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24 February 2009
Author: Giorgos Lazaridis
BJT Transistor theory

Small Signal operation

When we discussed about the transistor operation in AC, i mentioned the term "Small Signal". It is important to know what we call "Small Signal", and why it is necessary to know the difference. The following characteristic is a typical IC to VBE input characteristic. It shows the increment of IC current in relation to the VBE base voltage.

The collector current is zero as long as the VBE voltage is less than about 0.65 volts. This is something that we've talked before many times. The VBE voltage has to do with the material that the transistor is made of. Above this voltage level, the collector current (along with the emitter current of course) climbs up rapidly. This is the typical transistor operation. What you need to notice here is the region of the characteristic around the 0.7 volts. The line seems to be curved at that point. This is a typical problem that designers face, if they want to have an undistorted signal amplification. The curve becomes more intense as temperature increases. At sub-zero temperatures things are usually much better and the curve is not so intense. The above characteristic corresponds to a temperature of around 150oC. I chose this high temperature because the distortion is more obvious.

So, let's take a closer look at the region that the transistor will work at. That's usually above 0.68V for VBE. The following diagram is a portion from the input characteristic shown above, but only for a VBE range from 0.68 to 0.72 Volts. The transistor is biased with DC and the Q is set. At that point, the VBE is stable at around 0.7 volts. Then we apply a large AC signal at the base. This signal causes the Q point of VBE to oscillate:

Although the input AC signal is symmetrical, due to the curvature of the input characteristic, the output current change is not symmetrical. The result is a distorted amplified signal which in certain applications it is totally unwanted.

Now, suppose that we apply a smaller input signal:

The difference is obvious. Although the output signal is much smaller in terms of amplitude, it seems to have almost no distortion even at that high temperature. This is normal because now we used a much smaller portion of the characteristic, and this portion can be considered as a straight line. As a conclusion we can say that if the AC input signal is small, the AC current change at the collector is proportional to the AC voltage change at the base.

But, how do we define a signal as "small"? There is a general rule to define the small signal which states that:

The AC peak to peak current of the emitter must be smaller than 10% of the DC current of the emitter.

Although the distortion will not be eliminated, it will be radically limited. The amplifiers that satisfy this 10% rule are called small signal amplifiers. They are usually used to amplify small signals such as the TV or radio signals.

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