematic symbols for NPN (L) and PNP (R) transistors.
The bipolar transistor tries to follow two rules (analogous to the golden rules of the op-amp).
1)
2)
The base-emitter junction acts like a diode, with forward voltage of 0.7 V, and
The transistor will adjust the voltage difference from collector to emitter so that the
collector current (IC) will equal a constant (b) times the base current (IB), that is,
Eqn. 1
IC = β × I B
Note the arrowhead on the schematic symbols. It is meant to show you the direction of the baseemitter diode, and it is reversed between the NPN and the PNP transistors. The parameter b
(sometimes called hFE) is typically between 20 and 200 or so. Many characteristics are reversed
between NPN and PNP transistors, from here on we note the PNP performance in parenthesis.
The bipolar transistor is a current amplifier, that is, a small current flowing into (out of) the base
of the NPN (PNP) transistor, controls the much larger current--by a factor of b--into (out of) its
collector.
Rule 2 can fail if the circuitry attached to collector is incapable of supplying (consuming) that
much current to (from) the NPN (PNP) transistor.
1
An important class of FETs are the Metal-Oxide-Semiconductor (MOS) FETs, or MOSFETs.
MOSFETS also come in two types, called n-channel and p-channel. You can combine them into a complementary
pair of MOSFETs, making what is called a CMOS device.
2
Page 1 of 4
6.070
Transistors
Dr. James Bales
Transistor Man
The book, The Art of Electronics, by Horowitz and Hill, is a detailed electronics textbook
designed for Physics undergraduates. They introduce the “Transistor man” model of the
transistor:
“Transistor Man” sits inside the transistor and
measures the base current. He then adjusts a
variable resistor that lies between the collector
and the base and tries to adjust that resistance
so that
IC = β × I B .
(I think that, in the language of the Transistor
People, “b ” is spelled, “hFE”.)
The maximum value of that potentiometer is
effectively infinite. There is a minimum value,
at which point the transistor is letting the
maximum possible current flow between
collector and emitter.
When a transistor is in that state (i.e., maximum possible ICE) we say the transistor is “fully on”
(aka saturated), or that it “acts like a closed switch”. If we only use a transisor as “fully on” or
“fully of” we are using it as a switch. Digital electronics only uses transistors in this way
Alternatively we can use the transistor in the region where it gives current amplification. Analog
electronics usually use transistors in this way, but will also use them as switches as well.
Page 2 of 4
6.070
Transistors
Dr. James Bales
An Example
Suppose I’m building a night-light that, when the room is too dark, turns on an incandescent light
bulb (instead of an LED). Let us assume that my comparator gives low voltage when the room is
lit and a high voltage when it is dark. Table 1 shows the specifications of my components.
Table 1. Specifications of components for example.
Comparator
Light Bulb*
High = 5 V
Voltage
Voltage Required 4-6 Volts
Low = 0 V
150 mA (at 3 V)
Source 20 mA (at 5 V)
200 mA (at 4 V)
Current Capability
Current Required
250 mA (at 5 V)
Sink 20 mA (at 0 V)
300 mA (at 6 V)
*
You might notice that my “light bulb” acts like a 20 Ohm resistor. This is a bad model of a real light bulb!
I’d like to connect the output of the comparator directly to the light bulb, unfortunately, I can’t.
Why? Because the voltages match, but the currents don’t. The comparator can only provide some
30 mA - the bulb needs 250 mA at 5 V.
The way out of this bind is to partition the problem into three parts. First is the sensing part,
second is the deciding part (the comparator), whose job is to take the sensor output and turn it
into a clear, unambiguous, but relatively weak (meaning limited ability to drive current),
electrical signal. Last is the actuating part, the light bulb and a transistor that can take the
unambiguous but weak signal from the comparator and use it allow or prohibit current from
flowing through the light bulb. The circuit is shown in Figure 2.
Deciding Circuit
+5 V
10k
+5 V
+5 V
+5 V
Sensing
Circuit
+
-
C
1.1k
B
E
1k
Actuating Circuit
Figure 2. Comparator circuit to drive bulb.
Page 3 of 4
6.070
Transistors
Here we use an NPN transistor as our electrical switch, its
specifications are in Table 2. Let us analyze the circuit. We
begin by noting that the emitter is grounded, as is commonly
done with NPN transistors. Then, by Rule 1, the base must be
at 0.7 V when the comparator output is high. 3
Dr. James Bales
Table 2. Transistor Specifications
Example Specifications of Transistor
50
b
VSATCE
0.3 V
Maximum Power Dissipation 0.5 W
We now know the voltage at the ends of the 1.1 kW resistor, and can calculate the current
through it from Ohm’s law, namely,
I B = ΔV RB = 4.3V /1100Ω ≈ 4mA .
Eqn. 2
With IB known, we can then calculate the collector current, which is also the current passing
through the light bulb, as simply b ´ IB = 50 ´ 4 mA = 200 mA. This will light the lamp.
Finally, let us see where the voltage drops occur. The lamp has a 4 V drop across it when 200
mA passes through it, so the remaining 1 V must appear between the collector and emitter of the
transistor (VCE). Because there is a 1 V drop from collector to emitter, and 200 mA flow into the
collector and out of the emitter, the transistor dissipates 1 V x 0.2 A = 0.2 W, a significant
fraction of the power rating of the transistor.
This usage of the transistor is called a low-side switch.
In practice, we would make the base resistor much smaller than
1,000 Ohms. That way, the transistor would try to get even
more current to flow through the light bulb, and it would do so
by reducing the voltage drop from collector to emitter as low
as it possibly can. We call this saturation, and when we use
transistors as switches, we try to operate them in saturation
when we want them to look like a closed switch.
PNP Transistors
Figure 3 shows a sample PNP circuit that turns current on or
off through the 220 W resistor. Note that here the current flows
out of the base, and out of the collector. This is known as a
high-side switch.
+5 V
E
RB
B
C
220
The logic here is the reverse of the NPN, that is, applying a
high voltage to the base resistor (i.e., > 4.3 V) turns the
transistor off, while lowing that voltage below 4.3 V turns the
transistor on more and more.
Figure 3. A PNP transistor
as a high-side switch.
3
This shows why we need the base resistor, RB, for without it, the comparator would try to hold the base at 5 V
while the transistor would try to pull the base voltage down to 0.7 V. If you tried this, most likely the comparator
would lose and the magic blue smoke might come out of it.
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