Term Paper Analog FINAL

8/3/2019 Term Paper Analog FINAL

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ON

HYBRID PI CE TRANSISTOR MODEL

SUBJECT: ANALOG CIRCUITS AND LINEAR IC

SUBJECT CODE:ECE-210

SUDMITTED TO:MR MANDEEP SINGH

SUBMITTED BY:

Name:MOHAN VERMA

Roll no. :RB6803B48

Reg.no.:10802379

Program:B.Tech(ECE)-M.B.A


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Hybrid pi CE transistor model this is the topic of my term paper . Before knowing about it we

should know the following terms:-

what is a transistor?

what is common emitter configuration?

what is hybrid pi configuration?

So, before starting the main content of my term paper I am giving overview of these basic

terms.

Transistor:- A transistor is a semiconductordevice used to amplify and

switch electronic signals. It is made of a solid piece ofsemiconductormaterial, with at

least three terminals for connection to an external circuit. A voltage or current applied to

one pair of the transistor's terminals changes the current flowing through another pair of

terminals. Because the controlled (output) powercan be much more than the controlling

(input) power, the transistor provides amplification of a signal. Some transistors are

packaged individually but many more are found embedded in integrated circuits.

The transistor is the fundamental building block of modern electronic devices, and its

presence is ubiquitous in modern electronic systems.

The essential usefulness of a transistor comes from its ability to use a small signal

applied between one pair of its terminals to control a much larger signal at another pair

of terminals. This property is called gain. A transistor can control its output in proportion

to the input signal, that is, can act as an amplifier. Or, the transistor can be used to turn

current on or off in a circuit as an electrically controlled switch, where the amount of

current is determined by other circuit elements.

The two types of transistors have slight differences in how they are used in a circuit.

A bipolar transistorhas terminals labeled base, collector, and emitter. A small current at

the base terminal (that is, flowing from the base to the emitter) can control or switch a

much larger current between the collector and emitter terminals. For a field-effect
http://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Semiconductor_devicehttp://en.wikipedia.org/wiki/Electronic_amplifierhttp://en.wikipedia.org/wiki/Electronicshttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Electric_powerhttp://en.wikipedia.org/wiki/Gainhttp://en.wikipedia.org/wiki/Integrated_circuithttp://en.wikipedia.org/wiki/Electronic_devicehttp://en.wikipedia.org/wiki/Gainhttp://en.wikipedia.org/wiki/Amplifierhttp://en.wikipedia.org/wiki/Switchhttp://en.wikipedia.org/wiki/Semiconductor_devicehttp://en.wikipedia.org/wiki/Electronic_amplifierhttp://en.wikipedia.org/wiki/Electronicshttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Electric_powerhttp://en.wikipedia.org/wiki/Gainhttp://en.wikipedia.org/wiki/Integrated_circuithttp://en.wikipedia.org/wiki/Electronic_devicehttp://en.wikipedia.org/wiki/Gainhttp://en.wikipedia.org/wiki/Amplifierhttp://en.wikipedia.org/wiki/Switchhttp://en.wikipedia.org/wiki/Semiconductor


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transistor, the terminals are labeled gate, source, and drain, and a voltage at the gate

can control a current between source and drain.

The image to the right represents a typical bipolar transistor in a circuit. Charge will flow

between emitter and collector terminals depending on the current in the base. Since

internally the base and emitter connections behave like a semiconductor diode, a

voltage drop develops between base and emitter while the base current exists. The

amount of this voltage depends on the material the transistor is made from, and is

referred to as VBE.

Common Emitter configuration:- A transistor may be connected in any one ofthree basic configurations (Fig given below)common emitter (CE), common base (CB),

and common collector (CC). The term common is used to denote the element that is

common to both input and output circuits. Because the common element is often

grounded, these configurations are frequently referred to as grounded emitter,

grounded base, and grounded collector.


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Figure :-Transistor configurations.

Each configuration, as you will see later, has particular characteristics that make itsuitable for specific applications. An easy way to identify a specific transistorconfiguration is to follow three simple steps:

Identify the element (emitter, base, or collector) to which the input signal isapplied.

Identify the element (emitter, base, or collector) from which the output signal istaken.

The remaining element is the common element, and gives the configuration itsname.

Therefore, by applying these three simple steps to the circuit in figure 2-12, we canconclude that this circuit is more than just a basic transistor amplifier. It is a common-emitter amplifier.

Common Emitter

The common-emitter configuration (CE) shown in figure 2-16 view A is the arrangementmost frequently used in practical amplifier circuits, since it provides good voltage,current, and power gain. The common emitter also has a somewhat low input


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resistance (500 ohms-1500 ohms), because the input is applied to the forward-biasedjunction, and a moderately high output resistance (30 kilohms-50 kilohms or more),because the output is taken off the reverse-biased junction. Since the input signal isapplied to the base-emitter circuit and the output is taken from the collector-emittercircuit, the emitter is the element common to both input and output.

Since you have already covered what you now know to be a common-emitter amplifier(fig. 2-12), let's take a few minutes and review its operation, using the PNP common-emitter configuration shown in figure 2-16 view A.

When a transistor is connected in a common-emitter configuration, the input signal isinjected between the base and emitter, which is a low resistance, low-current circuit. Asthe input signal swings positive, it also causes the base to swing positive with respect tothe emitter. This action decreases forward bias which reduces collector current (IC) andincreases collector voltage (making VC more negative). During the negative alternationof the input signal, the base is driven more negative with respect to the emitter. Thisincreases forward bias and allows more current carriers to be released from the emitter,which results in an increase in collector current and a decrease in collector voltage

(making VC less negative or swing in a positive direction). The collector current thatflows through the high resistance reverse-biased junction also flows through a highresistance load (not shown), resulting in a high level of amplification.

Since the input signal to the common emitter goes positive when the output goesnegative, the two signals (input and output) are 180 degrees out of phase. Thecommon-emitter circuit is the only configuration that provides a phase reversal.

The common-emitter is the most popular of the three transistor configurations becauseit has the best combination of current and voltage gain. The term GAINis used todescribe the amplification capabilities of the amplifier. It is basically a ratio of outputversus input. Each transistor configuration gives a different value of gain even thoughthe same transistor is used. The transistor configuration used is a matter of design

consideration. However, as a technician you will become interested in this output versusinput ratio (gain) to determine whether or not the transistor is working properly in thecircuit.

The current gain in the common-emitter circuit is called BETA (b). Beta is therelationship of collector current (output current) to base current (input current). Tocalculate beta, use the following formula:

(D is the Greek letter delta, it is used to indicate a small change)For example, if the input current (IB) in a common emitter changes from 75 mA to100 mA and the output current (IC) changes from 1.5 mA to 2.6 mA, the current gain (b)will be 44.


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This simply means that a change in base current produces a change in collector currentwhich is 44 times as large.

You may also see the term hfe used in place of b. The terms hfe and b are equivalentand may be used interchangeably. This is because "h fe" means: h = hybrid (meaningmixture)

f = forward current transfer ratioe = common emitter configuration

The resistance gain of the common emitter can be found in a method similar to the oneused for finding beta:

Once the resistance gain is known, the voltage gain is easy to calculate since it is equalto the current gain (b) multiplied by the resistance gain (E = bR). And, the power gain is

equal to the voltage gain multiplied by the current gain b (P = bE).

Hybrid pi configuration:-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 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 basecurrent 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.
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is the trans conductance 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 constant k,

the charge of an electron q, and the transistor temperature in kelvins, T. At

300 K (approximately room temperature) VT is about 26 mV (Google

calculator).

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 eachtransistor, 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).

Hybrid pi CE transistor model:-Since common emitter circuit is considered themost important practical configuration, we seek a CE model suitable for highfrequencies. Hybrid pi or Giacoletto common emitter transistor model is given in the fig.below. This circuit is quite simple and analyses of circuit using this model are not toodifficult and give results which are in excellent agreement with experiment at allfrequencies for which the transistor gives reasonable amplification. Furthermore, theresistive components in this circuit may be derived from the low frequency h-parameters. All parameters (resistive or capacitive) in the model are assumedfrequency invariant. Parameters may vary with the quiescent operating point, but undergiven bias conditions they are reasonably constant for small variations. For high
http://en.wikipedia.org/wiki/Transconductancehttp://en.wikipedia.org/wiki/Siemens_(unit)http://en.wikipedia.org/wiki/Quiescenthttp://en.wikipedia.org/wiki/Boltzmann's_constanthttp://en.wikipedia.org/wiki/Elementary_chargehttp://en.wikipedia.org/wiki/Kelvinhttp://www.google.com/search?hl=en&q=300+kelvin+*+k+/+elementary+charge+in+millivolts+=http://www.google.com/search?hl=en&q=300+kelvin+*+k+/+elementary+charge+in+millivolts+=http://en.wikipedia.org/wiki/Ohm_(unit)http://en.wikipedia.org/wiki/Early_effecthttp://en.wikipedia.org/wiki/Transconductancehttp://en.wikipedia.org/wiki/Siemens_(unit)http://en.wikipedia.org/wiki/Quiescenthttp://en.wikipedia.org/wiki/Boltzmann's_constanthttp://en.wikipedia.org/wiki/Elementary_chargehttp://en.wikipedia.org/wiki/Kelvinhttp://www.google.com/search?hl=en&q=300+kelvin+*+k+/+elementary+charge+in+millivolts+=http://www.google.com/search?hl=en&q=300+kelvin+*+k+/+elementary+charge+in+millivolts+=http://en.wikipedia.org/wiki/Ohm_(unit)http://en.wikipedia.org/wiki/Early_effect


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frequency analysis, the transistor is replaced by high frequency hybrid- pi model andvoltage gain, current gain, input impedance etc. are determined.

Explanation of parameters:- The internal node B is not physically accessible. Theohmic base spreading resistance rbb is represented as a lumped parameter between theexternal base terminal and B. This resistance rbb, includes the base contact, base bulk

and base spreading resistances. The first is due to actual connection to the base, thesecond includes the resistance from the external terminal to the active region of thetransistor, while the last is the actual resistance within the active base region.

The increase in minority carriers in the base results in increased recombination basecurrent, and this effect is taken into account by inserting a resistance rbe betweeninternal node B and E.

The excess minority carrier storage in the base is accounted for by the diffusion

capacitance Cbe connected between B and E.The early effect indicates that the varying voltage across the collector to-emitter

junction results in base-width modulation. A change in the effective base width makesthe emitter and collector currents to vary because of change in the slope of the minority-carrier distribution in the base. The feedback effect between input and output is takeninto account by connecting rbc between B and C. resistance rce is the resistance presentbetween collector and emitter.

For small variations in the voltage Vbe, across the emitter junction, the excess minoritycarriers concentration injected into the base is proportional to Vbe, and therefore theresulting small-signal collector current with the collector shorted to the emitter isproportional to Vbe. this effect accounts for the current generator gmVbe in the circuit.

Lastly, the collector-junction barrier capacitance is included in the capacitance CBC. Itis to be noted that Cbc is a transition capacitance while Cbe is a diffusion capacitance.

References:-

http://en.wikipedia.org/wiki/Hybrid-pi_model

www.phas.ubc.ca/~quantmat/ARPES/PEOPLE/Andrea/.../lecture10.pdf

forum.allaboutcircuits.com
http://en.wikipedia.org/wiki/Hybrid-pi_modelhttp://www.phas.ubc.ca/~quantmat/ARPES/PEOPLE/Andrea/.../lecture10.pdfhttp://en.wikipedia.org/wiki/Hybrid-pi_modelhttp://www.phas.ubc.ca/~quantmat/ARPES/PEOPLE/Andrea/.../lecture10.pdf


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Books:-

Electronic Devices and Circuits by J B Gupta

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Term Paper Analog FINAL