Single-state Transistor Amplifier
A common emitter transistor amplifier provides high current, voltage and power gain.
This is an NPN transistor. We can tell this because in the electrical symbol, the arrow on the emitter terminal points outwards.
The goal of the amplifier is to produce an output signal that is a scaled up, or magnified version of the input signal. Here, the signal is also inverted.
Test it out
Tap the diagram for more information and click on the buttons to change a component and see what happens!
 Show Transistor Close-up Swop Transistor Make Common Collector Make Common Base
Example calculations
In order to work with Single-state Transistor Amplifiers, electrical engineers need to understand and work with a variety of calculations. Here are just a few:
For both NPN and PNP:
$I_E = I_C + I_B$
Current Gain ($A_i$):
$A_i = \frac{Output Current}{Input Current} = \frac{I_{out}}{I_{in}}$
Voltage Gain ($A_V$):
$A_V = \frac{Output Voltage}{Input Voltage} = \frac{V_{out}}{V_{in}}$
Power Gain ($A_p$):
$A_p = \frac{A_V}{A_i}$
 Collector $C$ Base $B$ Emitter $E$ Positive $P$ Negative $N$ Collector Current $I_C$ Base Current $I_B$ Emitter Current $I_E$
An amplifier is a piece of electronic equipment used to increase the magnitude of the voltage, power or current of an input signal. Almost all electronic instruments and appliances include some sort of signal amplification, from hearing aids and communication devices, to household equipment and the radio or air-conditioning in cars.
A single-state transistor amplifier refers to a circuit where only one transistor is used. In practice, it is more common to use multi-stage amplifiers, where several single-stage amplifiers are cascaded together. But, analysing the workings of a single-state circuit makes it easier to understand the workings of multi-state amplifier circuits.
To create such an amplifying circuit, a transistor and external power source work together to increase the input signal, or waveform, to produce a bigger, but identical waveform at the output. Extra energy used to boost the signal is supplied by the outside power source, such as a battery, but the transistor determines by how much.
The transistor itself is a small device that does this by controlling the current or voltage flow of the circuit in much the same way the head of a hosepipe controls the pressure and flow of water coming out the nozzle.
The most common transistor used in single-state amplifier circuits is made of three semi-conductive parts sandwiched together in either a NPN or PNP configuration. Each of these three parts is connected to a terminal or pin: the Emitter, Base, or Collector. Electricity flows between the Emitter and Collector terminals, while the Base is used to control how “open” the valve is.
There are also several different ways a single-state transistor amplifier circuit can be configured. The demo above demonstrates Common Emitter (CE), Common Collector (CC) and Common Base (CB) configurations.
Practice Questions
Test your new knowledge on single state transistor amplifiers by answering these questions.
1. How many transistors are needed for a single-stage amplifier?
2. What does BJT stand for?
3. Which terminal in the transistor has the greatest current value?