1	Bipolar Transistor Design 11/18/2007, David Farrell, CEPD, Inc.
2	NPN and PNP Transistors Use BJTs for small signal amplification, low current and voltage level shift applications Use MOSFETs for high current, low voltage drop and low “on resistance” applications BJTs are lower cost and more resilient than most FETs Low cost 40V BJTs: 2N(MMBT)2222, 2N(MMBT)2907 Low cost 300V BJTs MPS(MMBT)A42, MPS(MMBT)A92 Remember that the NPN symbol “never points in” Generally use NPN transistors in positive supply, single ended applications Use PNP transistors for high side switching or high side current sources
3	Topologies Common base Low impedance to controlled impedance matching Non-inverting level shifter Common Emitter High impedance to controlled impedance Inverting level shifter Emitter follower (common collector) High impedance to low impedance Non-inverting high current buffer
4	Symbols and pin labels Voltages are at the pin relative to ground and currents are into the pin Voltages and Currents at the base Vb, Ib Voltages and Currents at the collector Vc, Ic Voltages and Currents at the emitter Ve, Ie Voltages across pins Vbe, Vce
5	General Design Practices Good design practices will simplify the analysis there are exceptions to these Use gains of 10 or less per stage so that you are not dependent upon high Hfe’s exceeding 100. Don’t float the base (connectors may not be plugged in or uP or PLD pins can tristate), use a pull up or pull down resistor. Use “on” or active currents from 10uA (base) to 50mA (collector) choose coupling capacitors that have negligible impedance at the AC frequency of interest, i.e. an AC short Ensure the current into the base is less than 10mA In driver applications, use a large enough base resistor to prevent exceeding Iout ratings for the previous stage In switched applications, put the load on the collector, not the emitter You are not finished until you check the power dissipation, I*V
6	Simple methods for analysis DC analysis first: assume coupling capacitors are a DC open AC analysis: assume coupling capacitors are an AC short Assume the emitter is a voltage source with low source resistance Assume the collector is a high impedance current source Determine if emitter or base voltage is fixed (forced) If the emitter voltage is fixed, Vb is 0.65V DC above Ve, and Vb = Ve for AC Compute Ib DC, if (Hfe * Ib * (Rc+Re)) ≥ Vcc, Saturation has occurred and Vce = 0.2V If the base voltage is fixed, Ve is 0.65V DC below Vb and Ve = Vb for AC. Ie is Ve/Re, Ic = Ie, Ib = Ie/Hfe If the computation of Ib < 0, the transistor is off Ib = 0, Ic = 0 Rb = Re * Hfe re (Ac internal resistance = 26mV/Ie [dc]) Simulate
7	Applications Saturated drivers Opto-isolator/open collector Relay driver LED driver High voltage level shift Low V to High V HV High side current sense High side switch Small signal voltage amplifier High current linear drivers Buffer Darlington configuration Voltage regulator AM radio receiver
8	Opto-isolator / Open Collector
9	Opto-isolator Simulation
10	12V Relay Coil Driver
11	Relay Driver Simulation
12	Without the “catch” diode
13	LED Driver
14	LED Driver Simulation
15	Low V to High V level shifter (inverting)
16	Low V to High V level shifter inverting Simulation
17	Low V to High V level shifter (non-inverting)
18	Low V to High V non-inverting Simulation
19	HV High side current sense
20	High side switch
21	High side switch Simulation
22	Small signal voltage amplifier
23	Small signal voltage amplifier Simulation
24	High current buffer (10MHz 50 ohm line driver)
25	High current buffer Simulation
26	Darlington configuration
27	Darlington Simulation
28	Voltage Regulator
29	AM 2.5MHz WWV radio receiver
30	2.5MHz Radio Simulation
31	Summary of what was covered Uses of BJTs Topologies General Design Practices Analysis Applications Opto-isolator /open collector LED and Relay Drivers High voltage level shift High side switch Small signal voltage amplifier High current buffer and Darlington configuration Voltage Regulator AM radio receiver
32	References (oldies but goodies) Transistor Circuit Approximations, A. Malvino, 1980 McGraw Hill Integrated Electronics, Millman and Halkias, 1972 McGraw Hill