MALVINO Electronic PRINCIPLES SIXTH EDITION

Bipolar Transistors Chapter 6

N P N The bipolar junction transistor has 3 doped regions. COLLECTOR (medium doping) P BASE (light doping) N EMITTER (heavy doping)

In a properly biased NPN transistor, the emitter electrons diffuse into the base and then go on to the collector. RC N VCE RB P VCC N VBE VBB

IC IB IE IC IB IE Conventional flow Electron flow IE = IC + IB IC @ IE IB << IC adc = IC IE bdc = IC IB

RC RB VCE VCC VBE VBB The common emitter connection has two loops: the base loop and the collector loop. RC RB VCE VCC VBE VBB

Subscript notation When the subscripts are the same, the voltage represents a source (VCC). When the subscripts are different, the voltage is between two points (VCE). Single subscripts are used for node voltages with ground serving as the reference (VC).

The base circuit is usually analyzed with the same approximation used for diodes. VBB - VBE RC IB = RB VCE VCC RB VBE VBB

A graph of IC versus VCE 100 mA 14 12 80 mA 10 60 mA IC in mA 8 40 mA (Note that each new value of IB presents a new curve.) 100 mA 14 12 80 mA 10 60 mA IC in mA 8 40 mA 6 4 20 mA 2 0 mA 2 4 6 8 10 12 14 16 18 VCE in Volts This set of curves is also called a family of curves.

Regions of operation Cutoff - - - used in switching applications Active - - - used for linear amplification Saturation - - - used in switching applications Breakdown - - - can destroy the transistor

Transistor circuit approximations First: treat the base-emitter diode as ideal and use bIB to determine IC. Second: correct for VBE and use bIB to determine IC. Third (and higher): correct for bulk resistance and other effects. Usually accomplished by computer simulation.

The second approximation: VBE = 0.7 V bdcIB VCE

IB = VBB - VBE RB 5 V - 0.7 V IB = = 43 mA RC 100 kW 100 kW VCC RB VBE = 0.7 V VBB 5 V

IC = bdc IB IC = 100 x 43 mA = 4.3 mA RC 100 kW bdc = 100 VCC RB IB = 43 mA VBB 5 V

VRC = IC x RC VRC = 4.3 mA x 1 kW = 4.3 V 1 kW RC IC = 4.3 mA 100 kW 12 V VCC RB IB = 43 mA VBB 5 V

IC = 4.3 mA VCE = VCC - VRC VCE = 12 V - 4.3 V = 7.7 V 1 kW RC VCE 100 kW 12 V VCC RB IB = 43 mA VBB 5 V

Typical Breakdown Ratings VCB = 60 V VCEO = 40 V VEB = 6 V Note: these are reverse breakdown ratings

A graphic view of collector breakdown 14 12 10 IC in mA 8 6 4 2 50 VCE in Volts

Typical Maximum Ratings IC = 200 mA dc PD = 250 mW (for TA = 60 oC) PD = 350 mW (for TA = 25 oC) PD = 1 W (for TC = 60 oC)

Typical “On Characteristics” IC in mA hFE(min) hFE(max) 0.1 40 ___ 1 70 ___ 10 100 300 50 60 ___ 100 30 ___

Troubleshooting Look for gross voltage errors. First approximation and mental estimates will usually suffice. Resistors don’t short but circuit boards can. Circuit boards can and do open. Junctions can and do short. Junctions can and do open.