Electronics for physicists Chapter 5 Transistor The transistor

Electronics for physicists Chapter 5

Transistor • The transistor was one of most influential inventions of last century. • A transistor is an active, non-linear electronic component. Bipolar npn transistor December 2018 NMOS Electronics for physicists Marc Weber - KIT 2

Bipolar transistor basics npn transistor Collector-base-emitter Reverse bias Forward bias source: H. Spieler Base current (voltage) drives collector current. Key requirements: • • Base layer is shallow. December 2018 Electronics for physicists Marc Weber - KIT 3

Cross sections of realistic transistor Source: IMEC Source: Raytheon December 2018 Electronics for physicists Marc Weber - KIT 4

Moore‘s law Where else is technological progress so fast? Who/what is driving transistor technology? December 2018 Electronics for physicists Marc Weber - KIT 5

I-V curves • With 3 terminals need to distinguish IC vs UBE, IB vs UBE, IC vs UCE … • Knowledge of I-V curves is all what’s needed for circuit design • Watch power limit UCE IC < P max December 2018 Electronics for physicists Marc Weber - KIT 6

IC vs UCE • UBE drives collector current IC • Diode-like behaviour with large IC • Transistor is active device with amplification • December 2018 Electronics for physicists Marc Weber - KIT 7

IC vs IB • In good approximation: • ≈ 100 (Much larger can be realized in modern technologies. ) • A bipolar transistor is often considered as a current-driven current source. December 2018 Electronics for physicists Marc Weber - KIT 8

Review of IC vs UCE • Need to exceed minimum UCE for collector-emitter-current IC to flow • In active region IC does not vary much with UCE (differential resistance RCE is large) • IC >> IB Homework: relabel plot by replacing IB by UBE value December 2018 Electronics for physicists Marc Weber - KIT 9

Transistors in circuits Voltage amplifier configuration • Transistor current causes voltage drop RC IC . Thus d. Uaus = - RC d. IC • Current flows through RC and transistor (current source). No or little current goes into output node. Note that the absolute value of Uaus (the working point) is not defined yet. December 2018 Electronics for physicists Marc Weber - KIT 10

Current amplifier (voltage buffer) • Here IC may flow through either RC or the output node. • If RE >> load impedance, circuit acts as a buffer. • Uaus is related to UBase December 2018 (Uaus = UBase minus one diode drop). Electronics for physicists Marc Weber - KIT 11

1 T - circuits common emitter amplifier (voltage amplifier) common collector amplifier (buffer) common base amplifier (voltage amplifier) December 2018 Electronics for physicists Marc Weber - KIT 12

Transistor equations There are several valid alternative views of transistor operation leading to other (related) equations. differential base-emitter-resistance: differential collector-emitter resistance: Transconductance: December 2018 Electronics for physicists Marc Weber - KIT 13

Early voltage • Extrapolating RCE gives Early voltage UA : • UA ranges from 15 – 150 V. December 2018 Electronics for physicists Marc Weber - KIT 14

I-V curve overview Discuss the curves in each part of the diagram. December 2018 Electronics for physicists Marc Weber - KIT 15

Common emitter amplifier (This neglects RCE and RC. ) • Output voltage is inverted. • Large RC makes large voltage gain. • Large transconductance makes large gain. Recall December 2018 Electronics for physicists Marc Weber - KIT 16

Moore‘s law [nm] Source: Global. Foundries 100 80 60 metal 1/2 pitch 40 transistor gate length 20 node name 0 2005 2007 2009 2011 2013 • Moore: geometrical dimensions of transistors have been shrinking. • Non-planar transistors are gaining traction since a decade or so. • Intel introduced Fin. FET in 2011 (22 nm process node). December 2018 Electronics for physicists Marc Weber - KIT 17

Field-effect transistor topologies December 2018 Electronics for physicists Marc Weber - KIT 18

Published in April 2011 December 2018 Electronics for physicists Marc Weber - KIT 19

December 2018 Electronics for physicists Marc Weber - KIT 20

December 2018 Electronics for physicists Marc Weber - KIT 21

Small signal circuit analysis • Replace current and voltage sources by internal resistance current source ≙ open circuit (infinite impedance) voltage source ≙ short circuit (zero impedance) • Replace non-linear components by differential impedance at operation point. • Replace transistors by current- (or voltage-) driven current source. December 2018 Electronics for physicists Marc Weber - KIT 22

Small signal analysis December 2018 Electronics for physicists Marc Weber - KIT 23

Example Raus = and December 2018 Electronics for physicists Marc Weber - KIT 24

Emitter follower with and Emitter follower drives current into load. It does not amplify voltage. December 2018 Electronics for physicists Marc Weber - KIT 25

Emitter follower Large-signal analysis: Small signal analysis: December 2018 Electronics for physicists Marc Weber - KIT 26

Emitter follower input impedance December 2018 Electronics for physicists Marc Weber - KIT 27

Emitter follower output impedance with we get December 2018 Electronics for physicists Marc Weber - KIT 28

Common base amplifier Similar to common emitter amplifier, but with low input impedance: VU = S R C VI 1 Zaus ~ RC

Operating point So far we have neglected the amplifier bias. Have to chose values for R 1, R 2, Cein and RE. Choices depend on application. December 2018 Electronics for physicists Marc Weber - KIT 30

Common emitter amplifier with current feedback December 2018 Electronics for physicists Marc Weber - KIT 31

Common emitter amplifier with current feedback No feedback With feedback with Gain is independent of S ! December 2018 Electronics for physicists Marc Weber - KIT 32

Overview of BJT circuits Electronics for physicists 33

Current source • R 1 and R 2 set UB and thus IL • Uaus can vary largely since IL = IC does not depend (much) on UCE December 2018 Electronics for physicists Marc Weber - KIT 34

Miller effect I With Input impedance is reduced Input capacitance is much enhanced December 2018 Electronics for physicists Marc Weber - KIT 35

Miller effect II • Capacitive feedback looks like large capacitance at input. • Thus, slow-down of op-amp December 2018 Electronics for physicists Marc Weber - KIT 36

Input impedance Zein= RE ( + 1) (neglecting RL and RCE) Input impedance is given by RE and is large. Why? If RL is large, RE sets IC and thus IB. d. Uaus = d. Uein RE effectively sets input impedance December 2018 Electronics for physicists Marc Weber - KIT 37

Frequency dependence of amplifiers December 2018 Electronics for physicists Marc Weber - KIT 38

Appendix December 2018 Electronics for physicists Marc Weber - KIT 39

Output impedance • December 2018 Electronics for physicists Marc Weber - KIT 40