Basic Amplifier

We hope that you have gained sufficient knowledge on operating point, its stability and the compensation techniques in the previous chapter. Let us now try to understand the fundamental concepts of a basic amplifier circuit.

An electronic signal contains some information which cannot be utilized if doesn’t have proper strength. The process of increasing the signal strength is called as Amplification. Almost all electronic equipment must include some means for amplifying the signals. We find the use of amplifiers in medical devices, scientific equipment, automation, military tools, communication devices, and even in household equipment.

Amplification in practical applications is done using Multi-stage amplifiers. A number of single-stage amplifiers are cascaded to form a Multi-stage amplifier. Let us see how a single-stage amplifier is built, which is the basic for a Multi-stage amplifier.

Single-stage Transistor Amplifier

When only one transistor with associated circuitry is used for amplifying a weak signal, the circuit is known as single-stage amplifier.

Analyzing the working of a Single-stage amplifier circuit, makes us easy to understand the formation and working of Multi-stage amplifier circuits. A Single stage transistor amplifier has one transistor, bias circuit and other auxiliary components. The following circuit diagram shows how a single stage transistor amplifier looks like.

When a weak input signal is given to the base of the transistor as shown in the figure, a small amount of base current flows. Due to the transistor action, a larger current flows in the collector of the transistor. (As the collector current is β times of the base current which means I_C = βI_B). Now, as the collector current increases, the voltage drop across the resistor R_C also increases, which is collected as the output.

Hence a small input at the base gets amplified as the signal of larger magnitude and strength at the collector output. Hence this transistor acts as an amplifier.

Practical Circuit of a Transistor Amplifier

The circuit of a practical transistor amplifier is as shown below, which represents a voltage divider biasing circuit.

The various prominent circuit elements and their functions are as described below.

Biasing Circuit

The resistors R₁, R₂ and R_E form the biasing and stabilization circuit, which helps in establishing a proper operating point.

Input Capacitor C_in

This capacitor couples the input signal to the base of the transistor. The input capacitor C_in allows AC signal, but isolates the signal source from R₂. If this capacitor is not present, the input signal gets directly applied, which changes the bias at R₂.

Coupling Capacitor C_C

This capacitor is present at the end of one stage and connects it to the other stage. As it couples two stages it is called as coupling capacitor. This capacitor blocks DC of one stage to enter the other but allows AC to pass. Hence it is also called as blocking capacitor.

Due to the presence of coupling capacitor C_C, the output across the resistor R_L is free from the collector’s DC voltage. If this is not present, the bias conditions of the next stage will be drastically changed due to the shunting effect of R_C, as it would come in parallel to R₂ of the next stage.

Emitter by-pass capacitor C_E

This capacitor is employed in parallel to the emitter resistor R_E. The amplified AC signal is by passed through this. If this is not present, that signal will pass through R_E which produces a voltage drop across R_E that will feedback the input signal reducing the output voltage.

The Load resistor R_L

The resistance R_L connected at the output is known as Load resistor. When a number of stages are used, then R_L represents the input resistance of the next stage.

Various Circuit currents

Let us go through various circuit currents in the complete amplifier circuit. These are already mentioned in the above figure.

Base Current

When no signal is applied in the base circuit, DC base current I_B flows due to biasing circuit. When AC signal is applied, AC base current i_b also flows. Therefore, with the application of signal, total base current i_B is given by

$$i_B = I_B + i_b$$

Collector Current

When no signal is applied, a DC collector current I_C flows due to biasing circuit. When AC signal is applied, AC collector current i_c also flows. Therefore, the total collector current i_C is given by

$$i_C = I_C + i_c$$

Where

$I_C = \beta I_B$ = zero signal collecor current

$i_c = \beta i_b$ = collecor current due to signal

Emitter Current

When no signal is applied, a DC emitter current I_E flows. With the application of signal, total emitter current i_E is given by

$$i_E = I_E + i_e$$

It should be remembered that

$$I_E = I_B + I_C$$

$$i_e = i_b + i_c$$

As base current is usually small, it is to be noted that

$I_E \cong I_C$ and $i_e \cong i_c$

These are the important considerations for the practical circuit of transistor amplifier. Now let us know about the classification of Amplifiers.