Diodes, Transistors and Op-Amps, Oh My!

Suggested Reading: Horowitz and Hill: pp. 44-45 (diodes and rectification),  pp. 61-76 (tranisitors), pp. 175-179 (opamps)

1. The Diode

The Diode is essentially an electronic switch, which is on if the voltage is applied in one direction (diode in “forward bias”) and off in the other direction (diode in “reverse bias”). The resistance of the diode is, ideally, either zero or infinity. Thus, the diode is one of the many devices that do not obey Ohm’s Law: the voltage across the diode does not increase linearly with the current, i.e., the relationship V = IR does not apply to a diode. In forward bias, the diode drops always 0.6 V approximately.

The DMM can be used to test a diode: just connect the DMM as an Ohmmeter, but at the setting labeled with a diode symbol. In the forward direction, the diode should give a reading of approximately 0.6, and infinity (i.e., out of range) in the reverse direction. If the diode gives different readings from those, then it is probably bad or burned.

a) Connect the following circuit:

Turn on the 5V supply. As the pushbotton switch is depressed, current flows through the circuit, as shown by the Ammeter reading. Now, reverse the diode: no current flows as the pushbutton is depressed; the reverse biased diode acts as an open switch.

b) The LED (light emitting diode) is also a diode. Check that the LED lights up when in forward bias, and it remains off when in reverse bias, as the pushbutton is depressed.

c) Measure and report the voltage drop across both the diode and the LED.

d) Replace the diode and LED combination with a Zener Diode (e.g. 1N5233), and measure the current and voltage drop when it is forward biased. Also measure the current and voltage drop when it is reverse biased.

e)   For the reverse bias situation, increase the applied voltage to 10 V and measure the current and voltage drop. What is the difference between the normal diode and the Zener diode?

2. The Transistor

The transistor is an electronic device that has two main functions: in digital electronics it is used for switching (fast on/off function), or in analog electronics it is used for amplification of voltage or current. The most common type of transistor is a 3 pin device that comes in two varieties: NPN and PNP. Note that the 4123 transistor is int eh diagram below, but you can use any standard NPN transistor, such as the 3904, which we have plenty of.

a) Hookup the circuit below (this uses an NPN) and measure the base and collector currents:

Notice that instead of two different power supplies for base and collector, we are using the same power supply for both, using a resistor R1 to make the base current small.

In our application, the small base current controls a larger collector current. The current gain, b, is defined as:

b = I_c / I_b, and it is typically ~ 100.

The LED will turn on, but dimly. Little bias current flows through the transistor base. Use a DMM to measure Ib and Ic. Calculate beta for the above transistor circuit.

b) Now connect the following circuit and adjust the pot, which controls the current into the base of the transistor, to give several different values of I_b over a wide range. At each setting, measure both I_b and I_c and calculate the value of b. Be sure to vary the pot to make the LED as bright as possible.

3. Integrated Circuits: The Op-Amp

IC’s are chips containing many electronic components assembled in complete, miniaturized circuits. They are flexible devices, which can perform different functions depending on how they are connected to a few required external components. Some IC’s are called Operational Amplifiers (op-amps) because they can be used to perform mathematical operations on input signals (e.g., sum, difference, integration, differentiation, etc.); they also have many other uses, as amplifiers, filters, voltage clamps, etc. We will start by looking at a few uses of op-amps. For your edification, a standard op-amp circuit looks like this:

You do not have to build this circuit, as we have them already built!

Build the following inverting amplifier using +/-12 Volt DC power supplies to power the op-amp. Drive it with a 1 kHz sine wave (input), and use an oscilloscope to look at the output.

Note that the amplifier symbol only has three leads, but the amplifier you have has 8 pins. See the spec sheets to see which is the V+ (+12V), V- (-12V), non-inverting input (+), inverting input (-), and Vout pins. The rest of the pins may be left unattached.

a) What is the gain? Is it as expected? What is the maximum output voltage? At what maximum frequency sine wave does the amplifier stop working well? Make qualitative observations for triangular wave inputs.

b) We expect the input impedance of the ideal amplifier to be infinite (it draws no current, hence does not affect the input circuit) and the output impedance to be zero (whatever a load pulls from the amplifier circuit does not affect the properties of the circuit). Try adding an R (try, perhaps, 1kohm and 100 ohms) from the output to ground, and measure the voltage across the various loads. Analyze the circuit to determine the output impedance (see these notes for an alternate way of measuring impedance). Does the result imply the expected output impedance?

c) Build the non-inverting amplifier in Circuit Maker, as below, and measure the gain. In CM, be sure to add a load resistor to the output (you choose the value), and have that resistor go to ground, so you have something to measure output voltage over. Show your circuit and output in your writeup.

Next time, we build some circuits that actually do more than give us an output to compare to an input!