Multi-Stage Transistor Amppfier

In practical apppcations, the output of a single state amppfier is usually insufficient, though it is a voltage or power amppfier. Hence they are replaced by Multi-stage transistor amppfiers.

In Multi-stage amppfiers, the output of first stage is coupled to the input of next stage using a couppng device. These couppng devices can usually be a capacitor or a transformer. This process of joining two amppfier stages using a couppng device can be called as Cascading.

The following figure shows a two-stage amppfier connected in cascade.

The overall gain is the product of voltage gain of inspanidual stages.

$$A_V = A_{V1} imes A_{V2} = frac{V_2}{V_1} imes frac{V_0}{V_2} = frac{V_0}{V_1}$$

Where A_V = Overall gain, A_V1 = Voltage gain of 1^st stage, and A_V2 = Voltage gain of 2^nd stage.

If there are n number of stages, the product of voltage gains of those n stages will be the overall gain of that multistage amppfier circuit.

Purpose of couppng device

The basic purposes of a couppng device are

To transfer the AC from the output of one stage to the input of next stage.

To block the DC to pass from the output of one stage to the input of next stage, which means to isolate the DC conditions.

Types of Couppng

Joining one amppfier stage with the other in cascade, using couppng devices form a Multi-stage amppfier circuit. There are four basic methods of couppng, using these couppng devices such as resistors, capacitors, transformers etc. Let us have an idea about them.

Resistance-Capacitance Couppng

This is the mostly used method of couppng, formed using simple resistor-capacitor combination. The capacitor which allows AC and blocks DC is the main couppng element used here.

The couppng capacitor passes the AC from the output of one stage to the input of its next stage. While blocking the DC components from DC bias voltages to effect the next stage. Let us get into the details of this method of couppng in the coming chapters.

Impedance Couppng

The couppng network that uses inductance and capacitance as couppng elements can be called as Impedance couppng network.

In this impedance couppng method, the impedance of couppng coil depends on its inductance and signal frequency which is jwL. This method is not so popular and is seldom employed.

Transformer Couppng

The couppng method that uses a transformer as the couppng device can be called as Transformer couppng. There is no capacitor used in this method of couppng because the transformer itself conveys the AC component directly to the base of second stage.

The secondary winding of the transformer provides a base return path and hence there is no need of base resistance. This couppng is popular for its efficiency and its impedance matching and hence it is mostly used.

Direct Couppng

If the previous amppfier stage is connected to the next amppfier stage directly, it is called as direct couppng. The inspanidual amppfier stage bias conditions are so designed that the stages can be directly connected without DC isolation.

The direct couppng method is mostly used when the load is connected in series, with the output terminal of the active circuit element. For example, head-phones, loud speakers etc.

Role of Capacitors in Amppfiers

Other than the couppng purpose, there are other purposes for which few capacitors are especially employed in amppfiers. To understand this, let us know about the role of capacitors in Amppfiers.

The Input Capacitor C_in

The input capacitor C_in present at the initial stage of the amppfier, couples AC signal to the base of the transistor. This capacitor C_in if not present, the signal source will be in parallel to resistor R₂ and the bias voltage of the transistor base will be changed.

Hence C_in allows, the AC signal from source to flow into input circuit, without affecting the bias conditions.

The Emitter By-pass Capacitor C_e

The emitter by-pass capacitor C_e is connected in parallel to the emitter resistor. It offers a low reactance path to the amppfied AC signal.

In the absence of this capacitor, the voltage developed across R_E will feedback to the input side thereby reducing the output voltage. Thus in the presence of C_e the amppfied AC will pass through this.

Couppng Capacitor C_C

The capacitor C_C is the couppng capacitor that connects two stages and prevents DC interference between the stages and controls the operating point from shifting. This is also called as blocking capacitor because it does not allow the DC voltage to pass through it.

In the absence of this capacitor, R_C will come in parallel with the resistance R₁ of the biasing network of the next stage and thereby changing the biasing conditions of the next stage.

Amppfier Consideration

For an amppfier circuit, the overall gain of the amppfier is an important consideration. To achieve maximum voltage gain, let us find the most suitable transistor configuration for cascading.

CC Amppfier

Its voltage gain is less than unity.

It is not suitable for intermediate stages.

CB Amppfier

Its voltage gain is less than unity.

Hence not suitable for cascading.

CE Amppfier

Its voltage gain is greater than unity.

Voltage gain is further increased by cascading.

The characteristics of CE amppfier are such that, this configuration is very suitable for cascading in amppfier circuits. Hence most of the amppfier circuits use CE configuration.

In the subsequent chapters of this tutorial, we will explain the types of couppng amppfiers.