Common Emitter as an Amplifier is a configuration of the basic Bipolar Junction Transistor (BJT). As it consists of three basic terminals that are base, emitter and the collector but for the input and the output circuit connections it require minimum of four terminals. In order to overcome this drawback a terminal among those three terminals made common so that it behaves in common for both input and output circuitry connections. When emitter is made common to both input and output then it becomes Common Emitter Transistor.
This is one among the three configurations of these terminals. This configuration is the most widely preferred one because it has both current and the voltage gains which produces the high power gain value. When it operates in between cut-off and the region of saturation the transistor is said to be working as switch. In order to make function as amplifier it must be operating in the region that is active.
What is a Common Emitter Amplifier?
A transistor in which the emitter terminal is made common for both the input and the output circuit connections is known as common emitter configuration. When this configuration is provided with the supply of the alternating current (AC) and operated in between the both positive and the negative halves of the cycle in order to generate the specific output signal is known as common emitter amplifier.
In this type of configuration the input is applied at the terminal base and the considered output is to be collected across the terminal collector. By keeping emitter terminal is common in both the cases of input as well as output.
Working of Common Emitter Amplifier
Let us considered a CE circuit is provided with the divider circuit of the voltage such that it is provided with the two resistors connected at the input side. In this type of configuration the base is considered to be the input terminal whereas the collector is for collecting the output.
Circuit for Common Emitter Amplifier
Other than this there are various electronic components are to be included in this circuit. One is the resistor R1 that is the one to make the transistor to function in the forward biasing mode. The R2 is responsible to make the biasing possible. There is the load resistor and the resistor that is connected at the emitter so that it controls the stability related to thermal issue. The resistors R1 and R2 connected across the terminal base as it is the input side. The load resistor is connected at the output side that is across the collector terminal.
There are capacitors as well in the circuit. The capacitor C1 is at the input side and the capacitor C2 is connected across the emitter resistor. The C1 capacitor is responsible to separate the value of the AC signals from that of DC signals. There exists the inverse relation between the R1 resistor and the biasing. As R2 tends to increase the biasing tends to increase and vice-versa. Hence to the smaller or the weaker signals that are applied to the base gets amplified at the obtained output signals. Hence this is the reason it is known as CE amplifier.
Voltage Gain
The ratio of the output voltage generated when the input voltage applied decides the voltage gain of the common emitter amplifier. Further this ratio can be equated to the ratio of the resistance at the collector to the resistance value of the emitter.
Here the load resistance is the resistor connected across the collector. The ratio in between the output current to the applied input current gives the current gain. This is denoted as beta.
In this way the values of the voltage gain and the current gain are calculated for this amplifier configuration.
Characteristics
The characteristics of the common emitter configuration amplifier configuration are as follows
- The voltage gain value obtained for the common emitter amplifier is medium.
- It also consists of the current gain in the medium range.
- Because of both the voltage and the current gains the power gain value of this configuration is referred to be high.
- There is some resistance value at the inputs as well as the output but in this configuration it is maintained at the medium value.
- As the signals at the output generated because of the input signals applied are in 180 degree shift of the phase.
These all the characteristics of the common emitter configuration make it as a most widely used one among the two remaining configurations. As the signals of input and the output are maintaining 180 degree phase shift makes the transistor configuration as an inverting amplifier. But it has the high impedance value at the output by maintaining low impedance at the input.
As the applied base voltage increases the voltage produced at the output tends to decrease and vice-versa. This is the reason of the circuit acting in an inverting manner.
Applications
The applications of this CE amplifier are as follows
- These amplifiers are preferably used as the current amplifier than a voltage amplifier as it has more current gain than the voltage gain.
- In the radio frequency circuitry this configuration is preferred.
- For the lower values of noise and its amplification this configuration is preferred.
The above are the some of the applications of the amplifier circuit. This configuration is simple in design while operating as switch it operates under cut-off and the saturating regions. But while operating as an amplifier there must be certain point in it that must be lying at the center of the active region. This point decides the output signal quality.
Please refer to this link to know more Common Emitter Amplifier MCQs
This is the basic model of the amplifier circuitry. This type of configuration is preferred in various circuits based on its efficiency of the signal amplification. Now after discussing all the details regards to common emitter configuration can you tell which configuration functions well as an amplifier?