A study of low noise transistorized amplifier input stages



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The design of transistorized amplifiers for the special purpose of handling very small signals, at low frequencies, has always posed a problem to the circuit designer, due to the undesired presence of noise. Noise, as defined here, corresponds to any undesired signal that appears superposed to the wanted information, thus limiting one's ability to process that information with fidelity. Circuit-wise, noise can be classified in two major types: first - extraneous electrical disturbances (generally random in nature) generated outside the circuit, and second - internal electrical disturbances associated with almost all circuit components, producing unwanted signals at the output, therefore masking the desired signal. The present work concerns itself with the treatment of the second kind of noise just described; in other words, no attempt is made to eliminate or cancel any external type noise. Input stages of amplifiers play a decisive role when the design calls for low noise operation, since any noise generated inside it will appear at the output multiplied by the amplifier's total gain. The purpose of this thesis is to present a study of low noise transistorized amplifier input stages, using bipolar transistors and field effect transistors.