1. Kristin Ackerson, Virginia Tech EE Spring 2002 B C E

2. The Bipolar Junction Transistor_______________________________slide 3 BJT Relationships – Equations________________________________slide 4 DC  and DC  _____________________________________________slides 5 BJT Example_______________________________________________slide 6 BJT Transconductance Curve_________________________________slide 7 Modes of Operation_________________________________________slide 8 Three Types of BJT Biasing__________________________________slide 9 Common Base______________________slide 10-11 Common Emitter_____________________slide 12 Common Collector___________________slide 13 Eber-Moll Model__________________________________________slides 14-15 Small Signal BJT Equivalent Circuit__________________________slides 16 The Early Effect___________________________________________slide 17 Early Effect Example_______________________________________slide 18 Breakdown Voltage________________________________________slide 19 Sources__________________________________________________slide 20 Table of Contents Kristin Ackerson, Virginia Tech EE Spring 2002

3. The BJT – Bipolar Junction Transistor Kristin Ackerson, Virginia Tech EE Spring 2002  Note: It will be very helpful to go through the Analog Electronics Diodes Tutorial to get information on doping, n-type and p-type materials. The Two Types of BJT Transistors: npnpnp npn E B C pnp E B C Cross Section B C E Schematic Symbol B C E Collector doping is usually ~ 10 6Collector doping is usually ~ 10 6 Base doping is slightly higher ~ 10 7 – 10 8Base doping is slightly higher ~ 10 7 – 10 8 Emitter doping is much higher ~ 10 15Emitter doping is much higher ~ 10 15

4. BJT Relationships - Equations Kristin Ackerson, Virginia Tech EE Spring 2002 B C E IEIEIEIE ICICICIC IBIBIBIB - + V BE V BC + - +- V CE B C E IEIEIEIE ICICICIC IBIBIBIB - + V EB V CB + - +- V EC npn I E = I B + I C V CE = -V BC + V BE pnp I E = I B + I C V EC = V EB - V CB Note: The equations seen above are for the transistor, not the circuit.

5. DC  and DC  Kristin Ackerson, Virginia Tech EE Spring 2002  = Common-emitter current gain  = Common-base current gain  = I C  = I C  = I C  = I C I B I E I B I E The relationships between the two parameters are:  =   =   =   =   + 1 1 -   + 1 1 -  Note:  and  are sometimes referred to as  dc and  dc because the relationships being dealt with in the BJT are DC.

6. BJT Example Kristin Ackerson, Virginia Tech EE Spring 2002 Using Common-Base NPN Circuit Configuration +_ +_ Given: I B = 50  A, I C = 1 mA Find: I E, , and  Solution: I E = I B + I C = 0.05 mA + 1 mA = 1.05 mA b = I C / I B = 1 mA / 0.05 mA = 20  = I C / I E = 1 mA / 1.05 mA = 0.95238  could also be calculated using the value of  with the formula from the previous slide.  =  = 20 = 0.95238  =  = 20 = 0.95238  + 1 21  + 1 21 ICICICIC IEIEIEIE IBIBIBIB V CB V BE E C B

7. BJT Transconductance Curve Kristin Ackerson, Virginia Tech EE Spring 2002 Typical NPN Transistor 1 V BE ICICICIC 2 mA 4 mA 6 mA 8 mA 0.7 V Collector Current: I C =  I ES e V BE /  V T Transconductance: (slope of the curve) g m =  I C /  V BE I ES = The reverse saturation current of the B-E Junction. of the B-E Junction. V T = kT/q = 26 mV (@ T=300K)  = the emission coefficient and is usually ~1 usually ~1

8. Modes of Operation Kristin Ackerson, Virginia Tech EE Spring 2002 Most important mode of operationMost important mode of operation Central to amplifier operationCentral to amplifier operation The region where current curves are practically flatThe region where current curves are practically flatActive: Saturation: Barrier potential of the junctions cancel each other out causing a virtual shortBarrier potential of the junctions cancel each other out causing a virtual short Cutoff: Current reduced to zeroCurrent reduced to zero Ideal transistor behaves like an open switchIdeal transistor behaves like an open switch * Note: There is also a mode of operation called inverse active, but it is rarely used.

9. Three Types of BJT Biasing Kristin Ackerson, Virginia Tech EE Spring 2002 Biasing the transistor refers to applying voltage to get the transistor to achieve certain operating conditions. Common-Base Biasing (CB) :input = V EB & I E output = V CB & I C Common-Emitter Biasing (CE):input = V BE & I B output= V CE & I C Common-Collector Biasing (CC):input = V BC & I B output = V EC & I E

10. Common-Base Kristin Ackerson, Virginia Tech EE Spring 2002 Although the Common-Base configuration is not the most common biasing type, it is often helpful in the understanding of how the BJT works. Emitter-Current Curves Saturation Region IEIEIEIE ICICICIC V CB Active Region Cutoff I E = 0

11. Common-Base Kristin Ackerson, Virginia Tech EE Spring 2002 Circuit Diagram: NPN Transistor +_+_ ICICICIC IEIEIEIE IBIBIBIB V CB V BE EC B V CE V BE V CB Region of Operation ICICICIC V CE V BE V CB C-B Bias E-B Bias Active IBIBIBIB =V BE +V CE ~0.7V  0V Rev.Fwd. SaturationMax~0V~0.7V -0.7V<V CE <0 Fwd.Fwd. Cutoff~0 =V BE +V CE  0V  0V Rev. None /Rev. The Table Below lists assumptions that can be made for the attributes of the common-base biased circuit in the different regions of operation. Given for a Silicon NPN transistor.

12. Common-Emitter Kristin Ackerson, Virginia Tech EE Spring 2002 Circuit Diagram +_ VCCVCCVCCVCC ICICICIC V CE IBIBIBIB Collector-Current Curves V CE ICICICIC Active Region IBIBIBIB Saturation Region Cutoff Region I B = 0 Region of Operation Description ActiveSmall base current controls a large collector current SaturationV CE(sat) ~ 0.2V, V CE increases with I C CutoffAchieved by reducing I B to 0, Ideally, I C will also equal 0.

13. Common-Collector Kristin Ackerson, Virginia Tech EE Spring 2002 Emitter-Current Curves V CE IEIEIEIE Active Region IBIBIBIB Saturation Region Cutoff Region I B = 0 The Common- Collector biasing circuit is basically equivalent to the common-emitter biased circuit except instead of looking at I C as a function of V CE and I B we are looking at I E. Also, since  ~ 1, and  = I C /I E that means I C ~I E

14. Eber-Moll BJT Model Kristin Ackerson, Virginia Tech EE Spring 2002 The Eber-Moll Model for BJTs is fairly complex, but it is valid in all regions of BJT operation. The circuit diagram below shows all the components of the Eber-Moll Model: E C B IRIRIRIR IFIFIFIF IEIEIEIE ICICICIC IBIBIBIB RIERIERIERIE RICRICRICRIC

15. Kristin Ackerson, Virginia Tech EE Spring 2002 Eber-Moll BJT Model  R = Common-base current gain (in forward active mode)  F = Common-base current gain (in inverse active mode) I ES = Reverse-Saturation Current of B-E Junction I CS = Reverse-Saturation Current of B-C Junction I C =  F I F – I R I B = I E - I C I E = I F -  R I R I F = I ES [exp(qV BE /kT) – 1]I R = I C [exp(qV BC /kT) – 1]  If I ES & I CS are not given, they can be determined using various BJT parameters. BJT parameters.

16. Kristin Ackerson, Virginia Tech EE Spring 2002 Small Signal BJT Equivalent Circuit The small-signal model can be used when the BJT is in the active region. The small-signal active-region model for a CB circuit is shown below: iBiBiBiB rrrr iEiEiEiE iCiCiCiC iBiBiBiB BC E r  = (  + 1) *  V T I E I E @  = 1 and T = 25  C r  = (  + 1) * 0.026 I E I E Recall:  = I C / I B

17. The Early Effect (Early Voltage) Kristin Ackerson, Virginia Tech EE Spring 2002 V CE ICICICIC Note: Common-Emitter Configuration -V A IBIBIBIB Green = Ideal I C Orange = Actual I C (I C ’) I C ’ = I C V CE + 1 V A V A

18. Kristin Ackerson, Virginia Tech EE Spring 2002 Early Effect Example Given:The common-emitter circuit below with I B = 25  A, V CC = 15V,  = 100 and V A = 80. Find: a) The ideal collector current b) The actual collector current Circuit Diagram +_ V CC ICICICIC V CE IBIBIBIB b = 100 = I C /I B a) I C = 100 * I B = 100 * (25x10 -6 A) I C = 2.5 mA b) I C ’ = I C V CE + 1 = 2.5x10 -3 15 + 1= 2.96 mA V A 80 V A 80 I C ’ = 2.96 mA

19. Breakdown Voltage Kristin Ackerson, Virginia Tech EE Spring 2002 The maximum voltage that the BJT can withstand. BV CEO =The breakdown voltage for a common-emitter biased circuit. This breakdown voltage usually ranges from ~20-1000 Volts. BV CBO = The breakdown voltage for a common-base biased circuit. This breakdown voltage is usually much higher than BV CEO and has a minimum value of ~60 Volts. Breakdown Voltage is Determined By: The Base WidthThe Base Width Material Being UsedMaterial Being Used Doping LevelsDoping Levels Biasing VoltageBiasing Voltage

20. Sources Dailey, Denton. Electronic Devices and Circuits, Discrete and Integrated. Prentice Hall, New Jersey: 2001. (pp 84-153) 1 Figure 3.7, Transconductance curve for a typical npn transistor, pg 90. Liou, J.J. and Yuan, J.S. Semiconductor Device Physics and Simulation. Plenum Press, New York: 1998. Neamen, Donald. Semiconductor Physics & Devices. Basic Principles. McGraw-Hill, Boston: 1997. (pp 351-409) Web Sites http://www.infoplease.com/ce6/sci/A0861609.html Kristin Ackerson, Virginia Tech EE Spring 2002