transistor electronic technologies

November 8, 2001 g.hall@ic.ac.uk www.hep.ph.ic.ac.uk/~hallg/ 1 Transistor electronic technologies •Bipolar Junction Transistor discrete or integrated circuit •MOS (Metal-Oxide-Silicon) Field Effect Transistor mainly used in integrated circuits driven by digital applications but analogue •Junction Field Effect Transistor similar in many ways to MOS FET discrete, not easily implemented in ICs •so - is this a course on circuits? No but it is necessary to understand some basics to be able to deal with more complex elements, including some features of op-amps discrete = individual component November 8, 2001 g.hall@ic.ac.uk www.hep.ph.ic.ac.uk/~hallg/ 2 Bipolar transistor •pnp or npn semiconductor, usually Si, but also Ge heavily doped emitter, lightly doped base •Operation - npn base is biased more positive than emitter so a forward biased diode collector more positive than base = reverse biased diode majority carriers from emitter diffuse across base to collector small fraction combine with majority carriers in base current reaching collector is I C = αI E I B = (1-α)I E I C = βI B = [α/(1- α)]I B β = α/(1- α) = d.c current gain = h fe p+ n n++ emitter collector base I B I C I E I B I C I E npn arrows show direction of current flow eg α = 0.99 β = 99 I E ≈ I C November 8, 2001 g.hall@ic.ac.uk www.hep.ph.ic.ac.uk/~hallg/ 3 pnp transistor •Works like npn transistor bias arrangements different but if emitter is positioned at top easy to remember both pnp and npn most positive most negative I B I C I E pnp I B I C I E npn November 8, 2001 g.hall@ic.ac.uk www.hep.ph.ic.ac.uk/~hallg/ 4 Slightly more precise picture •to turn npn transistor on V B -V E = V BE > 0.6-0.7V (invert for pnp) •so we can use it as a switch by controlling V BE V BE ≈ 0 I C = I E = 0 •however, if transistor is ON V BE ≈ 0.7V this is a consequence of the diode behaviour - discuss in a moment •in contrast, β is not a reliable parameter for design NB both log scales 2n3906 npn transistor NB log scale for I C I C (mA) β November 8, 2001 g.hall@ic.ac.uk www.hep.ph.ic.ac.uk/~hallg/ 5 Ebers-Moll model •Transistor can be modelled as two back-to-back diodes I-V behaviour of diode I ≈ I 0 [exp(qV BE /kT)-1)] •Base-emitter diode is forward biased I E = I E0 .[exp(qV BE /kT)-1] ≈ I E0 exp(qV BE /kT) ie V BE ≈ (kT/q)log e I E •Base-collector diode is reverse biased I BC = I CO [exp(qV BC /kT)-1] ≈ I C0 - which is small so current arriving at collector is dominated by current from emitter, which has diffused across base •How does current vary with small change in V BE ? dI E /dV BE = i e /v be = (q/kT)I E0 exp(qV BE /kT) = (q/kT)I E i e r e = v be with r e = kT/qI = 25Ω/I E (mA) ie to ac current signals transistor looks like dynamic resistance this explains why V BE varies so little with I also basis of band-gap T reference NB we don’t usually need to distinguish between I C and I E - consider them equal November 8, 2001 g.hall@ic.ac.uk www.hep.ph.ic.ac.uk/~hallg/ 6 Emitter-follower +6V -6V V B V E V C R E v in v out •DC conditions ±6V are example values, but results don’t depend on them at all apply our rule that V BE ≈ 0.7V I E = [V E - (-6V)]/R E ≈ (V B + 5.3V)/R E •Now ac behaviour v in = v b = ∆V B = ∆(V E +0.7V) = v e = v out amplifier with gain = 1 - not very interesting!!?? •Input impedance R in = v in /i in i in = i b = i c /β i c = i e = v e /R E R in = βR E high, eg β ~ 100, R E ~ 1kΩ (more careful treatment => R in = β(R E + r e ) this is promising for a voltage buffer - what is the output impedance? November 8, 2001 g.hall@ic.ac.uk www.hep.ph.ic.ac.uk/~hallg/ 7 Emitter-follower output impedance •How to find it? Consider the black box… vary v out and see what happens to i out keep other conditions fixed •Use Ebers-Moll result V BE = (kT/q)log e I E dV BE /dI E = v be /i e =(kT/qI E ) •If V B is constant v out = v e i out = i e Z out = (kT/qI E ) = r e = 25Ω/I E (mA) v out R O i out small, as required for buffer +6V -6V V B V E V C R E v in v out November 8, 2001 g.hall@ic.ac.uk www.hep.ph.ic.ac.uk/~hallg/ 8 Short footnotes •In analysing circuits for small signal (AC) behaviour all fixed DC levels are equivalent to ground ie ac current does not need to distinguish voltage at other end of path •This is often useful in looking at circuits to tell if routes are in parallel •Calculations keep simple try to make approximations - 1% answers are almost never required if so better tools exist eg parallel resistances transistor β - assume β ≈ 100 - unless better value known or is critical 47Ω ≈ 51Ω ≈ 50Ω November 8, 2001 g.hall@ic.ac.uk www.hep.ph.ic.ac.uk/~hallg/ 9 Common-emitter amplifier •DC conditions ±6V are example values again V C = 6V -I C R C V E = -6V +I E R E Since V E = V B - 0.7V, I E & I C defined by bias network •small signal, AC behaviour v e = v b = v in = i e R E v out = -i c R C so v out /v in = -R C /R E amplifier with gain •Input impedance: signal sees bias network in parallel with transistor so R in = R 1 ||R 2 || β(R E +r e ) - typically a high value +6V -6V R E v in v out R C R 2 R 1 C what’s the purpose of C? November 8, 2001 g.hall@ic.ac.uk www.hep.ph.ic.ac.uk/~hallg/ 10 Common-emitter output impedance R E v out R C •Play same trick as emitter-follower but this time, from output terminal, the two paths for i out are collector-base junction reverse biased diode = high impedance R C usually much lower than r cb no need to worry about any source impedance driving amplifier so Z out ≈ R C usually relatively high [...]... Metal-Oxide-Silicon (MOS) devices •Principle of MOS Field Effect Transistor transistor operation Metal (poly) gate on oxide between source and drain Source and drain implants of opposite type to substrate Gate is biased to invert channel below oxide apply voltage bias to gate, which gives field across oxide PMOS modulates current in conducting channel transistor can be used as switch (digital) or amplifier... putting p-type "wells" into n-type wafer (or vice-versa) build nMOS transistors in locally p-type region •Why? NMOS inverter CMOS inverter VDD VDD VDD R VDD 0 0 NMOS consumes power in low state CMOS version consumes power only when switching basis of almost all modern logic In IC technologies, accurate resistors are harder to make than C and transistors g.hall@ic.ac.uk www.hep.ph.ic.ac.uk/~hallg/ 17 November... few nm in advanced technologies oxide breakdown field < 1000MV/m = 1V/nm •Human body can easily charge to 30-40kV walking across carpet on a dry day precautions: circuits designed with protection diodes stand on conductive pad and earth body with wrist strap 4000V ESD test g.hall@ic.ac.uk www.hep.ph.ic.ac.uk/~hallg/ 16 November 8, 2001 CMOS = Complementary MOS •Both pMOS and nMOS transistors on same... •Latch-up under certain conditions, parasitic bipolar transistors formed MOS circuits can go into high current states - destructive •ESD care needed in handling protection networks can degrade performance g.hall@ic.ac.uk www.hep.ph.ic.ac.uk/~hallg/ 20 November 8, 2001 Another building block - the current mirror (if time) •Q1 & Q2 are identical transistors VBE1 = VBE2 and VBE ≈ (kT/q)logeIE Rload so... matched transistors are easy to construct - useful to program currents •add a resistor Iout =(kT/qR)loge(Iref/Iout) eg R = 1kΩ, Iref = 1mA => Iout = 67µA add another resistor • VBE1 ≈ VBE2 I1/I2 = R2/R1 R also works for discrete R1 R2 circuits g.hall@ic.ac.uk www.hep.ph.ic.ac.uk/~hallg/ 21 November 8, 2001 Band-gap circuit + •To be more precise, VBE ~ log(current density) so in non-matched transistors... modulates current in conducting channel transistor can be used as switch (digital) or amplifier (analogue ) L W g.hall@ic.ac.uk www.hep.ph.ic.ac.uk/~hallg/ 12 p p n-type November 8, 2001 MOS Field Effect Transistor •Operation - input signal is voltage on gate very high input impedance > 1012Ω G S VG > VT to switch on, vary VDS source G body •I-V behaviour nMOS linear region D D IDS pMOS oxide body S nMOS... = (2kT/qR)ln(r) r = 8 R ≈ 1kΩ I ≈ 1µA/K @ 300K R should vary little with T •actual AD590 only slightly more complicated g.hall@ic.ac.uk www.hep.ph.ic.ac.uk/~hallg/ AD590 precision 22 November 8, 2001 Transistor differential amplifier (for the ambitious) VCC •DC R1 is large, to act as current source VE1 = VA + I1RE VE2 = VA + I2RE RC RC VA = VEE + IR1 (ignoring re) v1 •AC v1 = vA + i1RE = iR1 + i1RE... 2R1/RE R1 >> RE g.hall@ic.ac.uk www.hep.ph.ic.ac.uk/~hallg/ 23 November 8, 2001 vo v2 What's inside an op-amp… •Look for the building blocks MOS IC amplifiers look similar but currents determined by transistor aspect ratios g.hall@ic.ac.uk www.hep.ph.ic.ac.uk/~hallg/ 24 November 8, 2001 . www.hep.ph.ic.ac.uk/~hallg/ 1 Transistor electronic technologies •Bipolar Junction Transistor discrete or integrated circuit •MOS (Metal-Oxide-Silicon) Field Effect Transistor mainly used in integrated. = 99 I E ≈ I C November 8, 2001 g.hall@ic.ac.uk www.hep.ph.ic.ac.uk/~hallg/ 3 pnp transistor •Works like npn transistor bias arrangements different but if emitter is positioned at top easy to. picture •to turn npn transistor on V B -V E = V BE > 0.6-0.7V (invert for pnp) •so we can use it as a switch by controlling V BE V BE ≈ 0 I C = I E = 0 •however, if transistor is ON V BE