Chapter 4 bipolar junction transistors
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Chapter 4 Bipolar Junction Transistors. Objectives. Describe the basic structure of the bipolar junction transistor (BJT). Explain and analyze basic transistor bias and operation. Discuss the parameters and characteristics of a transistor and how they apply to transistor circuits.

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Chapter 4 Bipolar Junction Transistors

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Chapter 4 bipolar junction transistors

Chapter 4Bipolar Junction Transistors



  • Describe the basic structure of the bipolar junction transistor (BJT)

  • Explain and analyze basic transistor bias and operation

  • Discuss the parameters and characteristics of a transistor and how they apply to transistor circuits

  • Discuss how a transistor can be used as an amplifier or a switch

  • Troubleshoot various failures typical of transistor circuits



A transistor is a device that can be used to either switch flow on/offor “control flow” device.

Let’s first consider its operation in a simpler view as a flow switching device.








1 - open

2 - closed





On/Off Flow



Let’s first consider its operation in a simpler view as a current controlling










Variable Flow






Transistor structure

Transistor Structure

With diodes there is one p-n junction. With bipolar junction transistors (BJT), there are three layers forming two p-n junctions. Transistors can be either pnp or npn type.

Bjt construction

Physical construction of a BJT is deposited layers of “doped “ N and P semiconductor materials.

BJT Construction

Note: Epitaxial films may be grown from gaseous or liquid precursors. Because the substrate acts as a seedcrystal, the deposited film takes on a lattice structure and orientation identical to those of the substrate. This is different from other thin-film deposition methods which deposit polycrystalline or amorphous films, even on single-crystal substrates. If a film is deposited on a substrate of the same composition, the process is called homoepitaxy; otherwise it is called heteroepitaxy.

Basic transistor operation

Basic Transistor Operation

  • Look at this one circuit as two separate circuits:

  • The base-emitter (left side) circuit and

  • The collector-emitter (right side) circuit.

  • Note that the emitter leg serves as a conductor for both circuits.

  • The amount of current flow in the base-emitter circuit controls the amount of current that flows in the collector circuit.

  • Small changes in the transistor base-emitter current yields a large change in collector-current.

IE = IC + IB

Basic transistor operation1

Basic Transistor Operation

Below, is the proper bias configuration for the npn and the pnp transistor. Note that base-emitter junction is forward-biased, while the base-collector junction is reverse-biased (known as forward-reverse bias). For the npn, the forward-bias BE, narrows the BE junction depletion zone, while the reverse-bias BC depletion zone grows larger. Let’s examine this more closely (next slide).

IC = ß IB

Forward-Reverse bias of a bipolar transistor.

Basic transistor operation conventional flow

Basic Transistor Operation - Conventional Flow

  • Looking at a forward-reverse bias NPN

  • Forward-bias BE narrows “Depletion Zone” (see forward-biased diode).

  • Reverse-bias BC widens “Depletion Zone”.

  • Emitter (doped) region is “rich” with free (conduction-band) electrons.

  • These “conduction-band” electrons easily migrate across the BE junction to become “minority carriers” in the base.

  • Since there are few “holes, only a few recombine to form a small “valence electron current” thru RB to BE supply.

  • The “free electrons” are attracted across the BC junction into the Collector and thru to the +C supply as Ic (collector current).

  • The PNP transistor follows a similar logic.




“valence electron” current









Transistor currents conventional flow

Transistor Currents (Conventional Flow)

IE = IC + IB Note relative current sizes.

Regions of operation

Regions of operation

  • Applied voltages Mode

  • E < B < C Forward active

  • E < B > C Saturation

  • E > B < C Cut-off

  • Forward active: base higher than emitter, collector higher than base (in this mode the collector current is proportional to base current by βF).

  • Saturation: base higher than emitter, but collector is not higher than base.

  • Cut-Off: base lower than emitter, but collector is higher than base. It means the transistor

  • is not letting conventional current to go through collector to emitter.

Chapter 4 bipolar junction transistors

  • In terms of junction biasing: ('reverse biased base–collector junction' means Vbc < 0 for NPN, opposite for PNP)

  • Forward-active (or simply, active): The base–emitter junction is forward biased and the base–collector junction is reverse biased. Most bipolar transistors are designed to afford the greatest common-emitter current gain, βF, in forward-active mode. If this is the case, the collector–emitter current is approximately proportional to the base current, but many times larger, for small base current variations.

  • Saturation: With both junctions forward-biased, a BJT is in saturation mode and facilitates high current conduction from the emitter to the collector. This mode corresponds to a logical "on", or a closed switch.

  • Cutoff: In cutoff, biasing conditions opposite of saturation (both junctions reverse biased) are present. There is very little current, which corresponds to a logical "off", or an open switch.

Chapter 4 bipolar junction transistors

  • There are basically three possible ways to connect a Bipolar Transistor within an electronic

  • circuit with each method of connection responding differently to its input signal as the static characteristics of the transistor vary with each circuit arrangement.

  • 1. Common Base Configuration - has Voltage Gain but no Current Gain.

  • 2. Common Emitter Configuration - has both Current and Voltage Gain.

  • 3. Common Collector Configuration - has Current Gain but no Voltage Gain

Transistors characteristics and parameters

Transistors Characteristics and Parameters

For proper operation, the base-emitter junction is forward-biased by VBBand conducts just like a diode. IB is set.

The collector-base junction is reverse biased by VCC and blocks current flow through it’s junction just like a diode.

Current flow through the base-emitter junction will help establish the path for current flow from the collector to emitter.



IC + IB results in IE



Transistor characteristics and parameters

Transistor Characteristics and Parameters

Analysis of this transistor circuit to predict the dc voltages and currents requires use of Ohm’s law, Kirchhoff’s voltage law and the ß for the transistor.

Determine the base BIAS current.

Using Kirchhoff’s voltage law, subtract the 0.7VBE from VBB and the remaining voltage is dropped across RB. Determining the current for the base with this information is a matter of applying of Ohm’s law. VRB/RB = IB (ohm’s law)

The collector DC current is determined by multiplying the base current by beta.ICdc =  IBdc

orDC = IC/IB


0.7 VBE will be used in most analysis examples.

Transistor characteristics and parameters1

Transistor Characteristics and Parameters

What we ultimately determine by use of Kirchhoff’s voltage law for series circuits is that; In the base circuit, VBB is distributed across the base-emitter junction and RB in the base circuit.

In the collector circuit we determine that VCC is distributed proportionally across RCand the transistor (VCE).

Transistor characteristics and parameters2

Transistor Characteristics and Parameters

  • SmallΔIDCbase-emitteryieldsLargeΔIDCcollector-emitter

  • This relationship of current change (collector to base)

  • (ΔIDCcollector-base)is calledbeta. (  )

DC = IC/IB (Current Gain)

Data sheets will refer to hybrid (h) parameters hFE = DC (typical hFEvalues 20 – 200)

  • The relationship of current change (collector to emitter)

  • (ΔIDCcollector-emitter) is called alpha.(∝)

  • ∝DC = IC/ IE is always >1.

  • (Typical ∝ values0.95 – 0.99+)

See Ex. 4-1

Transistor characteristics and parameters3

Transistor Characteristics and Parameters

There are three key dc voltages and three key dc currents to be considered. Note that these measurements are important for troubleshooting.

Relationships of Key Parameters:

VBE= 0.7 V.




IB: dc base current

IE: dc emitter current

IC: dc collector current

VBE: dc voltage across base-emitter junction

VCB: dc voltage across collector-base junction

VCE: dc voltage from collector to emitter

See Ex 4-2

Collector characteristic curves

Collector Characteristic Curves

Collector characteristic curves give a graphical illustration of the relationship of: Ic (collector current), and VCE (Voltage-Collector to Emitter) with specified amounts of IB(base current).

With greater increases of VCC , VCE continues to increase until it reaches breakdown, but the current remains about the same in the linearregion from .7V to the breakdown voltage.




Linear Region





Cutoff region



Active Region

See Ex. 4-2

Transistor characteristics curves review

Transistor Characteristics Curves - Review

1.Vcc = 0

IB is thru the base & emitter.

Transistor is in Saturation

2. VCC , VCE and Ic with increase Vcc

3. VCE exceeds 0.7V

Base/Collector junction is


Transistor operates

in Linear region

4. “breakdown” or

Avalanche region.

……. Bad!!






See Ex. 4-3

Transistor characteristics and parameters4

Transistor Characteristics and Parameters

With no IB this transistor is in the cutoff region and just as the name implies there is practically no current flow in the collector part of the circuit. With the transistor in a cutoff state the full VCC can be measured across the collector and emitter (VCE). Rc≈0V.

Transistor characteristics and parameters5

Transistor Characteristics and Parameters

Current flow in the collector part of the circuit is, determined by IB multiplied by .

However, there is a limit to how much current can flow in the collector circuit regardless of additional increases in IB.

Once this maximum is reached, the transistor is said to be in saturation. Note that saturation can be determined by application of Ohm’s law. IC(sat)=VCC/RC

The measured voltage across the now “shorted” collector and emitter is VCE = 0V.

Transistor characteristics and parameters6

Transistor Characteristics and Parameters

The dc load line graphically illustrates IC(sat)and cutoff for a transistor.


This will be the area of operation for all out transistor applications.

See Ex.4-4

Transistor characteristics and parameters more about

Transistor Characteristics and ParametersMore about β

The beta for a transistor is not always constant. Temperature and collector current both affect β. Variances in the manufacture of the transistor may also result in variable β’s.

There are also maximum power ratings to consider.

The data sheet below, provides information on these characteristics.

Note:ΔβDC for various temperatures.

Note: ΔβDCfor ΔIC

See End of Chap. Q.4-20 & 21

Transistor characteristics and parameters maximum power ratings

Transistor Characteristics and ParametersMaximum Power Ratings

  • Maximun ratings are available from datesheets and on-line.

  • Typical max. ratings address:VCE max, VCB max, VEB max, IC max & Power.

  • A primary limitation is; Ic = PD(max) / VCE rearranged: VCE = PD(max)/IC

  • Power: P= EI, P= I2R, P=E2/R

Assuming PD(MAX) is 500mW:

VCEmax is 20V, IC(MAX) is 50mA.

The graph shows the transistor can’t be operated in the “shaded“ areas.




See Ex. 4-5 & 4-6

Transistor characteristics and parameters power derating data sheets

Power Derating – PDmax

3 factors decide Power Deratings: see the previous slide.

ICmax , VCEmax & PDmax -------- IC = PD(max) / VCE

See the Datasheet for Deratings

Transistor Characteristics and ParametersPower Derating/Data Sheets

See Ex. 4-7

  • Transistor Datasheets

  • Provide:

  • Physical details

  • Absolute Maximum ratings:

    • Voltage, Current, Temperature & Power

  • Thermal characteristics

  • Electrical Characteristics – hfe, Cutoff, Breakdown voltage, Gain, Capacitance, Switching speeds, etc.

See Ex. 4-8

Bjt transistor amplifier

BJT (Transistor) Amplifier

Amplification is the process of linearly increasing the amplitude of an electrical signal.

Some designation standards and conventions:

DC Currents:IC IEIB


External DC Resistance:RERCRB

AC Currents: Ic Ie Ib

AC Voltages:Vbe Vcb Vce Vb Vc Ve

ExternalAC Resistances: ReRcRb

Internal Transistor Resistances: r ’ r ’e

Transistor switch

Transistor Switch

A transistor when used as a switch is simply being biased so that it is in cutoff (switched off) or saturation (switched on). Remember that the VCE in cutoff is VCC and 0 V in saturation.

VCE(cutoff) = VCC

IC(sat) = VCC – VCE(sat)


IB(min) = Ic (sat)

See Ex 4-10


Transistor switch1

Transistor Switch

Application: Transistor Switch

Base of transistor is “pulsed” with a “squarewave”.

“Off” periods provide no base biasing….transistor remains OFF.

“On” periods provide DC bias, switching the transistor “ON”

IB is max. Ic is max.

LED lights!!

See Ex 4-11

Transistor types packaging

Transistor Types & Packaging



Same transistor type in metal packaging

Transistor types packaging1

Transistor Types & Packaging

RF Transistors

High-density, multi-transistor packaging

Power transistors



Troubleshooting a live transistor circuit requires us to be familiar with known good voltages, but some general rules do apply. Certainly a solid fundamental understanding of Ohm’s law and Kirchhoff’s voltage and current laws is imperative. With live circuits it is most practical to troubleshoot with voltage measurements.



Opens in the external resistors or connections of the base or the circuit collector circuit would cause current to cease in the collector and the voltage measurements would indicate this.

Internal opens within the transistor itself could also cause transistor operation to cease.

Erroneous voltage measurements that are typically low are a result of point that is not “solidly connected”. This called a floating point. This is typically indicative of an open.

More in-depth discussion of typical failures are discussed within the textbook.



Testing a transistor can be viewed more simply if you view it as testing two diode junctions. Forward bias having low resistance and reverse bias having infinite resistance.



The diode test function of a multimeter is more reliable than using an ohmmeter. Make sure to note whether it is an npn or pnp and polarize the test leads accordingly.

Troubleshooting bias testing

Troubleshooting – Bias Testing



  • The bipolar junction transistor (BJT) is constructed of three regions: base, collector, and emitter.

  • The BJT has two pn junctions, the base-emitter junction and the base-collector junction.

  • The two types of transistors are pnp and npn.

  • For the BJT to operate as an amplifier, the base-emitter junction is forward-biased and the collector-base junction is reverse-biased.

  • Of the three currents IB is very small in comparison to IE and IC.

  • Beta is the current gain of a transistor. This the ratio of IC/IB.



  • A transistor can be operated as an electronics switch.

  • When the transistor is off it is in cutoff condition (no current).

  • When the transistor is on, it is in saturation condition (maximum current).

  • Beta can vary with temperature and also varies from transistor to transistor.

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