Chapter3
This presentation is the property of its rightful owner.
Sponsored Links
1 / 89

Chapter3 Bipolar Junction Transistor (BJT) PowerPoint PPT Presentation


  • 92 Views
  • Uploaded on
  • Presentation posted in: General

Chapter3 Bipolar Junction Transistor (BJT). Outline. Introduction Operation in the Active Mode Analysis of Transistor Circuits at DC The transistor as an Amplifier Graphical Analysis Biasing the BJT for Discrete-Circuit Design Configuration for Basic Single Stage BJT Amplifier

Download Presentation

Chapter3 Bipolar Junction Transistor (BJT)

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


Chapter 3 bipolar junction transistor bjt

Chapter3

Bipolar Junction Transistor (BJT)

SJTU Zhou Lingling


Outline

Outline

  • Introduction

  • Operation in the Active Mode

  • Analysis of Transistor Circuits at DC

  • The transistor as an Amplifier

  • Graphical Analysis

  • Biasing the BJT for Discrete-Circuit Design

  • Configuration for Basic Single Stage BJT Amplifier

  • High frequency Model

SJTU Zhou Lingling


Introduction

Introduction

  • Physical Structure

  • Circuit Symbols for BJTs

  • Modes of Operation

  • Basic Characteristic

SJTU Zhou Lingling


Physical structure

Physical Structure

A simplified structure of the npn transistor.

SJTU Zhou Lingling


Physical structure1

Physical Structure

A dual of the npn is called pnp type. This is the simplified structure of the pnp transistor.

SJTU Zhou Lingling


Circuit symbols for bjts

Circuit Symbols for BJTs

The emitter is distinguished by the arrowhead.

SJTU Zhou Lingling


Modes of operation

Modes

EBJ

CBJ

Application

Cutoff

Reverse

Reverse

Switching application in digital circuits

Saturation

Forward

Forward

Active

Forward

Reverse

Amplifier

Reverse active

Reverse

Forward

Performance degradation

Modes of Operation

SJTU Zhou Lingling


Basic characteristics

Basic Characteristics

  • Far more useful than two terminal devices (such as diodes)

  • The voltage between two terminals can control the current flowing in the third terminal. We can say that the collector current can be controlled by the voltage across EB junction.

  • Much popular application is to be an amplifier

SJTU Zhou Lingling


Operation in the active mode

Operation in the Active Mode

  • Current flow

  • Current equation

  • Graphical representation of transistor’s characteristics

SJTU Zhou Lingling


Current flow

Current Flow

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

SJTU Zhou Lingling


Collector current

Collector Current

  • Collector current is the drift current.

  • Carriers are successful excess minority carriers.

  • The magnitude of collector current is almost independent of voltage across CB junction.

  • This current can be calculated by the gradient of the profile of electron concentration in base region.

SJTU Zhou Lingling


Base current

Base Current

  • Base current consists of two components.

    • Diffusion current

    • Recombination current

  • Recombination current is dominant.

  • The value of base current is very small.

SJTU Zhou Lingling


Emitter current

Emitter Current

  • Emitter current consists of two components.

  • Both of them are diffusion currents.

  • Heavily doped in emitter region.

  • Diffusion current produced by the majority in emitter region is dominant.

SJTU Zhou Lingling


Profiles of minority carrier concentrations

Profiles of Minority-Carrier Concentrations

SJTU Zhou Lingling


Current equation

Current Equation

  • Collector current

  • Base current

  • Emitter current

SJTU Zhou Lingling


Explanation for saturation current

Explanation for Saturation Current

  • Saturation current is also called current scale.

  • Expression for saturation current:

  • Has strong function with temperature due to intrinsic carrier concentration.

  • Its value is usually in the range of 10-12A to 10-18A.

SJTU Zhou Lingling


Explanation for common emitter current gain

Explanation for Common-Emitter Current Gain

  • Expression for common –emitter current gain:

  • Its value is highly influenced by two factors.

  • Its value is in the range 50 to 200 for general transistor.

SJTU Zhou Lingling


Explanation for common base current gain

Explanation for Common-Base Current Gain

  • Expression for common –base current gain:

  • Its value is less than but very close to unity.

  • Small changes in α correspond to very large changes in β.

SJTU Zhou Lingling


Recapitulation

Recapitulation

  • Collector current has the exponential relationship with forward-biased voltage as long as the CB junction remains reverse-biased.

  • To behave as an ideal constant current source.

  • Emitter current is approximately equal to collector current.

SJTU Zhou Lingling


Graphical representation of transistor s characteristics

Graphical Representation of Transistor’s Characteristics

  • Characteristic curve relates to a certain configuration.

  • Input curve is much similar to that of the diode, only output curves are shown here.

  • Three regions are shown in output curves.

  • Early Effect is shown in output curve of CE configuration.

SJTU Zhou Lingling


Output curves for cb configuration

Output Curves for CB Configuration

SJTU Zhou Lingling


Output curves for cb configuration1

Output Curves for CB Configuration

  • Active region

    • EBJ is forward-biased, CBJ is reverse-biased;

    • Equal distance between neighbouring output curves;

    • Almost horizontal, but slightly positive slope.

  • Saturation region

    • EBJ and CBJ are not only forward-biased but also turned on;

    • Collector current is diffusion current not drift current.

    • Turn on voltage for CBJ is smaller than that of EBJ.

  • Breakdown region

    • EBJ forward-biased, CBJ reverse-biased;

    • Great voltage value give rise to CBJ breakdown;

    • Collector current increases dramatically.

SJTU Zhou Lingling


Output curves for ce configuration

Output Curves for CE Configuration

(a) Conceptual circuit for measuring the iC–vCE characteristics of the BJT.

(b) The iC–vCE characteristics of a practical BJT.

SJTU Zhou Lingling


The early effect

The Early Effect

  • Curves in active region are more sloped than those in CB configuration.

  • Early voltage.

  • Effective base width and base width modulation.

SJTU Zhou Lingling


The early effect cont d

The Early Effect(cont’d)

  • Assuming current scale remains constant, collector current is modified by this term:

  • Narrow base width, small value of Early voltage, strong effect of base width modulation, strong linear dependence of on .

SJTU Zhou Lingling


Analysis of transistor circuit at dc

Analysis of Transistor Circuit at DC

  • Equivalent Circuit Models

  • Analysis Steps

  • Examples

SJTU Zhou Lingling


Equivalent circuit models

Equivalent Circuit Models

Large-signal equivalent-circuit models of the npn BJT operating in the forward active mode. In practical DC analysis, constant voltage drop model is popular used.

SJTU Zhou Lingling


Dc analysis steps

DC Analysis Steps

  • Using simple constant-voltage drop model, assuming , irrespective of the exact value of currents.

  • Assuming the device operates at the active region, we can apply the relationship between IB, IC, and IE, to determine the voltage VCE or VCB.

  • Check the value of VCE or VCB, if

    • VC>VB(or VCE>0.2V), the assumption is correct.

    • VC<VB (or VCE<0.2V), the assumption is incorrect. It means the BJT is operating in saturation region. Thus we shall assume VCE=VCE(sat) to obtain IC. Here the common emitter current gain is defined as forced=IC/IB, we will find forced< .

SJTU Zhou Lingling


Examples

Examples

  • Example 5.4 shows the order of the analysis steps indicated by the circled numbers.

  • Example 5.5 shows the analysis of BJT operating saturation mode.

  • Example 5.6 shows the transistor operating in cutoff mode.

SJTU Zhou Lingling


Examples cont d

Examples(cont’d)

  • Example 5.7 shows the analysis for pnp type circuit. It indicates the the current is affected by ill-specified parameter β. As a rule, one should strive to design the circuit such that its performance is as insensitive to the value of β as possible.

  • Example 5.8 is the bad design due to the currents critically depending on the value of β.

  • Example 5.9 is similar to the example 5.5 except the transistor is pnp type.

SJTU Zhou Lingling


Examples cont d1

Examples(cont’d)

  • Example 5.10 shows the application of Thévenin’s theorem in calculating emitter current and so on. This circuit is the good design for the emitter is almost independent of β and temperature.

  • Example 5.11 shows the DC analysis for two stage amplifier.

  • Example 5.12 shows the analysis of the power amplifier composed of the complimentary transistors.

SJTU Zhou Lingling


The transistor as an amplifier

The Transistor as an Amplifier

  • Conceptual Circuits

  • Small-signal equivalent circuit models

  • Application of the small-signal equivalent circuit models

  • Augmenting the hybrid π model.

SJTU Zhou Lingling


Conceptual circuit

Conceptual Circuit

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

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

SJTU Zhou Lingling


Conceptual circuit cont d

Conceptual Circuit(cont’d)

With the dc sources (VBE and VCC) eliminated (short circuited), thus only the signal components are present.

Note that this is a representation of the signal operation of the BJT and not an actual amplifier circuit.

SJTU Zhou Lingling


Small signal circuit models

Small-Signal Circuit Models

  • Transconductance

  • Input resistance at base

  • Input resistance at emitter

  • Hybrid π and T model

SJTU Zhou Lingling


Transconductance

Transconductance

  • Expression

  • Physical meaning

    gm is the slope of the

    iC–vBE curve at the bias point Q.

  • At room temperature,

SJTU Zhou Lingling


Input resistance at base and emitter

Input Resistance at Base and Emitter

  • Input resistance at base

  • Input resistance at emitter

  • Relationship between these two resistances

SJTU Zhou Lingling


The hybrid model

The Hybrid- Model

  • The equivalent circuit in (a) represents the BJT as a voltage-controlled current source (a transconductance amplifier),

  • The equivalent circuit in (b) represents the BJT as a current-controlled current source (a current amplifier).

SJTU Zhou Lingling


The t model

The T Model

  • These models explicitly show the emitter resistance re rather than the base resistance rp featured in the hybrid-p model.

SJTU Zhou Lingling


Augmenting the h ybrid model

Augmenting the Hybrid- Model

  • Expression for the output resistance.

  • Output resistance represents the Early Effect(or base width modulation)

SJTU Zhou Lingling


Models for pnp type

Models for pnp Type

  • Models derived from npn type transistor apply equally well to pnp transistor with no changes of polarities. Because the small signal can not change the bias conditions, small signal models are independent of polarities.

  • No matter what the configuration is, model is unique. Which one to be selected is only determined by the simplest analysis.

SJTU Zhou Lingling


Graphical analysis

Graphical Analysis

  • Graphical construction for the determination of the dc base current in the circuit.

  • Load line intersects with the input characteristic curve.

SJTU Zhou Lingling


Graphical analysis cont d

Graphical Analysis(cont’d)

Graphical construction for determining the dc collector current IC and the collector-to-emitter voltage VCE in the circuit.

SJTU Zhou Lingling


Small signal analysis

Small Signal Analysis

Graphical determination of the signal components vbe, ib, ic, and vce when a signal component vi is superimposed on the dc voltage VBB

SJTU Zhou Lingling


Effect of bias point location on allowable signal swing

Effect of Bias-Point Location on Allowable Signal Swing

  • Load-line A results in bias point QA with a corresponding VCEwhich is too close to VCCand 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.

SJTU Zhou Lingling


Biasing in bjt amplifier circuit

Biasing in BJT Amplifier Circuit

  • Biasing with voltage

    • Classical discrete circuit bias arrangement

      • Single power supply

      • Two-power-supply

    • With feedback resistor

  • Biasing with current source

SJTU Zhou Lingling


Classical discrete circuit bias arrangement

Classical Discrete Circuit Bias Arrangement

  • by fixing VBEby fixing IB.

SJTU Zhou Lingling


Classical discrete circuit bias arrangement1

Classical Discrete Circuit Bias Arrangement

  • Both result in wide variations in IC and hence in VCE and therefore are considered to be “bad.”

  • Neither scheme is recommended.

SJTU Zhou Lingling


Classical biasing for bjts using a single power supply

Classical Biasing for BJTs Using a Single Power Supply

  • Circuit with the voltage divider supplying the base replaced with its Thévenin equivalent.

  • Stabilizing the DC emitter current is obtained by considering the negative feedback action provided by RE

SJTU Zhou Lingling


Classical biasing for bjts using a single power supply1

Classical Biasing for BJTs Using a Single Power Supply

  • Two constraints

  • Rules of thumb

SJTU Zhou Lingling


Two power supply version

Two-Power-Supply Version

  • Resistor RB can be eliminated in common base configuration.

  • Resistor RB is needed only if the signal is to be capacitively coupled to the base.

  • Two constraints should apply.

SJTU Zhou Lingling


Biasing with feedback resistor

Biasing with Feedback Resistor

  • Resistor RB provides negative feedback.

  • IEis insensitive to β provided that

  • The value of RB determines the allowable signal swing at the collector.

SJTU Zhou Lingling


Biasing using current source

Biasing Using Current Source

  • Q1and Q2are required to be identical and have high β.

  • Short circuit between Q1’s base and collector terminals.

  • Current source isn’t ideal due to finite output resistor of Q2

SJTU Zhou Lingling


Application of the small signal models

Application of the Small-Signal Models

  • Determine the DC operating point of BJT and in particular the DC collector current IC(ICQ).

  • Calculate the values of the small-signal model parameters, such as gm=IC/VT, r=/gm=VT/IB, re=/gm=VT/IE.

  • Draw ac circuit path.

  • Replace the BJT with one of its small-signal models. The model selected may be more convenient than the others in circuits analysis.

  • Determine the required quantities.

SJTU Zhou Lingling


Basic single stage bjt amplifier

Basic Single-Stage BJT Amplifier

  • Characteristic parameters

  • Basic structure

  • Configuration

    • Common-Emitter amplifier

      • Emitter directly connects to ground

      • Emitter connects to ground by resistor RE

    • Common-base amplifier

    • Common-collector amplifier(emitter follower)

SJTU Zhou Lingling


Characteristic parameters of amplifier

Characteristic Parameters of Amplifier

  • This is the two-port network of amplifier.

  • Voltage signal source.

  • Output signal is obtained from the load resistor.

SJTU Zhou Lingling


Definitions

Definitions

  • Input resistance with no load

  • Input resistance

  • Open-circuit voltage gain

  • Voltage gain

SJTU Zhou Lingling


Definitions cont d

Definitions(cont’d)

  • Short-circuit current gain

  • Current gain

  • Short-circuit transconductance

SJTU Zhou Lingling


Definitions cont d1

Definitions(cont’d)

  • Open-circuit overall voltage gain

  • Overall voltage gain

SJTU Zhou Lingling


Definitions cont d2

Definitions(cont’d)

Output resistance of amplifier proper

Output resistance

SJTU Zhou Lingling


Definitions cont d3

Definitions(cont’d)

Voltage amplifier

Voltage amplifier

Transconductance amplifier

SJTU Zhou Lingling


Relationships

Relationships

  • Voltage divided coefficient

SJTU Zhou Lingling


Basic structure

Basic Structure

Basic structure of the circuit used to realize single-stage, discrete-circuit BJT amplifier configurations.

SJTU Zhou Lingling


Common emitter amplifier

Common-Emitter Amplifier

SJTU Zhou Lingling


Common emitter amplifier1

Common-Emitter Amplifier

Equivalent circuit obtained by replacing the transistor with its hybrid-p model.

SJTU Zhou Lingling


Characteristics of ce amplifier

Characteristics of CE Amplifier

  • Input resistance

  • Voltage gain

  • Overall voltage gain

  • Output resistance

  • Short-circuit current gain

SJTU Zhou Lingling


Summary of ce amplifier

Summary of CE amplifier

  • Large voltage gain

  • Inverting amplifier

  • Large current gain

  • Input resistance is relatively low.

  • Output resistance is relatively high.

  • Frequency response is rather poor.

SJTU Zhou Lingling


The common emitter amplifier with a resistance in the emitter

The Common-Emitter Amplifier with a Resistance in the Emitter

SJTU Zhou Lingling


The common emitter amplifier with a resistance in the emitter1

The Common-Emitter Amplifier with a Resistance in the Emitter

SJTU Zhou Lingling


Characteristics of the ce amplifier with a resistance in the emitter

Characteristics of the CE Amplifier with a Resistance in the Emitter

  • Input resistance

  • Voltage gain

  • Overall voltage gain

  • Output resistance

  • Short-circuit current gain

SJTU Zhou Lingling


Summary of ce amplifier with r e

Summary of CE Amplifier with RE

  • The input resistance Rin is increased by the factor (1+gmRe)

  • The voltage gain from base to collector is reduced by the factor (1+gmRe).

  • For the same nonlinear distortion, the input signal vican be increased by the factor (1+gmRe).

  • The overall voltage gain is less dependent on the value of β.

SJTU Zhou Lingling


Summary of ce amplifier with r e1

Summary of CE Amplifier with RE

  • The reduction in gain is the price for obtaining the other performance improvements.

  • Resistor RE introduces the negative feedback into the amplifier.

  • The high frequency response is significant improved.

SJTU Zhou Lingling


Common base amplifier

Common-Base Amplifier

SJTU Zhou Lingling


Common base amplifier1

Common-Base Amplifier

SJTU Zhou Lingling


Characteristics of cb amplifier

Characteristics of CB Amplifier

  • Input resistance

  • Voltage gain

  • Overall voltage gain

  • Output resistance

  • Short-circuit current gain

SJTU Zhou Lingling


Summary of the cb amplifier

Summary of the CB Amplifier

  • Very low input resistance

  • High output resistance

  • Short-circuit current gain is nearly unity

  • High voltage gain

  • Noninverting amplifier

  • Current buffer

  • Excellent high-frequency performance

SJTU Zhou Lingling


The common collector amplifier or emitter follower

The Common-Collector Amplifier or Emitter-Follower

SJTU Zhou Lingling


The common collector amplifier or emitter follower1

The Common-Collector Amplifier or Emitter-Follower

SJTU Zhou Lingling


The common collector amplifier or emitter follower2

The Common-Collector Amplifier or Emitter-Follower

SJTU Zhou Lingling


Characteristics of cc amplifier

Characteristics of CC Amplifier

  • Input resistance

  • Voltage gain

  • Overall voltage gain

  • Output resistance

  • Short-circuit current gain

SJTU Zhou Lingling


Summary for cc amplifier or emitter follower

Summary for CC Amplifier or Emitter-Follower

  • High input resistance

  • Low output resistance

  • Voltage gain is smaller than but very close to unity

  • Large current gain

  • The last or output stage of cascade amplifier

  • Frequency response is excellent well

SJTU Zhou Lingling


Summary and comparisons

Summary and Comparisons

  • The CE configuration is the best suited for realizing the amplifier gain.

  • Including RE provides performance improvements at the expense of gain reduction.

  • The CB configuration only has the typical application in amplifier. Much superior high-frequency response.

  • The emitter follower can be used as a voltage buffer and exists in output stage of a multistage amplifier.

SJTU Zhou Lingling


Internal capacitances of the bjt and high frequency model

Internal Capacitances of the BJT and High Frequency Model

  • Internal capacitance

    • The base-charging or diffusion capacitance

    • Junction capacitances

      • The base-emitter junction capacitance

      • The collector-base junction capacitance

  • High frequency small signal model

  • Cutoff frequency and unity-gain frequency

SJTU Zhou Lingling


The base charging or diffusion capacitance

The Base-Charging or Diffusion Capacitance

  • Diffusion capacitance almost entirely exists in forward-biased pn junction

  • Expression of the small-signal diffusion capacitance

  • Proportional to the biased current

SJTU Zhou Lingling


Junction capacitances

Junction Capacitances

  • The Base-Emitter Junction Capacitance

  • The collector-base junction capacitance

SJTU Zhou Lingling


The high frequency hybrid model

The High-Frequency Hybrid- Model

  • Two capacitances Cπand Cμ , where

  • One resistance rx. Accurate value is obtained form high frequency measurement.

SJTU Zhou Lingling


The cutoff and unity gain frequency

The Cutoff and Unity-Gain Frequency

  • Circuit for deriving an expression for

  • According to the definition, output port is short circuit

SJTU Zhou Lingling


The cutoff and unity gain frequency cont d

The Cutoff and Unity-Gain Frequency(cont’d)

  • Expression of the short-circuit current transfer function

  • Characteristic is similar to the one of first-order low-pass filter

SJTU Zhou Lingling


The cutoff and unity gain frequency cont d1

The Cutoff and Unity-Gain Frequency (cont’d)

SJTU Zhou Lingling


  • Login