Radio Frequency Amplifiers

Published: 01st July 2007
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There are many similarities between radio frequency amplifiers and audio frequency amplifiers, but there are also many differences. Radio frequency amplifiers perform the same amplifying functions at radio frequencies as do audio frequency amplifiers at audio frequencies. Voltage and power amplifiers are used in both types of systems. But since the signal frequencies are very different, and radio frequency amplifiers require higher selectivity than audio frequency amplifiers, the radio frequency amplifiers normally make use of tuned circuits or selective filters to achieve the required selectivity.



Radio frequency voltage amplifiers are used in transmitters to amplify radio frequency signals before modulation, in receivers to amplify received radio frequency signals, and as frequency multipliers. The two types of radio frequency voltage amplifiers are the standard broadband, multi frequency radio frequency amplifiers and the intermediate frequency amplifiers, a fixed frequency radio frequency amplifier commonly used in super heterodyne receivers. The intermediate frequency amplifier follows the mixer stage and precedes the detector stage. In the receiver the intermediate frequency amplifies a narrow band of frequencies between audio frequencies and radio frequencies ranges.



What distinguishes radio frequency amplifiers from audio frequency amplifiers is that the input and output circuits are tunable in radio frequency amplifiers. Tunable circuits provide improved selectivity and impedance matching for good transfer of power between stages. Transformers are tuned to accept the broadband radio frequencies. Adjustable capacitors are ganged to allow the selection of the desired incoming frequency. Adjusting the capacitors alters the frequencies input and output simultaneously, thus providing maximum transfer of power. In receivers, radio frequency voltage amplifiers are normally biased to operate class A thus enabling 360 degrees collector current flow with minimum distortion.



The intermediate frequency amplifiers are similar to radio frequency amplifiers except that intermediate frequency amplifiers normally use special slug-tuned transformers in the tank circuits. In typical intermediate frequency amplifier a tank circuit transformer are pretuned to a specific intermediate frequency, so that both the primary and secondary windings are resonant at the same frequency. For alignment purposes the tank resonant frequency can be changed slightly by adjusting slugs in the transformer.



Radio frequency power amplifiers are designed to deliver amplified current values to a load. In communication applications radio frequency power amplifiers are used to supply energy to another power amplifier or to a transmitting antenna.



The radio frequency power amplifier can use either a single ended arrangement or a parallel arrangement. Single ended arrangements are typically used as output stages in low power radio frequency devices. With small value emitter resistors, these circuits can operate either class B or class C. However the low output impedance of radio frequency power transistors usually makes it necessary to insert an impedance matching circuit between the output stage and the load.

Maximum power output and efficiency are obtained when the amplifier is tuned to the input frequency. The correct frequency selected in the pie network by filtering out the fundamental frequency from the collector current pulses, thus permitting the circuit output to be sinusoidal.





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Tymon Hytem has worked in the electronics feild for the past 15 years. He enjoys helping people decide on electronic gadgets from telephones to XM Radio and choosing the perfect XM Satellite Radio system for their needs.

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