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Basic BJT Amplifier Configurations

There are plenty of texts around on basic electronics, so this is a very brief look at the

three basic ways in which a bipolar junction transistor (BJT) can be used. In each

case, one terminal is common to both the input and output signal. All the circuitsshown here are without bias circuits and power supplies for clarity.

Common Emitter Configuration

Here the emitter terminal is common to both the input and output signal. Thearrangement is the same for a PNP transistor. Used in this way the transistor has the

advantages of a medium input impedance, medium output impedance, high voltage

gain and high current gain.

Common Base Configuration

Here the base is the common terminal. Used frequently for RF applications, this stage

has the following properties. Low input impedance, high output impedance, unity (or

less) current gain and high voltage gain.

Common Collector Configuration


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This last configuration is also more commonly known as the emitter follower. This isbecause the input signal applied at the base is "followed" quite closely at the emitter

with a voltage gain close to unity. The properties are a high input impedance, a very

low output impedance, a unity (or less) voltage gain and a high current gain. Thiscircuit is also used extensively as a "buffer" converting impedances or for feeding or

driving long cables or low impedance loads.

Transistor Configuration Comparison Chart(see Sedra & Smith and "Detailed Analysis" below)

AMPLIFIER

TYPECOMMO

N BASE

COMMO

N

EMITTER

COMMON

EMITTER

(Emitter

Resistor)

COMMON

COLLECTOR

(Emitter Follower)

INPUT/OUTPU

T PHASE

RELATIONSHIP

0 180 180 0

VOLTAGE GAIN

HIGH MEDIUM MEDIUM LOW

CURRENT GAINLOW

E

MEDIUM MEDIUM

F

HIGH

POWER GAIN LOW HIGH HIGH MEDIUM

INPUT LOW MEDIUM MEDIUM HIGH


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RESISTANCE

OUTPUT

RESISTANCE

HIGH MEDIUM MEDIUM LOW

Detailed Analysis

Common or Grounded Emitter Amplifier (actual circuit configuration)

CE Amplifier Small-Signal Equivalent Circuit

To analyze this configuration, we first set down the complete nodal equations:

Using the relationship , the nodal equations can be rewrite in a more

homogeneous form:


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Eliminating vo from the last two nodal equations we find that

and if we substitute this expression into the first nodal equation we find that

Finally, substituting these two expressions into the second nodal equation we find

the following expression for the voltage gain:

y When this expression reduces to

y When but it reduces to


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Common or Grounded Collector Amplifier (actual circuit configuration)

CE ("Emitter-Follower") Amplifier Small-Signal Equivalent Circuit

Again to analyze this configuration, we first set down the complete nodal

equations:

Again using the relationship , the nodal equations can be rewrite in a

more homogeneous form:

Substituting the second nodal equation into the first we find the following

expression for the voltage gain:


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A "trickly" calculation is required to obtain the output impedance. To do so we

first shut off the input voltage and then apply test voltage source, vx , to the

output terminal. Under these circumstances, the current into the output

terminal is given by:

Therefore, the relatively low output impedance is given by:

while the relatively high input impedance is given by:

Small-signal modelFrom Wikipedia, the free encyclopedia

(Redirected from Small signal)

Small-signal modeling is a common analysis technique in electrical engineering which is used to approximate

the behavior ofnonlinear devices with linear equations. This linearization is formed about the DCbias point of

the device (that is, the voltage/current levels present when no signal is applied), and can be accurate for small

excursions about this point.


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Motivation

Many electronic circuits, such as radio receivers, communications, and signal processing circuits,

generally carry small time-varying signals on top of a constant bias. This suggests using a method akin to

approximation by differentials to analyze relatively small perturbations about the bias point.

Any nonlinear device which can be described quantitatively using a formula can then be 'linearized' about

a bias point by taking partial derivatives of the formula with respect to all governing variables. These

partial derivatives can be associated with physical quantities (such

as capacitance, resistance and inductance), and a circuit diagram relating them can be formulated. Small-

signal models exist forelectron tubes, diodes, field-effect transistors (FET) and bipolar transistors, notably

the hybrid-pi model and various two-port networks.

[edit]Variable notation

Large-signal DC quantities are denoted by uppercase letters with uppercase subscripts. For example,

the DC input bias voltage of a transistor would be denoted VIN.

Small-signal quantities are denoted using lowercase letters with lowercase subscripts. For example,

the input signal of a transistor would be denoted as vin.

Total quantities, combining both small-signal and large-signal quantities, are denoted using lower

case letters and uppercase subscripts. For example, the total input voltage to the aforementioned

transistor would bev

IN(t) =

VIN +

vin(

t).

[edit]Example: PN junction diodes

Main article: Diode modelling#Small-signal modelling

The (large-signal) Shockley equation for a diode can be linearized about the bias point or quiescent point

(sometimes called Q-point) to find the small-signal conductance, capacitance and resistance of the diode.

This procedure is described in more detail underdiode modeling, which provides an example of the

linearization procedure followed in all small-signal models of semiconductor devices.

[edit]Differences between Small Signal and Large Signal

A small signal model takes a circuit and based on an operating point (bias) it linearizes all the

components. Nothing changes because the assumption is that the signal is so small that the operating

point (gain, capacitance etc) doesn't change.


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A large signal model on the other hand takes into account the fact that the large signal actually affects the

operating point and takes into account that elements are non-linear and that circuits can be limited by

power supply values. A small signal model ignores supply values.

Hybrid-pi modelFrom Wikipedia, the free encyclopedia

The hybrid-pi model is a popularcircuit model used for analyzing the small signal behavior of bipolar junction

and field effect transistors. The model can be quite accurate for low-frequency circuits and can easily be

adapted for higher frequency circuits with the addition of appropriate inter-electrode capacitances and other

parasitic elements.

Bipolar junction (BJT) parameters

The hybrid-pi model is a linearized two-port network approximation to the BJT using the small-signal

base-emitter voltage vbe and collector-emitter voltage vce as independent variables, and the small-signal

base current ib and collector current ic as dependent variables.[1]

Figure 1: Simplified, low-frequency hybrid-pi BJT model.

A basic, low-frequency hybrid-pi model for the bipolar transistoris shown in figure 1. The various

parameters are as follows.

is the transconductance in siemens, evaluated in a simple model[2]

where:

is the quiescent collector current (also called the collector bias or DC collector current)

is the thermal voltage, calculated from Boltzmann's constantk, the charge of an

electronq, and the transistor temperature in kelvins, T. At 295 K (approximately room

temperature) VT is about 25 mV (Google calculator).


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in ohms

where:

is the current gain at low frequencies (commonly called hFE). Here IB is the Q-

point base current. This is a parameter specific to each transistor, and can be found on a

datasheet; is a function of the choice of collector current.

is the output resistance due to the Early

effect (VA is the Early voltage).

[edit]Related terms

The reciprocal of the output resistance is named the output conductance

.

The reciprocal ofgm is called the intrinsic resistance

.

[edit]MOSFET parameters

Figure 2: Simplified, low-frequency hybrid-piMOSFET model.

A basic, low-frequency hybrid-pi model for the MOSFET is shown in figure 2. The

various parameters are as follows.


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is the transconductance in siemens, evaluated in the Shichman-Hodges model in terms

of the Q-point drain current ID by (see Jaeger and Blalock[3]

):

,

where:

ID is the quiescent drain current (also called the drain bias or DC drain current)

Vth = threshold voltage and VGS = gate-to-source voltage.

The combination:

often is called the overdrive voltage.

is the output resistance due to channel length

modulation, calculated using the Shichman-Hodges model as

,

using the approximation for the channel length

modulation parameter [4]

.

Here VE is a technology-related parameter (about 4 V/m for

the 65 nm technology node[4]

) and L is the length of the source-to-

drain separation.

The reciprocal of the output resistance is named the drain

conductance

.

AnalogElectronics_Lecture4_PartB_low and

high frequency model of CE BJT. Module by:Bijay_Kumar Sharma. E-mail the author Summary: Part B of Lecture 4 derives the Hybrid-pi model of CE BJT from T-model of CB BJT at low and

high frequencies.


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AnalogElectronics_Lecture4_PartB_low and high frequency model of CE BJT.

INCREMENTAL MODEL OF CE BJT FROM T MODEL OF CB BJT

Figure 1

Figure 11a. Low Frequency Incremental T Model of CB BJT.

Let us re-orient this as CE configuration.

Figure 2

Figure 11b. The reoriented T Model to represent CE BJT.


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It should be noticed that in T-Model under normal orientation has controlled current( fie) coming out of

collector node since base current is coming out of base node. But in reoriented T Model controlled

current( fie) coming into collector node since base current is coming into base node.

Input Mesh Equation:

Figure 3

Figure 4

Figure 5

Where gm = trans conductance = IC/VT whereas re = VT/ IE and IC= FIE

Figure 6

The output current is =

Figure 7

Here


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Figure 8

.

Figure 9

Figure 12. Bode Plot of beta and alpha and location of and .

Beta cutoff frequency = and alpha cutoff frequency = .

Cut-off frequency = -3dB frequency

= this is the frequency where parameter falls to 0.707 of its flat band value or midband value

= corner frequency

= half power frequency


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Figure 10

Figure 13. Low Frequency Hybrid- Model of CE BJT.

Figure 11

This is due to EARLY EFFECT or due to Base Width Modulation. A parameter Early Voltage VA is used for

determining the output impedance of the hybrid Model. This output impedance is 1/hoe . The definition of

Early Voltage VA is given in Figure 14.


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Figure 12

Figure 13

Figure 14. The definition of Early Voltage for a CE BJT.

COMPARISON BETWEEN HYBRID-pi MODEL OF CE BJT AND T MODEL OF CB BJT

Figure 14


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Figure 15

T-model of CB BJT Hybrid-pi Model of CE BJT

Unilateral model Non Unilateral model

hrb=TABLE 1

In CB BJT,if we consider base spreading resistance to be zero then

Figure 16


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Figure 17

For CE BJT if we consider r to be infinity then:

Figure 18

Figure 19


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Figure 20

Incremental model at high frequency


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Figure 21

Figure 16. High frequency T-Model of CB BJT.

Figure 22


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Figure 23

Figure 24

Here t is the transit t ime taken by the minority carriers to cross the base width. The mechanism of transit

is both diffusion and drift.


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Now let us consider CE BJT at high frequency:

Here f(short circuit current gain in CE BJT) is arrived at in exactly the same manner as fwas arrived at

in CB BJT.

Figure 25

Figure 17. High Frequency Hybrid- Model of CE BJT.


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Figure 26

Figure 27

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