An amplifier is used to convert a low-amplitude signal into a high-amplitude signal. The input characteristics of the transistor show that the BE voltage has a range where the current varies in a nearly linear fashion. This is where the input signal must be shifted by DC current to control the base of the transistor properly.
Similarly, the output must be set so that the amplifier always uses the transistor in the active range. If the signal is accidentally saturated or is in the cut-off region, where the transistor is fully open or closed, the information is lost and the amplifier is distorting. The DC component of the output is filtered by a capacitor.
The advantage of this circuit is that the operating point can be easily adjusted by the base resistor, but an important disadvantage is that this point shifts due to heat. This configuration is not stable, the heating of the transistor leads to distortion. In addition, even identical transistors may not have the same beta.
$ I_B = \dfrac{V_0 - V_{BE}}{R_B} $
$ I_C = \beta ⋅ \dfrac{V_0 - V_{BE}}{R_B} $
$ V_{CE} = V_0 - I_C ⋅ R_C $
Building in an emitter resistor increases the stability of the operating point, reducing the dependence on temperature and beta. If the collector current increases due to a rise in temperature, the emitter current and the emitter resistor voltage also increase. However, this increase in the voltage across the emitter resistor reduces the base-emitter voltage, thus weakening the gain, reducing the collector current. The circuit is essentially self-regulating. Similarly, the change in current gain is also compensated in this way.
$ I_B = \dfrac{V_0 - V_{BE}}{R_B + (1 + \beta ) ⋅ R_E} $
$ I_C = \beta ⋅ \dfrac{V_0 - V_{BE}}{R_B + (1 + \beta ) ⋅ R_E} $
$ V_{CE} = V_0 - I_C ⋅ R_C - (1 + \beta) ⋅ I_B ⋅ R_E $
By using a voltage divider instead of a single base resistor, the stability of the amplifier is further increased, with the divider adjusting the voltage between base and ground, thus minimising the operating point shift due to variations in transistor parameters. It is important to know that in all of the circuits the output signal is inverted relative to the input, or in other words, the amplifier causes a 180 degree phase shift.
$ I_B = \dfrac{V_{th} - V_{BE}}{R_{th} + (1 + \beta ) ⋅ R_E} $
$ I_C = \beta ⋅ \dfrac{V_{th} - V_{BE}}{R_{th} + (1 + \beta ) ⋅ R_E} $
$ V_{CE} = V_0 - I_C ⋅ R_C - (1 + \beta) ⋅ I_B ⋅ R_E $
Signals can be amplified using a number of different amplifier types. There are differential amplifiers, class A, class B, class AB and class C amplifiers, all of which have their uses. There is also a class D amplifier, but it works differently from conventional analog solutions. It converts the input signal into a high-frequency square wave using a triangular signal as a reference, amplifies and filters it. It is very efficient.