BJT can operate as a switch by utilizing its cut-off and saturation modes. In cut-off mode, the transistor is off with no collector current. In saturation mode, the transistor is on with low resistance between collector and emitter.
The small signal model of BJT represents the transistor as an equivalent circuit for analysis when a small signal is applied. It models the transistor as a voltage controlled current source with input resistance rπ at the base and re at the emitter.
A BJT amplifier circuit can be analyzed using the small signal model by first finding the DC operating point, then replacing the BJT with its small signal model and analyzing the resulting circuit to determine voltage gain, input and output imped
discussing differences faithful and un- faithful amplification
discussing stabilition in transistors
and how temperature affect collector current
discussing various methods of transistor biasing like
Base resister method ,Emitter Base method , Biasing with collector feedback method , Voltage divider bias
This document provides an overview of bipolar junction transistors (BJT) including:
- The basic structure of an NPN or PNP transistor with emitter, base, and collector layers.
- How transistors operate in cutoff, saturation, and active modes depending on biasing of the PN junctions.
- How a small base current controls a larger collector current in the active region, allowing transistors to function as electronic switches or amplifiers.
1) CE amplifiers use bias circuits to set an operating point for the transistor. Simple bias circuits vary with transistor parameters like beta.
2) Load line analysis graphs the interplay between circuit constraints and the transistor output characteristic to determine the operating point.
3) Stabilized bias circuits aim to fix the emitter current independently of beta using an emitter resistor and potential divider bias network. Negative feedback bias circuits also use feedback from the collector voltage.
This document discusses Bipolar Junction Transistors (BJTs) and their operating regions. It describes the basic components and operation of NPN and PNP BJTs. The three main operating regions for BJTs are discussed: active, cutoff, and saturation regions. Equations for calculating operating points in the active region are provided. The document also discusses biasing techniques, including using a four-resistor network to provide stable biasing and prevent variations in the operating point due to changes in transistor characteristics. An example calculation of bias points is included.
The document discusses bipolar junction transistors (BJTs) and their operation. It begins by introducing the npn and pnp transistor structures, which contain three doped regions: emitter, base, and collector. It describes the active, cutoff, and saturation regions of operation. The document then provides mathematical expressions to relate the emitter, base, and collector currents based on the common-emitter configuration. It also discusses the common-base current gain and common-emitter current gain. Examples are provided to calculate different transistor parameters like beta, alpha, and currents. Finally, the common-emitter configuration and its current-voltage characteristics are summarized.
Understand the “magic” of negative feedback and the characteristics of ideal op amps.
Understand the conditions for non-ideal op amp behavior so they can be avoided in circuit design.
Demonstrate circuit analysis techniques for ideal op amps.
Characterize inverting, non-inverting, summing and instrumentation amplifiers, voltage follower and first order filters.
Learn the factors involved in circuit design using op amps.
Find the gain characteristics of cascaded amplifiers.
Special Applications: The inverted ladder DAC and successive approximation ADC
This chapter discusses small-signal modeling and linear amplification using transistors. The goals are to understand transistors as linear amplifiers, small-signal models, and amplifier characteristics. A simple common-emitter BJT amplifier circuit is presented and analyzed using DC and AC equivalent circuits. Key points include defining the Q-point, constructing small-signal models, and calculating voltage gain. Capacitor selection criteria are provided to maintain linearity in the amplifier.
discussing differences faithful and un- faithful amplification
discussing stabilition in transistors
and how temperature affect collector current
discussing various methods of transistor biasing like
Base resister method ,Emitter Base method , Biasing with collector feedback method , Voltage divider bias
This document provides an overview of bipolar junction transistors (BJT) including:
- The basic structure of an NPN or PNP transistor with emitter, base, and collector layers.
- How transistors operate in cutoff, saturation, and active modes depending on biasing of the PN junctions.
- How a small base current controls a larger collector current in the active region, allowing transistors to function as electronic switches or amplifiers.
1) CE amplifiers use bias circuits to set an operating point for the transistor. Simple bias circuits vary with transistor parameters like beta.
2) Load line analysis graphs the interplay between circuit constraints and the transistor output characteristic to determine the operating point.
3) Stabilized bias circuits aim to fix the emitter current independently of beta using an emitter resistor and potential divider bias network. Negative feedback bias circuits also use feedback from the collector voltage.
This document discusses Bipolar Junction Transistors (BJTs) and their operating regions. It describes the basic components and operation of NPN and PNP BJTs. The three main operating regions for BJTs are discussed: active, cutoff, and saturation regions. Equations for calculating operating points in the active region are provided. The document also discusses biasing techniques, including using a four-resistor network to provide stable biasing and prevent variations in the operating point due to changes in transistor characteristics. An example calculation of bias points is included.
The document discusses bipolar junction transistors (BJTs) and their operation. It begins by introducing the npn and pnp transistor structures, which contain three doped regions: emitter, base, and collector. It describes the active, cutoff, and saturation regions of operation. The document then provides mathematical expressions to relate the emitter, base, and collector currents based on the common-emitter configuration. It also discusses the common-base current gain and common-emitter current gain. Examples are provided to calculate different transistor parameters like beta, alpha, and currents. Finally, the common-emitter configuration and its current-voltage characteristics are summarized.
Understand the “magic” of negative feedback and the characteristics of ideal op amps.
Understand the conditions for non-ideal op amp behavior so they can be avoided in circuit design.
Demonstrate circuit analysis techniques for ideal op amps.
Characterize inverting, non-inverting, summing and instrumentation amplifiers, voltage follower and first order filters.
Learn the factors involved in circuit design using op amps.
Find the gain characteristics of cascaded amplifiers.
Special Applications: The inverted ladder DAC and successive approximation ADC
This chapter discusses small-signal modeling and linear amplification using transistors. The goals are to understand transistors as linear amplifiers, small-signal models, and amplifier characteristics. A simple common-emitter BJT amplifier circuit is presented and analyzed using DC and AC equivalent circuits. Key points include defining the Q-point, constructing small-signal models, and calculating voltage gain. Capacitor selection criteria are provided to maintain linearity in the amplifier.
Transistors can be used as switches or amplifiers. The document discusses the basics of bipolar transistors including their structure, operation, and different configurations (common base, common emitter, common collector). It provides examples of calculating currents and voltages in transistor circuits using characteristics curves and explains how different classes of amplifiers (A, B, AB, C) determine the portion of the input signal cycle during which the transistor is active.
Bipolar Junction Transistors (BJTs) are three-terminal semiconductor devices that use both holes and electrons to conduct current. There are two types, NPN and PNP, which are constructed from alternating layers of N-type and P-type semiconductor material. BJTs can be used as amplifiers and switches by applying forward or reverse bias to the base-emitter and base-collector junctions. Key parameters specified in datasheets include maximum voltage, current, power dissipation, and current gain (beta). Proper biasing is required to operate the BJT in its active region for amplification applications.
Bipolar Junction Transistors (BJTs) are three-terminal semiconductor devices that use both holes and electrons to conduct current. There are two types, NPN and PNP, distinguished by the order of semiconductor layers. BJTs can be used as amplifiers and switches by applying forward or reverse bias to the base-emitter and base-collector junctions. Key parameters include current gain (beta), maximum voltage and current ratings, and power dissipation limits. BJTs are commonly configured as common-base, common-emitter, or common-collector amplifiers depending on the terminal used as the signal reference.
The document discusses the bipolar junction transistor (BJT). It begins by introducing the BJT, noting it has three terminals (collector, base, emitter) and exists in NPN and PNP types. It then discusses BJT construction, noting it consists of two N-P or P-N junctions. The document covers BJT operation, biasing, and the three common configurations - common-base, common-emitter, and common-collector. It discusses input/output characteristics and key parameters like alpha, beta, and power dissipation limits. In summary, the document provides an overview of BJT fundamentals, including its construction, operation, configurations, and performance parameters.
DC Biasing – Bipolar Junction Transistors (BJTs)ssuserb29892
This document discusses various methods of biasing bipolar junction transistors (BJTs) for proper operation, including voltage-divider bias, emitter bias, and collector feedback bias. It explains that transistors must be biased to establish a stable operating point between cutoff and saturation. Various bias circuits are analyzed using Kirchhoff's laws and the transistor model. Key aspects of establishing bias, such as determining the quiescent point and load lines, as well as sources of instability and techniques to improve stability, are covered. Examples are provided to illustrate calculating important bias parameters.
DC biasing applies fixed voltages to transistors to place them in an operating region for amplification. The operating point defines the transistor's quiescent operating conditions under DC. Stability refers to a circuit's insensitivity to parameter variations like temperature. Emitter-stabilized and voltage divider biasing improve stability over fixed biasing by incorporating an emitter or voltage divider resistor. Feedback biasing further increases stability by introducing negative feedback from collector to base.
This document discusses AC models and analysis of amplifiers. It introduces the concepts of capacitive coupling and bypassing for AC signals while blocking DC. It explains how to model transistors using the T model or p model for small signal AC analysis by replacing the transistor with its equivalent resistor and current source. It provides guidelines for amplifier analysis including performing a DC analysis first, replacing capacitors with shorts for AC, and using supply voltages as AC grounds to derive the AC equivalent circuit. Data sheet parameters like hfe, hie, bac, and re' are also defined for transistor modeling.
Transistors are semiconductor devices with three terminals - collector, base, and emitter. There are two main types, NPN and PNP, which differ in the doping of the semiconductor regions. Bipolar transistors can operate as amplifiers, switches, or oscillators depending on the biasing conditions. The common emitter configuration provides voltage gain but inverts the phase, while common base and common collector configurations do not invert phase. Transistor gain is determined by factors like current gain (beta) and varies based on operating point and temperature.
This document discusses transistor biasing circuits. It outlines different types of biasing circuits including fixed bias, emitter bias, collector feedback bias, and voltage divider bias. It explains how each circuit works, analyzing key parameters like collector current, collector-emitter voltage, and stability factor. The goal of biasing circuits is to set the transistor's operating point in the active region for faithful signal amplification while stabilizing it against temperature and other variations. Voltage divider bias is highlighted as the most widely used method due to its ability to stabilize the operating point through negative feedback.
The document discusses unijunction transistors (UJTs) and programmable unijunction transistors (PUTs). It describes their equivalent circuit models, characteristics such as intrinsic standoff ratio and peak voltage, and applications in relaxation oscillators and timing circuits. Examples are provided to illustrate how to calculate values like standoff voltage from given specifications. PUTs offer the advantage over UJTs of having programmable parameters set by external resistors.
The document discusses the modes of operation and analysis of a differential amplifier (DA). It contains the following key points:
1. A DA can operate in either differential mode or common mode. In differential mode, two input signals are 180 degrees out of phase, while in common mode they are in phase.
2. The transfer characteristic of a DA relates its differential input voltage (Vd) to the collector currents (IC1 and IC2) of its transistors. This shows the linear operating region of the circuit.
3. Techniques like using a constant current source or current mirror in place of the emitter resistor (RE) can improve the common mode rejection ratio (CMRR) of a DA
1. The document discusses bipolar junction transistors (BJTs), including their construction, operation, and uses. BJTs are made of n-type and p-type semiconductors and have three terminals - emitter, base, and collector.
2. There are two types of BJTs - npn and pnp. BJTs operate in different regions including cutoff, saturation, linear/active, and breakdown. Key equations relate currents and voltages at the terminals.
3. BJTs are used for amplification, switching, and detecting light. They can be configured in common-emitter, common-base, or common-collector circuits and operated in classes A or B for
Class 12th Solids and semiconductor devices part 3Arpit Meena
This document discusses NPN and PNP transistors. It describes the symbols, operation, and characteristics of transistors in common base and common emitter configurations. The key points are:
1. NPN transistors contain an N-type semiconductor sandwiched between two P-type semiconductors. PNP transistors contain a P-type semiconductor sandwiched between two N-type semiconductors.
2. In common base configuration, the input and output signals are in phase. In common emitter configuration, the input and output signals are 180 degrees out of phase.
3. Important gains include current gain (α or β), voltage gain, power gain, and transconductance.
This document discusses oscillators and their various types. It begins with an introduction to oscillators and their characteristics. It then describes different types of linear oscillators, including Wien bridge, RC phase-shift, and LC oscillators. It also discusses oscillator stability and applications such as generating signals for receivers, transmitters, and digital clocks. Specific oscillator circuits like Colpitts and Hartley are analyzed.
The document discusses integrated circuits and operational amplifiers. It begins by defining an integrated circuit and listing its advantages. It then describes the two main types of integrated circuits - linear and digital ICs. The document goes on to explain operational amplifiers in detail, including their ideal characteristics, block diagram, equivalent circuit model, open-loop configurations, and applications. It also provides information about specific op-amps like the 741 and TL082, discussing their features, input and bias currents, and common mode rejection ratio.
The document provides information about bipolar junction transistors (BJTs), including:
1) BJTs have three doped semiconductor regions (emitter, base, collector) separated by two pn junctions and operate using both holes and electrons.
2) For a BJT to operate as an amplifier, the base-emitter junction must be forward-biased and the base-collector junction must be reverse-biased.
3) Changes in base current cause much larger changes in collector current, allowing BJTs to amplify signals.
Transistors can be used as switches or amplifiers. The document discusses the basics of bipolar transistors including their structure, operation, and different configurations (common base, common emitter, common collector). It provides examples of calculating currents and voltages in transistor circuits using characteristics curves and explains how different classes of amplifiers (A, B, AB, C) determine the portion of the input signal cycle during which the transistor is active.
Bipolar Junction Transistors (BJTs) are three-terminal semiconductor devices that use both holes and electrons to conduct current. There are two types, NPN and PNP, which are constructed from alternating layers of N-type and P-type semiconductor material. BJTs can be used as amplifiers and switches by applying forward or reverse bias to the base-emitter and base-collector junctions. Key parameters specified in datasheets include maximum voltage, current, power dissipation, and current gain (beta). Proper biasing is required to operate the BJT in its active region for amplification applications.
Bipolar Junction Transistors (BJTs) are three-terminal semiconductor devices that use both holes and electrons to conduct current. There are two types, NPN and PNP, distinguished by the order of semiconductor layers. BJTs can be used as amplifiers and switches by applying forward or reverse bias to the base-emitter and base-collector junctions. Key parameters include current gain (beta), maximum voltage and current ratings, and power dissipation limits. BJTs are commonly configured as common-base, common-emitter, or common-collector amplifiers depending on the terminal used as the signal reference.
The document discusses the bipolar junction transistor (BJT). It begins by introducing the BJT, noting it has three terminals (collector, base, emitter) and exists in NPN and PNP types. It then discusses BJT construction, noting it consists of two N-P or P-N junctions. The document covers BJT operation, biasing, and the three common configurations - common-base, common-emitter, and common-collector. It discusses input/output characteristics and key parameters like alpha, beta, and power dissipation limits. In summary, the document provides an overview of BJT fundamentals, including its construction, operation, configurations, and performance parameters.
DC Biasing – Bipolar Junction Transistors (BJTs)ssuserb29892
This document discusses various methods of biasing bipolar junction transistors (BJTs) for proper operation, including voltage-divider bias, emitter bias, and collector feedback bias. It explains that transistors must be biased to establish a stable operating point between cutoff and saturation. Various bias circuits are analyzed using Kirchhoff's laws and the transistor model. Key aspects of establishing bias, such as determining the quiescent point and load lines, as well as sources of instability and techniques to improve stability, are covered. Examples are provided to illustrate calculating important bias parameters.
DC biasing applies fixed voltages to transistors to place them in an operating region for amplification. The operating point defines the transistor's quiescent operating conditions under DC. Stability refers to a circuit's insensitivity to parameter variations like temperature. Emitter-stabilized and voltage divider biasing improve stability over fixed biasing by incorporating an emitter or voltage divider resistor. Feedback biasing further increases stability by introducing negative feedback from collector to base.
This document discusses AC models and analysis of amplifiers. It introduces the concepts of capacitive coupling and bypassing for AC signals while blocking DC. It explains how to model transistors using the T model or p model for small signal AC analysis by replacing the transistor with its equivalent resistor and current source. It provides guidelines for amplifier analysis including performing a DC analysis first, replacing capacitors with shorts for AC, and using supply voltages as AC grounds to derive the AC equivalent circuit. Data sheet parameters like hfe, hie, bac, and re' are also defined for transistor modeling.
Transistors are semiconductor devices with three terminals - collector, base, and emitter. There are two main types, NPN and PNP, which differ in the doping of the semiconductor regions. Bipolar transistors can operate as amplifiers, switches, or oscillators depending on the biasing conditions. The common emitter configuration provides voltage gain but inverts the phase, while common base and common collector configurations do not invert phase. Transistor gain is determined by factors like current gain (beta) and varies based on operating point and temperature.
This document discusses transistor biasing circuits. It outlines different types of biasing circuits including fixed bias, emitter bias, collector feedback bias, and voltage divider bias. It explains how each circuit works, analyzing key parameters like collector current, collector-emitter voltage, and stability factor. The goal of biasing circuits is to set the transistor's operating point in the active region for faithful signal amplification while stabilizing it against temperature and other variations. Voltage divider bias is highlighted as the most widely used method due to its ability to stabilize the operating point through negative feedback.
The document discusses unijunction transistors (UJTs) and programmable unijunction transistors (PUTs). It describes their equivalent circuit models, characteristics such as intrinsic standoff ratio and peak voltage, and applications in relaxation oscillators and timing circuits. Examples are provided to illustrate how to calculate values like standoff voltage from given specifications. PUTs offer the advantage over UJTs of having programmable parameters set by external resistors.
The document discusses the modes of operation and analysis of a differential amplifier (DA). It contains the following key points:
1. A DA can operate in either differential mode or common mode. In differential mode, two input signals are 180 degrees out of phase, while in common mode they are in phase.
2. The transfer characteristic of a DA relates its differential input voltage (Vd) to the collector currents (IC1 and IC2) of its transistors. This shows the linear operating region of the circuit.
3. Techniques like using a constant current source or current mirror in place of the emitter resistor (RE) can improve the common mode rejection ratio (CMRR) of a DA
1. The document discusses bipolar junction transistors (BJTs), including their construction, operation, and uses. BJTs are made of n-type and p-type semiconductors and have three terminals - emitter, base, and collector.
2. There are two types of BJTs - npn and pnp. BJTs operate in different regions including cutoff, saturation, linear/active, and breakdown. Key equations relate currents and voltages at the terminals.
3. BJTs are used for amplification, switching, and detecting light. They can be configured in common-emitter, common-base, or common-collector circuits and operated in classes A or B for
Class 12th Solids and semiconductor devices part 3Arpit Meena
This document discusses NPN and PNP transistors. It describes the symbols, operation, and characteristics of transistors in common base and common emitter configurations. The key points are:
1. NPN transistors contain an N-type semiconductor sandwiched between two P-type semiconductors. PNP transistors contain a P-type semiconductor sandwiched between two N-type semiconductors.
2. In common base configuration, the input and output signals are in phase. In common emitter configuration, the input and output signals are 180 degrees out of phase.
3. Important gains include current gain (α or β), voltage gain, power gain, and transconductance.
This document discusses oscillators and their various types. It begins with an introduction to oscillators and their characteristics. It then describes different types of linear oscillators, including Wien bridge, RC phase-shift, and LC oscillators. It also discusses oscillator stability and applications such as generating signals for receivers, transmitters, and digital clocks. Specific oscillator circuits like Colpitts and Hartley are analyzed.
The document discusses integrated circuits and operational amplifiers. It begins by defining an integrated circuit and listing its advantages. It then describes the two main types of integrated circuits - linear and digital ICs. The document goes on to explain operational amplifiers in detail, including their ideal characteristics, block diagram, equivalent circuit model, open-loop configurations, and applications. It also provides information about specific op-amps like the 741 and TL082, discussing their features, input and bias currents, and common mode rejection ratio.
The document provides information about bipolar junction transistors (BJTs), including:
1) BJTs have three doped semiconductor regions (emitter, base, collector) separated by two pn junctions and operate using both holes and electrons.
2) For a BJT to operate as an amplifier, the base-emitter junction must be forward-biased and the base-collector junction must be reverse-biased.
3) Changes in base current cause much larger changes in collector current, allowing BJTs to amplify signals.
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3. BJT as a switch:
• To operate BJT as a switch, we utilize the cut off and saturation modes of
operation.
• For Vin less than about 0.5 V, the transistor will be cut off
• Thus, IB=0, IC=0 and VCE =VCC. In this state node C is disconnected from ground
and the switch is in open position.
4. BJT as a switch:
• To operate BJT as a closed switch, it must operate in the saturation.
• Practically VCEsat ≈ 0.2 V
• So, ICsat =
VCC−VCEsat
RC
• IB =
ICsat
β
• In saturation , the switch
is closed with a low
closure resistance RCEsat
• Then Vin = IBRB +VBE
• RB =
Vin−VBE
IB
5. Numerical:
• The transistor is specified to have β in the range of 50 to 150. find the
value of RB that results in saturation with an overdrive factor of at
least 10.
• Soln:
• Icsat = 9.8 mA =
VCC
−VCEsat
RC
• IB(EOS) =
ICsat
βmin
= 0.19 mA
• Overdrive factor = 10
• IB = 1.9mA
• RB =
Vin
−VBE
IB
= 2.1 K
6. Small signal operation and models:
• The input signal to be amplified is represented by the voltage source
vbe that is superimposed on VBE. We consider first the dc bias
conditions by setting the signal vbe to zero.
• We can write following relationships for the dc current and voltage
• IC = Is e
VBE
nVT
• IE = IC/α
• IB = IC/β
7. The collector current and the transconductance
• If a signal vbe is applied , the total instantaneous base –emitter voltage
vBE becomes
• vBE = VBE +vbe
• Correspondingly the collector current becomes
• iC = Is 𝑒
𝑣𝐵𝐸
𝑛𝑉𝑇 = Is 𝑒
𝑉𝐵𝐸
+𝑣𝑏𝑒
𝑛𝑉𝑇
• iC =IC𝑒
𝑣𝑏𝑒
𝑛𝑉𝑇
• If vbe<< nVT, We may expand the exponential of equation and truncate
the series after first two terms
iC ≈ IC (1 +
v𝑏𝑒
nVT
) ( 𝑒𝑥
= {1+
𝑥
1!
+
𝑥2
2!
+
𝑥3
3!
+……..} if x<<1
8. The collector current and the transconductance
• This approximation, which is valid only for vbe less than approximately
10 mV (for n=1) is referred to as the small signal approximation.
• Then , iC = IC +
𝐼𝐶
𝑛𝑉𝑇
vbe
• But collector current is composed of the dc value IC and a signal
component ic
• ic =
𝐼𝐶
𝑛𝑉𝑇
vbe.
• It can be rewritten as
• ic= gmvbe
• Where gm is called transconductance
• gm =
IC
nVT
=
IC
VT
(if n=1)
9. Small signal …
• Above analysis suggests that for small signal , transistor behaves as a voltage
controlled current source. The input port of this controlled source is between
base and emitter and the output port is between collector and emitter.
10. The base current and the input resistance at
the base.
• To determine the resistance seen by vbe we first evaluate the total base
current iB: (taking n=1)
• iB =
𝑖𝐶
β
=
𝐼𝐶
β
+
𝐼𝐶
𝑣𝑏𝑒
β𝑉𝑇
• Comparing it with ; iB = IB+ib
• ib=
𝐼𝐶
𝑣𝑏𝑒
β𝑉𝑇
=
𝑔𝑚
𝑣𝑏𝑒
β
• The small signal input resistance
• between base and emitter,
• looking into the base is denoted by rπ
rπ =
𝑣𝑏𝑒
𝑖𝑏
=
β
𝑔𝑚
11. The emitter current and the input resistance at the
emitter
• The total emitter current iE can be determined from
• iE =
𝑖𝐶
α
=
𝐼𝐶
α
+
𝑖𝑐
α
• Thus, iE = IE + ie
• The signal current ie is given by
• ie =
𝑖𝑐
α
=
𝐼𝐶
𝑣𝑏𝑒
α𝑉𝑇
=
𝐼𝐸
𝑣𝑏𝑒
𝑉𝑇
• The small signal resistance between base and emitter, looking into the emitter is
denoted by re
re =
𝑣𝑏𝑒
𝑖𝑒
=
𝑉𝑇
𝐼𝐸
And gm =
𝐼𝐶
𝑉𝑇
So, re =
α
𝑔𝑚
≅
1
𝑔𝑚
12. Relation Between rπ and re:
• vbe = ibrπ = iere
• rπ = (ie/ib) re
• rπ = (β+1) re
• Since β>> 1
• rπ ≅ β re
16. Application of small signal model: analysis of
transistor amplifier circuit.
• Determine the dc operating point of BJT and in particular the dc collector
current IC.
• Calculate the values of small signal model parameters: gm, rπ and re
• Eliminate the dc sources by replacing each dc voltage source with a short
circuit and each d current source with an open circuit.
• Replace all capacitors by a short circuit.
• Remove all elements bypassed by the short circuit.
• Redraw the network in a more convenient and logical form
• Replace BJT with one of its small signal equivalent circuit models
• Analyze the resulting circuit to determine the required quantities ie voltage
gain, input impedance and output impedance.
18. Fixed bias …
• Input resistance (Zin):
• Zin =RB//rπ
• Voltage gain (Av):
• Vo = -gm Vπ(ro//RC)
• From circuit Vin = Vπ
• Av = Vo/Vπ = -gm(ro//RC)
• Av ≅ −gmRC (ro >> RC)
• Av = −
𝑅𝐶
𝑟𝑒
19. • Output resistance (Zo):
• Output resistance of any system is defined as the resistance Zo
determined when Vin =0. when Vin = 0 , IB = 0 and IC= 0 resulting in
open ckt equivalent for the current source.
Zo = RC//ro
20. Phase relationship:
• The negative sign in voltage gain equation reveals that a 1800 phase
shift occurs between the input and output signals.
24. Voltage gain (Av):
• Neglecting the effect of ro,
• Vo = -icRC
• Vo = -gmVπRC
• Vo = -gmRC
𝑉𝑖𝑛
1+𝑔𝑚𝑅𝐸
• Av =
𝑉𝑜
𝑉𝑖𝑛
=−
𝑔𝑚
𝑅𝐶
1+𝑔𝑚𝑅𝐸
• Av= −
𝑅𝐶
𝑟𝑒
+𝑅𝐸
25. Output resistance (Zo):
• Input is short circuited and a test voltage Vx is applied at output so
that Ix current flows in circuit.
30. Input resistance (Zin):
Zib =
𝑉𝑖𝑛
𝑖𝑏
= rπ(1+gmRE)
Zin = R1//R2//Zib
Voltage Gain (Av):
• Av =
𝑉𝑜
𝑉𝑖𝑛
=−
𝑔𝑚
𝑅𝐶
1+𝑔𝑚𝑅𝐸
• Av =−
𝑅𝐶
𝑅𝐸
+𝑟𝑒
Output resistance (Zo):
• Zoc = (1+gmRE’)ro
• Zo = Zoc//RC
31. Voltage gain of CE amplifier;
• Av = −
𝑡𝑜𝑡𝑎𝑙 𝑟𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑎𝑡 𝑐𝑜𝑙𝑙𝑒𝑐𝑡𝑜𝑟 𝑠𝑒𝑐𝑡𝑖𝑜𝑛
𝑡𝑜𝑡𝑎𝑙 𝑟𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑎𝑡 𝑒𝑚𝑖𝑡𝑡𝑒𝑟 𝑠𝑒𝑐𝑡𝑖𝑜𝑛
32. Common Collector Configuration/ emitter
follower:
• Here output is always slightly
less than input signal. The fact
that Vo follows the magnitude of
Vi with an in –phase relationship
accounts for the terminology
emitter follower.
33. • Neglecting the effect of ro
• Input resistance (Zin):
• Using KVL at input side;
• Vi = ibβre + (β+1) ib RE
• Zb =
𝑣𝑖
𝑖𝑏
=βre + (β+1) RE
• Zb ≅ β(re + RE)
• Zin = RB//Zb
34. • We can write; ie = (β+1) ib
• And ib =
𝑉𝑖
𝑍𝑏
• So, ie = (β+1)
𝑉𝑖
β(re + RE)
• ie =
𝑉𝑖
(re + RE)
• Constructing network from above
equation
• Voltage gain (Av):
• Using voltage divider rule;
• Vo = Vi
𝑅𝐸
𝑅𝐸
+𝑟𝑒
• Av =
𝑉𝑜
𝑉𝑖
=
𝑅𝐸
𝑅𝐸+𝑟𝑒
• RE is usually much greater
than re so, Av ≅ 1
• Output resistance (Zo):
• To determine Zo, Vi is set to 0
and
• Zo = RE // re
37. Input resistance (Zin): (neglecting the effect of ro)
• Using KCL at emitter node ,
• iin +ie = i
• iin = -ie +i
• iin = -
𝑣𝑏𝑒
𝑟𝑒
+
𝑉𝑖𝑛
𝑅𝐸
• From circuit ;
• Vin =-vbe, so
• iin =
𝑉𝑖𝑛
𝑟𝑒
+
𝑉𝑖𝑛
𝑅𝐸
• Zin =
𝑉𝑖𝑛
𝑖𝑖𝑛
= RE //re
• Since RE>> re
• Zin ≅ 𝑟𝑒
38. Output resistance and voltage gain:
• By taking Vin =0;
• Zo = Rc
• Vo = -αieRC
• And ie =
𝑣𝑏𝑒
𝑟𝑒
=−
𝑉𝑖𝑛
𝑟𝑒
• Av =
𝑉𝑜
𝑉𝑖𝑛
=
α𝑅𝐶
𝑟𝑒
= gm RC
39. Determining current gain
• For each transistor configuration,
the current gain can be
determined directly from the
voltage gain, the defined load and
the input impedance
• Applying Ohm’s law to the input
and output circuits results in:
• Ii =
𝑉𝑖
𝑍𝑖
and IO =−
𝑉𝑂
𝑅𝐿
• The current gain
• Ai =
𝐼𝑂
𝐼𝑖
= −
𝑉𝑂
𝑉𝑖
𝑍𝑖
𝑅𝐿
= −Av
𝑍𝑖
𝑅𝐿
• The value of RL is defined by
the location of Vo and Io.
42. Analysis:
• No load voltage gain :
• AVNL =
𝑉𝑜
𝑉𝑖
=−
𝑅𝐶
𝑟𝑒
• Voltage gain with load RL:
• R’L = ro//RC//RL ≅ RC//RL
• Vo = -βIbR’L
• Ib =
𝑉𝑖
β𝑟𝑒
• AVL =
𝑉𝑜
𝑉𝑖
=−
𝑅𝐶
∕∕𝑅𝐿
𝑟𝑒
• Input impedance (Zin) = RB//βre
• Output impedance (Zo) = RC//ro
• Voltage gain with load and source
resistance:
• Vi =
𝑍𝑖𝑛𝑉𝑠
𝑍𝑖𝑛+𝑅𝑆
•
𝑉𝑖
𝑉𝑠
=
𝑍𝑖𝑛
𝑍𝑖𝑛+𝑅𝑠
• AVS =
𝑉𝑜
𝑉𝑠
=
𝑉𝑜
𝑉𝑖
𝑉𝑖
𝑉𝑠
• AVS = AVL
𝑍𝑖𝑛
𝑍𝑖𝑛+𝑅𝑠
43. Analysis shows that :
• For the same configuration, AVNL > AVL >AVS
• For particular design, the larger the value of RL, greater is the level of
ac gain.
• For particular amplifier, the smaller the internal resistance of the
signal source, the greater is the overall gain
45. Dc Analysis:
• βRE≥10R2
• since above condition is satisfied, we can use approximate method
• VB =
𝑅2
𝑉𝐶𝐶
𝑅1
+𝑅2
=1.754 V
• Using KVL at input side
• VB= VBE + IERE
• IC≅ IE =
𝑉𝐵
−𝑉𝐵𝐸
𝑅𝐸1+
𝑅𝐸2
=1.12 mA
• gm=IC/VT= 0.0448 mho
• re = VT/IC= 22.32Ω
• rπ= βre= 3.34 KΩ
46. Ac analysis:
Input resistance (Zin):
Zib =
𝑉𝑖𝑛
𝑖𝑏
= rπ(1+gmRE1)
=73.67 K
Zin = R1//R2//Zib
=7.41 K
Voltage Gain with load (AVL):
• AVL =
𝑉𝑜
𝑉𝑖𝑛
=−
𝑔𝑚(𝑅𝐶/ 𝑅𝐿)
1+𝑔𝑚𝑅𝐸1
• AVL =−
𝑅𝑐/∕𝑅𝐿
𝑅𝐸1
+𝑟𝑒
=
• -8.67
• Overall voltage gain (AVS)
• AVS = AVL
𝑍𝑖𝑛
𝑍𝑖𝑛+𝑅𝑠
=
• -8.029
Output resistance (Zo):
• Zoc = (1+gmRE’)ro = 1945.85K
Zo = Zoc//RC = 4.68 K ; let ro = 100 K ; RE’ = (rπ+Rs)//RE1
47. Numerical:
Vcc = IBRB+VBE+IERE = IBRB +VBE +(β+1)IBRE
IB =0.0358 mA
IC =βIB =4.3 mA
gm = IC/VT = 0.172 mho
re = VT/IC = 5.813 Ω
48. rπ = βre =698.4Ω
Zib= rπ(1+gmRE) =67.96 KΩ
Zin = Zib//RB = 59.31KΩ
• Voltage gain Av =−
𝑅𝐶
𝑅𝐸
+𝑟𝑒
= -3.89
• Output impedance Zo =Zoc//RC =2.19K , let ro= 50K Ω
• Zoc = (1+gmRe’)ro =2722.83 K ; Re’ = RE// rπ = 310.8 Ω
Ai = −Av
𝑍𝑖
𝑅𝐿
= −Av
𝑍𝑖
𝑅𝑐
= 104.97
49. Numerical:
• Design voltage divider bias CE amplifier having gain of -140.
• Given VCC = +9 V and β = 295.
• With emitter by pass capacitor
• Av =- Rc/re
• Without emitter by pass capacitor
• Av = - Rc/(re+RE)
50. Solution: we are using voltage divider bias
with emitter by pass capacitor
• Av= −
𝑅𝑐
𝑟𝑒
=-140
• Assuming IC = 1 mA
• re =VT/IC = 25 Ω
• RC =3.5KΩ
• VRC = ICRC =3.5 V
• VCE = 0.5 Vcc =4.5V
• VE = Vcc-VRC-VCE = 1 V
VE = IERE = ICRE
RE= 1K
51. • Using firm biasing
• βRE≥10R2
• R2 =29.5 K
• VB = VBE + VE = 1.7 V
• VB =
𝑅2
𝑅2+𝑅1
𝑉𝐶𝐶
• R1 =126.68 K
52.
53. The Ebers –Moll (EM) Model:
• Ebers and Moll have shown that this composite model ( combination
of both active mode and reverse active mode) can be used to predict
the operation of BJT in all of its possible mode.
• Using KCL at each node:
• iE = iDE-αRiDC ……………i
• iC = -iDC +αFiDE ………….ii
• iB = (1-αF)iDE + (1-αR) iDC …………..iii
• We use diode equation to express iDE and iDC
• iDE = ISE (𝑒
𝑉𝐵𝐸
𝑉𝑇 -1) …………………iv
• iDC = ISC (𝑒
𝑉𝐵𝐶
𝑉𝑇 -1) …………………v
54. • A simple and elegant formula relates the scale currents as:
• αFISE = αRISC = IS …………………vi
• From above equations:
• iE =
𝐼𝑆
αF
(𝑒
𝑉𝐵𝐸
𝑉𝑇 -1) – IS (𝑒
𝑉𝐵𝐶
𝑉𝑇 -1)
• iC = IS (𝑒
𝑉𝐵𝐸
𝑉𝑇 -1) −
𝐼𝑆
αR
(𝑒
𝑉𝐵𝐶
𝑉𝑇 -1)
• iB =
𝐼𝑆
β𝐹
(𝑒
𝑉𝐵𝐸
𝑉𝑇 -1) +
𝐼𝑆
β𝑅
(𝑒
𝑉𝐵𝐶
𝑉𝑇 -1)
Where βF =
α𝐹
1−α𝐹
• And βR =
α𝑅
1−α𝑅
55. For active region:
• Here VBE is positive and in the range of 0.5 V to 0.8 V and VBC is
negative. So the terms containing 𝑒
𝑉𝐵𝐶
𝑉𝑇 will be negligibly small and can
be neglected to obtain:
• iE ≅
IS
αF
e
VBE
VT +IS (1−
1
αF
)
• iC ≅ IS e
VBE
VT +IS(
1
αR
-1)
• iB ≅
IS
βF
e
VBE
VT - IS(
1
βF
+
1
βR
)