TDC 311

1. TDC 311 Basic Electronic Circuits

2. Voltage • A battery / generator has + and - posts. Electrons flow through a circuit from the - post to the + post. • The potential difference between the two posts is V, voltage. V is the push of electrons. • The higher the voltage, the higher the push. (Figures 1 and 2)

3. Current • The current (I) is the charge passing through a given cross section of wire per unit time: • I = Δ Q / Δ t = charge / time • Consider a hose versus a fire hydrant. • The direction of a current is in the direction of positive charge motion. Since only electrons (the negative charge) move within metal, the direction of current is opposite the flow of electrons. • For example • House current • Typical CPU • Lightning strike

4. Resistance • All systems experience resistance (R). • Ohm’s Law: V = I*R where R = Ohms (Ω) • or R = V / I • As temperatures increase in metal, resistance increases. In semiconductors, resistance generally decreases. (Figures 3 and 4)

5. Watt • The measure of electric work and power is the watt. • Power = V*I • A current of 0.50 A flows through a 200 ohm resistor. How much power is lost in the resistor? (Expression 1)

6. AC and DC • All above examples are direct current (DC). The push of electrons is always in one direction. • AC - alternating current: The push of electrons is first from one direction and then from the opposite direction, over and over. (Figure 5)

7. Two ways to control power • 1. Control the amount of power put into the circuit (harder to do) • 2. Control the power at some point other than at the source (more common). (Figure 6) • Then we can switch it on or off, or regulate/vary the resistance. • Circuits are made of hundreds / thousands / ... of points that need switching and regulating. What device does this? • A vacuum tube? Yes, but something better

8. Transistor • The transistor is made of either germanium or silicon and has three distinct sections (Figure 7) • The NPN transistor can act as a variable resistor or as a switch (conduct current, throttle it partially, or block it entirely).

9. Consider the following circuit + - Emitter (N) Base (P) Collector (N) Microphone Figure 8 Speaker Base P blocks flow from emitter to collector. Need to add a wire from base so electrons have some place to flow.

10. More appropriate figure + - Microphone Speaker Figure 9

11. Basic logic gates • How were relays used to create basic logic gates? (Figures 11 and 12) • How are transistors used to create basic logic gates? (Figures 13 and 14)

12. Equivalent Transistors

14. Consider the numbers • 1GB flash drive • 230 bits (about 1 billion) • Each bit requires 2 transistors • About 2 billion transistors

15. Future Trends • Semiconductors are approaching fundamental physical size limits • Technologies that may improve performance beyond semiconductor limitations • Optical processing • Hybrid optical-electrical processing

16. Optical Processing • Could eliminate interconnection and simplify fabrication problems; photon pathways can cross without interfering with one another • Eliminating wires would improve fabrication cost and reliability • Not enough economic incentive to be a reality yet

17. Electro-Optical Processing • Devices provide interface between semiconductor and purely optical memory and storage devices • Gallium arsenide (both optical and electrical properties) • Silicon-based semiconductor devices (encode data in externally generated laser light)

18. Inductor • An inductor is basically a coil of wire. While it looks simple, it has some interesting properties (Figure 15) • If you remove the inductor from the circuit, this is just a flashlight. • What happens when you insert the inductor into the circuit?

19. Inductor • When you close the switch, the bulb burns brightly and then gets dimmer. • When you open the switch, the bulb burns brightly, then quickly goes out. • Why?

20. Inductor • The wire in the coil (inductor) has less resistance than the light bulb. But the coil wants to build up a magnetic field. While the field is building, the coil inhibits the flow of current. Once the field is built, the current can flow normally through the coil. • When the switch is opened, the magnetic field around the coil keeps current flowing in the coil until the field collapses. This keeps the bulb lit for a short period of time after the switch is open.

21. Inductor • The capacity of an inductor is controlled by four factors: • number of turns of wire • material the coils are wrapped around • cross-sectional area of the coil • length of the coil

22. Inductor • Putting iron in the core of an inductor gives it much more inductance than air would. • The standard unit of inductance is the henry: H=(4*pi*NumTurns2*AreaOfCoil*m) / (LengthOfCoil * 10,000,000) where m=permeability of core (air=2; steel=2000) • Common application area: traffic signals

23. Capacitor • A capacitor is a little like a battery. It holds a charge, but only for a brief moment. • A capacitor has two metal plates separated by a dielectric (such as air, paper, plastic, or anything else that does not conduct electricity). • What happens when you connect a capacitor to a battery? (Figure 16)

24. Capacitor • Once the capacitor is charged, it has the same voltage as the battery. • Let’s hook up a capacitor, a battery, a bulb, and a switch in the following fashion: (Figure 17)

25. Capacitor • When the switch closes, the light bulb lights up as current flows from the battery to the capacitor and the capacitor charges up. • The bulb gets progressively dimmer and finally goes out once the capacitor reaches it full charge. • When you open the switch, the light bulb brightens momentarily and then dims and goes out. The capacitor is now discharged.

26. Capacitor • The farad is the unit of capacitance. • A farad is one coulomb (6.25e 18 electrons) of charge at 1 volt. That is a lot of charge! • Most capacitors are rated in microfarads. • Common capacitor applications include storing charges for high speed use, such as in a flash of a camera. • Capacitors can also smooth (eliminate) ripples in current. • A capacitor can block DC voltage and let AC voltage through.

27. Diode • A diode is a simple device that allows electrons to flow in one direction only (Figure 18) • Diodes can be used to make sure current flows in one direction only, so if you put your batteries in backwards, it does not ruin your electronic device. • Diodes can also be used to smooth out AC voltages (Figure 19)

28. Serial vs parallel circuits • Circuits can be either serial or parallel. • Serial circuits: Figures 20 and 21 • For example, resistors in series add directly • Parallel circuits: Figure 22 • Resistors in parallel: add their reciprocals and take reciprocal of total

29. Magnetic Fields and Wires • Oersted discovered that currents in wires produce magnetic fields. • Right-hand rule: Grasp wire with thumb pointing in direction of current. Fingers point in direction of magnetic field. • Furthermore, a changing magnetic field passing through a wire will induce a current in that wire. • Crosstalk

30. Memristor? • Concept introduced in a paper in 1971; first memristor created in lab April 30, 2008 • First basic element since resistor, capacitor and inductor • Very small; can hold a 1 or 0 with or without power • Can be fashioned into non-volatile solid state memory • 100 gigabits in a square centimeter at one tenth the speed of DRAM