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Why does a thyristor as an amplifier not work ?

Thyristors, specifically silicon-controlled rectifiers (SCRs), are not typically used as linear amplifiers. The primary reason lies in the inherent characteristics of thyristors and their mode of operation. Thyristors are semiconductor devices designed for switching applications rather than amplification. Let’s explore the reasons why thyristors are not suitable for use as amplifiers:

1. Binary Switching Operation:

  • On/Off States: Thyristors, including SCRs, operate in a binary fashion—they are either in an “on” state (conducting) or an “off” state (non-conducting). This on/off characteristic is suitable for applications where switching between states is desired, such as in rectifiers and power control circuits.
  • Lack of Continuous Control: Unlike linear amplifiers that can smoothly vary their output in response to a continuously changing input, thyristors lack the ability to provide continuous control. The absence of intermediate states between fully conducting and fully non-conducting limits their application in linear amplification.

2. Regenerative Feedback:

  • Hysteresis: Thyristors have hysteresis in their voltage-current characteristics. Once a thyristor is triggered into conduction, it remains in that state until the current falls below a certain level, called the holding current. This regenerative feedback characteristic is unsuitable for linear amplification, where precise control is required.

3. Unidirectional Conduction:

  • Current Flow Direction: Thyristors conduct current in only one direction. While this is advantageous for applications like rectification in power supplies, it limits their versatility in linear amplification, which often requires bidirectional current flow.

4. Nonlinear Voltage-Current Characteristics:

  • Voltage Drop: The voltage-current characteristics of thyristors exhibit a nonlinear behavior, particularly during turn-on and turn-off. This nonlinearity makes it challenging to achieve linear amplification with a thyristor.

5. Limited Frequency Response:

  • Switching Speed: Thyristors, being designed for switching applications, have limitations in terms of their switching speed. This limitation in frequency response makes them unsuitable for applications that require high-speed signal amplification.

6. Distortion and Harmonics:

  • Distortion: Thyristors can introduce distortion and harmonics into the output signal due to their abrupt switching behavior. In linear amplifiers, minimizing distortion is essential for faithful signal reproduction.

7. Alternative Devices for Amplification:

  • Amplification Devices: For linear amplification purposes, other semiconductor devices like bipolar junction transistors (BJTs), field-effect transistors (FETs), and operational amplifiers (op-amps) are more commonly used. These devices offer linear amplification with fine control over the output in response to a varying input.

8. Controlled Rectification and Power Control:

  • Preferred Applications: Thyristors, especially SCRs, are widely used in applications where controlled rectification and power control are essential, such as in motor drives, phase-locked loops, and certain types of power supplies.

9. Thyristor as a Switch:

  • Switching Power Applications: While thyristors may not serve as linear amplifiers, their ability to switch high-power loads on and off with efficiency makes them suitable for applications like phase-angle control in AC power circuits.

In summary, thyristors, particularly SCRs, are not designed for linear amplification due to their binary switching behavior, regenerative feedback characteristics, unidirectional conduction, and limited frequency response. Other semiconductor devices, such as transistors and op-amps, are more suitable for linear amplification where precise and continuous control of the output is required. Thyristors find their primary application in switching and power control circuits, where their unique characteristics are beneficial.

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