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Chapter 3 Bipolar Junction Transistor (BJT)

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Chapter 3 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

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slide1
Chapter 3

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

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introduction
Introduction
  • Physical Structure
  • Circuit Symbols for BJTs
  • Modes of Operation
  • Basic Characteristic

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physical structure
Physical Structure

A simplified structure of the npn transistor.

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physical structure1
Physical Structure

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

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circuit symbols for bjts
Circuit Symbols for BJTs

The emitter is distinguished by the arrowhead.

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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

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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

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operation in the active mode
Operation in the Active Mode
  • Current flow
  • Current equation
  • Graphical representation of transistor’s characteristics

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current flow
Current Flow

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

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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.

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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.

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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.

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current equation
Current Equation
  • Collector current
  • Base current
  • Emitter current

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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 .

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analysis of transistor circuit at dc
Analysis of Transistor Circuit at DC
  • Equivalent Circuit Models
  • Analysis Steps
  • Examples

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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.

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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

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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.

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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.

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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.

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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.

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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.

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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.

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small signal circuit models
Small-Signal Circuit Models
  • Transconductance
  • Input resistance at base
  • Input resistance at emitter
  • Hybrid π and T model

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transconductance
Transconductance
  • Expression
  • Physical meaning

gm is the slope of the

iC–vBE curve at the bias point Q.

  • At room temperature,

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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

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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).

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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.

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augmenting the h ybrid model
Augmenting the Hybrid- Model
  • Expression for the output resistance.
  • Output resistance represents the Early Effect(or base width modulation)

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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.

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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.

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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.

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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

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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.

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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

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classical discrete circuit bias arrangement
Classical Discrete Circuit Bias Arrangement
  • by fixing VBE by fixing IB.

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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.

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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

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classical biasing for bjts using a single power supply1
Classical Biasing for BJTs Using a Single Power Supply
  • Two constraints
  • Rules of thumb

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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.

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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.

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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

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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.

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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)

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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.

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definitions
Definitions
  • Input resistance with no load
  • Input resistance
  • Open-circuit voltage gain
  • Voltage gain

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definitions cont d
Definitions(cont’d)
  • Short-circuit current gain
  • Current gain
  • Short-circuit transconductance

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definitions cont d1
Definitions(cont’d)
  • Open-circuit overall voltage gain
  • Overall voltage gain

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definitions cont d2
Definitions(cont’d)

Output resistance of amplifier proper

Output resistance

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definitions cont d3
Definitions(cont’d)

Voltage amplifier

Voltage amplifier

Transconductance amplifier

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relationships
Relationships
  • Voltage divided coefficient

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basic structure
Basic Structure

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

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common emitter amplifier
Common-Emitter Amplifier

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common emitter amplifier1
Common-Emitter Amplifier

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

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characteristics of ce amplifier
Characteristics of CE Amplifier
  • Input resistance
  • Voltage gain
  • Overall voltage gain
  • Output resistance
  • Short-circuit current gain

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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.

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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

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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.

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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.

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common base amplifier
Common-Base Amplifier

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common base amplifier1
Common-Base Amplifier

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characteristics of cb amplifier
Characteristics of CB Amplifier
  • Input resistance
  • Voltage gain
  • Overall voltage gain
  • Output resistance
  • Short-circuit current gain

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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

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characteristics of cc amplifier
Characteristics of CC Amplifier
  • Input resistance
  • Voltage gain
  • Overall voltage gain
  • Output resistance
  • Short-circuit current gain

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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

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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.

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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

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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

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junction capacitances
Junction Capacitances
  • The Base-Emitter Junction Capacitance
  • The collector-base junction capacitance

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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.

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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

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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

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