Schedule

• Last Lab today

• No class next week

• “makeup” following week. A great time to come in and practice for the exam

• Exam on May 10. see my web site for old exams. I don’t believe in creativity in lab exams.

Lab #7: Diode and Rectifier Circuits

• learn what diode is

• learn some simple circuits that use diodes

Diode•Energy levels of electrons in atoms are quantized (1s, 2s, 2p, etc)

•When these atoms are brought together in crystals, these energy levels become energy bands with gaps between them.

• The distributions of electron energies depends on the temperature. The highest possible energy when T=0 is called the Fermi Energy (EF).

• some materials can be made into semi conductors by adding small amounts of impurities. p-type (one fewer valence e than Si) and n-type (one more valence than Si).

• semi conductors do not conduct at T=0.

1 eV

Diode

Electrons migrate from n to the holes in p, giving a net charge to each side (neg on p side, pos on n side)

104 – 106 V/cm, 0.3-0.6 V

In steady state, there are conduction electrons who have enough thermal energy to go over the potential barrier and make a (positive) current from p to n. There is an equal and opposite current of electrons that get enough thermal energy to break their bonds in the p region, go up into the conduction band, and then slide down the barrier back to the n region.

Diode

+ - - +Here, the external bias aids the electrons trying to go from n-p and you get a net current in the p to n direction.

/0( 1)qV kTI I e

Here, the external voltage stops the electrons going from n to p. The ones going from p-n can still go and you get a (small) net current from n to p (the prob to get enough thermal energy to break the bond is unchanged)

Diode

/0( 1)qV kTI I e

symbol Idealized I vs V

Realistic I vs V

Half-wave rectifier

Full wave rectifier

RC differentiator

See pg 36

For t>>RC. Voltage across resistor is derivative of VIN

frequency to voltage converter

Hints• lab begins pg 70

• no error analysis this lab

• IV.A. Do 10 measurements

• IV.A be careful. RMS, not peak-to-peak

• IV.A. Try pulling/pushing the amplitude knob on the scope. You should be able to go up to about 7.5V and down to 0.1 V.

• IV.A fit the upper points to a straight line, extrapolate the line down to the lower points. Include a graph with all the points and the line extrapolated from the high voltage points.

• IV.B. take measurements for 3 different generator voltages and compare ratios of gen to vload for the various quantities for the 3 generator voltages

• IV.C. Be careful! Not the normal cap

• IV.C. USE CURSORS NOT MEASURE

• IV.C take at least 8 points

Hints

• skip V-A, V-B, and V-C