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    Triode based audio and electronic amplifiers are very commonly found in many walks of life. Although, the transistor based amplifiers created the danger of obsolescence of tube amplifiers, the tube amplifiers have succeeded in maintaining a cult following amongst the audiophiles. The main reason behind this is the warm and crunchy sound response of tube amplifiers.

    The main component in tube amplifiers is the triode. Triode is essentially an amplifying vacuum tube which consists of three electrodes inside a glass casing. The electrodes are known as anode, cathode, and grid respectively. Triodes were widely used in all types of electronic circuits until they got replaced by transistors. [1]

    According to the electronic circuit configuration, there are three main types of amplifiers. These types include common emitter, common collector, and common base amplifiers. Following sub-sections provide an ample discussion on these amplifier types.

    Common Emitter Amplifier

    Common emitter amplifiers are the most widely used type of amplifiers. Common emitter amplifiers can be identified easily by the grounded emitter terminal. Like all other amplifiers, the CE amplifier also operates on an AC input. The common emitter amplifier is a single-stage amplifier which uses a BJT transistor or a triode as an amplifying element. The circuit of common emitter amplifier is given as following:

    Figure 1: Common Emitter Amplifier Circuit


    The resistors R1 and R2 form a voltage divider circuit which is used for biasing the transistor. The resistor RE provides thermal stability to the amplifier. A coupling capacitor is present on the input side of the transistor which filters out DC component from the signal. [2]

    Major advantages of common emitter amplifier include low input impedance, high output impedance, high power gain, low noise, and high current gain. Main disadvantages of common emitter amplifier include unsuitability for high frequencies, unstable voltage gain, high thermal instability, and high output resistance. The CE amplifiers find their applications in low frequency voltage amplifiers, RF circuits, and low noise amplifiers. [2]

    Common Collector Amplifier

    The common collector amplifier can be identified from the grounded collector terminal of the triode or the transistor. The common collector amplifiers are mostly used as buffers in multi-stage amplifier circuits. The CC amplifier circuit is given as following:

    Figure 2 Common Collector Amplifier or Emitter Follower Circuit


    The input signal is introduced via the base of the triode whereas the output is taken from the emitter terminal. The main advantages of CC amplifier include high current gain, high input resistance, and low output resistance. The disadvantages of CC amplifier include low voltage gain. The CC amplifiers find their applications as impedance matching amplifiers, isolation amplifiers, and buffer amplifiers in cascade or multi-stage amplifier systems. [3]

    Common Base Amplifier

    The common base amplifier configuration is not as widely used as the CE and CC amplifiers. They are mostly used in high frequency circuits. In a common base amplifier the base terminal of the triode is connected to the ground, the input signal is applied to the emitter, and the output is taken from the collector terminal. The circuit diagram of the CB amplifier is given as following:

    Figure 3 Common Base Amplifier using an NPN Transistor


    The CB amplifiers are used in high frequency circuits where low input impedance is required. They are used in applications such as moving coil microphone pre-amplifiers, UHF, VHF, and RF amplifiers. The advantages of CB amplifier include decent voltage gain and current buffering capability. The disadvantages include need for dual power supply, low input impedance, low current gain, and high output impedance. [4]

    Works Cited

    [1] Lee Forest. (2019, August) www.allaboutcircuits.com. [Online]. https://www.allaboutcircuits.com/textbook/semiconductors/chpt-13/the-triode/
    [2] Tarun Agarwal. (2019, June) www.elprocus.com. [Online]. https://www.elprocus.com/common-emitter-amplifier-circuit-working/
    [3] Dave Moldenhauer. (2019, March) www.watelectrical.com. [Online]. https://www.watelectrical.com/working-and-applications-of-common-collector-amplifier/
    [4] (2019, August) www.electronics-notes.com. [Online]. https://www.electronics-notes.com/articles/analogue_circuits/transistor/transistor-common-base-circuit.php


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