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Transistor as CB amplifier

  • ac current gain
    VB (constant)
  • dc current gain, 107148.png
    valve of αdc lies between 0.95 to 0.99
  • Voltage gain, 107154.png
    ⇒ Av = αac × Resistance gain
  • 107161.png

Transistor as CE amplifier

  • —ac current gain, 107175.png VCE = constant
  • —dc current gain, 107183.png
  • —Voltage gain: 107191.png
  • —107197.png
  • —Transconductance (gm): The ratio of the change in collector current to the change in emitter base voltage is called transconductance, i.e., 108893.pngAlso gm = AV / RL, where RL = load resistance.
    Relation between α and β: 107218.png or 107224.png

Transistor as an Oscillator

It is defined as a circuit which generates an ac output signal without any externally applied input signal.
Audio frequency oscillators generates signals of frequencies ranging from a few hertz to 20 kHz and radio frequency oscillators have a range from a few kilohertz to megahertz.
In an oscillator, the frequency, waveform, and magnitude of ac power generated is controlled by circuit itself. An oscillator may be considered as amplifier which provides its own input signal. The essentials of a transistor oscillator are as follows:
  • Tank circuit: Parallel combination of L and C. This network resonates at a frequency 107232.png.
  • Amplifier: It receives dc power from the battery and converts into ac power. The amplifier increases the strength of oscillations.
  • Feed back circuit: This circuit supplies a part of the collector energy to the tank circuit.
Fig. 36
A basic common-emitter NPN oscillator is shown in Fig. 37.
A tank circuit (L-C circuit) is connected in the base-emitter circuit, in which the capacitance C is kept variable. By changing C, oscillations of a desired frequency can be obtained. An inductance coil L’ connected in the collector-emitter circuit is coupled to coil L.
On completion of the circuit, electrical oscillations are developed in the tank circuit. The circuit amplified these oscillations. A part of the amplifies signal in the collector circuit is fed back, in the base circuit by the coupling between L and L’. Due to this feed back amplitude of the oscillation builds up till power dissipation in the oscillatory circuit becomes equal to power feed-back. In this state, the amplitude of oscillations becomes constant.
Fig. 37
The oscillations can be transferred to an external circuit by mutual induction in a coil connected in that circuit.

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