PHYS225 Lecture 6. Electronic Circuits

Transcription

1 PHYS225 Lecture 6 Electronic Circuits

2 Transistors History Basic physics of operation Ebers-Moll model Small signal equivalent Last lecture

3 Introduction to Transistors A transistor is a device with three separate layers of semiconductor material stacked together The layers are made of n type or p type material in the order pnp or npn The layers change abruptly to form the pn or np junctions A terminal is attached to each layer (The Art of Electronics, Horowitz and Hill, 2 nd Ed.) (Introductory Electronics, Simpson, 2 nd Ed.)

4 Transistors Heat sink

5 Introduction to Transistors When a transistor is off it behaves like a two diode circuit A transistor operates (or turns on) when the base emitter junction is forward biased and the base collector junction is reversed biased ( biasing ) (Electronic Devices and Circuits, Bogart, 1986) (The Art of Electronics, Horowitz and Hill, 2 nd Ed.)

6 Transistor Biasing (npn Transistor) Electrons are constantly supplied to the emitter V EE These electrons can: 1. Recombine with holes in the base, giving rise to I B 2. Diffuse across base and be swept (by electric field at base emitter junction) into collector, then diffuse around and eventually recombine with holes injected into collector, giving rise to I C Since the base region is designed so thin, process 2 dominates (no time for #1 to occur as often) In an actual npn transistor, 98 or 99% of the electrons that diffuse into the base will be swept into the collector

7 Current Flow Inside a Transistor Current flow for an npn transistor (reverse for pnp): From conservation of current (I E = I B + I C ) we can obtain the following expressions relating the currents (where b ) I b I E b 1 I B (and thus I C I E ) C I B b increases as I E increases (for very small I E ) since there is less chance that recombination will occur in the base b decreases slightly (10 20%) as I E increases beyond several ma due to increased base conductivity resulting from larger number of charge carriers in the base Thus b is not a constant for a given transistor! An average value of 100 is typically used

8 Transistor Current Amplification If the input current is I B and the output current is I C, then we have a current amplification or gain Happens because base emitter junction is forward-biased Forward bias ensures that the base emitter junction conducts (transistor is turned on) Reverse bias ensures that most of the large increase in the base emitter current shows up as collector current Thus small gains in I B result in large gains in I E and hence I C (Student Manual for The Art of Electronics, Hayes and Horowitz, 2 nd Ed.)

9 Basic Transistor Switch Circuit Transistor switch circuit: (BC junction forward biased) (The Art of Electronics, Horowitz and Hill, 2 nd Ed.) V B 0.6 V 0.2 V 0 V V E V C With switch open, transistor is off and lamp is off With switch closed, I B = (10 0.6) V / 1k = 9.4 ma However, I C = bi B 940 ma (assuming b = 100) When collector current I C = 100 ma, lamp has 10V across it To get a higher current, collector would need to be below ground Transistor can t do this, so it goes into saturation Collector voltage gets as close to emitter voltage as it can (about 0.2 V higher) and I C remains constant (I C is maxed out )

10 Emitter Follower Output follows the input: only difference is a 0.6 V diode drop True for V in > 0.6 V If V in < 0.6 V, transistor turns off and V out = 0 Data with R E = 3.3k: B C E E (The Art of Electronics, Horowitz and Hill, 2 nd Ed.) V out V in

11 Emitter Follower By returning the emitter resistor to a negative supply voltage, you can obtain negative voltage swings as well Data with R E = 3.3k: (The Art of Electronics, Horowitz and Hill, 2 nd Ed.)

12 Emitter Follower Biasing You must always provide a DC path for base bias current, even if it is just through a resistor to ground (The Art of Electronics, Horowitz and Hill, 2 nd Ed.)

13 Emitter Follower Biasing With R B included in the previous circuit: f = 1 khz

14 Emitter Follower Biasing Without R B included in the previous circuit: (Here there is no DC base bias current, so transistor is off.)

15 Emitter Follower Biasing To obtain symmetric output waveforms without clipping, provide constant DC bias using a voltage divider Capacitors block outside DC current, which may affect quiescent (no input) values ( AC-coupled follower ) (The Art of Electronics, Horowitz and Hill, 2 nd Ed.)

16 Emitter Follower Impedance The usefulness of the emitter follower can be seen by determining its input and output impedance: Input impedance (i.e. the impedance looking into the base of the transistor): Zin Z 1 b bz load load Output impedance (i.e. the impedance looking into the emitter of the transistor): source Thus the input impedance is much larger than the output impedance Z out Zsource 1 b Z b

17 Emitter Follower Impedance Thus the input and output sees what it wants to see on the other side of the transistor: (Student Manual for The Art of Electronics, Hayes and Horowitz, 2 nd Ed.) Using an emitter follower, a given signal source requires less power to drive a load than if the source were to drive the load directly Very good, since in general we want Z out (stage n) << Z in (stage n + 1) (by at least a factor of 10) An emitter follower has current gain, even though it has no voltage gain The emitter follower has power gain

18 Emitter Follower Impedance When measuring the input and output impedance of the emitter follower, it is useful to think about the Thévenin equivalent circuit as seen at the input and the output: Input impedance seen by the source: V in Z source Z in V B V B Z Z source in Z in V in Output impedance seen by the load: V out, no load ~ V out, load Z out Z load V out, load (Student Manual for The Art of Electronics, Hayes and Horowitz, 2 nd Ed.) Z out Zload Z load V out,noload

19 Emitter Follower With Load Consider the following circuit: (The Art of Electronics, Horowitz and Hill, 2 nd Ed.) V in I E V out V out and V in waveforms: V in V out V in (V) V out (V) I E (ma)

20 Emitter Follower With Load The npn emitter follower can only source current (supply current to something like a load) It cannot sink current (draw current from something like a load) In this example, the transistor turns off when V in = 4.4 V (V out = 5.0 V) Then I E = 0 and the base emitter junction becomes reverse biased As V in increases further, a rather large reverse bias develops across this junction which could result in breakdown The output could swing more negative than 5 V by reducing the R E = 1k resistor, but this increases power consumption in both the resistor and transistor

21 Transistors as Current Sources A transistor can be used as a current source I E I C V R E E V B 0.6 R E Note that I C is independent of V C as long as V C > V E V (i.e., the transistor is not saturated) The output voltage (V load or V C ) range over which I load (= I C ) is (nearly) constant is called the output compliance

22 Deficiencies of Current Sources The load current will still vary somewhat, even when the transistor is on and not in saturation There are two kinds of effects that cause this: V BE varies somewhat with collector-to-emitter voltage for a given collector current (Early effect), as does b DV BE DV CE We assume V BE = constant = 0.6 V in the basic transistor model V BE and b depend on temperature DV BE 2.1 mv/ 0 C We neglect changes in b by assuming I C = I E To minimize DV BE from both effects, choose V E large enough ( 1V) so that DV BE 10 mv will not result in large fractional changes in the voltage across R E V E too large will result in decreased output compliance, however (V C range for transistor on state decreases)

23 Common Emitter Amplifier Consider a transistor current source with a resistor R C as load, and block unwanted DC at the base input (V in is an AC signal): f 3dB 1 2 R eq C C 1 2 f 3dB Now imagine we apply a base signal v B The emitter follows the wiggle so v E = v B Then the change in the emitter current is: i E v R E E R eq v R B E R 1 i C R 2 R eq b R E (Note DC quiescent output voltage of 10 V) lower-case letters represent small changes

24 Common Emitter Amplifier V C = V CC I C R C so v C = i C R C = v B (R C / R E ) Since v in = v B and v out = v C, we have a voltage amplifier, with a voltage gain of: vout RC G v R Minus sign means that a positive change at the input gets turned into a negative change at the output Input and output impedance: Z in = R 1 R 2 br E 8k Z out = R C (impedance looking into collector) = R C (high Z current source) R C = 10k Be careful to choose R 1 and R 2 correctly so that design is not b dependent (R 1 R 2 << br E ) in E

25 Transistor topologies Switch Current source Emitter follower Common-emitter amplifier Common-base amplifier Useful at higher frequencies No Miller effect 5pf at 100MHz is 320Ω! Other interesting combinations also exist Usually can be replaced by OpAmps or other devices

26 Darlington Transistors Allow for much greater gain in a circuit β = β 1 * β 2