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Budapest University of Technology and Economics Department of Electron Devices Microelectronics, BSc course Bipolar transistors 3 http://www.eet.bme.hu/~poppe/miel/en/08-bipolar3.ppt http://www.eet.bme.hu Budapest University of Technology and Economics Department of Electron Devices Characteristics of the ideal BJT 24-10-2011 Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011 2 Budapest University of Technology and Economics Department of Electron Devices Common base setup input output Also called grounded base setup 24-10-2011 Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011 3 Budapest University of Technology and Economics Department of Electron Devices Common base setup input Input characteristic: output Output characteristic: normal active saturation IE normal active closed saturation closed inverse active 24-10-2011 Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011 4 Budapest University of Technology and Economics Department of Electron Devices The process of amplification common base setup rd U T 26 mV 2,6 I E 10 mA ube in i rd 4 mA 2,6 10,4 mV uout uki 2000 ki i Rt 4 mA 500 2000mV Au 200 ube 10 24-10-2011 Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011 5 Budapest University of Technology and Economics Department of Electron Devices Common emitter setup Also called as grounded emitter setup 24-10-2011 Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011 6 Budapest University of Technology and Economics Department of Electron Devices Common emitter setup I C AN I E I CB 0 I C AN ( I B I C ) I CB 0 AN I CB 0 IC IB 1 AN 1 AN I C BN I B I CE 0 I E I B IC AN BN 1 AN I CE 0 I CB 0 1 AN B : common emitter, large signal current gain 24-10-2011 Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011 7 Budapest University of Technology and Economics Department of Electron Devices Common emitter setup is constant charge increases No current flows. 24-10-2011 Part of the base current is spent on accumulating the base chrage. UBE voltage increases, the emitter current starts to flow. The other part of the base current is spent on recombination with some part of the emitter current. charge is constant The charge in the base is not increased any longer. Any increase of the base current recombines with a given part of the emitter current, thus, the emitter current will also increase. Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011 8 Budapest University of Technology and Economics Department of Electron Devices Common emitter setup Input characteristic: Output characteristic: saturation saturation inverse active normal active normal active closed 24-10-2011 Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011 9 Budapest University of Technology and Economics Department of Electron Devices IE I ES exp(U BE / U T ) 1 AI I CS exp(U BC / U T ) 1 I C AN I ES exp(U BE / U T ) 1 I CS exp(U BC / U T ) 1 X saturation UCE Threshold of saturation: saturation UBC = 0 inverse active UBE = UCEnormal active U CES 0 24-10-2011 1 U T ln AI normal active closed Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011 10 Budapest University of Technology and Economics Department of Electron Devices Characteristics of real BJTs: secondary effects ► Parasitic CB diode ► Series resistances ► Early effect ► Operating point dependence of the gain 24-10-2011 Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011 11 Budapest University of Technology and Economics Department of Electron Devices Effect of the parasitic CB diode U CES 0 inner transistor 1 U T ln AI parasitic junction No emitter region opposite to it, thus, in inverse oparation the electrons injected from the collector into the base will be lost: inverse active current gain is worsened. 24-10-2011 Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011 12 Budapest University of Technology and Economics Department of Electron Devices Effect of series resistances Base contact Where is it exactly? The "inner base" – good approximation: RBB' E C B' RBB' B 24-10-2011 Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011 13 Budapest University of Technology and Economics Department of Electron Devices Effect of series resistances Collector contact n+ emitter collector p base chip carrier (collector lead) ICRCC' adds to UCE characteristics can be only on the right hand side of the 1/RCC' line 24-10-2011 reduction of RCC' in case of discrete transistors: epitaxial structure (like in case of diodes) Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011 14 Budapest University of Technology and Economics Department of Electron Devices The Early effect Backlash: The output voltage influences the input characteristic 24-10-2011 Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011 15 Budapest University of Technology and Economics Department of Electron Devices The Early effect 24-10-2011 Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011 16 Budapest University of Technology and Economics Department of Electron Devices The Early effect The Early voltage dU CE U E rout ki dI C IC 24-10-2011 Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011 17 Budapest University of Technology and Economics Department of Electron Devices Early effect at common base setup 24-10-2011 Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011 18 Budapest University of Technology and Economics Department of Electron Devices Early effect: the backlash ~exp(UBE/UT) CB setup 24-10-2011 CE setup Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011 19 Budapest University of Technology and Economics Department of Electron Devices The Early effect Problem What is the output resistance of the transistor in common emitter setup is the Early voltage is 80V and the collector current in the operating point is 5mA? UE rout ki IC 24-10-2011 80 V rout 16 k ki 5 mA Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011 20 Budapest University of Technology and Economics Department of Electron Devices Op.p. dependence of current gain Voltage dependence: due to the Early effect dAN dAN dwB dU BC dwB dU BC D p wB N B e 1 Dn wE N E 1 wB tr 1 2 Ln 2 wB wBM const U CB ' SC dAN / wB 1 (1 e ) 2(1 tr ) dU BC 2 U CB 24-10-2011 Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011 21 Budapest University of Technology and Economics Department of Electron Devices Op.p. dependence of current gain Voltage dependence: due to the Early effect Current dependence: high level 24-10-2011 Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 2008-2011 22
