Transistors are can be configured in three different ways depending on whether the common terminal in between the input and output ports is base, collector or emitter and are named common base, common collector and common emitter, accordingly. Further, by choosing an appropriate biasing point, one may make the device suitable for either amplification or for switching i.e. the transistor circuits when made to operate between cut-off and saturation regions will be suitable to be used as switches while act as amplifiers when made to operate in their active region.
In addition, it is to be kept in mind that the transistors are inherently nothing but the current-controlled devices wherein a small change in the base current, IB results in a large variation in the collector current, IC.
Figure 1 shows a simple common emitter circuit which uses an npn transistor whose
- Collector terminal (output terminal) is connected to supply voltage VCC through the collector resistor RC.
- Base terminal is provided with the AC signal which needs to be amplified.
- Emitter terminal is grounded (hence also referred to as Grounded Emitter configuration).
In this kind of arrangement, as the input voltage Vi increases, the base current IB also increases which inturn increases the collector current IC.
This causes an increase in the voltage drop across the collector resistor, RC which results in a decreased output voltage V0 as emphasized by the following relationship
Similarly as the input voltage goes on decreasing, IB and hence IC decrease, due to which the voltage drop across RC also decreases thereby increasing the output voltage. This indicates that for the positive half-cycle of the input waveform, one would get amplified negative half-cycle while for the negative input pulse, the output would be a amplified positive pulse. Hence there exists a phase-shift of 180o between the input and the output waveforms of the common emitter amplifier for which it is also referred to as Inverting Amplifier.
However inorder to obtain an undistorted amplified version of the input waveform, nothing but faithful amplification, transistor needs to be biased properly by setting a suitable operating point (Q-point). This indicates that practically one has to resort to a stable network (Figure 2) which will be resistant to the changes in temperature and other transistor parameters.
In the circuit shown by Figure 2, the resistors R1 and R2 are used to provide bias for the base of the transistor (voltage-divider transistor biasing) while the emitter resistor RE is used to ensure that proper DC conditions are maintained for the circuit by regulating the amount of DC feedback. Further the circuit also employs the capacitors Ci and Co which are the decoupling capacitors used to provide AC coupling between the amplifier stages. The values of these capacitances are chosen to such that they provide negligible reactance at the frequency of operation. In particular, the value of the input capacitance Ci should be chosen to be equal to the resistance of the input circuit at the lowest frequency such that it results in a -3dB fall at this frequency. In addition, the value of the output capacitor Co is chosen so that it is equal to the circuit resistance at the lowest operating frequency.
Further the emitter voltage VE is chosen to be 10% of the supply voltage VCC to ensure a good level of DC stability and the current through R1 which is I1 is chosen to be 10 times the required base current. Here it is to be noted that, even I2 will be of almost the same value as the base current IB will be negligible. The emitter bypass capacitor CE when added into the circuit, increases its gain considerably by short-circuiting the emitter resistance RE for high frequency signals, which results in the reduction of the overall transistor load. The value of this CE is chosen such that the capacitor offers a reactance value which is equal to the 1/10th of RE at the lowest operating frequency.
Having known the design strategies for the common emitter amplifier, one would be interested to know the mathematical expressions for its current and the voltage gains which are given as
These common emitter amplifiers are most widely used, say for example as low noise amplifiers and radio frequency amplifiers, as they offer medium input resistance, medium output resistance, medium voltage gain, medium current gain and high power gain.