Common Emitter Amplifier

Transistors can be configured in three ways based on the common terminal: common base, common collector, or common emitter. By choosing the right biasing point, transistors can be used for amplification or switching. When operating between cut-off and saturation, they act as switches, and when in the active region, they act as amplifiers.

It’s important to remember that transistors are current-controlled devices. A small change in the base current (I_B) leads to a large change in the collector current (I_C).

Figure 1 shows a simple common emitter circuit which uses an npn transistor whose

• Collector terminal (output terminal) is connected to supply voltage V_CC through the collector resistor R_C.
• 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 setup, as the input voltage (V_i) increases, the base current (I_B) also increases, which in turn increases the collector current (I_C).

This increase causes a larger voltage drop across the collector resistor (R_C), leading to a decreased output voltage (V₀).

Similarly as the input voltage goes on decreasing, I_B and hence I_C decrease, due to which the voltage drop across R_C 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 180^o 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 R_E 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 C_i and C_o 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 C_i 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 C_o is chosen so that it is equal to the circuit resistance at the lowest operating frequency.
The emitter voltage (V_E) is set to 10% of the supply voltage (V_CC) for good DC stability. The current through R₁ (I₁) is ten times the base current. Adding an emitter bypass capacitor (C_E) increases gain by short-circuiting the emitter resistance (R_E) for high-frequency signals, reducing the overall transistor load. The value of C_E is chosen so its reactance is 1/10^th of R_E 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.