Department of Information Engineering286 Transistor 3-layers device –npn (more common) –pnp (less common) N P N e b c P N P e b c

Department of Information Engineering287 Mode of operation For both pnp and npn transistors –base-emitter - forward biased –base-collector -reverse biased example –pnp transistor P N P e b c

Department of Information Engineering288 PNP transistor P N P e b c v x Forward biased reduce barrier Before bias Reverse biased increase barrier

Department of Information Engineering289 PNP transistor – key point Base-emitter is forward biased –Lots of holes diffused from emitter to base and electrons from base to emitter Base-collector is reverse biased –Attracts all injected holes from emitter to the collector

Department of Information Engineering290 Base-emitter is forward biased pnp transistor – emitter (P) is much more heavily doped than the base (N) –lots more holes in the emitter than free electrons in the base –majority of the current is carried by the holes P N P holes e e b c

Department of Information Engineering291 Base-collector is reverse biased All holes from emitter are sucked to the collector by the reverse biased potential barrier !! P N P holes e e b c

Department of Information Engineering292 PNP transistor Number of holes moving from emitter to base depends on V BE (forward biased, like a diode) Collector voltage is NOT critical, as long as it is negative (so as to attract the holes) P N P holes e e b c Negative collector voltage

Department of Information Engineering293 Transistor model v BE is like the knob that controls a valve Small change in voltage (V BE ) causes large change in current (I C ) (or V BE )

Department of Information Engineering294 How a transistor works Transistor is a voltage-control current source –Collector current (I C ) is controlled by base-emitter voltage (V BE ) P N P holes e b c Current is controlled by V BE ICIC

Department of Information Engineering295 Holes distribution in the base region –At base-emitter junction High concentration due to the injected holes –At base-collector junction Zero (all holes are sucked by the collector) Holes move across the base region by diffusion P N P holes e b c 0

Department of Information Engineering296 Recombination current Some holes recombine with the electrons in the base –Base becomes more +ve, attract electrons from outside –This constitutes the base current P N P holes e b c Base current Electrons for recombination

Department of Information Engineering297 Base current electrons move from base to emitter –lower the barrier, more electrons can cross the base- emitter junction –Electrons are supplied externally from the base P N P holes e b c Electrons Base current Electrons

Department of Information Engineering298 Base current P N P holes e b c recombination electrons IEIE IBIB ICIC

Department of Information Engineering299 Current gain I E = I C + I B If  is the proportion of current that flows from emitter to collector – The current gain  is defined as – –e.g. if  = 0.99, then  = 100

Department of Information Engineering300 Current gain  = transport factor –the efficiency of collecting the carriers emitted by the emitter –The higher the better (the larger the current gain  ) To have high current gain –make the width of the base as narrow as possible, so as to reduce recombination –Heavily doped the emitter to make the current from emitter to base much larger than from base to emitter –Physical construction of the transistor

Department of Information Engineering301 Physical construction of the transistor –To capture as many carriers from emitter as possible –Base is made very thin –Emitter is made smaller than collector collector emitter base holes PNP transistor

Department of Information Engineering302 Charge stored in the base and the speed of CPU conductingnon-conducting conducting –holes injected to base from emitter non-conducting –No more holes injected from emitter, gradient is flat e b c Stored charge

Department of Information Engineering303 Base base when transistor is conducting (ON state) –charge is stored in the base region when the transistor is non-conducting (OFF) –needs to discharge the stored charge Why this is important? –CPU’s speed depends on how fast you can turn a transistor from ON (logic 1) to OFF (logic 0) and vice versa –smaller cell size, smaller capacitance, faster discharge time, faster CPU clock

Department of Information Engineering304 npn transistor –similar to pnp transistor, but the majority carrier at the emitter (n-type) is electrons –forward biased the base-emitter, free electrons are injected from emitter to base –reverse biased the base-collector, the injected electrons are attracted to the collector npn transistor is more popular than pnp transistor because electrons move faster than holes, hence better performance (faster switching speed)

Department of Information Engineering305 NPN transistor Number of holes moving from emitter to base depends on V BE (forward biased, like a diode) Collector voltage is NOT critical, as long as it is positive (so as to attract the electrons) N P N e e b c Positive collector voltage

Department of Information Engineering306 Transistor model 1.V BE ~ 0.6 V –like a forward biased diode 2.V C > V E for npn –collector is more positive than the emitter, so that it can suck in the electrons from the emitter V C < V E for pnp –Collector is more negative than the emitter, so that it can suck in the holes from the emitter

Department of Information Engineering307 Basic property of a transistor Graph of I C vs V BE –I E vs V BE is the diode characteristic –  ~ 1, therefore I C ~ I E –I C vs V BE is also like the diode characteristic Eber-Moll model (the diode equation) – –where V T =25.3mV at room temperature –I S is the saturation current of the transistor

Department of Information Engineering308 What the equation means A small change in V BE leads to large change in I C If V BE is increased by 18mV, what is the change in I C ? – –18 mV doubled I C –60 mV for a 10X change in I C

Department of Information Engineering309 The curve of I C vs V BE is so steep that a narrow range of V BE (0.5V-0.8V) can cover a wide range of I C –Can assume V BE ~0.6V (like the diode)

Department of Information Engineering310 Transistor model Simple model –V BE ~ 0.6V –I C =  I B –I E = I C + I B = (  I B

Department of Information Engineering311 Transistor model An even simpler model –V BE ~ 0.6V –I E ~ I C 0.6V B C E ICIC I E =I C IBIB

Department of Information Engineering312 Large and small signal model of transistor Large V BE (large signal) is difficult to analyze because I C vs V BE is non-linear (exponential curve)  is not useful because different transistor has different  Small V BE (small signal) –The small section of the curve can be assumed to be linear –Linear relation  i C = g m  v BE –g m is known as the transconductance

Department of Information Engineering313 Transconductance (g m ) g m = = the slope of I C vs V BE –

Department of Information Engineering314 Transconductance (g m ) g m is constant for small signal (since I C is constant), but not a constant for large signal (I C can be changed wildly) g m can be known more easily than  because –if we know I C, we know g m, but  varies from transistor to transistor

Department of Information Engineering315 Two basic transistor circuits Emitter follower Amplifier

Department of Information Engineering316 Emitter follower R E 3.3k V IN V OUT 15V

Department of Information Engineering317 AC output = AC input What if V IN  V IN +  V ? V OUT  V OUT +  V (because V IN - V OUT =0.6V) Hence :  V OUT =  V IN If input is up by  V, output also goes up by  V Output AC voltage follows input AC voltage –That is why it is called a follower

Department of Information Engineering318 DC offset voltage The difference in DC voltage between input and output What is the DC offset voltage of a follower? –0.6V (the V BE ) Ideal case – zero offset voltage, so the follower is not perfect

Department of Information Engineering319 What is the use of a follower?  V OUT =  V IN –Not much use But a follower has –large input impedance –small output impedance –Super glue, good at joining circuits together

Department of Information Engineering320 Input impedance of a follower  is typically large (>100), so R IN > 330k  V IN  V OUT RERE

Department of Information Engineering321 Output impedance Thevenin model –Z OUT =  V OUT /  I OUT V B is fixed, V OUT drops by  V OUT –  V BE =  V OUT –Small increase in  V BE gives large  C Since  V E is small, I E remains the same –  I OUT =  C e.g. if I C = 1mA, then Z OUT = 25  V B is fixed  V OUT RERE IEIE  I OUT

Department of Information Engineering322 Why follower has a small output impedance? Increase output current  a tiny drop in output voltage  V OUT due to output impedance R thev  tiny drop in causes  V OUT a tiny increase V BE  because of the exponential relationship between I C and V BE, a small increase in V BE gives a very large increase in I C  The output sees that even a tiny drop in output voltage would produce large amount of output current, therefore the output impedance is small

Department of Information Engineering323 Important properties of a follower Output voltage follows the input voltage Large input impedance = (  +1)R E –(e.g. Z IN =330k  ) Small output impedance = 1/g m –(e.g. Z OUT =25  )

Department of Information Engineering324 The use of a follower Problem in joining the voltage dividers together –Loading effect –Input impedance of B is too small –Output impedance of A is too large V OUT =v IN /4 ? A B

Department of Information Engineering325 magic glue –A now sees a large input impedance –B sees a small source impedance follower B A V OUT =V IN /4

Department of Information Engineering326 All practical electronic circuits use followers as the input and output stage follower Device A follower Device B

Department of Information Engineering327 Biasing What is wrong with the bias of this circuit? The circuit only works if the input voltage > 0V if input voltage < 0V, base-emitter diode is no longer forward biased

Department of Information Engineering328 What is the quiescent (DC) voltage of this circuit? V B ~ 8V – since the input impedance of the follower is large quiescent output voltage (V E ) ~ 7.5V For small ac input, V BE is always forward biased even the ac input is -ve

Department of Information Engineering329 Use a smaller value (13k & 15k) for the divider Can we use a smaller resistors for the divider? From the ac signal point of view, the 13k and 15k resistors appear in parallel

Department of Information Engineering330 Use a smaller value (13k & 15k) for the divider 13k//15k ~ 14k//14k ~ 7k  Input impedance of transistor = 750k (assume  = 100) The 7k resistor is in parallel with 750k (of the follower) –the resultant resistance ~ 7k Input impedance of the follower is reduced significantly (from 750k to 7k) Conclusion –small input impedance (BAD)

Department of Information Engineering331 Use a larger value (1.3M & 1.5M) for the divider large resistors => large source resistance (BAD)!! –the dc voltage is no longer at 8V to see why, use a Thevinin equivalent model –V B is now only 4V !!

Department of Information Engineering332 Increase R E How about increase R E so that we have a larger input impedance? Increase R E from 7.5k to 75k? –I E (~ I C ) is reduced from 1mA to 0.1mA –output impedance R OUT ~ 25mV/I C R OUT increases from 25  to 250  Input impedance is increased at the cost of increase in output impedance (BAD) Engineering is about compromise !

Department of Information Engineering333 Recapitulation bias at 7.5V so as to allow max swing

Department of Information Engineering334 A simple constant current source What is I OUT ? (10V)

Department of Information Engineering335 Constant current source The compliance of the current source (the range of load that the current source can perform properly) –V CE > 0V (base-collector reverse biased) –Max voltage drop across load = V + -5V = 10V-5V = 5V = I OUT R LOAD(max) –R LOAD(max) = 5k  Compliance of the current source = 0 to 5k 

Department of Information Engineering336 Amplifier

Department of Information Engineering337 To design an amplifier DC output voltage – Great! Sitting right at the middle between 0V and 20V –Max ac output swing (+/- 10V)

Department of Information Engineering338 AC gain of an amplifier Gain = output signal voltage/ input signal voltage Let input signal =  v IN

Department of Information Engineering339 Gain of an amplifier IMPORTANT –The gain depends on R E and R C only, not on the characteristics of the transistor ! –Can build amplifier of fixed gain using any transistor (REALLY GREAT) –input and output signal are 180 o out of phase

Department of Information Engineering340 Intrinsic emitter resistance Gain of our amplifier : What if R E =0? =10V =5.1k

Department of Information Engineering341 Intrinsic emitter resistance Alternatively, we can model the base-emitter junction by a resistor (r e ) –What is it value? r e = V BE / I E – V BE IEIE

Department of Information Engineering342 Intrinsic emitter resistance r e so that But Large input -> large I C -> large gain

Department of Information Engineering343 Gain is not constant, output is distorted !

Department of Information Engineering344 Feedback resistor Remedy: emitter resistor –we cannot remove r e, but we can reduce its effect by adding a large emitter resistor R E r e varies from 13 – 50 ohms

Department of Information Engineering345 Emitter resistor Advantages –reduce the impact of r e on overall gain –increase input impedance –easier to bias bias point determined by resistors and not by current gain  (highly variable) –reduce the impact of temperature instability Disadvantage –Smaller overall gain