Considering the voltage loop base-emitter, we can write the voltage V 2 across the resistance R 2 such as : V 2=R E.I E+V BE, with V BE the voltage between the base and emitter of the transistor. First of all, since the input current I in before amplification is very low, we assume that I 1=I 2. To understand why the stability is improved, a few lines of basic math are required here. The most popular and best biasing architecture in terms of stability is the voltage divider biasing method.

Any variation of the output current is thus controlled by a negative feedback to the input which will counterbalance the fluctuations of I out. The consequence is that the input current I in decreases, which вЂњcloses the tapвЂќ and makes the current I out to decrease. We can indeed understand without any formula that if I out increases due to a temperature fluctuation, the voltage across the resistance R C increases so that V out decreases. However, it is rarely employed since the stabilization in temperature of the operating point and the transistor parameters cannot be controlled.ДЄ very small modification of the previous method leads to more stability and is known as the collector to base method. This method is very simple to realize since it involves very few components. In a base resistor biasing architecture, the base resistance R B is very high and chosen such as R B=V supply/I out. The first one, known as the base resistor method is exactly the same as presented in Figure 1 only that the same power supply is shared so that V supply1=V supply2=V supply. This process of supplying a DC voltage to the base is commonly known as biasing and is very important to force the transistor to work in his active region.ДЇor the common emitter amplifier, many different biasing methods exist, but some present poor temperature stability : the operating point and parameters of the transistors vary too much with variations of temperature. This makes the circuit more simple and decreases the cost. Only one power supply is usually found in CEA amplifiers and delivers a voltage to both the base and the collector so that Equations 1 can be simplified with only one V supply. When considering only the base/emitter and collector/emitter, we can clearly establish the following loops : fig 3 : Voltage loops on a CEA configurationДЇrom Figure 3 and using KirchhoffвЂ™s law of potential, we can easily write that : First of all, we consider that no AC input signal is delivered to the amplifier, we thus only study the behavior of the CEA in DC mode. The name вЂњ Common EmitterвЂќ comes from the fact that in this configuration, the emitter electrode is linked to the ground and thus the input V in, I in and output V out, I out are measured between the emitter and the blue dot with the mention V out represented on Figure 1. The transistor acts as an opened switch.Д«iasing of a Common Emitter Transistor Configuration An increase in the base current I B does not affect the output current anymore.

Saturation region: The вЂњtapвЂќ is fully opened.In this region, the transistorвЂ™s behavior is linear and the output current is amplified by a base current amplification factor ОІ that is very dependent from the external temperature : Active region: It is the вЂњnormalвЂќ working region where the вЂњtapвЂќ is neither fully closed nor opened.Cutoff region: In this region, the вЂњtapвЂќ is closed, thus the emitter, collector and base currents are equal to zero I E=I C=I B=0.Three different working regions can be distinguished for the bipolar transistors : fig 2 : Analogy of the tap water for bipolar transistors The base acts like a tap : it controls the flow of electron through the current I out between the collector and emitter electrodes. General characteristics of bipolar transistorsДЄ bipolar transistor has three electrodes represented in Figure 1 by the letters B, C and E, respectively for Base, Collector and Emitter. A CEA configuration always presents a resistance linked to the collector where the output current and voltage are extracted.

Bipolar transistor