Budapest University of Technology and Economics Department of Electron Devices Microelectronics, BSc course Bipolar transistors 3

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET Characteristics of the ideal BJT

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET Common base setup Also called grounded base setup input output

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET Common base setup Input characteristic: Output characteristic: IEIE normal active saturation closed normal active closed inverse active saturation inputoutput

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET The process of amplification common base setup in out

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET Common emitter setup Also called as grounded emitter setup

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET Common emitter setup B : common emitter, large signal current gain

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET Common emitter setup No current flows. Part of the base current is spent on accumulating the base chrage. U BE 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. 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. charge increases charge is constant is constant increasing

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET Common emitter setup Input characteristic: Output characteristic: normal active saturation closed inverse active saturation normal active

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET X U CE normal active saturation closed inverse active saturation Threshold of saturation: U BC = 0 U BE = U CE normal active

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET Characteristics of real BJTs: secondary effects ► Parasitic CB diode ► Series resistances ► Early effect ► Operating point dependence of the gain

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET Effect of the parasitic CB diode 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. inner transistor parasitic junction

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET Effect of series resistances Base contact E C Where is it exactly? R BB' The "inner base" – good approximation: R BB' B' B

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET Effect of series resistances Collector contact I C  R CC' adds to U CE  characteristics can be only on the right hand side of the 1/R CC' line reduction of R CC' in case of discrete transistors: epitaxial structure (like in case of diodes) n + emitter p base collector chip carrier (collector lead)

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET The Early effect Backlash: The output voltage influences the input characteristic

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET The Early effect

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET The Early effect The Early voltage out

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET Early effect at common base setup

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET Early effect: the backlash ~exp(U BE /U T ) CB setup CE setup

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET 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? out

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET Op.p. dependence of current gain Voltage dependence: due to the Early effect

Budapest University of Technology and Economics Department of Electron Devices Microelectronics BSc course, Bipolar transistors 3 © András Poppe & Vladimír Székely, BME-EET Op.p. dependence of current gain Voltage dependence: due to the Early effect Current dependence: high level