Amplification is an essential task in all electronic circuits and devices. Amplifiers are used to amplify low level dc and ac signals. A transistor can be used for amplification. For achieving special tasks, some special amplifiers can be designed. Some examples for specially designed amplifiers are difference amplifiers, chopper amplifiers, electrometer amplifiers and feedback amplifiers. Difference amplifiers which are also known as balanced amplifiers make use of two matched transistors working under same operating conditions. Here design is such that the amplifier response to the difference signal between the input terminals is excellent response to any interference and unwanted signal is very weak. Electrometer amplifiers are designed by employing different devices like electrometer valves, negative f\feedback converters, dc-to-ac converters and operational amplifiers. The principle of negative feedback is used in feedback amplifiers. Chopper stabilized amplifier features the properties of both chopper amplifier and feedback amplifier. Operational amplifiers are dc-coupled differential amplifiers which offer high gain. Commonly used operational amplifiers are bipolar differential amplifiers, FET differential amplifiers, chopper stabilized amplifiers, varactor bridge amplifiers etc.
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