A transistor circuit in which a number of amplifier stages are used in succession is called multi-stage or cascaded amplifier.

Gain

  1. The ratio of output to the input of an amplifier is called its gain.

Decibel

  1. The common logarithm of power gain is known bel power gain.

Band Width

  1. The range of frequency over which the gain of an amplifier is equal to or greater than 70.7% of its maximum gain is known as band width.
  2. The frequency of f­1 or f2 ­is also called frequency or half power frequency.

Function of a coupling device

  1. To transfer only ac output of one stage to the input of the next stage.
  2. Another function of coupling is to block dc components and isolate the dc conditions of one stage from the other stage.

RC (Resistance-Capacitance) Coupled Transistor Amplifier

  1. At low frequencies (below 50Hz), the gain is small.
  2. At mid-frequencies (50Hz to 20 KHz) also called audio frequency range, the gain is almost constant.
  3. At high frequencies (above 20 KHz), the voltage gain is reduced.
  4. Light in weight
  5. RC coupled amplifiers provide excellent audio-fidelity over a wide range of frequencies. Therefore, they are widely used as voltage amplifier for example in the initial stage of public address system.

Transformer Coupled Transistor Amplifier

  1. These are also used at the output stage fir amplification because of their excellent impedance matching properties.
  2. Higher gain can be obtained using step-up coupling transformer.
  3. These amplifiers give very poor frequency response.
  4. They produce hum due to nearby power rating.

Direct Coupled Amplifiers

  1. When extremely low frequency (below 10Hz) signals are required to be amplified, direct-coupled transistor amplifiers are employed.
  2. Cheap in cost.
  3. It cannot be used to amplify high frequency signals.
  4. Temperature may shift the operating point.