Lecture - 18: Small Signal CE Amplifiers

Small Signal CE Amplifiers:

CE amplifiers are very popular to amplify the small signal ac. After a transistor has been biased with a Q point near the middle of a dc load line, ac source can be coupled to the base. This produces fluctuations in the base current and hence in the collector current of the same shape and frequency. The output will be enlarged sine wave of same frequency.

The amplifier is called linear if it does not change the wave shape of the signal. As long as the input signal is small, the transistor will use only a small part of the load line and the operation will be linear.

On the other hand, if the input signal is too large. The fluctuations along the load line will drive the transistor into either saturation or cut off. This clips the peaks of the input and the amplifier is no longer linear.

The CE amplifier configuration is shown in fig. 1.

Fig. 1

The coupling capacitor (CC ) passes an ac signal from one point to another. At the same time it does not allow the dc to pass through it. Hence it is also called blocking capacitor.

Fig. 2

For example in fig. 2, the ac voltage at point A is transmitted to point B. For this series reactance XC should be very small compared to series resistance RS. The circuit to the left of A may be a source and a series resistor or may be the Thevenin equivalent of a complex circuit. Similarly RL may be the load resistance or equivalent resistance of a complex network. The current in the loop is given by

As frequency increases,decreases, and current increases until it reaches to its maximum value  vin / R. Therefore the capacitor couples the signal properly from A to B when XC<< R. The size of the coupling capacitor depends upon the lowest frequency to be coupled. Normally, for lowest frequency XC £ 0.1R is taken as design rule.

The coupling capacitor acts like a switch, which is open to dc and shorted for ac.

The bypass capacitor Cb is similar to a coupling capacitor, except that it couples an ungrounded point to a grounded point. The Cb capacitor looks like a short to an ac signal and therefore emitter is said ac grounded. A bypass capacitor does not disturb the dc voltage at emitter because it looks open to dc current. As a design rule XCb £ 0.1RE at lowest frequency.

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