TRANSISTOR

INTRODUCTION TO TRANSISTORS  The discovery of the first transistor in 1948 by a team of physicists at the Bell Telephone Laboratories sparked an interest in solid-state research that spread rapidly.  Transistor demonstrated amplification in solids possible.  Before the transistor, amplification was achieved only with electron tubes.  In many cases, transistors are more desirable than tubes coz they are small, rugged, require no filament power, and operate at low voltages with comparatively high efficiency.  The development of transistors made possible the miniaturization of electronic circuits.

INTRODUCTION TO TRANSISTORS Transistors have infiltrated virtually every area of science and industry, from the family car to satellites, Military etc. The ever increasing uses for transistors have created an urgent need for sound and basic information regarding their operation.

TRANSISTOR FUNDAMENTALS Semiconductor devices that have-three or more elements are called TRANSISTORS. The term transistor was derived from the words TRANSfer and resISTOR Term adopted coz it best describes the operation of the transistor - the transfer of an input signal current from a low-resistance circuit to a high- resistance circuit. Basically, the transistor is a solid-state device that amplifies by controlling the flow of current carriers through its semiconductor materials.

TRANSISTOR FUNDAMENTALS  Same theory to explain operation of transistor used with the PN-junction diode.  The three elements of the two-junction transistor are (1) the EMITTER, which gives off, or emits," current carriers (electrons or holes); (2) the BASE, which controls the flow of current carriers; (3) the COLLECTOR, which collects the current carriers.

NPN Transistor Bias

The voltage on the collector must be more positive than the base: In summary, the base of the NPN transistor must be positive with respect to the emitter, and the collector must be more positive than the base.

Introduction Recall forward-biased(FB) PN junction is EQU to a low- resistance circuit element. A reverse-biased (RB) PN junction is EQU to a high- resistance circuit element. Ohm's law formula for power(P = I 2 R) & assuming current is held constant, can conclude that the power developed across a high resistance is greater than that developed across a low resistance. If crystal were to contain two PN junctions (one FB and the other RB), a low-power signal could be injected into the FB junction and produce a high-power signal at the RB junction = Power Gain across Crystal This concept, is the basic theory behind how the transistor amplifies.

NPN FORWARD-BIASED JUNCTION Important point to bring out at this time is the fact that the N material on one side of the forward-biased junction is more heavily doped than the P material. This results in more current being carried across the junction by the majority carrier electrons from the N material than the majority carrier holes from the P material. Therefore, conduction through the forward- biased junction is mainly by majority carrier electrons from the N material (emitter)

The forward biased junction in the NPN Transistor

NPN FORWARD-BIASED JUNCTION With the emitter-to-base junction biased in the forward direction, electrons leave the negative terminal of the battery and enter the N material (emitter). Since electrons are majority current carriers in the N material, they pass easily through the emitter, cross over the junction, and combine with holes in the P material (base). For each electron that fills a hole in the P material, another electron will leave the P material (creating a new hole) and enter the positive terminal of the battery.

NPN REVERSE-BIASED JUNCTION The second PN junction (base-to-collector), or reverse-biased junction as it is called, blocks the majority current carriers from crossing the junction. There is a very small current that does pass through this junction. This current is called minority current, or reverse current. This current was produced by the electron-hole pairs. The minority carriers for the reverse-biased PN junction are the electrons in the P material and the holes in the N material. These minority carriers actually conduct the current for the reverse-biased junction when electrons from the P material enter the N material, and the holes from the N material enter the P material. However, the minority current electrons play the most important part in the operation of the NPN transistor.

Reverse biased junction in the NPN Transistor

NPN REVERSE-BIASED JUNCTION At this point you may wonder why the second PN junction (base-to-collector) is not forward biased like the first PN junction (emitter-to-base). If both junctions were forward biased, the electrons would have a tendency to flow from each end section of the N P N transistor (emitter and collector) to the center P section (base). In essence, we would have two junction diodes possessing a common base, thus eliminating any amplification and defeating the purpose of the transistor. A word of caution is in order at this time. If you should mistakenly bias the second PN junction in the forward direction, the excessive current could develop enough heat to destroy the junctions, making the transistor useless. Therefore, be sure your bias voltage polarities are correct before making any electrical connections.

NPN JUNCTION INTERACTION

The bias batteries in the above figure have been labeled V CC for the collector voltage supply, and V BB for the base voltage supply. Also notice the base supply battery is quite small, as indicated by the number of cells in the battery, usually 1 volt or less. However, the collector supply is generally much higher than the base supply, normally around 6 volts. This difference in supply voltages is necessary to have current flow from the emitter to the collector.

Common–Base Current Gain  The ratio of collector current to emitter current is called α, which is also named as h-parameter h FB.  This parameter is commonly known as common base gain. α = I C /I E (2)  The typical value of α ranges from from 0.95 to  For a good transistor, its α value is close to one.  If the collector leakage current is included then the collector current is given by  I C = αI E + I CBO (3)  The ratio of collector current to base current is β, which also denoted as h parameter-h FE.  This parameter is commonly known as common emitter gain. β = I C /I B (4)

The Common Emitter Configuration. As well as being used as a semiconductor switch to turn load currents "ON" or "OFF" by controlling the Base signal to the transistor in ether its saturation or cut-off regions, NPN Transistors can also be used in its active region to produce a circuit which will amplify any small AC signal applied to its Base terminal with the Emitter grounded. If a suitable DC "biasing" voltage is firstly applied to the transistors Base terminal thus allowing it to always operate within its linear active region, an inverting amplifier circuit called a single stage common emitter amplifier is produced.

The Common Emitter Configuration One such Common Emitter Amplifier configuration of an NPN transistor is called a Class A Amplifier. A "Class A Amplifier" operation is one where the transistors Base terminal is biased in such a way as to forward bias the Base-emitter junction. The result is that the transistor is always operating halfway between its cut-off and saturation regions, thereby allowing the transistor amplifier to accurately reproduce the positive and negative halves of any AC input signal superimposed upon this DC biasing voltage. Without this "Bias Voltage" only one half of the input waveform would be amplified. This common emitter amplifier configuration using an NPN transistor has many applications but is commonly used in audio circuits such as pre-amplifier and power amplifier stages.

The Common Emitter Configuration A family of curves known as the Output Characteristics Curves, relates (Ic) to (Vce) when different values of (Ib) are applied to the transistor for transistors with the same β value. A DC "Load Line" to show all the possible operating points when different values of base current are applied. It is necessary to set the initial value of Vce correctly to allow the output voltage to vary both up and down when amplifying AC input signals and this is called setting the operating point or Quiescent Point, Q-point.

Single Stage Common Emitter Amplifier Circuit

Output Characteristics Curves for a Typical Bipolar Transistor

Common-Emitter CE Configuration The most important factor to notice is the effect of Vce upon the collector current Ic when Vce is greater than about 1.0 volts. We can see that Ic is largely unaffected by changes in Vce above this value and instead it is almost entirely controlled by the base current, Ib. When this happens we can say then that the output circuit represents that of a "Constant Current Source".

By using the output characteristics curves in our example above and also Ohm´s Law, the current flowing through the load resistor, (RL), is equal to the collector current, Ic entering the transistor which inturn corresponds to the supply voltage, (Vcc) minus the voltage drop between the collector and the emitter terminals, (Vce) and is given as:

Common-Emitter CE Configuration Transistor connected with emitter as the common or ground is called common-emitter configuration as shown in Fig below

dc Analysis C-E Config. there are three currents and three voltages, which are base current I B, emitter current I E, collector current I C, base-to-emitter voltage V BE, collector-to-base voltage V CB, and collector-to-emitter voltage V CE. For any other dc biasing configuration, there always have these currents and voltages.

Common-emitter current and voltage

C-E Config.

C-E Config

dc Operating Point The dc operating point is referred to Q-point (quiescent point). It is a point on the transistor characteristic curve. If one chooses collector current I C versus collector-to- emitter voltageV CE characteristics curve then Q-point is the point on the curve determined by collector current I C and collector-to-emitter voltageV CE for a fixed value of base current I B derived from the biasing of circuit. Using the transistor biasing circuit shown in Fig. overleaf, the Q-point on the characteristics curve can be determined by finding the values of I C and V CE for a given base current I B determined by the circuit. The line joining the Q-point is known as dc load line.

Biasing circuit for determining Q-point

Q-point and dc load line