In these laboratory exercises, students will examine the operation of diodes and use them in circuits that will confirm the fact that the current can essentially flow in one direction only. The operation of BJT and MOSFET transistors will also be demonstrated.

Background material for this experiment can be found in Chapter 3 of this manual

Parts List:

• Protoboard with built-in power supply
• Digital multimeter
• Light bulb
• Resistors and a potentiometer
• Rectifier diode (1N4001), LEDs, BJT (2N3904 npn) and n-type MOSFET (IRF510)

Part 1 - Diodes: Forward and reverse current

Set the voltage on the DC power supply to 5V. Connect a light bulb in series with a diode and connect them to the power supply terminal through an ammeter as seen in Figure L3.2 (note the silver band around the 1N4001, representing the cathode). Measure the current with the diode connected in the forward direction (the bulb lights up) and the reverse directions (see Figure L3.3, the lamp is dark). To change the diode polarity, simply exchange the diode terminals on the protoboard (flip the diode position). What is the ratio of the reverse to the forward current? In an ideal diode it should be zero.

Repeat the experiment using an LED (note that an LED is a diode that lights up when it is forward biased) in series with a 220 Ω resistor (you should have removed the light bulb!). This circuit is shown in Figure L3.4. The cathode of the LED can be recognized by the shorter lead, or a lead closest to the flat side. The resistor is necessary to limit the current, preventing the diode from breaking down. Which polarity is needed for emitting light, forward or reverse?

Part 2 – Operation of a BJT transistor

In this laboratory exercise we will examine both switching operation and amplification of transistors. Schematics of connections to MOS and BJT transistors are shown in Figure L3.5

The BJT-based circuit, shown in Figure L3.6, and the MOSFET-based circuit will be assembled as a demonstration. However, you are encouraged to put them together for individual confirmation of the fact that both BJT and MOSFET transistors can generate substantial currents to a load given an input current for a BJT, or an input voltage for a MOSFET (remember that a BJT is current driven and a MOSFET is voltage driven). Both circuits (in this section, the BJT-based circuit is shown below using the npn BJT 2N3904) are used to switch on and off a small incandescent lamp. Resistor R1 should be substantial enough to generate a base current that is not excessive to harm the transistor but will allow a substantial current to flow through the load because of the large current gain of the BJT.

In the BJT, the current flows between the collector (C), which is positive, and the emitter (E) while the base (B) is the control electrode. Note that the schematic really represents two circuits (or circuit loops) connected by a transistor. One of them, the base circuit, consists of the power supply, the switch (S), the resistor R1, and the base and the emitter of the transistor, which is connected to ground closing the loop to the power supply. The other circuit uses the same 5V power supply, connected to the lamp and through the collector (C) and emitter (E) of the BJT, closing the loop to ground. The switch turns on or off only the current to the base (in the first circuit) but the current through the lamp (in the second loop) responds to the condition of the first circuit.

After successful demonstration of the circuit, measure and compare currents in the two loops (the base current and the collector current. How many times greater is the collector current then the base current? You should be able to demonstrate GAIN, a great asset of the transistor. Big currents (for example, motors of an electric train) can be adjusted by small currents from electronic control circuits.

A simple demonstration of such control can be accomplished using our lamp and a potentiometer in the circuit shown in Figure L3.6 above. The potentiometer (RP) in the base circuit controls the light output from the lamp in the collector circuit. Resistor R1 (3300 ohm) protects the base circuit from excessive current when the potentiometer is turned all the way up. As the arm of the potentiometer is moved downward, the voltage applied to the circuit becomes smaller (voltage division). The current applied to the base becomes smaller. Hence the collector current becomes smaller, which leads to the dimming of the light bulb. Note that this is not a circuit that is appropriate for creating a dimmer for home usage because you will still pay for the full power (the full power is dissipated in the combination of the pot and the light bulb) even though the light bulb consumes less power.

Part 3 – MOSFETs

In the circuit represented in Figure L3.7, we replaced the BJT with an n-channel enhancement mode MOSFET: the gate terminal replaces the base, the source replaces the emitter and the drain the collector, so that the drain is positive. The switch can be omitted in this part, but more importantly, the resistor R1 is not needed because the current entering the gate is zero, and there is no need to protect the transistor from an excessive current. Turn the lamp “on” and “off” by connecting the wire from the transistor gate either to the positive power supply terminal or to the ground.

Now a surprise: touch the end of the wire from the MOSFET gate terminal with one hand and touch your other hand to the ground and then to the positive terminal. This demonstrates a big advantage of MOSFET. Do you need much current to turn the MOSFET “on” (remember, the MOSFET is _______ driven)?

In the report, address questions shown in bold print in this part of the manual. Comment on the role of diodes and transistors in electrical circuits and on the important difference between BJT and MOS transistors.