Lab 4: Transistors 1

4-1 Transistor Junctions are Diodes

 

Introduction:

In this lab, we are to check to see if we have a bad transistor. In doing so, we will become familiar with the structure and components of a 2N3904 NPN transistor. A transistor is a semiconductor device used to amplify and switch electronic signals and electrical power. It is composed of semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor’s terminals changes the current through another pair of terminals. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal.

Procedure:

Identify the leads of a 2N3904 NPN transistor. Connect the base and emitter leads to the DVM using the diode plug. Test the voltage by applying the ‘diode test function’ and record the voltage voltage at this node. Do the same measurement but connect the base and collector leads instead.

Circuit:

Data and Calculations:

VBE = 0.6 VBC = 0.2

Conclusion:

The voltage drop from the base to emitter is similar to that of a diode drop. A smaller drop was detected in the base to collector junction because of its larger size, which creates a lower current density and less resistance, hence a smaller voltage drop.

 

4-2 Emitter Follower

 

Introduction:

 

The purpose of this lab is to test the output resulting from the transistor in the circuit and to observe how it behaves under a given input signal and DC input. An emitter follower is one of three basic single-stage bipolar junction transistor (BJT) amplifier topologies, typically used as a voltage buffer. In this circuit the base terminal of the transistor serves as the input, the emitter is the output, and the collector is common to both.

 

Procedure:

 

The function generator is set to deliver a 1k Hz sine wave to the base of the 2N3904 transistor after going through a 270 ohm resistor. The collector of the transistor receives a 15 V direct current while the emitter is connected to ground after going through a 3.3k resistor. Channel 1 of the scope (5 ms timescale and 5.00 V voltage scale) will measure the input signal of the function generator while channel 2 will measure the voltage across the 3.3k resistor to ground.

 

In the 2nd part of this section, we will connect a -15V voltage source to the emitter and compare the new output results. This change should improve the quality of the output signal.

 

Circuit:

Data and Calculations:

 

Ch.1pp=5.4 V Ch.2pp= Started at 4.5 V and went to 6.8 V

 

Graph:

The difference in peak voltage between the input and output is about .72 V which is very close to a standard ‘diode drop’.

 

Conclusion:

4-3 Input and Output Impedance of Follower

 

Introduction:

 

The purpose of this lab is to become familiar with the concept of input and output impedance of a circuit with a transistor involved. We will measure the input and output impedance of the following circuit.

 

Procedure:

 

Drive the function generator through a 100k resistor into the base of a 2N3904 transistor. Apply a 15 V voltage source to the collector and -15 V voltage source to the emitter through a 3.3k resistor. The output, which is between the 3.3k resistor and transistor, will lead to a 4.7 Fblocking capacitor to block out the DC signal.

 

Circuit:

Data and Calculations:

 

Ch.1pp (VT)= 8.4 V Ch.2pp (VL)= 7.2 V RE = 3300

 

Zin = RT[VL/(VT-VL)] = 100000(7.2/1.2) = 600000

 

(1+)RE = Zin = 180.82 True = 161

Zout = RT[(V’T-V’L)/V’L] = 1000(0.44/5.16) = 85.27

 

RB/(1+) = Zout =116.28

 

Note: Slight changes in measured values of V will have large effect on calculation of

 

Conclusion:

4-4 Single Supply Follower

 

Introduction:

 

The purpose of this lab is to observe the output signal of a single supply follower circuit. Using a single power supply instead of 2, for positive and negative, this type of circuit is great for reducing attenuation (not necessarily amplifying in this case) when driving a heavy load with an already reduced input signal.

 

Procedure:

 

Begin by connecting a DC power supply to the collector of a 2N3904 transistor and to a separate lead connected to a 130k resistor. This resistor leads to a node that connects to 3 other points, the base of the transistor, a 150k resistor that leads to ground, and the positive side of a 1F capacitor which is connected to the function generator on the negative side. The emitter will lead to the output and to a 7.5k resistor into ground. Channel 1 of scope will measure the input signal (function generator) and channel 2 will measure the signal from the output node to ground. The generator will drive a sine wave of 1k Hz. We will compare the input and output signals at both a “high” and “low” amplitude.

 

Circuit:

Data and Calculations:

 

Lower amplitude graph

High amplitude graph

Conclusion:

 

At higher amplitudes, the base voltage tries to climb above the positive supply so it becomes “clipped”.

 

4-5 Transistor Current Gain

 

Introduction:

Procedure:

Circuit:

Data and Calculations:

Conclusion:

4-6 Current Source

 

Introduction:

Procedure:

Circuit:

Data and Calculations:

Conclusion:

4-7 Common Emitter Amplifier

 

Introduction:

Procedure:

Circuit:

Data and Calculations:

Conclusion:

Handout

 

Introduction:

Procedure:

Circuit:

Data and Calculations:

Conclusion:

 

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