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About Transistor Amplifiers

Paper type: Essay
Pages: 10 (2286 words)
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Table of ContentsIntroduction42. Common emitter configuration52.1 Base Current Amplification factor (±)52.2 Knee voltage 62.3 Characteristics of CE configuration 72.4 Common emitter detailed analysis 72.5 LT-Spice Simulation of common emitter transistor circuit 83. Common collector configuration93.1 Current amplification Factor (і)103.2 Common collector detailed analysis 113.3 LT-Spice Simulation of Common Collector transistor circuit 124. Common base configuration134.1 Current amplification Factor (±)134.2 Characteristics of CB configuration 144.3 LT-Spice Simulation of common base transistor circuit 155.

Conclusion15Reference16center407797001.IntroductionBipolar JunctionssTransistor (BJT) is a Semiconductorxdevice constructed with three doped semiconductor regions known as Base (B), Collector (C) and Emitter (E) separated by two p-n Junctions.

The p-n Junctionxbetween the Base and the Emitter has a Barrier Voltage (V0) of about 0.6 V, which is an important parameter of a BJT. Unlike the Field Effect Transistor (FET), in which Current is produced only by one type of Charge Carrier either Electrons or Holes, While in BJT, Current is produced by both types of Charge Carriers both Electrons and Holes, hence the name Bipolar.

right129921000There are two Types of BJT: npn and pnp. The npn Type consists of two n-Regions separated by a p-Region. The pnp Type consists of twoxp-Regions separated by an n-Region. Figures 1 is their schematicxsymbols respectively. The following explanationxfocuses on the npn BJT.97282022694900-11938017780000Figure 1: Schematic symbol of NPN and PNP, BJTFigure 2: IC-VCE Characteristics Curves of a NPN BJTThe BJT operates in three different modes: Cut-offxmode, Linear Amplification mode and Saturation mode, Figure 2. BJT is very important in electronics. They are used extensively in other Exhibits, especially as Amplifiers in analoguexcircuit and Electronic Switches in digital circuit.2.

Common emitter (CE) configurationcenter1678305In common emitter configuration the emitter terminal is taken as common terminal for both input and output of the transistor. The common emitter connection for NPN transistor is as shown in the following figure. In the configuration, the emitter junction is forward biased and the collector junction is reverse biased. The flow of electrons is controlled in the same manner. The input current is the base current IB and the output current is the collector current IC.Fig 3: Common emitter configuration2.1 Base current Amplification factor (І)The ratio of change in collector current (”IC) to the change in base current (”Ib) is known as Base Current Amplification Factor. It is denoted by ІІ=”Ic”Ib ( SEQ Equation * ARABIC 1 )Relation between base current (І) and emitter current amplification factor (±)±=”Ic”IE ( SEQ Equation * ARABIC 2 )IE=IB+IC ( SEQ Equation * ARABIC 3 )”IB=”IE-”Ic ( SEQ Equation * ARABIC 4 )І=”IC”IE-”IC ( SEQ Equation * ARABIC 5 )Dividing by,”IE І=”Ic”IE”IE”IE-”Ic”IE , ±=”Ic”IEІ=±1-± ( SEQ Equation * ARABIC 6 )From equation (6) as, ± approaches 1, І reaches infinity. Which result in high current gain in Common Emitter connection. That makes CE configuration favourable for most transistor applications.From equation (3), IC=±IE+ICBO=±IB+IC+ICBO ( SEQ Equation * ARABIC 7 )IC1-±=±IB+ICBOIC=±1-±IB+11-±ICBO ( SEQ Equation * ARABIC 8 )when base circuit is open, IB = 0Then collector emitter current ICBO isICEO=11-±ICBO ( SEQ Equation * ARABIC 9 )From equation (9) Equation (8) becomes, IC=±1-±IB+ICEOAnd from equation (6), IC=ІIB+ICEO ( SEQ Equation * ARABIC 10 )2.2 Knee voltageIn CE configuration, VCE is varied, while keeping the base current IB constant, when IC increases close to 1V of VCE and remain constant after that. The value of VCE up to which collector current IC changes with VCE is called the Knee Voltage. The transistors in CE configuration are operated above the knee voltage.2.3 Characteristics of CE configurationCommon emitter configuration provides good current gain and voltage gain. Keeping VCE constant, with a small increase in VBE the base current IB increases rapidly.

For values of VCE above knee voltage. while, IC is approximately-equal to ІIB. The input resistance ri is calculated-as the ratio of change in base emitter voltage ( ”VBE) to the change in base current (”IB) at constant-collector emitter voltage vCE.ri=”VBE”IB, at constant vBE ( SEQ Equation * ARABIC 11 )when the input resistance is of very low value, large value of base-current IB is produced for a small change in base-emitter voltage vBE. Similarly, the output resistance r0 is calculated-as the ratio of change in collector-emitter voltage (”VCE) to the change in collector current (”IC ) at constant IB.r0=”VCE”IC at constant IB ( SEQ Equation * ARABIC 12 )The outputr resistance of CB configuration is less so it is usually used for bias stabilization methods and audiosfrequency applications.2.4 Common emitter detailed analysisright487392600-635486082500In voltagennode analysis of small-signal equivalent circuit of CE configuration. In Medium Frequency ranges the decoupling capacitors C1 and C2 are set to a large value while value of parasitencapacitor is set to be negligibly small.Figure 4: small signal equivalent diagram of CE configuration.The nodal equation for each node can be written as,B node: VBRs+rRsr=Vi1Rs+VE1v ( SEQ Equation * ARABIC 13 )C node: VE1r0=VORc‹…r0Rc+r0+gmVB-VE ( SEQ Equation * ARABIC 14 )E node: VERe‹…r0‹…rRe+r0+r=VB1r0+VB1r+gmVB-VE ( SEQ Equation * ARABIC 15 )Using the relationship gmr=І, the nodal equations can be written as,B node: VBR0rxR0+rx=Vi1R0+VE1rx ( SEQ Equation * ARABIC 16 )C node: VC1r0+Іr=Vo‹…Rcr0Rc+r0+VBІr ( SEQ Equation * ARABIC 17 )E Node: VEREr0rRE+r0+r+Іr=V01r0+VBІ+1r ( SEQ Equation * ARABIC 18 )Eliminating v0 from equation (17) and (18), VEREr02RE+r0-RCRC+r0+r0R1+Іr0RC+r0 = VBr0R1+Іr0(RC+r0) ( SEQ Equation * ARABIC 19 )Substituting equation (19) in equation (16) provide,V0=ViRs+rRs2‹…rREr02RE+r0-RCRC+r0+r0R1+Іr0RC+r0REr02RE+r0-RCRC+r0+RRs+rr0R1+Іr0(RC+r0)-1 ( SEQ Equation * ARABIC 20 ) Ve=Vir0(Rs+r)Rs2‹…r21+І‹…r0RC+r0REr02RE+r0-RCRC+r0+RRs+rr0R1+Іr0(RC+r0)-1 ( SEQ Equation * ARABIC 21 ) substituting equation (20) and (21) to Equation (17) we expression for the voltage gain as,Av=V0Vi=І(Rc+ro)(Rs+r)Rsr01-І-1І+1ІRE+r0Rcr021-RE+r0RC+r0Rc2r04+RE+r0Rcr02RRs2rr0R1+ІRC+r0Rcr02-1 ( SEQ Equation * ARABIC 22 )When RE=0, this expression reduces to, Av=V0Vi=-ІRs+r0Rcr0Rs+r ( SEQ Equation * ARABIC 23 ) when RE‰ 0 butr0’€ћ it reduces to, Av=V0Vi=-ІRcRs+І+1RE+re ( SEQ Equation * ARABIC 24 ) 2.5 LT-Spice simulation of common emitter transistor circuitCommon emitter transistor amplifier gives output which is 180° phase shift be-tween input and output oftthe circuit. Figure 5 shows LT-Spice circuit for common emitter transistor circuit. The circuit is used toaamplify the input signal.

Due to overall gain for circuit amplitude of output voltage is amplified with gain factor. In figure 5 V(n004) isthe inputnsignal applied to transistor amplifier and V(n003) is the output acrossleft3693360resistor R7.Figure 5: Circuit diagram of CE Transistor amplifier and Input and output waveforms.13430254260850003. Common collector (CC) configurationFigure 6: Common base configurationThe collectortterminal is taken as common terminal for both input and output of the transistor. The common collector connection for PNP transistors is as shown in the following figure 6. The configurationthas a forward biased emitter junction ad reverse biased collector junction. The input current is the base current IB and the output current is the emitter current IE here.3.1 Current amplification Factor (і)The ratio of change in emittertcurrent (”IE) to the change in base current (”IB) is known as current amplification factor (і) in commontcollector (CC) configuration. It is denoted by і.і=”IE”IB ( SEQ Equation * ARABIC 25 )The Relation between і and ±,Substituting equation (25) in equation (4) gives, і=”IE”IE-”IC ( SEQ Equation * ARABIC 26 )Dividing by,”IE і=”IE”IE”IE”IE-”Ic”IE , we have, ±=”Ic”IE. Gives, і=11-± ( SEQ Equation * ARABIC 27 )To obtain expression for collector currentIC=±IE+ICBO ( SEQ Equation * ARABIC 28 )From equation (3) in equation (28) we can obtain, IC=IB+(±IE+ICBO) ( SEQ Equation * ARABIC 29 )IB1-±=IB+ICBO ( SEQ Equation * ARABIC 30 )IE=IB1-±+ICBO1-± ( SEQ Equation * ARABIC 31 )substituting equation (6), IC‰…IC=І+1IB+І+1ICBO ( SEQ Equation * ARABIC 32 )The current gain in common collector and emitter configurations are both same while the voltage gain (Av) is less than 1. The inputtresistance is high and the output resistance is low in the case of CC configuration. The sum of collector current and base current equals emitter current. This configuration works as non-inverting amplifier outputsince the output and inputtvoltages are in phase to each other and voltage levels aretapproximately equal neglecting the voltage drop across the transistor. The circuit can drive a low impedance load from a hightimpedance source.

Therefore, itstcommonly used for impedance matching.-11430462280003.2 Common collector detailed analysisFigure7: small signal equivalent diagram of CC configuration.For B node: VBRs+rRsr=Vi1Rs+Vo1r ( SEQ Equation * ARABIC 33 )For E node: VoRe‹…r0‹…rRe+r0+r=VB1r+gmVB-VE ( SEQ Equation * ARABIC 34 )Again, using the relationship gmr=І, the nodal equation can be written as,For B node: VBRsrRs+r=Vi1Rs+Vo1r ( SEQ Equation * ARABIC 35 )For E Node: VCREr0rRE+r0+r+Іr=VBІ+1r ( SEQ Equation * ARABIC 36 )Substituting the equation (36) in equation (35) we find expression for the voltage gain_Av=V0Vi=І+1RL€ҐrO Rs+rІ+1RL€ҐrO = І+1RL€ҐrO Rs+І+1re+RL€ҐrO ( SEQ Equation * ARABIC 37 )the input voltage istremoved and test voltage source (Vx) is applied to the output terminal to calculate output impedance (R0).The current into the output terminal is given by,Ix= VxrO€Ґ(RS+r)-gmV=VxrO€ҐRS+r-Іrr(Rs+r)Vx ( SEQ Equation * ARABIC 38 )Therefore, output impedance is given by,1R0=IxVx=1r0+1r+RSІ+1 ( SEQ Equation * ARABIC 39 )While input impedance (Ri) given by,1Ri=IiVi=iivivi-v0Rs+r= 1r+RS1-V0Vi= 1r+RSІ+1RL€ҐrO Rs+І+1re+RL€ҐrO ( SEQ Equation * ARABIC 40 )3.3 LT-Spice simulation of Common Collector transistor circuit283654589979500left89674900The figure 8 shows LT-Spice circuit for common collector transistor configuration. Both the input and output signals are in same phase. Where the input signal is denoted as V(n002) and output signal as V(n005) measured across load resistor R7.Figure 8: Circuit diagram of CC Transistor amplifier and Input and output waveforms.4.

Common base (CB) configurationIn common base configurationtbase terminal is taken as common terminal for bothcenter5263470input and output of the transistor. The common base connection for PNP transistor is as shown in the following figure 9. when emitter voltage is applied ittforward Figure 9: Common base configurationBiased emitter base junction and thetelectrons from the negative terminal repel the emitter electrons and currenttflows through the emitter and base to the collector to contribute collector current. The collector voltage VCB is kept constant throughout. In the common base configuration, the emitter current IE is the input current and the collector current IC is the output current.4.1 Current amplification Factor (±)The ratio of change in collector current (”IC ) to thetchange in emitter current (”IE)As the collector voltage VCB is kept constant, is called as Current amplification factor.It is denoted by ±.±=”IC”IE, at constant VCB ( SEQ Equation * ARABIC 41 )Due to electron hole recombination a small amount of base current IB which flows through the basetterminal. While the collector-base junction is reversetbiased, there is another currenttleakage current which can be understood as I leakage flowing which is due to minority charge carriers in the PN junction, which is negligibly small for calculations.The emitter current at collector terminal is, ±IETherefore, total collector current, IC=±IE+Ileakage ( SEQ Equation * ARABIC 42 )There flows a smalltleakage current when the output is open, which can be termed as ICBO. That is the emitter-base voltage, VCB=0.

The expression for collector current is same both in case of common base and common emittertconfigurations,Equation (8), IC=±1-±IB+11-±ICBOHence the value of collector current depends on base current and leakage current along with the currenttamplification factor of that transistor used.4.2 Characteristics of CB configurationThe CB configuration have good stability versus temperature and provides voltage gain but on the otherthand no current gain. Being VCB constant, with a smalltincrease in the Emitter-base voltage VEB, Emitter current IE gets increased.Emitter Current IE is independent of collectortvoltage VCB. Collector Voltage VCB can affect the collector current IC only at low voltages, when VEB is kept constant. The input resistance Ri is the ratio of change in emitter-base voltage (”VEB) to the change in emitter current (”IE ) at constant collector base voltage VCB is,·=”VEB”IE, at constant VCB ( SEQ Equation * ARABIC 43 )

As the input resistance is of very low value, a small value of VEB is enough toproduce a large current flowtof emitter current IE and output resistance Rois the ratio of change in the collector base voltage (”VCB) to the change in collectortcurrent (IC) at constant emitter current IE.Ro=”VEB”IE, at constant VCB ( SEQ Equation * ARABIC 44 )Due to the high value of outputtresistance, a large change in VEB produces only asmall change intcollector current IC.Due to this CB configurated circuits are use for high frequency applications.4.3 LT-Spice simulation of common base transistor circuitright1145982The figure shows LT-Spice circuit for common base transistor configuration. CB configuration is a non-invertingtamplifier hence, both input and output signals are in same phase. Where the input signal is denoted V(n008) is the input signaltapplied to transistor amplifier and V(n003) is the output across resistor R6.Figure 10: Circuit diagram of CB Transistor amplifier and Input and output waveforms.5. Conclusioncenter169174800The figure 12 shows the detailed comparisons of different transistor configuration characteristics. From the table shown in the figure we can find the CC configuration has high input resistance compared to low characteristic output impedance which make it suitable to be used for impedance matching purposes. This configuration is also suitable for amplification purposes of this high current gain attributed to the placement of load resistance at receiving end.Figure12: Comparison of important characteristics of different configurations.

In CE configuration the input impedance is connected to forward biased PN junction while output impedance to reverse biased PN junction the current and power gain is highest compared to other two configurations which make it ideal for amplification purposes and hence most of the transistor configurations are in this configuration due to this characteristic. For CB configuration have remarkably high voltage characteristics. Hence it is not an ideal choice for many applications. This configuration has a high input to output ratio due to the input characteristics are similar to forward biased diode and output characteristics similar to regular diode.


Website: “Basic bjt amplifier configurations “, 19 January 2019.Prof. Dr.-Ing. Werner Bogner: lab. 4 transistor amplifier, advanced circuits lab scripts, 10 October 2018.Website: ” Introduction to bjt (bipolar junction transistor)”, 19 January 2019.

Website: “Common collector amplifier”, 19 January 2019.Website: “Bipolar transistor cookbook”, 19 January 2019.

Website: “Basic electronics – transistor configurations”, 19 January 2019.Website: ” Introduction to Bipolar Junction Transistors (BJT)”, 19 January 2019.

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About Transistor Amplifiers. (2019, Aug 20). Retrieved from https://studymoose.com/about-transistor-amplifiers-essay

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