Transistors

Transistors are current operated devices which perform two functions:

• An electronic switch
• Amplify current - so are used as transducer drivers

They are made up of layers of semi-conductive material which means that they will allow current to pass through them under certain conditions - in this case if a current is present at the base terminal. Depending on these layers transistors could be either NPN, or PNP but in Intermediate 2 we only need to look at NPN transistors.

Transistors have 3 legs:

The base leg is the "trigger" leg - if there is a current here, the transistor will saturate and allow current to flow between the collector and the emitter (in the direction of the arrow on the emitter).

The Transistor as a Switch

If there is no current at the base leg, the transistor material will act as an insulator and no current will flow.

If there is a current at the base the transistor material will act as a conductor and current will flow from the collector to the emitter.  (This is just like you pushing a switch with your finger, allowing current to flow through a switch.)

It only takes a small base current to switch on the transistor.  When it is fully switched on we say that the transistor is saturated.  The voltage required to produce a current which will saturate a transistor is 0.7V.

The Transistor as an Amplifier

The nature of the transistor will see that a small base/input current, will be amplified and the collector current will be a lot greater.  Technically the base current and the collector current join at the emitter but because IB is so small in comparison to IC we say that IC ~ IE

Consider this circuit.

Because the lamp is on, the transistor must be saturated and VBE = 0.7V.

The gain of the transistor is found using the equation HFE = IC / IB

So if IB was 8mA and the IC was 800mA, the gain of the transistor would be 100.  That means that it has amplified the input current by 100x.

Transistor Calculations:

Calculate the current gain of the transistor in this circuit:

First we can work out what we know:

VBE = 0.7V because the lamp is on, the transistor must be saturated.
VR = Vsignal - VBE
      = 5.3V

We can then work out the base current:

IB = VR/R
    = 5.3/1000
    = 5.3mA

And the collector current:

IC = VL/RL
    = 6/150
    = 40mA

Now that we know both the base current and the collector current we can work out the transistor gain:

HFE = IC/IB
        = 40 / 5.3
        = 7.55

Note that there is no unit for current gain.

Calculate the voltage dropped over the lamp in this circuit:

First we can work out what we know:

VBE = 0.7V because the lamp is on, the transistor must be saturated.
VR = Vsignal - VBE
      = 4.3V

 If we know the voltage dropped over the base resistor we can work out the base current:

IB = VR/R
    = 4.3/1000
    = 4.3mA

If we know the base current and the current gain we can work out the collector current:

IC = HFE x IB
     = 10 x 4.3x10-3
     = 43mA

If we know the current flowing through the lamp and its resistance we can work out the voltage dropped over it:

V = ICR
   = 43x10-3 x 150
   = 6.45V