lecture 07 transistor re model

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The emitter follower configuration is frequently used forimpedance matching purposes . It presents a high impedance atthe input & a low impedance at the output , which is the directopposite of the standard fixed biased configuration. Thereresulting effect is much the same as that obtained with atransformer , where a load is matched to the source impedancefor maximum power transfer through the system .

Figure 2.21 Substituting the re equivalent into the acequivalent network of Figure 2.20Substituting the re equivalent circuit into the network of figure3.1 will result in the network of figure 2.21 .Zi : The input impedance is determined as below :Zo :The output impedance is best described by first writing theequation for the current Ib:

Objective:To understand the transistor re model (CC & CB configuration).Hello! Students I hope now you are well versed with theconcepts of transistor re-model and the different configurationof the common emitter mode that we have studied in the lastsemester. Now in today’s class we are going to discuss the re-model for the remaining two configuration i.e. emitter followerconfiguration and common base configuration.I hope all of you know what is emitter follower configuration& why it is so called as you.Have already studied it in basic electronics but still let’s revise itwell its actually a common collector transistor in which thevoltage gain is unity that means the output terminal isfollowing the input terminal as we can say that emitter isfollowing the base as the emitter is the follower of base hencethe name emitter followerSo let us start with the emitter follower configuration.

Emitter Follower Configuration

Figure 2.20 Emitter-Follower ConfigurationWhen the output is taken from the emitter terminal of thetransistor as shown in figure 2.20 the network is referred to asan emitter follower. The output voltage is always slightly lessthen the input signal, due to the drop from base to emitter ,but the approximation Av ≅ 1 is usually a good one .Unlike thecollector voltage , the emitter voltage is in phase with a signal Ai. That is both V0 & Vi will attain their positive & negative peakvalue at the same time . The fact that V0 follows the magnitudeof Vi with an inphase relationship accounts for that terminol-ogy emitter followerThe most common emitter follower configuration appears infigure 2.20.Infact because the collector is grounded for ACAnalysis it is actually a common collector configuration. Othervariations of figure 2.20 that draw the output off the emitterwith V0 ≅ Vi will appear later in this section.

LESSON 7:TRANSISTOR RE MODEL

withorand

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2.31

2.322.33

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Phase relationship :As revealed by equation (2.37) and earlier discussion of thissection, Vo and Vi are in phase for the emitter follower configu-ration.

Common Base ConfigurationCommon base configuration is characterized as having arelatively low input and high output impedance and the currentgain less than 1. The voltage gain, however, can be quite large.The standard configuration appears in Fig. 2.23 with thecommon base re equivalent model substituted in figure 2.24.The transistor output impedance ro is not included for thecommon base configuration because it is typically in mega-ohms range and can be ignored in parallel with resistance RC.

The Hybrib Equivalent ModelThe manufacturers specified the characteristics of a transistor interms of its h-paramrters. The word hybrid is used with theseparameters because they are a mixture of constants in differentunits.Transistor is a three terminal device. For each pair of terminals,there are two variables (current & voltage). There are a numberof ways in which these variables can be related.

with 4.211

1 −≅−== αIv

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V1= h11 I1 + h12 V2

I2 = h21 I1 + h22 V2

Parameters h11 , h12 , h21 & h22 which relates the four variables ofthe two port system are called hybrid parameters.Theseparameters can be defined from the above equations by firstputting V2 = 0 (S.C. the output terminal) and then putting I 1 =0 (Opening the input terminal)The suffix e is added to the symbols of h-parameters toindicate that a transistor is used in CE-mode. In this modeterminal 1 is the base terminal and 2 is the collector.V1 and I 1 becomes Ve and I e and at the output port V2 and I 2becomes Vc and I c

Vb= h ie Ib + h re Vc ……………………………..(2.45)Ic = h fe Ib + hoe Vc …………………………….(2.46)

Each term in equation 2.45 has units of volts therefore we canuse K.V.L. to find the circuit that fits in the equation (Fig. 2.26).Each term in equation 2.46 has units of current.Therefore, the complete AC equivalent of transistor using h-parameters can be drawn as shown in Fig. 2.28.Comparing the hybrid equivalent circuit with the transistor remodel we find, The only difference is the presence of a voltagesource h reVc in the input of the hybrid model. The magnitudeof this voltage source depends on the output voltage Vc .Theparameter h re therefore represents a feedback of the outputvoltage to the input circuit. In the normal operation of thetransistor this effect is very small, it will make practically nodifference if the term h reVc is neglected from the hybridequivalent circuit.

Figure 2.26 Hybrid inputequivalent circuit.

Figure 2.27 Hybrid output equivalent circuit.

hie = 1K , h re = 305 x 10-4

hfe = 50 , hoe = 25ms.


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Figure 2.28 Complete hybrid equivalent circuit of atransistor


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YThe typical values of the hybrid parameters areThe topics covered during this lecture :1. Emitter follower configuration2. Common base configuration3. Hybrid equivalent model

Now Answer these Questions:1. Why is CC configuration is used in Impedance matching

purpose?2. Give the basic characteristics of CB configuration.3. What are hybrid parameters?4. What is hre and how does it effect transistor operation?

Notes

lecture 07 transistor re model

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