THE MIGHTY TRANSISTOR Kristen A. McIntyre K6WX ORIGINS • • I love using transistors • used to be mysterious • wonderfully versatile • visual thinking Bay Area Radio Builders • at the TechShop • chalk-talk WHAT IS A TRANSISTOR? • A small tube? • A three legged bug? • A demonstration of quantum mechanics? • A little slice of heaven? • A voltage or current controlled current source WHY BOTHER? • There’s a chip for that • appliance ready to go • 3 wires is hard • Calculations are needed • So many choices • what’s right? LET’S BOTHER • Learn some fundamentals • Extremely useful for small circuits • Basic building block • Just what the doctor ordered • • easier than chips in many cases • control more power Sorry for random schematic styles SOME ASSUMPTIONS • You know what Current and Voltage • You know how Ohm’s Law works • You know what resistors do • interacts with Ohm’s Law • Understand Kirchhoff ’s Current Law • You have a rough idea of how capacitors work • Currents are ‘conventional’ in direction SEMICONDUCTOR PHYSICS: THE DIODE • N-type, P-type • doping = flaws • quantum interference effects • Create junction - PN touch • Charge carriers • • electrons Reverse voltage • depletion layer - insulator EXTENDING THE DIODE • NPN & PNP • C-B is reverse biased • B-E is forward biased • Base injects ‘holes’ • emitter has excess electrons • e move toward collector • electrons pulled by E-field NPN & PNP • Order of doped material • Voltage / Current reversal • NPN is usually faster • NPN is usually convenient • Unlike tubes, you have choice! ANOTHER APPROACH: ELECTRIC FIELDS • Field Effect Transistor • E-field changes depletion region • Gate current very low • Symmetric / reversible (in principle) • Types • enhancement mode • depletion mode • junction • MOS CIRCUIT MODEL: DC • Ebers-Moll • current source • base current control • ignore reverse current • NPN easier to visualize • Consistent config is easier CIRCUIT MODEL EXTENDED: EBERS-MOLL FOR AC • • Add in capacitance • Base-Emitter • Collector-Base (Miller) Add in resistors • non-ideal junctions A SIMPLIFYING ASSUMPTION • • Replace the B-E diode • assume linearity • use resistor • only in ‘linear region’ Hybrid Pi model ISSUES AT AC = Voltage • Miller capacitance • same as for tubes • B-C feedback for AC • reduces gain • causes oscillation (phase delay) = Current MORE SIMPLIFYING ASSUMPTIONS • Vbe is 0.6V always • Base current zero • collector current == emitter current • Collector can supply infinite current • Transistor ‘works’ to make these assumptions hold 3 CIRCUITS FOR TRANSISTORS • Common Emitter • Common Base • Emitter Follower • • a.k.a. Common Collector What does “Common” mean? COMMON EMITTER • • • +V Simplest amplifier voltage amplification Output Prone to gain problems • emitter ‘degeneration’ • easier to model • little gain dependency • Medium impedance out, medium in • Miller capacitance Input 0.6V -V COMMON BASE • Voltage shifter • Can have gain • Miller capacitance non-issue • fast • Impedance depends on resistors • PNP easier to visualize EMITTER FOLLOWER (COMMON COLLECTOR) • Emitter follows Base • Thermal sensitivity • Current amplifier • high impedance in • low impedance out • Asymmetry in current • Some Miller effect EQUIVALENCE WITH TUBES • Each of the 3 circuits can be realized with a tube • C, D cathode = emitter, source • grid = base, gate • plate = collector, drain • Similar to FETs • Voltages higher, currents lower B, G E, S WHAT DEVICE TO USE? • • • 2N3904, 2N3906 • garden variety • 2N2222 TIP-22, TIP-120 • medium power • TIP-22 Darlington PNP 2N3055 • high power, slow KEY SPECS • Max collector current, voltage • Max power dissipation • Secondary breakdown • Beta β (current gain) - careful • • ft • don’t depend - careful can run faster, oscillation MORE CIRCUITS • • Variants of the 3 basic circuits • usually combined for interesting results • special properties used Some of my favorites • a random collection CURRENT SOURCE • • Use two properties • Vbe is constant 0.6V (sort of) • collector is naturally a current source Diode voltage is constant (sort of) • • Zener diode Control current with emitter resistor DIFFERENTIAL PAIR • Vout = g(V+ - V-) • Needs current source • Emitter degeneration preferred • 2 outputs, mutually inverted DARLINGTON • Extreme current amplification • Base current multiplied by first transistor’s β • Sometimes hard to turn off • Slow out of saturation • Miller effect can be extreme VBE MULTIPLIER • Very clever circuit! • Multiplies Vbe at Vce by g • • who cares? • inverse with temperature • negative thermal feedback Common in audio amps VOLTAGE BUFFER • Most typical emitter follower circuit • Gain of 1 in voltage • • current gain, but asymmetric • can fix, but there are complications Watch out for Vbe variations LEVEL SHIFTER • Common base configuration • No appreciable Miller effect • Replicate AC signals with or without gain • could use a capacitor, but no DC Vin Common Base V+ Emitter Follower Vout V- COMPLEMENTARY PAIR • Fix the asymmetric emitter follower problem • Replace that with crossover problem • • fix that with bias • thermal problems Use Vbe multiplier for thermal compensation BASIC CLASS-A AMPLIFIER • Typical voltage gain circuit • Sensitive to Miller effect • Current asymmetry • Bias must be managed • DC points may not be optimal CASCODE • Not “Cascade”!! • common emitter + common base • Wonderfully clever • • current control, not voltage • cancel Miller effect • keep upper base still • keeps lower collector still Stable! OSCILLATOR (COLPITTS) • Very simple • Classic ham oscillator • output -> input via LC circuit • Must be biased correctly OSCILLATOR (PHASE SHIFT) • Idea: delay, delay, delay • Create a phase inversion • • use R-C circuits, or equiv. • with gain = oscillation • works with any gain element If R’s and C’s same, calculation is easy 2 POLE FILTER (SALLEN-KEY) • 2-pole low-pass, high-pass, bandpass filter • Just needs a gain element • op-amp or transistor • Unity gain is easy • Can cascade ADD AN OP-AMP PASS REGULATOR • Use emitter follower • Include the transistor in the feedback loop • • loads vary, so … • compensate for β, Vbe, and thermal drift Feedback can also produce over unity gain, if needed HIGH CURRENT AMPLIFIER • Some designs use common emitter • high impedance - hard to stabilize • not recommended • Emitter follower is most stable • Use Vbe multiplier to bias with thermal compensation Vbe Multiplier Complementary Pair SATURATION • Take the device well out of the linear region - full-on • Vce at a minimum • Saturated with charge carriers • • slow to drain • transistor is briefly ‘stuck’ Acts like a switch INVERTING SWITCH • Use saturation property • Use V/I inversion • Use current multiplication • Simple version has current asymmetry • Might be slow to turn off V+ Out In V0 TIMER • Exploit capacitor V/I relationship • I = C dv/dt, I/C = dv/dt • current source • linear voltage rise • Reset with transistor • Use transistor as comparator OR GATE • Use saturated inverters • ‘Wire OR’ the collectors • Buffer and invert again to get the right output • Extend with more inverters for more inputs • RTL - resistor-transistor logic AND GATE • More saturated inverters • This time, invert twice • A ‘zero’ brings down the common connection • Can buffer and invert if needed • More RTL TOUCH SWITCH • Use body as capacitor • Slows down input transistor • oscillator makes dV/dt • Amplify pulses from slowed base • Pump charge into capacitor • • resistor drains charge When at threshold, turn on MORE CAPABLE AMPLIFIER • Uses all three basic topologies • Complementary pair • Includes Vbe multiplier • Can replace op-amp with differential pair HOMEOSTATIC FAN CONTROLLER • Many interesting sub-circuits • Uses Vbe to measure temperature • Transistor comparator & current source • Used every Friday for 9am talknet • Designed board RF USAGE • • Frequency dependent calculations • tuned circuits common • not exclusively tuned, though • input & output impedance Use Ls, Cs for coupling • • AC coupled Transformers allow device symmetry for class A/B CONCLUSIONS • Transistors are cool! • Transistors are hot! • 3 circuits form the basis of most everything • Quantum physics in a small box • Can understand more about our radios • they’re on the license exam QUESTIONS? THANK YOU!