radio-frequency amplifier[′rād·ē·ō ¦frē·kwən·sē ′am·plə‚fī·ər]
A tuned amplifier that amplifies the high-frequency signals commonly used in radio communications. The frequency at which maximum gain occurs in a radio-frequency (rf) amplifier is made variable by changing either the capacitance or the inductance of the tuned circuit. A typical application is the amplification of the signal received from an antenna before it is mixed with a local oscillator signal in the first detector of a radio receiver. The amplifier that follows the first detector is a special type of rf amplifier known as an intermediate-frequency (i-f) amplifier. See Amplifier, Intermediate-frequency amplifier
An rf amplifier is distinguished by its ability to tune over the desired range of input frequencies. The shunt capacitance, which adversely affects the gain of a resistance-capacitance coupled amplifier, becomes a part of the tuning capacitance in the rf amplifier, thus permitting high gain at radio frequencies. The power gain of an rf amplifier is always limited at high radio frequencies, however.
Two typical rf amplifier circuits are shown in the illustration. The conventional bipolar transistor amplifier of illus. a uses tapped coils in the tuned circuits to provide optimum gain-bandwidth characteristics consistent with the desirable value of tuning capacitance. Inductive coupling provides the desired impedance transformation in the input and output circuits. The tuning capacitors are usually ganged so as to rotate on a single shaft, providing tuning by a single knob. Sometimes varactor diodes are used to tune the circuits, in which case the tuning control is a potentiometer that controls the diode voltage. Automatic gain control (AGC) is frequently used on the rf amplifier, as shown. AGC voltage controls the bias and hence the transconductance of the amplifier. In the field-effect transistor (FET) circuit (illus. b), tapped coils are not required because of the very high input and output resistances of the FET. See Semiconductor, Transistor