The N-P-N and P-N-P Junctions

In this section we will look into two-junction devices, generally called transistors. Transistors were developed in the late 1940's by researchers in the Bell Telephone Laboratories. The discovery was based on earlier works by Lilienfeld. The dual junction crystal structures of the transistor provides the fundamental solid state architecture that allows flow-control to be placed on the flow of electrons. When transistors came on the scene, these three-legged devices provided the innovation that fueled a sweeping change in electronic technology. They started replacing vacuum tubes (Flimming valve) in the 1950's and early 1960's. No longer were we confined to simple circuits taking up the space of a shoe box or two. The shoe box could hold a large array of technology. Engineering started thinking about electronic miniaturization. Before long, this technology gave way to intergrading tens, then hundreds, then thousands, then millions of the NPN, and PNP devices into a single wafer device.

In this section we will study transistors to understand how they work and play together. Regardless of how powerful your computer, your controller, your stereo, or your device is, eventually you may want to output the results to do actual work, and that interface is based on the theories of transistorized IO devices. Who knows, you might want to drive a robot, build a sun tracking solar array, a DYI wind turbine for the cabin, an HO train controller, or just make some yard art like a wooden Owl with flashing eyes.

These are the schematic drawing symbols for the basic NPN and PNP transistors. In both cases the emitter arrow is pointing toward the negative portion of the circuit The named NPN and PNP are based on the construction materials, thus the way each is placed into a circuit. The three legs of the transistor are called the Emitter, Base, and Collector. The base lead is the control lead in the transistor circuit. What that means is that it takes very little change in base current (emitter-base current) to cause a large change in the current flowing between the Emitter-Collector junction. The base is an "electron" valve or gate that manages how many electrons flow through the transistor at a time.

The NPN Circuit

1. Quick look at the NPN transistor
2. When there is no base current flowing in the
emitter-base junction, the circuit is off.
3. With a small emitter-base potential (0.7 V)
the current starts to flow from the emitter
to the base and the transistor turns on.
4. Activating the base current reduces the
emitter-collector junction barrier and
emitter-collector current starts to flow.
5. Small change in base current gives large
change in collector current.

Controlling an LED with an NPN Transistor

Experiment with the NPN transistor and an LED:
Review the Resistor Color Code chart and wire up the kit as described. Perform the lab, and record your results.

Parts List:
1 9-volt battery
1 Battery power clip
1 2n3904 transistor - Q1
1 1 k ohm resistor - R1
1 100 k ohm resistor - R2
1 red light emitting diode
1 experimenters board
Miscellaneous:
hookup wire


In this experiment we will be using a 2n3904 NPN transistor to control the power for a light emitting diode (LED). By touching the loose end of R2, the 100K resistor, to the V+ side of the 9V battery, the transistor emitter-base junction will be activated. This will turn on the emitter-collector junction and current will start to flow through the transistor and LED. The LED will start emitting photon energy.

The PNP Circuit

1. Quick look at the PNP transistor circuit.
2. The same basic rules apply, however the PNP
sits in the circuit as a complement to the NPN.
3. When there is no base current flowing in the
emitter-base junction, the circuit in off.
Please Note: The emitter is tied to the more
positive portion of the circuit.
3. Lowering the emitter-base potential (0.7 V),
starts current flowing in the emitter-base
junction, turning on the transistor.
4. Activating the base current reduces the emitter-collector junction
barrier and emitter-collector current starts to flow.
5. Small change in base current gives large change in collector current.


The Push Pull Power Amplifier:
OK, If all is clear to this point, it is time to place both transistors into a common circuit.

In this circuit the output voltage at B follow the input voltage A with respect to common ground. We will see that a small change at Test Point A, will give a larger change at test point B. This circuit is a current amplifier.

There is a negative power supply and a positive power supply voltage with respect to ground (0 Volts). Both the input and output voltage can go through the 0 voltage point both in a positive and in a negative direction. For this example, consider the junction voltages for both transistors to be about 0.7 volts, resistors R1, R2, and R3 are 1K and R4 is 10K, and both batteries are about 5 volts.
At the start, consider the voltage at Test Point A to be 0 volts. This means both transistor base voltages are 0 volts. Test Point B is also at 0 V, and no current is flowing through R3. NOTE: R3 is tied between power supply common (0 V) and Test Point B.
With both Q1 and Q2 off the collector of Q1 is +5 volts and the collector of Q2 is -5 volts.

Hang on, because here we go!
We will provide an external potential and adjust Test Point A of +2.0V. Just at that moment, transition Q1 will see its Emitter-Base (E-B) voltage attempt to raise. Q1 will react, wanting to keep this junction voltage at a proper level (around 0.7 V). To do this it will turn on and allowing current to flow, creating a voltage drop across R3 suitable enough to restore the Q1 junction voltage back to the desired point. Q1 will also have emitter to collector current flowing. At the same time Q2 will not see any need to turn on, so its E-B junction will be off (Q2 will be acting as an insulator).

When Test Point A is set to -2.0V then Q2 will see the change in its Emitter-Base (E-B) voltage and attempt to correct the junction voltage back to about negative 0.7 V. In a PNP the Emitter it the most positive voltage. At the same time Q1 will shut off and not see any need to turn on so its where it E-B junction will be off (Q1 will be acting as an insulator). Now Q1 will have no current flowing and Q2 will have emitter to collector current flowing. In the Q2 circuit the ground or 0 voltage level is the highest voltage in the circuit and the battery is providing a -5 volt supply to the circuit.
The basic output voltage swing is about +1.3 volts to -1.3 volts. If the values of R1, R2 and R3 were reduced in resistance the circuit would work about the same but would drive more current through the load resistor R3.

This push-pull amplifier arrangement called emitter-follower which means the emitter for each transistor follows its base. The current amplification function for push-pull circuits is to supply power to its output over that of signal amplification.

Many times a signal amplifier will be needed to front-end and drive a Push-Pull amplifier being a power amplifier takes a small amount of power at its input and provides a larger amount of power at its output.


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