ELECTRONIC DESIGN NOTES #7 - TRANSISTORS
The transistor was discovered in 1947
at Bell laboratories. Beginning in 1960s, the transistor takes on a spectacular
ascending ramp in electronic design, and it
continues to do so up to these very days! Even more, the future looks very bright for transistors, since we are moving
now towards the
circuits--this means, electronic circuits printed on paper, and other materials,
using special semiconductor/conductor inks.
In this page are presented only a few instances of using transistors, some schematics,
a few graphs, and a general
classification. The structure employed is:
1. Biasing PNP and NPN BJT Transistors
BJT TRANSISTORS Functions
3. Biasing JFET Transistors
4. JFET Transistors Functions
5. Types of Transistors
The basic notions highlighted in this page are related to
a few electronic design topics presented in the first part, Hardware Design, of
FIRMWARE AND SOFTWARE DESIGN.
1. BIASING PNP AND NPN BJT TRANSISTORS
Bipolar Junction Transistors (BJT) work in two modes:
1. as amplifiers;
2. as digital switches in the saturation/cutoff states.
Many people do not understand this: bipolar transistors are current controlled electronic devices. Of
course that we do need specific voltages to bias a bipolar transistor,
except those voltages have the polarity, and the
required magnitudes, according to the currents they need to generate.
That misunderstanding is nobody's fault, because there are very many books where this issue is unclear and/or
To start, let's analyze a few voltage biasing
please remember that all biasing voltages
are generated by the needed currents. Transistors' graphs corresponding to the two functioning modes
mentioned above are related to Ib, Ie, and Ic only, according to the formula:
Ie = Ib + Ic
Ie = emiter current
Ib = base current
Ic = collector current
To start, let's see how we saturate transistors, when they are used as logic components in
the ON/OFF states. Please note: the saturation/cutoff digital
the only situation when transistors behave similar to voltage controlled relays.
Unfortunately, this situation brings some
confusion, therefore any analogy to "transistors' voltage control" should better be avoided.
SATURATING BIPOLAR JUNCTION TRANSISTORS
(IN DIGITAL MODE)
||Fig1: Saturating PNP transistors
The red trace is the base voltage
blue trace is the voltage in point A
The graph is moved downwards
for exemplification; the voltage variation is in fact from +5V to 0V.
PNP transistors are saturated when the current Base-Emitter is
at a maximum. However, due
to the PNP special biasing requirements, that happens when base voltage is 0 V in the above picture.
PNP transistors are in the cutoff state when the current Base-Emitter is
at a minimum
(zero). That happens
when base voltage is the same as the emitter one--in the above example it is +5V.
In the PNP biasing instance presented above, collector's voltage is a fixed value: zero volts.
||Fig 2: Saturating NPN transistors
Red trace is the base voltage
The graph is moved downwards for exemplification; the voltage variation is in fact from 0V to +5V.
Blue trace is the voltage in point A. Note that in this case collector's
voltage is +12 V
NPN transistors are saturated when the current Base-Emitter is
at a maximum. However, due
to the NPN biasing requirements, that happens when base voltage is greater than 0.7 V--otherwise, there is no
NPN transistors are in the cutoff state when the current Base-Emitter is
at a minimum
(zero). That happens
when the base voltage is the same as the emitter one--in the above example it is 0 V.
In the NPN biasing case presented above, collector's voltage may have any positive value supported by
the silicon. It can also be zero, although in that case we have no collector current.
As digital components in logic circuits, transistors are employed either as "control switches" or as "power drivers".
Excellent working examples of both implementations are thoroughly presented in our LHFSD book.
Now, there are a few good methods to bias a BJT
transistor when it works in "analog amplification mode", or in "digital/logic mode", as they are listed further down--though, only for the NPN
transistor; for PNP you should reverse the polarities (somehow).
Always remember that a PNP transistor is biased
according to the formula P-N-NN as follows:
1. Emitter = Positive (P)
2. Base = Negative (N)
3. Collector = More Negative than the Base (NN)
that an NPN transistor is biased
according to the formula N-P-PP as follows:
1. Emitter = Negative (N)
2. Base = Positive (P)
3. Collector = More Positive than the Base (PP)
The minimum amount of formulas needed to work with bipolar transistors are:
β (Beta) = Ic / Ib
α (Alpha) = Ic / Ie
Ie = (β + 1) * Ib
Ie = Ic + Ib
The junction Base-Emitter is directly biased, while the junction Base-Collector is reverse biased. That means,
there is current flowing from Base to Emitter (naturally), but there is (for certain)
no current exchange between
Base-Collector or Collector-Base.
For details about working with transistors in digital mode please consult
FIRMWARE AND SOFTWARE DESIGN.
2. BJT TRANSISTORS FUNCTIONS
The previously presented BJT saturation/cutoff states are specific to transistor's digital functionality.
[That also defines BJT
transistors as being perfect current controlled relays--which is one of
their main functions.] Further, BJT transistors are considered DC logic elements, and
they work as the very building bricks of all logic ICs (including
It needs to be pointed out that BJT transistors have nicer
switching characteristics compared to the MOS-FET ones. Even
more, the Isolated Gate Bipolar Transistors (IGBT) have the nicest
switching characteristics of all transistors--no ringing, or the minimum
On the other hand, BJT transistors have been used since they were invented in
analog circuits as
amplifiers in linear mode. There are three main schematics employed
to wire BJT transistors as analog amplifiers--they are presented, again,
only for the NPN instance. What we are looking for is:
1. voltage gain
2. current gain
3. power gain
The three most common schematics employed are:
for voltage, current, and power gain
Common-Base, for voltage and power gain
Common-Collector, for current
and power gain
The article "Driving Automotive Injectors"
Amazing Articles] presents a few interesting topics
about using power transistor drivers.
3. BIASING JFET TRANSISTORS
FET (Field Effect Transistor) is a high-input impedance (100 MOhms and better), low noise, voltage
controlled, solid-state semiconductor device. The first FET discovered was JFET (Junction Field Effect
Transistor) followed a few years later by IGFET (Isolated Gate Field Effect Transistor) which was later renamed MOS-FET
(Metal Oxide Semiconductor Field Effect Transistor).
The MOS technology is very cheap and perfectly suited for mass production, therefore it is used in most ICs today.
For hardware designers, however, discrete FET components are rather expensive to procure, and they may be easily damaged by a simple
hand touch (by our body-generated electrostatic voltage).
However, more problematic is biasing those FET transistors, therefore we present
only a few common schematics.
Before working with transistors (BJT or FET), you need to study their output curve. For that, you have to get their
Data Sheets. Particularly to FETs, their output curve is fairly complex (not presented here). You need to get
one, because it is possible there will be a few (unexplained) references to it.
FET transistors behave similar to voltage controlled relays.
Three schematics are commonly employed to bias N-JFET transistors:
1. Self Biased
Two Power Supply
A practical method of working with FET transistors is, always test their output curve before using
them. FETs are "third order semiconductors", and it is not easy to control
and to calculate them. First, decide on using one or two FET functions, for example
"switching" and/or "variable resistor", and then use a simulator program, or a
test stand to discover the right values need for the proper biasing resistors.
The test stand looks (generally, in principle) as follows:
Fig 13: Test stand used to discover the right biasing
resistors required by various FET configurations
Once you calculate and measure the needed voltages and currents, you should implement one of the above
biasing schematics. It is easy to find the right gate voltage/resistor using:
R = V / I
4. JFET TRANSISTORS FUNCTIONS
All FET transistors have three main functions. They are used as:
2. analog/digital switches
3. voltage-controlled resistors
If you want digital FET switches/relays in your application, please consider one of the biasing schematics presented
above. The voltage-controlled resistor function is left for you to discover. Further is presented only the amplification
5. TYPES OF TRANSISTORS
It is possible transistors are the electronic components coming in the wildest variety possible
(this is, considering their technical characteristics, not their shape
which is standard), and that clearly indicates they are very much used.
Above are presented only the BJT and the JFET ones. More or less, all other existing types of transistors are
similar in functionality.
It is possible we will attempt to develop this "Types of transistors" topic one
glorious day. Meanwhile, we do encourage you
FIRMWARE AND SOFTWARE DESIGN. In addition to presenting
all the schematics you need to start working with dsPIC controllers (or with any other
type of Microchip controller), this
book presents 12 firmware plus 7 software Source-Code applications, each of them being a practical working project,
fairly easy to understand. [These Source Code applications are priceless!]
Please believe this: complete, working, and thoroughly explained firmware Source Code programs in a book is unheard of!
Even more, the really exceptional aspect is, all firmware and software programs presented in LHFSD are the essence of
simplicity, and they are truly logic. Always remember that it is the firmware/software that
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