Even though single-state amplifiers are voltage or power amplifiers, their output is usually insufficient in practical applications. Hence, transistor amplifiers with multiple stages are replacing them. Why are we using different stages of amplifiers?
A coupling device couples the output of the first stage to the input of the next in multistage amplifiers. In most cases, these coupling devices are capacitors or transformers. Using a coupling device, two amplifier stages are joined using cascading. Voltage gains AV1 and AV2 are the voltage gains of the first and second stages, respectively. The overall gain of a multistage amplifier circuit will be calculated by taking the product of the voltage gains of each of the n stages.
Purpose of the coupling device
Coupling devices serve the following purposes:
- To transfer AC from one stage’s output to the next stage’s input.
- Isolate DC conditions by blocking DC from passing from one stage to the next.
Types of Coupling
Why are we using different stages of amplifiers? Multistage amplifiers are formed by cascading amplifier stages with coupling devices. These coupling devices, such as resistors, capacitors, transformers, etc., can be used in four primary ways. We should get an idea of what they are.
Using a simple resistor-capacitor combination is the most commonly applied coupling method. Capacitors allow AC and block DC. Why are we using different stages of amplifiers?
The coupling capacitor transfers AC from one stage to the next. They block DC bias voltages to affect the next stage while blocking DC bias voltages on DC components. This coupling method will be discussed in more detail in the following chapters.
Impedance coupling networks use inductance and capacitance as coupling elements.
This impedance coupling method is based on the coupling coil’s inductance and signal frequency. Despite its popularity, this method is seldom used.
Transformer coupling is a coupling method that uses a transformer as the coupling device. Because the AC component is transmitted directly to the base of the second stage, there is no need for a capacitor to be used in this method of coupling.
A transformer’s secondary winding provides a base return path, so there is no need for a base resistance. Due to its efficiency and impedance matching, it is a popular coupling.
Direct coupling occurs when the previous amplifier stage is directly connected to the next amplifier stage. Amplification stages can be directly connected with such action due to the biased conditions of each stage. Why are we using different stages of amplifiers?
In direct coupling, the load is connected in series with the output terminal of the active circuit element. An example includes headphones, loudspeakers, etc.
Role of Capacitors in Amplifiers
Few capacitors are used explicitly in amplifiers for purposes other than coupling. To understand this, it is essential to understand the role of capacitors in amplifiers.
The Input Capacitor Cin
A capacitor Cin, located at the base of the transistor, couples the AC signal to the base of the amplifier at the beginning. If capacitor Cin, the signal source will be parallel to resistor R2, and the bias voltage of the transistor base will change.This way, Cin allows the AC signal from the source to flow into the input circuit without affecting the bias conditions.
The Emitter By-pass Capacitor Ce
Parallel to the emitter resistor is the emitter by-pass capacitor Ce. AC signals are amplified through a low-reactance path.If this capacitor is not present, the voltage developed across RE will feed back to the input side, reducing the output voltage. In the presence of Ce, an AC will pass through it.
Coupling Capacitor CC
CC is the coupling capacitor between two stages that prevents DC interference and controls the operating point. It is also referred to as a blocking capacitor since it does not allow DC voltage to pass through it. Why are we using different stages of amplifiers?
Due to the absence of this capacitor, RC will come into parallel with the resistance R1 of the next stage’s biasing network, thereby changing its biasing conditions.
It is critical to consider the overall gain of an amplifier circuit. Let’s use cascading transistors to find the most effective transistor configuration for maximum voltage gain.
- Voltage gain is less than unity.
- Intermediate stages should not use it.
- Voltage gain is less than unity.
- Therefore, it cannot be cascaded.
- It has a voltage gain greater than unity.
- The voltage gain is further increased by cascading.
Why are we using different stages of amplifiers? Considering the characteristics of CE amplifiers, this configuration is very suitable for cascading in amplifier circuits. Therefore, most amplifier circuits use CE configurations. Following this tutorial, we will describe the types of coupling amplifiers.