Lab no. 4 Bipolar Transistor (NPN and PNP)

Lab no. 4 Bipolar Transistor (NPN and PNP)

Transistors are semiconductor devices that enable to control the flow of large current by much smaller current. Bipolar transistor consists of three areas of semiconductor that have opposite type of conductivity (n-p-n or p-n-p). These areas create two junctions: n-p and p-n or p-n and n-p. That is how we distinguish two types of bipolar transistors npn and pnp. Tranzystor PNP Tranzystor NPN

Structure of bipolar transistor [http://home.agh.edu.pl/~maziarz/labpe/bipolarne.html]

KOLEKTOR BAZA U BE Tranzystor PNP EMITER U BE Tranzystor NPN When in NPN transistor voltage on base (BAZA) is higher than voltage on emitter (EMITER) by about 0,7V then transistor starts conducting current. When in PNP transistor voltage on base (BAZA) is lower than voltage on emitter (EMITER) by about 0,7V then transistor starts conducting current.

Characteristics and modes of work of a transistor [http://home.agh.edu.pl/~maziarz/labpe/bipolarne.html]

Check how does the transistor work. Set the step for 1kOhm potentiometer on 1% (Increment)

At the begining voltage on collector is practically equal to supply voltage (transistor does not conduct current). When voltage on base increases collector voltage is decreasing until it reaches few hundred of milivolts. It is called the saturation of transistor. Large current that is flowing through it is causing almost whole supply voltage to be placed on R2 resistor.

The I C / I B ratio is constant. It means that some value of current in the base is connected to defined value of current in the collector I C. The current in the base can be changed in order to obtain b-times higher changes in collector s current. This way the input signal of the transistor (in the base) is amplified by the transistor. Higher power of the signal in collector s circuit is obtained in the expense of the power from supply. b parameter (current amplification is a crucial parameter of the transistor) For silicon NPN transistors this parameter is equal to from about few hundred (low power transistors) to few tens (high power transistors)

Transistor as a switch Build a circuit like in the picture. After starting the simulation please compare input signal in the base of the transistor with voltage on the collector. High voltage on the base (above 0,7V) causes the transistor to switch on. Current is flowing through R1 resistor, voltage on collector is equale to 0V.

This circuit operates as NOT element. It is used in digital electronics: Voltage on the input is low, then voltage on the output is equal to 5V. And the opposite way 5V on input, then almost 0V on the output. By putting voltage on the base of transistor we can turn it on and then voltage on the collector is almost equal to the potential on the emitter.

To observe these dependencies please put a ligth bulb in the collector s circuit (from Indicators library). Place similar bulb in base circuit. Both bulbs should have one pole connected to 5V.

Build the circuit with PNP transistor

Transistor keys are used to control motors of direct current or relays. Relays are inductive elements in which the energy is stored. When the key changes the state there appear backward voltages which can damage the transistor. Build circuit as in figure and observe these overvoltages. Relays can be found in Basic -> Relay.

Overvoltage is marked red

Transistor can be secured by placing a diode in paralel with the coil of relay. Modify the schematic. Overvoltages dissapeared.

Change the schematic using PNP transistor and place the security diode.

Transistor as voltage amplifier: common emitter

Build the circuit

Measure the ratio between ampitudes of output and input signals voltage gain. Modify values of the elements as in the figure and measure the gain again. Which element was changed and how it influenced the voltage gain.

Increase the amplitude of input signal and observe the distorted output signal. For different amplitude output signal will be a sinusoid.

Super Alpha or the Darlington transistor Build the circuit as shown and notice the position of the potentiometer when the lamp turns on. The resistance value depends on the gain of the transistor.

Super Alpha or the Darlington transistor The combination of the two transistors makes the gain of the circuit higher the lamp turns on for a different setting of the potentiometer.

Super Alpha or the Darlington transistor In the Darlington circuit the gain of current in both transistors (altogether) is equal to the product of the gain factors of each transistor. Darlington enables the construction of touch buttons. The transistor key can be turned on simply by touching the base of the Darlington and the positive terminal of the voltage source.

Practice with NI ELVIS

Bipolar transistor Outputs of BC 180 transistor PNP Transistor NPN Transistor

BC548 Transistor

Outputs of LED Cathode Anode Cathode Anode

Transistor as a switch Set the generator to rectangle wave with 5V amplitude, 2.5V offset and f=1khz. Use the osciloscope to notice how the output signal is placed with respect to the input signal.

Logical negation Rectangle wave on generator with f = 1Hz (voltage settings should not be changed). Observe how the LEDs behave.

The main parameter of bipolar transistor is the current gain. It is the ratio of collector current (current that is controlled) to the base current (current that controls). The combination of the two transistors in the so-called Darlington configuration (Super Alfa) ensures that the gain factor of such a system is the product of the amplification factors of both transistors.

Build the circuit. Use two wires as the test points. Watch how LED behaves when you squeeze the wires with your hands. Common application for the Darlington transistors was the touch switch.