Transistors as Amplifiers (Common Emitter Configuration)

In 1947, J. Bardeen and W.H. Brattain were the first to invent something the world called a transistor. This type of transistor was purely point-contact. 

In 1951, scientist William Shockley invented the bipolar junction transistor. The transistor which represented the junction transistor had the presence of p-n junctions in them. However, it was not the first time a transistor came into existence. 

Now let’s delve into the Transistors as Amplifiers (Common Emitter Configuration) study material.

Transistor as an Amplifier

Transistors act as amplifiers when increasing the strength of weak signals. When transistors are biassed in such a way, they are said to be in an active region. An active region should be of V0 and Vi, where V is the voltage. The rate of change of output with the input is the slope of the linear part of the curve. Output is Vcc- IcRc not IcRc, and because of that, it is negative and input voltage of CE amplifiers increases, output voltage decreases. So, in this condition output is out of phase with input.

Now consider that a small change in  ΔVo and ΔVi in the output and the input voltage then,

ΔV0 / ΔVi is said to be a signal voltage gain Av of the amplifier.

The circuit will behave as a CE amplifier with the voltage gain ΔV0 / ΔVi when Vbb voltage has a fixed value corresponding to the midpoint of the active region. In terms of resistors in a circuit we can express voltage gain Av.

We have, V0 = VCC

But, delta VBE is negligibly small when compared to delta IBR B in this circuit. So, the voltage gain in this CE amplifier.

Base current and the collector current would be constant when VBB has a fixed value that is corresponding to the middle point of the linear part of the transfer curve. VCE= VCC -I CR C  would be constant.

If Vi = 0 then,

Vcc = VCE + IC R L

Transistor as an Oscillator

It is an electronic device that produces a periodic, oscillating signal, often a square wave and sine wave. Basically, it converts direct current to an alternating current from a power supply. It also produces a continuously alternating wavefront without any input.

Functions of Transistor

Transistors have two different functions, meaning it acts in two distinct ways. The charge carrier moves across different regions of the transistor when the proper voltage is applied to its terminals.

• As we know, the transistor was invented to function as an amplifier which produces an enlarged copy of a signal by this device. Transistor as an amplifier is quite an interesting topic we study in detail in this article.
• Secondly, later on, it acts as a switch of equal importance.

Transistor Characteristics & Its Applications

There are only three terminals in the transistor : emitter, base, and collector.

The transistor can be configured in three ways: Common emitter (CE), Common Base (CB), Common Collector (CC).

In CE configuration, the transistor is most widely used.

CE Configuration

In CE configuration, there are two type of characteristics:

• INPUT CHARACTERISTICS – Input is between the base and the emitter. The variation between base current Ib and the base-emitter voltage Vbe is called input characteristic.
• OUTPUT CHARACTERISTICS – Output is between the collector and the emitter. The variation between collector current Ic and the collector-emitter voltage Vce is called output characteristic.

For the dependence of Ib on Vbe, the common-emitter voltage Vce should be kept fixed. For obtaining the input characteristic, the transistor should be in an active state and also the collector-emitter voltage Vce should be kept large enough so that the base-collector junction is in reverse bias. The reverse bias across the base-collector junction is high to operate the transistor as an amplifier. So, an increase in common emitter voltage appears as an increase in common base voltage and also its effect on base current (Ib) is negligible.

Conclusion 

This amplifier is used in communication at long distances because of its high intensity obtained by the output. It is also used in radio signals. n-p-n and p-n-p junctions are used as the best transistor as an amplifier. There are three types of transistor amplifiers: a common emitter, a common base, a common collector. It acts as an amplifier only when it is in the active region of a linear circuit. It is also used to generate electric signals, for amplifying purposes, and also for controlling signals. In simple words, an amplifier is a type of device which amplifies the power or voltage of signals.