A transistor circuit in which a number of amplifier stages are used in succession is called multi-stage or cascaded amplifier.
- The ratio of output to the input of an amplifier is called its gain.
- The common logarithm of power gain is known bel power gain.
- 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.
- The frequency of f1 or f2 is also called frequency or half power frequency.
Function of a coupling device
- To transfer only ac output of one stage to the input of the next stage.
- 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
- At low frequencies (below 50Hz), the gain is small.
- At mid-frequencies (50Hz to 20 KHz) also called audio frequency range, the gain is almost constant.
- At high frequencies (above 20 KHz), the voltage gain is reduced.
- Light in weight
- 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
- These are also used at the output stage fir amplification because of their excellent impedance matching properties.
- Higher gain can be obtained using step-up coupling transformer.
- These amplifiers give very poor frequency response.
- They produce hum due to nearby power rating.
Direct Coupled Amplifiers
- When extremely low frequency (below 10Hz) signals are required to be amplified, direct-coupled transistor amplifiers are employed.
- Cheap in cost.
- It cannot be used to amplify high frequency signals.
- Temperature may shift the operating point.