Capacitors, diodes, transistors

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1 Capacitors, diodes, transistors capacitors charging and time response filters (impedance) semi-conductor diodes rectifiers transformers transistors CHM6158C - Lecture 3 1

2 Capacitors Symbol 2

Capacitors Device consisting of two conducting plates separated by an insulating material By e.g. pulling away electrons from one side (Q+), build up electric field, which pulls electrons onto Q-.

3 Capacitors Device consisting of two conducting plates separated by an insulating material By e.g. pulling away electrons from one side (Q+), build up electric field, which pulls electrons onto Q-. Unit of capacitance is Farad (F) = C/V (electrical charge that can be built up for a particular voltage) Typically µf (10-6 ), nf (10-9 ), pf (10-12 ) Dielectric material = insulating material that contains polar molecules that orient with the electric field (energy storage) What is the resistance of a capacitor? 3

4 Charging up a capacitor takes time RC circuit Every circuit has a resistance, which slows down the flow of electrons What will be the DC potential here? V = Vi ( 1 exp( t / RC)) Why? What will the current plot as a function of time look like? 4

5 Capacitors in parallel Electron flow from the same circuit has access to both sides of the capacitors Capacitances in parallel add up like resistors in series. N 1 C total = ( C i ) e.g. C 1 = µf, C 2 = µf and C 3 = µf, what is the total capacitance? 5

6 Capacitors in series Electron flow from the same circuit do NOT have access to both sides of the capacitors Capacitances in series add up like resistors in parallel. N 1 1 = Ctotal 1 C i Total capacitance lower than sum of capacitances. capacitors adversely affect each others ability to store charge. e.g. C 1 = µf, C 2 = µf, what is the total capacitance? 6

7 AC and capacitors In alternating current, orientation of electron flow keeps changing. What will happen if RC time constant is slower than AC frequency? i = C dv dt Note that current leads voltage by 90 o. 7

8 Voltage response At any moment in time, total voltage V r + V c = E, but relative voltages for V r and V c change as a function of time. Remember that V c as a function of time looks like this: What will be corresponding voltage across resistor as a function of time? What is RC in this case? 8

9 Voltage response Initially, total voltage on resistor why? What will happen for an ac voltage source, if frequency is low, frequency is high? Topics/DC-Current/gifs/Circuits50.gif 9

10 High- and low- pass filter For high-frequency signal, where will signal be transmitted? For low-frequency signal, where will signal be transmitted? More on noise and filters later 10

11 Note on impedance: AC equivalent of DC resistance Impedance = measure of opposition to alternating current What does voltage response compared to current look like? AC voltage source 11

12 Pure resistance circuit: Impedance For capacitance-only circuit: Current leads voltage i = C dv dt Does this make sense based on formula? More on impedance in lecture 17 12

13 + - (Semi- conductor) Diodes What happens here? doped material causes local charge imbalance between P (holes) and N (electrons) in forward bias, holes and electrons combine at P-N junction in reverse bias, a depletion region is created, which impedes conductance current flow occurs in one direction only 13

14 ApplicaQon: RecQfier circuits: AC DC Only ac phase in forward direction leads to flow of current Draw which way does electricity flows (i.e., holes) What will the DC voltage look like? How can this voltage be made more continuous? 14

15 RC circuit to create smoother DC RC >> 1/f 15

16 Transformer: converqng ac voltage No. of coils V S N = S V N P P Principle of. 115 V ac converted to 12 VDC: Notice that current in secondary winding flows in opposite direction 115 ac 16

17 BJT (Bipolar JuncQon Transistor) Transistor = device that switches or amplifies electronic signals NPN transistor = two diodes with a shared anode (+) 17

18 BJT (Bipolar JuncQon Transistor) Bipolar junction transistor (BJT): pnp or npn Base current I B controls collector current I C. I C = β I B Common emitter circuit: Note design of pnp junction to allow much larger current flow from emitter to collector (i.e., very narrow n layer) 6th Ed., Douglas A. Skoog, F. James Holler and Stanley R. Crouch, Thomson Brooks/Cole, Gain 18

19 NPN transistor: common base Smaller current (base-emitter) acts as a gate to allow a larger current (collector-emitter). Collector-emitter current α base-emitter current Gate: base-emitter Larger current: collector-emitter Draw this circuit diagram Source: Wikipedia 19

20 Transistor amplifies current Collector current I C is h FE times larger than base current I B I C = h FE I B Art of Electronics 20

21 Other examples Common collector circuit Which way does I B flow? Transistors are essential units in integrated circuits (IC s), which are at the basis of computer operation. 21

Lecture 12. Bipolar Junction Transistor (BJT) BJT 1-1

Lecture 12. Bipolar Junction Transistor (BJT) BJT 1-1 Lecture 12 Bipolar Junction Transistor (BJT) BJT 1-1 Course Info Lecture hours: 4 Two Lectures weekly (Saturdays and Wednesdays) Location: K2 Time: 1:40 pm Tutorial hours: 2 One tutorial class every week