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CCE201 Solid State Electronic DevicesEEC223 Electronics 1 Lecture 07Bipolar Junction Transistors 2 Prepared By.

Dr Eng Sherif HekalAssistant Professor CCE departmentLecture 01 01 03 2021 1 Applying the BJT inAmplifier Design.

01 03 2021 2 Applying the BJT inAmplifier Design an amplifier may be designed by transistor andseries of resistances .

However it is necessary to model the voltagetransfer characteristic VTC Appropriate biasing is important to ensure lineargain and appropriate input voltage swing 01 03 2021 3.

Applying the BJT inAmplifier Design01 03 2021 4 Obtaining a Voltage The BJT is a voltage controlled current source that can.

serve as a transconductance amplifier A simple way to convert a transconductance amplifierto a voltage amplifier is to pass the output currentthrough a resistor and take the voltage across theThe outputasvoltage.

the output is given byThus it is an inverted version note theminus sign of iCRC that is shifted bythe constant value of the supply.

voltage which will be removed by a 01 03 2021 5coupling capacitor The Voltage TransferCharacteristic VTC 01 03 2021 6.

The Voltage TransferCharacteristic VTC A very useful tool that yields great insight into the operationof an amplifier circuit is its voltage transfer characteristic The VTC in Fig 7 1 b indicates that the segment of greatest.

slope and hence potentially the largest amplifier gain is thatlabeled YZ An expression for the segment YZ can be obtained by This is obviously a nonlinear relationship Nevertheless linearamplification can be obtained by using the technique of.

biasing the BJT 01 03 2021 7 Biasing the BJT to Obtain LinearAmplification6 32 Biasing the BJT amplifier at a point Q located.

01 03 2021 8the active mode segment of the VTC Biasing the BJT to ObtainLinear Amplification Biasing enables us to obtain almost linear.

amplification from the BJT A dc voltage VBE is selected to obtain operation at apoint Q on the segment YZ of the VTC Point Q is known as the bias point or the dcoperating point Also since at Q no signal.

component is present it is also known as thequiescent point How to select an appropriate location for the biaspoint Q will be discussed shortly 01 03 2021 9.

Biasing the BJT toObtain LinearAmplification01 03 2021 10Figure 7 3.

Biasing the BJT to ObtainLinear AmplificationGood BiasingBad Biasing01 03 2021 11.

The Small Signal VoltageIf the input signal vbe is kept small thecorresponding signal vce at the output will be nearlyproportional to with the constant of proportionalitybeing the slope of the almost linear segment of the.

VTC around Q 01 03 2021 12 Locating the Bias Point QThe bias point Q is determined by the value ofVBE and that of the load resistance RC .

01 03 2021 13 Locating the Bias Point QEffect of bias point location on allowable signal swing Load line A results in bias point QA with a corresponding VCEthat is too close to VCC and thus limits the positive swing of.

At the other extreme load line B results in an operating01 03 2021 point QB too close to the saturation region thus limiting thenegative swing of vCE Small Signal Operation and.

consider once more the conceptual amplifier circuit shown inDC analysis01 03 2021 15 Small Signal Operation and ModelsWe consider first the dc bias conditions by setting the.

signal vbe to zerofor active mode operation VC should be greater than VB 0 4 by an amount that allows for the required signalswing at the collector 01 03 2021 16.

Small Signal Operation and ModelsIf a signal vbe is applied as shown in Figure the totalinstantaneous base emitter voltage vBE becomesNow if vbe VT peak of vbe 10 mV wemay approximate the above Equation as.

where gm is called the transconductance 01 03 2021 17 Small Signal Operation and ModelsTo determine the resistance seen by vbe we firstevaluate the total base current iB usingSubstituting for IC VT by gm gives.

The small signal input resistance between base and emitter looking into the base is denoted by r and is defined as01 03 2021 18 Small Signal Operation and ModelsThe total emitter current iE can be determined from.

where IE is equal to IC and the signal current ie isIf we denote the small signal resistance between base and emitterlooking into the emitter by re it can be defined asre is called the emitter resistance and given by01 03 2021 19.

Small Signal Operation and ModelsVoltage GainHere the quantity VCE is the dc bias voltage at the collector and the signal voltage is given by01 03 2021 20.

Small Signal Operation and ModelsThe Hybrid Modelthe hybrid model for the small signal operation of the BJT Theequivalent circuit in a represents the BJT as a voltage controlled current source a transconductance amplifier and.

that in b represents the BJT as a current controlled currentsource a current amplifier 01 03 2021 21 Small Signal Operation and ModelsThe T model.

T model of the BJT The circuit in a is a voltage controlled currentsource representation and that in b is a current controlled currentsource representation 01 03 2021 Small Signal Operation and Models.

The relationship between r and reCheck compatibility of the two modelsThe Hybrid Model The T model01 03 2021 23 Small Signal Models of.

Augmenting the Small Signal Models to Accountfor the Early Effect Application of the Small SignalEquivalent Circuits.

Analysis of transistor amplifier circuits is a systematic process The processconsists of the following steps 1 Eliminate the signal source and determine the dc operating point of theBJT and in particular the dc collector current IC 2 Calculate the values of the small signal model parameters gm IC VT r .

gm and re VT IE gm 3 Eliminate the dc sources by replacing each dc voltage source with a shortcircuit and each dc current source with an open circuit 4 Replace the BJT with one of its small signal equivalent circuit models Although any one of the models can be used one might be more.

convenient than the others for the particular circuit being analyzed Thispoint will be made clearer later in this chapter 5 Analyze the resulting circuit to determine the required quantities e g 01 03 2021 25voltage gain input resistance .

Example 7 1We wish to analyze the transistor amplifier shown in the figurebelow to determine its voltage gain vo vi Assume 100 DC analysisVac S C.

Aac O C01 03 2021 26 Example 7 1AC analysisVdc S C.

Adc O C01 03 2021 27 Example 7 1The first step in the analysis consists of determining the quiescentoperating point .

Having determined the operating point we can now proceed todetermine the small signal model parameters 01 03 2021 28 Example 7 1Analysis of the equivalent circuit model .

Thus the voltage gain will be01 03 2021 29 Example 7 2In Example 7 1 assume that vi has a triangularwaveform First determine the maximum amplitude that.

vi is allowed to have Then with the amplitude of vi set tothis value give the waveforms of the total quantities iB t vBE t To and small signalsatisfy vC t .

approximation vbe should notexceed about 10 mV peak Test if you are still in active region with vi 0 91 VCBVshould be 0 4 VThe voltage at the collector will consist of a triangular wave vo.

superimposed on the dc value VC 3 1 V Example 7 2Maximum available swing at outputVCC vCE VCE SatVC reaches a minimum of 3 1 2 77 V0 33.

0 37V VCB 0 4 Vto be on the safe side we will use a somewhat lower value for viof approximately 0 8 V as shown in the figure below Example 7 2 Example 7 3.

Calculate the base collector and emitter currents and the C Evoltage for a common emitter circuit by considering VBB 4 V RB 220k RC 2 k VCC 10 V VBE on 0 7 V and 200 Example 7 4 0 99.

KVL at BE loop 0 7 IERE 4 0IE 3 3 3 3 1 mAHence IC IE 0 99 mAIB IE IC 0 01 mAKVL at CE loop ICRC VCE IERE 10 0.

VCE 10 3 3 4 653 2 047 V Example 7 5For the circuit shown in Figure thetransistor parameters are 100and VA Design the circuit such.

that ICQ 0 25 mA and VCEQ 3 V Findthe small signal voltage gain Av vo vs Find the input resistance seen bythe signal source vs 01 03 2021 35.

Example 7 501 03 2021 36 Example 7 501 03 2021 37 Example 7 5.

For small signal ac analysis all dc voltages and capacitors actas short circuit The following expressions are obtained The input resistance Ri seen by the signal source vs is 01 03 2021 38 Example 7 6.

Consider the circuit shown in Figure The transistorparameters are 100 and VA 100 V Determine Ri Av vo vs and Ai io is 01 03 2021 39 Example 7 6.

A dc analysis is performed to determine the dc operatingpoint by treating all capacitors as open circuit 01 03 2021 40 Example 7 6The small signal parameters are .

01 03 2021 41 Example 7 601 03 2021 42 Example 7 6The input resistance is.

01 03 2021 43 Example 7 701 03 2021 44 Example 7 7 a A dc analysis is performed to determine the dc operating.

point by treating all capacitors as open circuit 01 03 2021 45 Example 7 7 b Given VCEQ is desired to be 3 5 V hence c The small signal parameters are .

01 03 2021 46 Example 7 7Using the small signal ac equivalent circuit the followingexpressions are obtained 01 03 2021 47.

Example 7 7 d If the source resistor is changed to 500 the new valueTherefore the voltage gain Av decreases as the sourceresistance RS increases due to a larger voltage drop acrossthe source resistor .

01 03 2021 48 Summary of small signalmodel parameters01 03 2021 49.

Biasing the BJT amplifier at a point Q located on the active-mode segment of the VTC. 12/2/2017. Biasing the BJT to Obtain Linear Amplification. 7.2. Biasing enables us to obtain almost-linear amplification from the BJT. The technique is illustrated in Fig. 6.32(a). A dc voltage VBE is selected to obtain operation at a point Q on the segment YZ ...

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