BJT Bipolar Junction Transistors (BJT) Presented by D.Satishkumar Asst. Professor, Electrical & Electronics Engineering Contact No:

Introduction A transistor is a device that can be used as either an amplifier or a switch. Transistor is current controlling device. Transistors can be either npn or pnp type.

Introduction The three layers of BJT are called Emitter, Base and Collector Base is very thin compared to the other two layers Base is lightly doped. Emitter is heavily doped. Collector is moderately doped NPN – Emitter and Collector are made of N-type semiconductors; Base is P-type PNP – Emitter and Collector are P-type, Base is N-type Both types (NPN and PNP) are extensively used, either separately or in the same circuit BJT has two junctions – Emitter-Base (EB) Junction and Collector-Base (CB) Junction The device is called “bipolar junction transistor” because current is due to motion of two types of charge carriers – free electrons & holes Transistor Analogous to two diodes connected back-to-back: EB diode and CB diode

Transistor Structure In diodes there is one p-n junction. In Bipolar junction transistors (BJT), there are three layers and two p-n junctions. Note: Arrow Direction from P to N ( Like Diode)

Figure showing different transistor packages

Transistor Operation Operation of NPN transistor is discussed here For normal operation (amplifier application) – EB junction should be forward biased – CB junction should be reverse biased Depletion width at EB junction is narrow (forward biased) Depletion width at CB junction is wide (reverse biased)

Transistor- Normal Operation

When EB junction is forward biased, free electrons from emitter region drift towards base region Some free electrons combine with holes in the base to form small base current Inside the base region (p-type), free electrons are minority carriers. So most of the free electrons are swept away into the collector region due to reverse biased CB junction Three currents can be identified in BJT 1. Emitter current; 2. Base current; 3. Collector current

Current directions in NPN and PNP transistors: For both varieties: (1)

collector current has two components: α dc is the fraction of charge carriers emitted from emitter, that enter into the collector region I CBO is the reverse saturation current in CB diode (2) (3) As approximation, we can neglect ICBO in eq( 2) & (3) compared to IE and IC Hence approximate equations are: The parameter α dc is called common-base dc current gain Value of α dc is around 0.99

We know SubstitutingIn above eq, we get (4) where &

Since value of α dc is around 0.99, I CEO >> I CBO Hence approximation of eq(4) gives: Parameter βdc is called common emitter dc current gain Values of αdc and βdc vary from transistor to transistor. Both αdc and βdc are sensitive to temperature changes

Transistor Configurations BJT has three terminals For two-port applications, one of the BJT terminals needs to be made common between input and output Accordingly three configurations exist: – Common Base (CB) configuration – Common Emitter (CE) configuration – Common Collector (CC) configuration

Common Base ( CB ) configuration Base is common between input and output – Input voltage: V EB Input current: I E – Output voltage: V CB Output current: I C (Resistors are not shown here for simplicity)

CB Input characteristics – A plot of IE versus VEB for various values of VCB – It is similar to forward biased diode characteristics – As VCB is increased, IE increases only slightly – Note that second letter in the suffix is B (for base)

Input resistance r i Voltage amplification factor A V Note: Both can be determined from the CB input characteristics

CB Output characteristics – A plot of IC versus VCB for various values of IE – Three regions are identified: Active, Cutoff, Saturation

Active region : E-B junction forward biased C-B junction reverse biased I C is positive, V CB is positive I C increases with I E Cut off Region: When I E = 0, I C = I CBO I CBO is collector to base current with emitter open – Below this line we have cut-off region – Here both junctions are reverse biased Saturation Region: Region to the left of y-axis (V CB negative) Here both junctions are forward biased I C decreases exponentially, and eventually changes direction

Output resistance r o Current amplification factor A I or α ac Note: Both can be measured from output characteristics

Common Emitter configuration (Resistors are omitted for simplicity) Emitter is common between input and output – Input voltage: V BE ; Input current: I B – Output voltage: V CE ; Output current: I C

CE input characteristics Plot of I B versus V BE for various values of V CE Similar to diode characteristics As V CE is increased, I B decreases only slightly Note that second suffix is E (for emitter)

CE output characteristics – A plot of I C versus V CE for various values of IB – Three regions identified: Active, Cut-off, Saturation

– Active region: Linear region in the output characteristics E-B junction forward biased C-B junction reverse biased I C increases with I B – Cut off region: Region below I B =0 line (or I C =I CEO ) – Saturation Region: Region to the left of the vertical line V CE =V CE (sat)=0.3V ( for Silicon)

Input resistance r i Output resistance r o Voltage gain A V Current gain A I or β ac Note: All these parameters can be determined from CE characteristics

Experimental setup for determining CE characteristics

Transistor Biasing Applying external dc voltages to ensure that transistor operates in the desired region Which is the desired region? – For amplifier application, transistor should operate in active region – For switch application, it should operate in cut-off and sat.

Quiescent point (Q-point) The point we get by plotting the dc values of I C, I B and V CE (when ac input is zero) on the transistor characteristics Q-point is in the middle of active region.

Types of biasing: Fixed bias Self bias 1.Fixed bias: – Equations to consider are:

Pros: 1) Simple circuit 2) Uses very few resistors Cons: 1.Q-point is unstable If temperature increases, then β increases, and hence I CQ and V CEQ vary (effectively Q-point shifts) If the transistor is replaced with another transistor having different β value, then also Q- point shifts

Load Line Characteristics: Variation in load line with circuit parameters V CC, R C and R B

Load Line – We have: – This is an equation of straight line with points V CC /R C and V CC lying on y-axis and x-axis respectively – This line is called “Load line” because it depends on resistor R C considered as “Load” and V CC – Intersection of load line and transistor characteristic curve is the Q-point or operating point – This point is the common solution for characteristics and load line equation

Voltage divider bias or Self bias – Resistor R E connected between emitter and ground – Voltage-divider resistors R1 & R2 replace R B – Circuit can be analyzed in two methods: Exact method (using Thevenin’s theorem) Approximation method (neglecting base current)

Exact method: – Input side of self-bias (Fig. a) transformed into Thevenin’s equivalent circuit (Fig. b) where, RTH is the resistance looking into the terminals A & B (Fig. c) and VTH is given by:

Self-bias circuit redrawn with input side replaced by Thevenin’s equivalent : Since β >> 1 and (β+1)R E >> R TH Since IC is almost independent of β, Q-point is stable

Approximate analysis: – Carried out only if βR E ≥ 10R 2 IB is negligible compared to I1 and I2

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