Analogue amplifier

The ideal rectifier or diode is a two-terrninal device that allows current flow in only one direction. The transistor is a three-terminal device in which current flow through two terminals is controlled by the third. Transistors can be used as analogue amplifiers or digital switches. 

Soimd Amplifiers. Gar stereos, home theaters, and electric guitars all have one thing in common They all contain an amplifier. In the past, these have heen purely analogue devices, but since the late twentieth century, digital amplifiers have supplanted their analogue brethren due to their ease of operation and size. Audiophiles, however, claim that analogue amplifies sound better. 

The analogue amplifier can respond immediately at its output (delay time within the microsecond range), whereas the time required for charging and discharging the coil in a switching amplifier (usually based on a clock frequency) usually causes a signal delay. 

Depending on the requirements of output signal quality, one has to connect a filter to the amplifier output, which will even further reduce the already low dynamic response of the switching amphfier. The impedance of the load influences the filter properties and/or the properties (i. e. the cut-off frequency) of the amplifiers switching stage. Subsequently, the admissible range of values of the load impedance is considerably smaller compared to that of an analogue amplifier. 

Photodiodes are the modem analogues to photocells. They increase their electrical resistance under light impact which, as part of an electric circuit, can be measured easily. Many current instruments display diode arrays instead of a single diode. Tens of photodiodes are arranged in a tight area. They are exposed to the sample bound spectrum where they respond to the color that corresponds to their positions in the diode array. A rapid, periodically performed electrical interrogation of all diodes (sequence periodicity in the order of milliseconds) reveals a quasi-stationary stable spectrogram. More sophisticated than photodiodes are phototransistors. They amplify internally the photoelectric effect, but the sensitivity of a photomultiplier cannot be achieved. 

This technique is of high accuracy and is meant to be used in precision measurement instrumentation, for it is inherently insensitive to the DC-offset and the AC-noise in the sinusoidal signal which can be substantially reduced by a great variety of electronic devices ranging from various electronic analogue filters, and digital filters to the most effective lock-in amplifiers. 

Analogue noise measurements have been made using high gain amplifier/ filter circuits which permit examination of low frequency fluctuations on a real-time basis. 

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