In FM radio receiver, the received signal amplitude reaching the receiver antenna can be low as 1uV and if noise level can also be low as 1uV. This presents a problem such low level signal enters the RF mixer. So RF amplifier are used to amplify the weak signal detected by the antenna. The RF amplifier or low noise amplifier(LNA) should be able to detect and amplify such low amplitude to at least above 20uV. Here RF amplifier design with JFET transistor is illustrated which can be used for FM signal detection(actually any signal not only radio signals).
Below is the schematic diagram of the RF amplifier with JFET 2N3819.
In the above RF amplifier circuit diagram, the received signal(assumed carrier frequency of 50KHz) comes from antenna and is coupled into the LC tank circuit(L1 and C1) tuned to 50khz via the coupling capacitor Cc. After the tuned circuit the signal enters the gate of the 2N3819 JFET transistor. Another LC tank made up of L2 and C2 also tuned to the carrier frequency 50KHz is connected to the drain. A RFC(Radio Frequency Choke) is connected between the output LC tank and the power supply. The RFC has two main functions. First it allows dc bias signal to flow from the power supply because the RFC impedance is low if the signal frequency is low(XL = 2πfL). Second, it block any high frequency generated by the switching transistor or high frequency signal to enter into the power supply circuit because the RFC impedance is high at high frequency. The amplified signal is passed through the output coupling capacitor into the next stage which is usually the RF mixer or RF modulator.
Below video demonstrates the working of this RF amplifier.
