Coupling in Amplifier, Types, Application, Advantages

In simple words, Coupling in an Amplifier means the method of connecting multiple stages of an amplifier in a cascade. If the gain of a single amplifier is low or insufficient to drive the load then we need to use multiple stages in a cascade.
However, we cannot directly connect the output of one stage to the input of the next stage. If we connect two stages directly, the DC biasing of the amplifier will be affected and noise will occur.

Need of Coupling in Amplifier

1. Coupling in an amplifier is needed to connect multiple successive stages in a cascade.

2. Proper amplifier coupling is needed to avoid the effect in DC biasing when multiple amplifier stages are connected.

3. Coupling is needed to increase the overall gain of the amplifier.

4. Coupling is needed to reduce noise when multiple amplifier stages are connected.

5. Proper coupling is needed to reduce the wastage of power.

Different Types of Amplifier Coupling

There are different types of coupling that can be used to connect multiple amplifier stages according to gain, frequency level, distortion, and application requirements. Here different types of amplifier coupling are explained with applications and advantages.

Direct Coupling in Amplifier

When the output of an amplifier stage is directly connected to the input of the next stage then it is called Direct Coupling. In the Direct Coupling technique, no coupling device(such as a resistor, capacitor, or inductor) is used.

Here, you can see in the below figure a three-stage direct-coupled amplifier is shown. Here, the output of Transistor 1 is directly connected to the input of transistor 2, and the output of transistor 2 is directly connected to the input of transistor 3.

Here, you can also see the first transistor is NPN and the second transistor is PNP, again the third transistor is NPN. This is because the variations in one transistor will cancel the variations in others. If the same transistor is used then gain will decrease, and distortion will come.

Direct Coupled Amplifier Applications

1. Direct Coupled Amplifiers are used for low-frequency applications, such as sensors, transducers, etc.

2. Direct Coupled Amplifiers are used to amplify DC signals also.

3. They are used for low-current applications such as Buzzer, Tonner, etc.

Advantages of Direct Coupled Amplifier

1. It does not use any coupling elements that's why the circuit is very simple and easy to make.

2. This circuit is very low-cost.

3. This circuit can amplify both the AC and DC signals.

Disadvantages of Direct Coupled Amplifier

1. It is suitable for low-frequency applications only.

2. It is suitable for low-current applications only.

3. It has a very low bandwidth.

4. The Q-point is not stable due to temperature variations.

RC Coupling in Amplifier

In the below figure, you can see a Two-stage RC Coupled amplifier is shown. Here you can see, there are two transistors are connected. The output of the first transistor is connected to the input of the second transistor through a capacitor Cc which is called a coupling capacitor.

Here, you can also see a capacitor Ce is connected in parallel with resistor Re, this is the bypass capacitor used to provide the low reactance path for unwanted noise signals. The resistance RL is used as a load Impedance. So as the stages of the amplifier are connected with coupling devices resistors and capacitors that's why it is called an RC Coupled Amplifier.

RC Coupled Amplifier Applications

1. RC-coupled amplifiers are used for RF signal and audio signal amplifying purposes.

2. RC-coupled amplifiers are used for the preamplification of signals which generated from microphones, audio devices, and signal transmission and distribution systems.

3. These amplifiers are used in Radio and TV communication systems as a small-signal amplifiers.

4. RC-coupled amplifier is used as voltage amplifiers.

Advantages of RC Coupled Amplifier

1. It is suitable for high-frequency applications.

2. Its Q-factor is stable and has a smooth bandwidth.

3. It provides constant gain over a wide frequency range.

4. As it uses cheaper coupling devices such as resistors, and capacitors, it is low-cost and economical.

Disadvantages of RC Coupled Amplifier

1. The main disadvantage is it has very poor impedance-matching characteristics.

2. Due to the effective load resistance, it provides low voltage and power gain.

3. It is not suitable for Low-frequency applications.

4. These amplifiers become noisy with increasing their age.

Transformer Coupling in Amplifier

When the output of the amplifier stage is connected to the input of the next stage through a transformer, then it is called a transformer-coupled amplifier. The transformer used to couple two stages is called a coupling transformer.

Here, you can see in the below figure, a two-stage transformer-coupled amplifier is shown.

Here, you can see the collector load of the first transformer is replaced by the primary winding of the transformer. The secondary of the transformer is connected between the potential divider and the base of the next transistor.

Transformer Coupled Amplifier Applications

1. Transformer Coupling in amplifier is mainly used for impedance matching.

2. Transformer-coupled amplifiers are used as power amplifiers to drive high-power speaker load.

3. Transformer coupling is used in high-power audio amplifiers.

Advantages of Transformer Coupled Amplifier

1. It provides a very high gain.

2. It provides a very good impedance-matching property.

3. These amplifiers have high efficiency and low losses.

Disadvantages of Transformer Coupling Amplifier

1. These amplifiers have poor frequency response, the gain decreases with an increase in frequency.

2. These amplifiers are costly because of using the transformer as a coupling device.

3. Humming noise occurs in the transformer.

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