Output characteristics curves are applied to the transistor for transistors with the same β value.Ī DC “Load Line” can also be drawn onto the output characteristics curves to show all the possible operating points when different values of base current are applied. With reference to the common emitter configuration shown below, a family of curves known as the Output Characteristics Curves, relates the output collector current, ( Ic ) to the collector voltage, ( Vce ) when different values of Base current, ( Ib ). This common emitter amplifier configuration using an NPN transistor has many applications but is commonly used in audio circuits such as pre-amplifier and power amplifier stages. Without this “Bias Voltage” only one half of the input waveform would be amplified. The result is that the transistor is always operating halfway between its cut-off and saturation regions, thereby allowing the transistor amplifier to accurately reproduce the positive and negative halves of any AC input signal superimposed upon this DC biasing voltage. A “Class A Amplifier” operation is one where the transistors Base terminal is biased in such a way as to forward bias the Base-emitter junction. One such Common Emitter Amplifier configuration of an NPN transistor is called a Class A Amplifier. If a suitable DC “biasing” voltage is firstly applied to the transistors Base terminal thus allowing it to always operate within its linear active region, an inverting amplifier circuit called a single stage common emitter amplifier is produced. The Common Emitter Configuration.Īs well as being used as a semiconductor switch to turn load currents “ON” or “OFF” by controlling the Base signal to the transistor in either its saturation or cut-off regions, Bipolar NPN Transistors can also be used in its active region to produce a circuit which will amplify any small AC signal applied to its Base terminal with the Emitter grounded. Note that Beta has no units as it is a ratio.Īlso, the current gain of the transistor from the Collector terminal to the Emitter terminal, Ic/Ie, is called Alpha, ( α ), and is a function of the transistor itself (electrons diffusing across the junction). The value of β can be large up to 200 for standard transistors, and it is this large ratio between Ic and Ib that makes the bipolar NPN transistor a useful amplifying device when used in its active region as Ib provides the input and Ic provides the output. The current in a bipolar NPN transistor is the ratio of these two currents ( Ic/Ib ), called the DC Current Gain of the device and is given the symbol of hfe or nowadays Beta, ( β ).

However, this only happens when a small biasing current ( Ib ) is flowing into the base terminal of the transistor at the same time thus allowing the Base to act as a sort of current control input. Then we can see that the transistor is a current operated device (Beta model) and that a large current ( Ic ) flows freely through the device between the collector and the emitter terminals when the transistor is switched “fully-ON”. This link between the input and output circuits is the main feature of transistor action because the transistors amplifying properties come from the consequent control which the Base exerts upon the Collector to Emitter current. So in a NPN Transistor it is the movement of negative current carriers (electrons) through the Base region that constitutes transistor action, since these mobile electrons provide the link between the Collector and Emitter circuits. The Base supply voltage V B is connected to the Base resistor R B, which again is used to limit the maximum Base current. The Collector is connected to the supply voltage V CC via the load resistor, RL which also acts to limit the maximum current flowing through the device. Then the voltage sources are connected to an NPN transistor as shown.

Npn vs pnp mosfet