Transistors and transistor circuits Lecture (9, 10) Transistors and transistor circuits

Introduction Transistors are three terminal devices that replaced vacuum tubes. They are solid state devices that are used for… Amplification Switching Detecting Light The three terminals are the… Emitter, Base, Collector (BJT) Source, Gate, Drain (FET)

The BJT Bipolar Junction Transistors are made with n-type and p-type semiconductors. There are two types: npn and pnp. Circuit Symbols Transistor Analogy

The three layers of the sandwich are conventionally called the Collector, Base, and Emitter. A Bipolar Transistor essentially consists of a pair of PN Junction Diodes that are joined back-to-back. This forms a sort of a sandwich where one kind of semiconductor is placed in between two others. There are therefore two kinds of Bipolar sandwich, the NPN and PNP varieties.

The BJT – Bipolar Junction Transistor The Two Types of BJT Transistors: npn pnp n p n p n p E C E C C C Cross Section Cross Section B B B B Schematic Symbol Schematic Symbol E E Collector doping is usually ~ 106 Base doping is slightly higher ~ 107 – 108 Emitter doping is much higher ~ 1015

BJT Relationships - Equations IE IC IE IC - VCE + + VEC - E C E C - - + + VBE VBC IB VEB VCB IB + + - - B B npn IE = IB + IC VCE = -VBC + VBE pnp IE = IB + IC VEC = VEB - VCB Note: The equations seen above are for the transistor, not the circuit.

Some of the basic properties exhibited by a Bipolar Transistor are immediately recognizable as being diode-like. However, when the 'filling' of the sandwich is fairly thin some interesting effects become possible that allow us to use the Transistor as an amplifier or a switch. To see how the Bipolar Transistor works we can concentrate on the NPN variety.

What are the primary uses of transistors? Switching (Logic Gates) Amplification (Op-Amps)

Architecture of BJTs The bipolar junction transistor (BJT) is constructed with three doped semiconductor regions separated by two pn junctions Regions are called emitter, base and collector

Architecture of BJTs There are two types of BJTs, the npn and pnp The two junctions are termed the base-emitter junction and the base-collector junction The term bipolar refers to the use of both holes and electrons as charge carriers in the transistor structure In order for the transistor to operate properly, the two junctions must have the correct dc bias voltages the base-emitter (BE) junction is forward biased(>=0.7V for Si, >=0.3V for Ge) the base-collector (BC) junction is reverse biased

FIGURE Transistor symbols.

Basic circuits of BJT

Operation of BJTs BJT will operates in one of following four region Cutoff region (for digital circuit) Saturation region (for digital circuit) Linear (active) region (to be an amplifier) Breakdown region (always be a disaster)

Operation of BJTs

DC Analysis of BJTs Transistor Currents: IE = IC + IB alpha (DC) IC = DCIE beta (DC) IC = DCIB DC typically has a value between 20 and 200

Transistor Equations Alpha Factor Current Gain Kirchhoff’s Current Rule

DC Analysis of BJTs DC voltages for the biased transistor: Collector voltage VC = VCC - ICRC Base voltage VB = VE + VBE for silicon transistors, VBE = 0.7 V for germanium transistors, VBE = 0.3 V

DC  and DC   = Common-emitter current gain  = Common-base current gain The relationships between the two parameters are: Note:  and  are sometimes referred to as dc and dc because the relationships being dealt with in the BJT are DC.

BJT Example Using Common-Base NPN Circuit Configuration C Given: IB = 50  A , IC = 1 mA Find: IE ,  , and  Solution: IE = IB + IC = 0.05 mA + 1 mA = 1.05 mA = IC / IB = 1 mA / 0.05 mA = 20  = IC / IE = 1 mA / 1.05 mA = 0.95238  could also be calculated using the value of  with the formula from the previous slide. IC VCB + _ IB B VBE IE + _ E

BJT Transconductance Curve Typical NPN Transistor 1 Collector Current: IC =  IES eVBE/VT Transconductance: (slope of the curve) gm = IC / VBE IES = The reverse saturation current of the B-E Junction. VT = kT/q = 26 mV (@ T=300K)  = the emission coefficient and is usually ~1 IC 8 mA 6 mA 4 mA 2 mA VBE 0.7 V

Modes of Operation Active: Saturation: Cutoff: Most important mode of operation Central to amplifier operation The region where current curves are practically flat Saturation: Barrier potential of the junctions cancel each other out causing a virtual short Current reduced to zero Ideal transistor behaves like an open switch Cutoff: * Note: There is also a mode of operation called inverse active, but it is rarely used.

Three Types of BJT Biasing Biasing the transistor refers to applying voltage to get the transistor to achieve certain operating conditions. Common-Base Biasing (CB) : input = VEB & IE output = VCB & IC Common-Emitter Biasing (CE): input = VBE & IB output = VCE & IC Common-Collector Biasing (CC): input = VBC & IB output = VEC & IE

Common-Base Although the Common-Base configuration is not the most common biasing type, it is often helpful in the understanding of how the BJT works. Emitter-Current Curves IC Active Region IE Saturation Region Cutoff IE = 0 VCB

Orange = Actual IC (IC’) The Early Effect (Early Voltage) IC Note: Common-Emitter Configuration IB -VA VCE Green = Ideal IC Orange = Actual IC (IC’)

Early Effect Example Given: The common-emitter circuit below with IB = 25A, VCC = 15V,  = 100 and VA = 80. Find: a) The ideal collector current b) The actual collector current Circuit Diagram a) IC VCE + _ VCC IB b)

Breakdown Voltage The maximum voltage that the BJT can withstand. BVCEO = The breakdown voltage for a common-emitter biased circuit. This breakdown voltage usually ranges from ~20-1000 Volts. BVCBO = The breakdown voltage for a common-base biased circuit. This breakdown voltage is usually much higher than BVCEO and has a minimum value of ~60 Volts. Breakdown Voltage is Determined By: The Base Width Material Being Used Doping Levels Biasing Voltage

BJT as an amplifier Class A Amplifiers Class B Amplifiers

BJT Class A Amplifiers In a class A amplifier, the transistor conducts for the full cycle of the input signal (360°) used in low-power applications The transistor is operated in the active region, between saturation and cutoff saturation is when both junctions are forward biased the transistor is in cutoff when IB = 0 The load line is drawn on the collector curves between saturation and cutoff

BJT Class A Amplifiers

BJT Class A Amplifiers Three biasing mode for class A amplifiers common-emitter (CE) amplifier common-collector (CC) amplifier common-base (CB) amplifier

BJT Class A Amplifiers A common-emitter (CE) amplifier capacitors are used for coupling ac without disturbing dc levels

BJT Class A Amplifiers A common-collector (CC) amplifier The Common-Collector biasing circuit is basically equivalent to the common-emitter biased circuit except instead of looking at IC as a function of VCE and IB we are looking at IE. Also, since voltage gain  ~ 1, but current gain is greater than 1 and  = IC/IE that means IC~IE Emitter-Current Curves

BJT Class A Amplifiers The third configuration is the common-base (CB) the base is the grounded (common) terminal the input signal is applied to the emitter output signal is taken off the collector output is in-phase with the input voltage gain is greater than 1 current gain is always less than 1

BJT Class B Amplifiers When an amplifier is biased such that it operates in the linear region for 180° of the input cycle and is in cutoff for 180°, it is a class B amplifier A class B amplifier is more efficient than a class A In order to get a linear reproduction of the input waveform, the class B amplifier is configured in a push-pull arrangement The transistors in a class B amplifier must be biased above cutoff to eliminate crossover distortion

BJT Class B Amplifiers

The BJT as a Switch When used as an electronic switch, a transistor normally is operated alternately in cutoff and saturation A transistor is in cutoff when the base-emitter junction is not forward-biased. VCE is approximately equal to VCC When the base-emitter junction is forward-biased and there is enough base current to produce a maximum collector current, the transistor is saturated

The BJT as a Switch