Transistor

Transistors:

  • There are two types:
    • Bipolar transistors which shall be explored here.
    • Field effect transistors which shall be explored later.
  • Properly known as bipolar junction transistors (BJTs).
  • They are analogue components by nature but can be digital.
    • Remember the transistor circuit in the Capacitors section.
  • It is a switch which has no moving parts and which uses electricity to turn ON or OFF.
  • When a sensor is connected to the base pole of the transistor the transistor becomes a signal amplifier. This is because the current flowing through the base is what determines whether the transistor is conductive or not and so if even a very small current flows from the sensor the transistor will still be able to sense the current and so switch on:
    • It requires a minimum voltage of 0.6V through the base pole to switch ON.
  • The current flowing through the base wire is always small and determines whether current is allowed to pass from the collector to the emitter by determining whether or not the semi-conductor is conductive.
  • Transistors are very sensitive.
  • Transistors can be used as switches in many ways, predominately by constructing a bistable circuit with two transistors within (below).

Transistor Circuit

  • NPN transistors have a negative collector and a negative emitter as well as a positive base. This is the most common type of transistor currently used:
    • The collector pole of the transistor is always connected with the positive pole of the power source and is the pole “collecting” the current going into the transistor.
    • The emitter pole of the transistor is always connected with the negative or neutral pole of the power source and is the pole “emitting” the current that is collected by the transistor.
  • PNP transistors have a positive collector and a positive emitter as well as a negative base. This transistor sees less use than the more popular NPN transistor.
  • The base pole collects a small amount of current which determines the state of the transistor: whether it is ON or OFF. (No current through base, the transistor is OFF and vice versa).
  • A current only flows through the base pole of the transistor when the voltage across the base pole and the emitter pole is 0.6V or greater.
    • This small base current is what controls the larger current flowing through the collector and emitter.
  • Transistors can also be used to amplify current:
    • Ic = hFE × Ib
    • hFE is the DC current gain:
      • It is a ratio and so has no units.
    • Ic is the current at the collector.
    • Ib is the current at the base.
    • The emitter current is equal to:
      • Emitter Current
  • A Darlington pair involves having one transistor’s emitter leg connected to another transistor’s base leg. Such an arrangement allows the amplification of very small electrical signals that would not normally have been amplified enough to activate a certain output if only 1 transistor were used. The gain of such a transistor is equal to the individual gains of the transistors in the Darlington pair multiplied together.

Astable Multivibrator:

  • Two transistors can be used to create an astable circuit like the one below.
  • Presumably this was the precursor to the 555 astable before the invention of the 555 IC. (It is best not to use this circuit in the exam.)

Astable Multivibrator

  • If the values of the components are reduced it should be possible to cause the outputs to oscillate very rapidly:
    • This makes a high frequency oscillator if it is capable of producing a note on a loudspeaker.
      • It is possible to turn ON or OFF the alarm by adjusting a variable resistor, or a sensor that behaves like one, which is connected in series to the loudspeaker.
      • Thus you can make a burglar alarm by using an LDR:
        • If the burglar turns ON the lights when he enters your house the burglar alarm will sound:
          • This is a pathetic excuse for a burglar alarm.
    • Since bulbs cannot flash on and off fast enough to show such oscillations, we must use a loudspeaker.

Bistable Multivibrator:

  • Two transistors can also be used to make a bistable circuit like the one below.

Bistable Multivibrator

  • It is best not to use this circuit in the exam.

Monostable Multivibrator:

  • Two transistors can even be used to make a monostable circuit like the one below.

Monostable Multivibrator

  • It is best not to use this circuit in the exam.
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