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]

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.

Tranzystor PNPTranzystor NPN

UBE

UBE

EMITER

BAZA

KOLEKTOR

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 IC/ IB ratio is constant. It means that some value of current in the

base is connected to defined value of current in the collector IC. 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

Cathode

Anode

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.