Corollary Theorems: Transistors

 

ELECTRONIC DESIGN NOTES #7

Transistors
 
 

 
Back to Design page:

 
Back to Design Notes page

 


The transistor was discovered in 1947 at Bell laboratories. Starting with 1960s, the transistor takes on a spectacular ascending ramp, and it continues to do so these days. Even more, the future looks very bright for transistors, since we are moving now towards the "printed" electronic circuits--this means, electronic circuits printed on paper, and other materials, using special semiconductor/conductor inks.

In this page are presented just a few instances of using transistors, some schematics, a few graphs, and a general classification. The structure employed is:

1. Biasing PNP and NPN Bipolar Transistors
2. Bipolar Transistors Functions
3. Biasing JFET Transistors
4. JFET Transistors Functions
5. Types of Transistors

 
NOTE
The basic notions highlighted in this page are related to a few electronic design topics presented in the first part, Hardware Design, of
LEARN HARDWARE FIRMWARE AND SOFTWARE DESIGN.
 

 
 BIASING PNP AND NPN BIPOLAR TRANSISTORS


NPN TransistorThe Bipolar Junction Transistors (BJT)
work in two modes:
1. as amplifiers
2. as digital switches in the saturation/cutoff states


Many designers 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 many books where this issue is unclear and/or incorrectly presented. To start, let's analyze a few voltage biasing schematics, though 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. Please note: the saturation/cutoff digital states represent 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 TRANSISTORS
(DIGITAL MODE)
 


Schematic
 
Graph Fig
Saturating PNP Saturating PNP Graph Fig1: Saturating PNP transistors
The red trace is the base voltage

The 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.

ATTENTION
PNP transistors are saturated when the current Base-Emitter is at a maximum. 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.

 
Saturating NPN Saturating NPN Graph 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

ATTENTION
NPN transistors are saturated when the current Base-Emitter is at a maximum. Due to the NPN biasing requirements, that happens when base voltage is greater than
0.7 V--otherwise, there is no current flow.

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.

 



There are a few good methods to bias the transistor, as they are listed further down--though, only for the NPN transistor; for PNP you should reverse the polarities.
 
 


 BIASING BIPOLAR NPN TRANSISTORS
 


Schematic
 
Description
NPN biasing: Base biasing Fig 3: Base Biasing

This schematic is very much dependant on transistor's β (Ic/IB). If β changes with the temperature, the output varies. The formulas used to calculate this schematic are:

Ib = (+5V - Vbe) / Rb

Ic = (+5V - Vbe) / (Rb / β)

+5V is Vc
NPN biasing: Collector Feedback Fig 4: Collector Feedback Biasing

This schematic is very much similar to the one above.
NPN biasing: Universal Fig 5: Universal Biasing

This schematic is more stabile when β changes, therefore it is also the most used one. The formulas employed to calculate this schematic are:

 Ie = (Vre / Re)

 Ie = (Vrb2 -Vbe) / Re

 Ic = Ie

NPN biasing: 2 Power Supply

Fig 6: Two Power Supply (Emitter Biasing)

This schematic is dependant on the two voltage levels: if they are stable so will be the output. The formulas used to calculate this schematic are:

 Ib = (-5V - Vbe) / [Rb + (β+1) * Re]

 Ie = (-5V - Vbe) / { [Rb / (β+1)] + Re}


NOTE

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
)

NOTE
Always remember 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


ATTENTION
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
LEARN HARDWARE FIRMWARE AND SOFTWARE DESIGN.


 

 BIPOLAR TRANSISTORS FUNCTIONS


We have seen the BJT saturation and the cutoff states (the particular digital states of transistor's functionality). That also defines bipolar transistors as being perfect current controlled relays--which is also their main function. Further, BJT transistors are DC logic elements, and they are also the very building bricks of all logic ICs (including processors).

It needs to be pointed out that bipolar transistors (BJT) have nicer switching characteristics compared to the MOS-FET transistors. Even more, the Isolated Gate Bipolar Transistors (IGBT) have the nicest switching characteristics of all transistors--no ringing, or the minimum amount of al transistors.

Now, bipolar transistors have been used, since they were invented, in analog circuits as amplifiers in linear mode. There are three main schematics used to wire bipolar transistors as amplifiers--they are presented, again, only for the NPN case. What we are looking for is:
1. voltage gain
2. current gain
3. power gain


The three most common schematics used are:
1. Common-Emitter, for voltage, current, and power gain
2. Common-Base, for voltage and power gain
3. Common-Collector, for current and power gain




NPN TRANSISTORS LINEAR AMPLIFICATION TECHNIQUES
 


Schematic
 
Graph Description
NPN amplification: Common Emitter NPN amplification: Common Emitter Graph Fig 7: Simplified Common-Emitter amplifier

The red trace is the In FM (Frequency Modulation) signal (transposed down)

The blue trace is the amplified Out signal


This schematic is used for: 
1. voltage gain
2. current gain
3. power gain.
NPN amplification: Common Base NPN amplification: Common Base Graph Fig 8: Simplified Common-Base amplifier

Red trace is the In FM (Frequency Modulation) signal (transposed down)

Blue trace is the amplified Out signal


This schematic is used for:
1. voltage gain
2. power gain
NPN amplification: Common Collector NPN amplification: Common Collector Graph Fig 9: Simplified Common-Collector amplifier

Red trace is the In FM (Frequency Modulation) signal (transposed down)

Blue trace is the amplified Out signal


This schematic is used for:
1. current gain
2. power gain



The article "Driving Automotive Injectors" [A25 in Amazing Articles] presents a few topics about using power (transistor) drivers.
 

 

 BIASING JFET TRANSISTORS


The 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, FET are rather expensive to procure, and they may be easily damaged by a simple hand touch (by electrostatic voltages).

More problematic is biasing the FET transistors; therefore, we will try presenting a few schematics.

ATTENTION
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.

ATTENTION
FET transistors behave similar to voltage controlled relays.

Three schematics are commonly employed to bias N-JFET transistors:
1. Self Biased
2. Universally Biased
3. Two Power Supply




BIASING N-CHANNEL JFET
 

Schematic
 
Description
JFET Biasing: Self Biased Fig 10: Self Biased N-JFET

We use the following formulas to calculate this schematic:

 Id = Idss[1-(Vgs/Vgsoff)]2
Where Idss, and Vgsoff are given in the Data Sheet.
Id = the Drain to Source current
 Vrd = Id * Rd
 Vrs = Id * Rs
 Vd  = Vdd - Vrd
 Vds = Vd - Vs
 Vgs = Vg - Vs
JFET Biasing: Universal Fig 11: Universally Biased N-JFET

We use the following formulas to calculate this schematic:

 Id = Idss[1-(Vgs/Vgsoff)]2
Where Idss, and Vgsoff are given in the Data Sheet. In addition:

 Vrg2 = (Vdd * Rg2) / (Rg2 + Rg1)
 Vrd = Id * Rd
 Vrs = Id * Rs
 Vd = Vdd - Vrd
 Vds = Vd - Vs
 Vgs = Vg - Vs

JFET Biasing: 2 Power Supply Fig 12: Two Power Supply Biased N-JFET

We use the following formulas to calculate this schematic:

 Id = Idss[1-(Vgs/Vgsoff)]2
Where Idss, and Vgsoff are given in the Data Sheet. In addition:

 Vrd = Id * Rd
 Vrs = Id * Rs
 Vd = Vdd - Vrd
 Vds = Vd - Vs
 Vgs = Vg - Vs



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 biasing resistors.

The test stand looks (generally, in principle) as follows:

JFET: Test Stand 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


 JFET TRANSISTORS FUNCTIONS


All FET transistors have three main functions. They are used as:
1. amplifiers
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 function.

 


N-CHANNEL JFET AMPLIFICATION TECHNIQUES
 


Schematic
 
Graph Fig
JFET amplification: Common Source JFET amplification: Common Source Graph Fig 14: Common Source JFET amplifier

Red trace is the In FM (Frequency Modulation) signal (transposed down)

Blue trace is the amplified Out signal

JFET amplification: Common Drain JFET amplification: Common Drain Graph Fig 15: Common Drain JFET amplifier

Red trace is the In FM (Frequency Modulation) signal (transposed down)

Blue trace is the amplified Out signal

JFET amplification: Common Gate JFET amplification: Common Gate Graph Fig 16: Common Gate JFET amplifier

Red trace is the amplified Out signal

Blue trace is the In FM (Frequency Modulation) signal
(transposed down)


 

 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), which 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 to read
LEARN HARDWARE FIRMWARE AND SOFTWARE DESIGN. In addition to presenting all schematics you need to start working with dsPIC controllers (or with any other type of Microchip controller), this book presents 12 firmware and 7 software source-code applications, each of them being a practical working project, fairly easy to understand.

Please believe this: complete, working 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 truly logic. Always remember that it is the firmware/software that drives the hardware.

 
Send your comments regarding this page using support@corollarytheorems.com 
Page last updated on:
November 01, 2014
SC COMPLEMENT CONTROL SRL. All rights reserved.
 
OUR CANADIAN FLAG

Valid HTML 4.01!

Page valid according to W3C