This is a good approach for an error less than 5%. Previously, we made the assumption that the voltage divider current I VD is many times bigger than the base current I B, about 20 times as big. Moreover, since V BE is many times smaller than V B and V B remains unchanged all the time, the emitter voltage V E remains also unchanged, hence maintaining a stable emitter current. Two transistors with different current gains can operate as amplifiers with exactly the same amplification, only because they are biased with a voltage divider. This is an amazing and unexpected result. V CE = V CC - I C x R C - I E x R E = V CC - I C (R C + R E)Īs you see, we can calculate everything we need without using any hybrid parameter. The emitter current is calculated using the Ohm's law:Īnd since the collector current is practically equal to the emitter current, we can calculate all the transistor currents and voltages: The current that flows through the voltage divider is:įrom the base voltage we can calculate the emitter voltage and the collector-emitter voltage drop as follows: Or using the classic voltage divider equation: Now we can safely calculate the base voltage as follows: This condition allows us to exclude I B from our calculations, with an error less than 5%. This means that the base current must be at least 20 times smaller than the voltage divider current. We start with the assumption that the base current (I B) is many times smaller than the current through the voltage divider (I VD). The collector and emitter currents change just a few, and the Q point remains practically stable. Due to the fact that the base voltage remains unchanged, the negative feedback works very effectively and any unwanted increment of the current gain produces an -almost- equal negative feedback. The resistor Re provides the negative feedback as explained before (Emitter Feedback Bias). The idea is that the voltage divider maintains a very stable voltage at the base of the transistor, and since the base current is many times smaller than the current through the divider, the base voltage remains practically unchanged. Therefore, let's take some time to explain this method thoroughly. In the next chapter, we will analyze each transistor connection in detail and we will be using always this biasing method. The most effective method to bias the base of a transistor amplifier is using a voltage divider.
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