Impedance Matching in Amplifiers for Minimum Power Loss

Key Takeaways

• Impedance matching in multi-stage amplifiers is achieved through coupling networks. 

• In resistance-capacitance coupling, two consecutive amplifier stages are coupled through a resistance-capacitance network.

• By selecting a suitable turns ratio of the transformer, the input impedance of the succeeding amplifier stage is matched with the output impedance of the preceding amplifier stage. 

In practical electronic systems, an amplifier is a definite element that amplifies signals and most electronics use multi-stage amplifiers to meet the required gain.

In multi-stage amplifiers, several single-stage amplifiers are cascaded together to achieve sufficient gain for the application. An n number of single-stage amplifiers are cascaded in succession, but not directly connected to form the n-stage amplifier. The cascading follows the principle of impedance matching in amplifiers while connecting between consecutive amplifier stages. The impedance matching in amplifiers helps reduce losses and transfer maximum power from the input side to the load through n amplifier stages. 

Impedance Matching in Amplifiers  

In a two-stage amplifier, the output impedance of the first amplifier stage forms the input impedance of the second stage amplifier, and the same pattern is followed in a multi-stage amplifier until the last amplifier stage. As we are interested in reducing power losses and achieving maximum power transfer from amplifier-to-amplifier stages, the input impedance of the second stage should match the output impedance of the first stage in a two-stage amplifier. The same principle of impedance matching is applicable to the rest of the stages in a multi-stage amplifier.

Impedance matching is essential for transferring maximum power from the amplifier stages to the load. Impedance matching is also a solution to reduce distortions and noise coupling in amplifier stages. The low impedances in the load side draw excessive power from the active devices to meet load requirements.

Apart from power loss, improper impedance matching affects the performance, gain, and efficiency of multistage amplifiers. 

Coupling in Multi-Stage Amplifiers

Impedance matching in multi-stage amplifiers is achieved through coupling networks. Two consecutive single-stage amplifiers are connected through a coupling network in multi-stage amplifiers. As the signal passes through the coupling network from one stage to another, they experience minimum losses.

Next, we will discuss the different types of coupling schemes used in multi-stage amplifiers. 

Resistance-Capacitance Coupling

In resistance-capacitance coupling, two consecutive amplifier stages are coupled through a resistance-capacitance network. This is the most commonly used, convenient, and cheap scheme in multi-stage amplifiers. This type of coupling scheme is used in audio small-signal amplifiers, public addressing systems, television receivers, CD players, etc.

Transformer Coupling

A transformer used to couple two amplifier stages depicts a transformer-coupled multi-stage amplifier. In fixed bias amplifier stages, the collector resistor in each stage is replaced by the primary winding of the transformer and secondary winding replaces the connecting wires between the biasing network and the base of the second stage active device (transistor).

Transformer coupling saves a lot of power and helps in implementing proper impedance matching. By selecting a suitable turns ratio of the transformer, the input impedance of the succeeding amplifier stage is matched with the output impedance of the preceding amplifier stage. Transformer coupled amplifiers are suitable for the amplification of radio frequency signals in radio and TV receivers. 

Direct Coupling

To amplify slowly varying signals, direct-coupled amplifiers are used. In direct-coupled amplifiers, connecting wires are employed to cascade two amplifier stages. When the frequency of the signals to be amplified is low, direct coupling is best. However, both AC and DC signals get coupled to the succeeding stages.

The impedance matching in amplifiers is crucial for achieving the best performance in multi-stage amplifiers. Cadence software can help in the proper layout and design of multi-stage amplifiers and offers tools to analyze the behavior of impedance matching networks. 

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