Transistors

The transistor was discovered in 1947 at Bell labouratories. Starting with 1960s, the transistor has a spectacular development, and it continues to do so. Even more, the future looks very bright for transistors, since we are moving towards "printed" electronic circuits--this means printing on paper and other materials, using special inks.

Presented here are just few instances of using transistors, few schematics, some Graphs, and a general classification.
1. Biasing PNP and NPN bipolar transistors
2. Bipolar transistors functions
3. Biasing JFET transistors
4. JFET transistors functions
5. Types of transistors

BIASING PNP AND NPN BIPOLAR TRANSISTORS

Bipolar Junction Transistors work in two modes:
1. as amplifiers
2. as digital switches in saturation/cutoff

Many designers do not understand this: bipolar transistors are current controlled electronic devices. Of course, we need specific voltages to bias a bipolar transistor, but those voltages have the polarity and the required magnitude according to the currents they need to generate.

That misunderstanding is nobody's fault, because I have many books where the issue is unclear and/or incorrectly presented. To start, let's analyze those voltage biasing, but remember that all biasing voltages are generated by the needed currents. Transistors graphs corresponding to the two functioning modes are related to I_b, I_e, and I_c, only, according to the formula:

I_e = I_b + I_c

I_e = emiter current
I_b = base current
I_c = collector current

To start let's see how we saturate transistors. Please note: the saturation/cutoff digital mode of functioning is the only situation when transistors behave similar to voltage controlled relays. However, this idea brings confusion, and any analogy to voltage control should better be avoided.

SATURATING BIPOLAR TRANSISTORS (DIGITAL MODE)
Fig1: Saturating PNP transistors Red trace is Base voltage Blue trace is voltage in point A (The graph is transposed downwards for exemplification and voltage variation is in fact from +5V to 0V)

NOTE:
PNP transistors are saturated when the current Base-Emitter is maximum. Due to the PNP special bias requirements, that happens when Base voltage is 0 V in above picture.

PNP transistors are in cutoff state when the current Base-Emitter is 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 case presented above Collector's voltage is a fixed value: zero volts.

Fig 2: Saturating NPN transistors Red trace is Base voltage (The graph is transposed downwards for exemplification and voltage variation is in fact from 0V to +5V) Blue trace is voltage in point A. Note that in this case collector's voltage is +10 V

NOTE:
NPN transistors are saturated when the current Base-Emitter is maximum. Due to the NPN bias requirements, that happens when Base voltage is greater than 0.7 V--otherwise there is no current flow.

NPN transistors are in cutoff state when the current Base-Emitter is minimum (zero). That happens when 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 few good methods to bias the transistor and they are listed them further down, but only for the NPN transistor. For PNP you should reverse polarities.

BIPOLAR NPN TRANSISTORS BIASING
	Fig 3: Base Biasing This schematic is very much dependant on transistor's β. If β changes with temperature, the output will also change. The formulas used to calculate this schematic are: Ib = (+5V - Vbe) / Rb Ic = (+5V - Vbe) / (Rb / β) +5V is Vc
	Fig 4: Collector Feedback Biasing This schematic is similar to the above one.
	Fig 5: Universal Biasing This schematic is more stabile with β change, and it is also one of the most used one. The formulas used to calculate this schematic are: Ie = (Vre / Re) Ie = (Vrb2 -Vbe) / Re Ic = Ie
	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 1
Always remember that PNP 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 2
Always remember NPN 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) = I_c / I_b
α (Alpha) = I_c / I_e
I_e = (β - 1) * I_b
I_e = I_c + I_b

ATTENTION
The junction Base-to-Emitter is directly biased, while the junction Base-to-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-to-Collector or Collector-to-Base.

BIPOLAR TRANSISTORS FUNCTIONS

We have already seen the saturation and cutoff states (digital mode). That defines bipolar transistors as being perfect current controlled switches, which is their main function. That also makes them DC logic elements, and they are the building bricks of all logic ICs (and processors).

Bipolar transistors (BJT) have nicer switching characteristics than the MOS-FET ones. The Isolated Gate Bipolar Transistors (IGBT) have the nicest switching characteristics--no ringing, or the minimum amount possible.

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, and I will present them 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
Fig 7: Simplified Common-Emitter amplifier Red trace is the In FM (Frequency Modulation) signal (transposed down) Blue trace is the amplified Out signal This schematic is used for: * voltage gain * current gain * power gain.
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: * voltage gain * power gain
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: * current gain * power gain

BIASING JFET TRANSISTORS

FET (Field Effect Transistor) is 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 few years later by IGFET (Isolated Gate Field Effect Transistor) which was later named MOS-FET (Metal Oxide Semiconductor Field Effect Transistor).

The MOS technology is fairly cheap and nicely 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. They are also more problematic to bias.

Before working with transistors (BJT or FET), you need to study their output curve. For that you have to get their Data Sheet. Particularly to FET, their output curve it is fairly complex, and I do not present it here. You need to get one, because it is possible I will make few (unexplained) references to it.

FET are voltage controlled devices.

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

BIASING N-CHANNEL JFET
	Fig 10: Self Biased N-JFET We use the following formulas to calculate the voltages and currents: Id = Idss[1-(Vgs/Vgsoff)]2 Where Idss, and Vgsoff are given in Data Sheet. Id = Drain to Source current Vrd = Id * Rd Vrs = Id * Rs Vd = Vdd - Vrd Vds = Vd - Vs Vgs = Vg - Vs
	Fig 11: Universally Biased N-JFET We use the following formulas to calculate the voltages and currents: Id = Idss[1-(Vgs/Vgsoff)]2 Where Idss, and Vgsoff are given in Data Sheet. In addition: Vrg2 = (Vdd * Rg2) / (Rg2 + Rg1) Vrd = Id * Rd Vrs = Id * Rs Vd = Vdd - Vrd Vds = Vd - Vs Vgs = Vg - Vs
	Fig 12: Two Power Supply Biased N-JFET We use the following formulas to calculate the voltages and currents: Id = Idss[1-(Vgs/Vgsoff)]2 Where Idss, and Vgsoff are given in Data Sheet. In addition: Vrd = Id * Rd Vrs = Id * Rs Vd = Vdd - Vrd Vds = Vd - Vs Vgs = Vg - Vs

When working with FET transistors is, it is always a good idea test their output curve first before using them. FETs are second order semiconductors and it is not easy to control them. Usually, use one or two FET functions, like switching and variable resistors, and use a simulator program, or a test stand to discover the right values needed for the biasing resistors.

The test stand looks, in principle, like this:

Fig 13: Test stand used to discover the right biasing resistors required by various FET configurations Once you calculate and measure proper voltages and currents, you should implement one of the above biasing schematics. It is easy to find the right 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

When you need analog or digital FET switches in your application, please consider one of the biasing schematics presented above. The voltage-controlled resistor function I will let it for you to discover. I will present only the amplification function.

N-CHANNEL JFET AMPLIFICATION TECHNIQUES
Fig 14: Common Source JFET amplifier Red trace is the In FM (Frequency Modulation) signal (transposed down) Blue trace is the amplified Out signal
Fig 15: Common Drain JFET amplifierRed trace is the In FM (Frequency Modulation) signal (transposed down) Blue trace is the amplified Out signal
Fig 16: Common Gate JFET amplifier Red trace is the amplified Out signal Blue trace is the In FM (Frequency Modulation) signal (transposed down)