BJT Biasing Electronic Devices & Circuits/Unit II (BJT Biasing) Electronics & Telecommunication Engineering

Bipolar Junction Transistors (BJT) A bipolar transistor essentially consists of a pair of PN Junction diodes that are joined back-to-back. There are therefore two kinds of BJT, the NPN and PNP varieties. The three layers of the sandwich are conventionally called the Collector, Base, and Emitter.

Modern Transistors

BJT Structure - Discrete Early BJTs were fabricated using alloying - an complicated and unreliable process. The structure contains two p-n diodes, one between the base and the emitter, and one between the base and the collector.

Circuit Symbols

How the BJT works Figure shows the energy levels in an NPN transistor under no externally applying voltages. In each of the N-type layers conduction can take place by the free movement of electrons in the conduction band. In the P-type (filling) layer conduction can take place by the movement of the free holes in the valence band. However, in the absence of any externally applied electric field, we find that depletion zones form at both PN- Junctions, so no charge wants to move from one layer to another. NPN Bipolar Transistor

How the BJT works What happens when we apply a moderate voltage between the collector and base parts. The polarity of the applied voltage is chosen to increase the force pulling the N-type electrons and P- type holes apart. This widens the depletion zone between the collector and base and so no current will flow. In effect we have reverse- biassed the Base-Collector diode junction. Apply a Collector-Base voltage

Charge Flow This 'pushes' electrons from the Emitter into the Base region and sets up a current flow across the Emitter-Base boundary. Once the electrons have managed to get into the Base region they can respond to the attractive force from the positively- biassed Collector region. Hence a Emitter-Collector current magnitude is set by the chosen Emitter- Base voltage applied. Hence an external current flowing in the circuit. Apply an Emitter-Base voltage

Charge Flow As a result, the Base region loses one of its positive charges (holes). The Base potential would become more negative (because of the removal of the holes) until it was negative enough to repel any more electrons from crossing the Emitter- Base junction. Some electron fall into a hole

Charge Flow For most practical BJT only about 1% of the free electrons which try to cross Base region get caught in this way. Hence a Base current, I B, which is typically around one hundred times smaller than the Emitter current, I E. Some electron fall into a hole

Terminals & Operations Three terminals:  Base (B): very thin and lightly doped central region (little recombination ).  Emitter (E) and collector (C) are two outer regions sandwiching B. Normal operation (linear or active region):  B-E junction forward biased; B-C junction reverse biased.  The emitter emits (injects) majority charge into base region and because the base very thin, most will ultimately reach the collector.  The emitter is highly doped while the collector is lightly doped.  The collector is usually at higher voltage than the emitter.

Terminals & Operations

Operation Mode

Active:  Most importance mode, e.g. for amplifier operation.  The region where current curves are practically flat. Saturation:  Barrier potential of the junctions cancel each other out causing a virtual short.  Ideal transistor behaves like a closed switch. Cutoff:  Current reduced to zero  Ideal transistor behaves like an open switch.

Operation Mode

BJT in Active Mode Operation  Forward bias of EBJ injects electrons from emitter into base (small number of holes injected from base into emitter)  Most electrons shoot through the base into the collector across the reverse bias junction (think about band diagram)  Some electrons recombine with majority carrier in (P-type) base region

Circuit Configuration

I-V Characteristics Base-emitter junction looks like a forward biased diode Collector-emitter is a family of curves which are a function of base current.

I-V Characteristics

Early Effect  Current in active region depends (slightly) on v CE  V A is a parameter for the BJT (50 to 100) and called the Early voltage  Due to a decrease in effective base width W as reverse bias increases

Early Effect  Increasing V CB causes depletion region of CBJ to grow and so the effective base width decreases (base-width modulation)  Shorter effective base width  higher dn/dx

BJT Electronic Devices & Circuits/Unit II/Biasing (BJT)

Introduction BJTs amplifier requires a knowledge of both the DC analysis (large signal) and AC analysis (small signal). For a DC analysis a transistor is controlled by a number of factors including the range of possible operating points. Once the desired DC current and voltage levels have been defined, a network must be constructed that will establish the desired operating point.

Operating Point For transistor amplifiers the resulting dc current and voltage establish an operating point on the characteristics that define the region that will be employed for amplification of the applied signal. Operating point  quiescent point or Q-point The biasing circuit can be designed to set the device operation at any of these points or others within the active region.

Various operating points within the limits of operation of a transistor Q-point A: I=0A, V=0V Not suitable for transistor to operate Q-point B: The best operating point for linear gain and largestpossible voltage and current It is a desired condition for a small signal analysis Q-point C: Concern on nonlinearities due to I B curves is rapidly changesin this region.

Load-Line Analysis Refer to figure below (output loop) one straight line can be draw at output characteristics. This line is called load line.The process to plot the load line as follows

Load-Line Analysis Step 1: Refer to circuit, V CE =V CC – I C R C (1) Choose I C =0 mA. Subtitute into (1), we get V CE =V CC (2)  located at X axis Step 2: Choose V CE =0V and subtitute into (1), we get I C =V CC /R C (3)  located at Y-axis Step 3: Joining two points defined by (2) + (3), we get straight line that can be drawn as Fig.

Load-Line Analysis

Fixed-Bias Circuit

Forward Bias of Base-Emitter Write KVL equation in the clockwise direction of the loop : +V CC – I B R B – V BE =0 Solving the equation for the current I B results :

Collector-Emitter Loop The magnitude of the IC is related directly to IB through I C =βI B Apply KVL in the clockwise direction around the indicated close loop results: V CE +I C R C -V CC =0 V CE = V CC -I C R C

Emitter Bias The DC bias network below contains an emitter resistor to improve the stability level of fixed-bias configuration

Base-Emitter Loop

Collector-Emitter Loop

Voltage Divider Bias Two method for analyzed the voltage-divider bias configuration: - Exact method - Approximate method

Exact Analysis Step 1: The input side of the network can be redrawn for DC analysis. Step 2: Analysis of Thevenin equivalent network to the left of base terminal

Approximate Analysis Step 1:  R E  10R 2 Step 2: The input section can be represented by the network of figure below and R 2 can be considered in series by assuming I 1  I 2 and I B = 0A.

Bias stabilization Stability of a system is a measure of the sensitivity of a network to variation in its parameter.  β increases with increase in temperature  V BE decreases 7.5mV every degree celcius  I CO doubles every 10 o C increase in temperature

Stability factors

thermal runaway as a transistor heats, its junction temperature increases. This increases the collector current, which forces the junction temperature to increase further, producing more collector current, etc., until the transistor is destroyed. Another definition of thermal runaway: as a transistor heats, its junction temperature increases. This increases the collector current, which forces the junction temperature to increase further, producing more collector current, etc., until the transistor is destroyed.