EEE 312 Lecture1

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Lecture 1

February 9, 2015

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EEE 312 Electronic Circuits

BJT AC Analysis

• Lecture:– Taner İnce

– Office Hours: Tuesday 10:00-11:00 AM

• Textbook– R. Boylestad and L. Nashelsky, Electronic Devices and

Circuit Theory, Pearson. 11e

• Grading– First Midterm (20%)

– Second Midterm (20%)

– Laboratory (20%)

– Final (40 %)

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Course Information

• Become familiar with the re, hybrid, and hybrid π models for

the BJT transistor.

• Learn to use the equivalent model to find the important ac

parameters for an amplifier.

• Understand the effects of a source resistance and load resistor

on the overall gain and characteristics of an amplifier.

• Become aware of the general ac characteristics of a variety of

important BJT configurations.

• Begin to understand the advantages associated with the two-

port systems approach to single- and multistage amplifiers.

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BJT AC Analysis (Chapter 5)

AC response of the BJT amplifier is examined by using the models most frequently used to represent the transistor in the sinusoidal ac domain.

There are three models commonly used in the small-signal ac analysis of transistor networks:

• re model

• hybrid π model

• hybrid equivalent model

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BJT AC Analysis (Chapter 5)

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• The superposition theorem is applicable for theanalysis and design of the dc and ac components ofa BJT network, permitting the separation of theanalysis of the dc and ac responses of the system.

• In other words, one can make a complete dcanalysis of a system before considering the acresponse. Once the dc analysis is complete, the acresponse can be determined using a completely acanalysis.

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BJT AC Analysis

• The key to transistor small-signal analysis is the use of theequivalent circuits (models).

• A model is an equivalent circuit that represents the ACcharacteristics of the transistor.

• A model uses circuit elements that approximate thebehavior of the transistor.

• Once the ac equivalent circuit is determined, the schematicsymbol for the device can be replaced by this equivalentcircuit and the basic methods of circuit analysis applied todetermine the desired quantities of the network.

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BJT TRANSISTOR MODELING

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In summary, therefore, the ac equivalent of atransistor network is obtained by:

1. Setting all dc sources to zero and replacing them bya short-circuit equivalent

2. Replacing all capacitors by a short-circuitequivalent

3. Removing all elements bypassed by the short-circuit equivalents introduced by steps 1 and 2

4. Redrawing the network in a more convenient andlogical form

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• BJTs are basically current-controlled devices;therefore the re model uses a diode and a currentsource to duplicate the behavior of the transistor.

• One disadvantage to this model is its sensitivity tothe DC level. This model is designed for specificcircuit conditions.

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The re Transistor Model

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Common-Emitter Configuration

• The result is that the impedance seen “looking into” the base ofthe network is a resistor equal to beta times the value of r

e, as

shown in Figure. The collector output current is still linked to theinput current by beta as shown in the same figure.

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• We now have a good representation for the inputcircuit, but aside from the collector output currentbeing defined by the level of beta and IB, we do nothave a good representation for the outputimpedance of the device.

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Common-Emitter Configuration14


• For situations where the Early voltage is notavailable the output impedance can be found fromthe characteristics at any base or collector currentusing the following equation

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• In any event, an output impedance can now bedefined that will appear as a resistor in parallel withthe output as shown in the equivalent circuit of Fig.5.16

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Common-Base Configuration

• In general, common-base configurations have verylow input impedance because it is essentiallysimply re. Typical values extend from a few ohmsto perhaps 50 . The output impedance ro willtypically extend into the megohm range.

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• The transistor models just introduced will now be used toperform a small-signal ac analysis of a number of standardtransistor network configurations.

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COMMON-EMITTER FIXED-BIAS CONFIGURATION

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• The small-signal ac analysis begins by removing the dceffects of VCC and replacing the dc blocking capacitors C1

and C2 by short-circuit equivalents, resulting in the networkof Fig. 5.21.

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• Substituting the re model for the common-emitter configuration of Fig.5.21 results in the network of Fig. 5.22.

• The next step is to determine β, re, and ro. The magnitude of β is typicallyobtained from a specification sheet or by direct measurement using a curvetracer or transistor testing instrument. The value of re must be determinedfrom a dc analysis of the system, and the magnitude of ro is typicallyobtained from the specification sheet or characteristics. Assuming that β, re,and ro have been determined will result in the following equations for theimportant two-port characteristics of the system.

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• Voltage Gain, Av = V0 / Vi

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• Phase Relationship The negative sign in the resulting equationfor Av reveals that a 180° phase shift occurs between the inputand output signals, as shown in Fig. 5.24. This is a result of thefact that βIb establishes a current through RC that will result in avoltage across RC, the opposite of that defined by Vo.

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VOLTAGE-DIVIDER BIAS

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• Unbypassed

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CE EMITTER-BIAS CONFIGURATION

• Unbypassed RE

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• Unbypassed RE

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• Unbypassed RE- Effect of ro

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• bypassed RE

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• bypassed RE

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Example

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Solution

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Solution (cont’d)

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Example (cont’d)

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Example (cont’d)

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Example (cont’d)

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Example (cont’d)

EEE 312 Lecture1

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