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Chapter 4 – Bipolar Junction Transistors (BJTs)

Introduction

http://engr.calvin.edu/PRibeiro_WEBPAGE/courses/engr311/311_frames.html

Physical Structure and Modes of Operation

A simplified structure of the npn transistor.

Physical Structure and Modes of Operation

A simplified structure of the pnp transistor.

Physical Structure and Modes of Operation

Mode EBJ CBJ

Active Forward Reverse

Cutoff Reverse Reverse

Saturation Forward Forward

Operation of The npn Transistor Active Mode

Current flow in an npn transistor biased to operate in the active mode, (Reverse current components due to drift of thermally generated minority carriers are not shown.)

Operation of The npn Transistor Active Mode

Profiles of minority-carrier concentrations in the base and in the emitter of an npn transistor operating in the active mode; vBE 0 and vCB 0.

Operation of The npn Transistor Active Mode

The Collector Current

The Base Current

Physical Structure and Modes of Operation

Equivalent Circuit Models

Large-signal equivalent-circuit models of the npn BJT operating in the active mode.

The Constant n

- The Collector-Base Reverse Current
- The Structure of Actual Transistors

The pnp Transistor

Current flow in an pnp transistor biased to operate in the active mode.

The pnp Transistor

Two large-signal models for the pnp transistor operating in the active mode.

The Graphical Representation of the Transistor Characteristics

The Graphical Representation of the Transistor Characteristics

Temperature Effect (10 to 120 C)

Dependence of ic on the Collector Voltage

The iC-vCB characteristics for an npn transistor in the active mode.

Dependence of ic on the Collector Voltage – Early Effect

VA – 50 to 100V

(a) Conceptual circuit for measuring the iC-vCE characteristics of the BJT. (b) The iC-vCEcharacteristics of a practical BJT.

The Transistor As An Amplifier

(a) Conceptual circuit to illustrate the operation of the transistor of an amplifier.

(b) The circuit of (a) with the signal source vbe eliminated for dc (bias) analysis.

The Collector Current and The Transconductance

The Base Current and the Input Resistance at the Base

The Emitter Current and the Input Resistance at the Emitter

The Transistor As An Amplifier

Linear operation of the transistor under the small-signal condition: A small signal vbe with a triangular waveform is superimpose din the dc voltage VBE. It gives rise to a collector signal current ic, also of triangular waveform, superimposed on the dc current IC. Ic = gm vbe, where gm is the slope of the ic - vBE curve at the bias point Q.

Small-Signal Equivalent Circuit Models

Two slightly different versions of the simplified hybrid- model for the small-signal operation of the BJT. The equivalent circuit in (a) represents the BJT as a voltage-controlled current source ( a transconductance amplifier) and that in (b) represents the BJT as a current-controlled current source (a current amplifier).

Small-Signal Equivalent Circuit Models

Two slightly different versions of what is known as the T model of the BJT. The circuit in (a) is a voltage-controlled current source representation and that in (b) is a current-controlled current source representation. These models explicitly show the emitter resistance rerather than the base resistance r featured in the hybrid- model.

Fig.4.30 Example 4.11: (a) circuit; (b) dc analysis; (c) small-signal model; (d) small-signal analysis performed directly on the circuit.

Fig.4.34 Circuit whose operation is to be analyzed graphically.

Fig.4.35 Graphical construction for the determination of the dc base current in the circuit of Fig.4.34.

Fig. 4.36 Graphical construction for determining the dc collector current IC and the collector-to-emmiter voltage VCE in the circuit of Fig. 4.34.

Fig.4.37 Graphical determination of the signal components vbe, ib, ic, and vce when a signal component viis superimposed on the dc voltage VBB(see Fig.4.34).

Fig.4.38 Effect of bias-point location on allowable signal swing: Load-line A results in bias point QA with a corresponding VCE which is too close to VCC and thus limits the positive swing of vCE. At the other extreme, load-line B results in an operating point too close to the saturation region, thus limiting the negative swing of vCE.

Fig.4.44 The common-emitter amplifier with a resistance Re in the emitter. (a) Circuit. (b) Equivalent circuit with the BJT replaced with its T model (c) The circuit in (b) with ro eliminated.

Fig.4.45 The common-base amplifier. (a) Circuit. (b) Equivalent circuit obtained by replacing the BJT with its T model.

Fig.4.46 The common-collector or emitter-follower amplifier. (a) Circuit. (b) Equivalent circuit obtained by replacing the BJT with its T model. (c) The circuit in (b) redrawn to show that ro is in parallel with RL.(d) Circuit for determining Ro.

A General Large-Signal Model For The BJT:

The Ebers-Moll Model

ISC > ISE (2-50)

An npn resistor and its Ebers-Moll (EM) model. ISC and ISE are the scale or saturation currents of diodes DE (EBJ) and DC (CBJ).

More General – Describe Transistor in any mode of operation.

Base for the Spice model.

Low frequency only

A General Large-Signal Model For The BJT:

The Ebers-Moll Model

A General Large-Signal Model For The BJT:

The Ebers-Moll Model – Terminal Currents

A General Large-Signal Model For The BJT:

The Ebers-Moll Model – Forward Active Mode

Since vBC is negative and its magnitude

Is usually much greater than VT the

Previous equations can be approximated

as

A General Large-Signal Model For The BJT:

The Ebers-Moll Model – Normal Saturation

A General Large-Signal Model For The BJT:

The Ebers-Moll Model – Reverse Mode

Note that the currents indicated have positive values. Thus, since ic = -I2 and iE = -I1, both iC and IE will be negative. Since the roles of the emitter and collector are interchanged, the transistor in the circuit will operate in the active mode (called the reverse active mode) when the emitter-base junction is reverse-biased. In such a case

I1 = beta_R . IB

This circuit will saturate (reverse saturation mode) when the emitter-base junction becomes forward-biased.

I1/IB < beta_R

I1

IB

I2

A General Large-Signal Model For The BJT:

The Ebers-Moll Model – Reverse Saturation

We can use the EM equations to find the expression of VECSat

From this expression, it can be seen that the minimum VECSat is obtained when I1 = 0. This minimum is very close to zero.

The disadvantage of the reverse saturation mode is a relatively long turnoff time.

A General Large-Signal Model For The BJT:

The Ebers-Moll Model – Example

A General Large-Signal Model For The BJT:

The Ebers-Moll Model – Example

A General Large-Signal Model For The BJT:

The Ebers-Moll Model – Transport Model npn BJT

The transport model of the npn BJT. This model is exactly equivalent to the Ebers-Moll model. Note that the saturation currents of the diodes are given in parentheses and iTis defined by Eq. (4.117).

Basic BJT Digital Logic Inverter.

vi high (close to power supply) - vo low

vi low vo high

Basic BJT digital logic inverter.

Basic BJT Digital Logic Inverter.

Sketch of the voltage transfer characteristic of the inverter circuit of Fig. 4.60 for the case RB = 10 k, RC = 1 k, = 50, and VCC = 5V. For the calculation of the coordinates of X and Y refer to the text.

The Voltage Transfer Characteristics

(a) The minority-carrier concentration in the base of a saturated transistor is represented by line (c). (b) The minority-carrier charge stored in the base can de divided into two components: That in blue produces the gradient that gives rise to the diffusion current across the base, and that in gray results in driving the transistor deeper into saturation.

Complete Static Characteristics, Internal Impedances,

and Second-Order Effects – Common Base

Avalanche

Saturation

Slope

The ic-vcb or common-base characteristics of an npn transistor. Note that in the active region there is a slight dependence of iC on the value of vCB. The result is a finite output resistance that decreases as the current level in the device is increased.

Complete Static Characteristics, Internal Impedances,

and Second-Order Effects – Common Base

The hybrid- model, including the resistance r, which models the effect of vc on ib.

Complete Static Characteristics, Internal Impedances,

and Second-Order Effects – Common-Emitter

Common-emitter characteristics. Note that the horizontal scale is expanded around the origin to show the saturation region in some detail.

Complete Static Characteristics, Internal Impedances,

and Second-Order Effects – Common-Emitter

An expanded view of the common-emitter characteristics in the saturation region.

The Spice BJT Model and Simulation Examples

.model Q2N2222-X NPN(

Is=14.34f

Xti=3

Eg=1.11

Vaf=74.03

Bf=200

Ne=1.307

Ise=14.34f

Ikf=.2847

Xtb=1.5

Br=6.092

Nc=2

Isc=0

Ikr=0

Rc=1

Cjc=7.306p

Mjc=.3416

Vjc=.75

Fc=.5

Cje=22.01p

Mje=.377

Vje=.75

Tr=46.91n

Tf=411.1p

Itf=.6

Vtf=1.7

Xtf=3

Rb=10)

*National pid=19

case=TO18 88-09-07 bam creation

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