RF amplifier – Working, Circuit Diagram, and Advantages


In this lecture, we are going to learn about the one block of Superheterpdyne Receiver called an RF amplifier. We will learn about why an RF amplifier requires, and its circuit diagram, and in the end, we will see the advantages of an RF amplifier. So let’s discuss it one by one.

RF Amplifier

  • RF amplifier is a small signal tuned amplifier with a tuned circuit both on the input side and the output side. Both this input and output tuned circuits are tuned to the desired incoming carrier frequency.
  • Accordingly, the tuned circuits select the desired carrier frequency and reject all undesired frequencies including the image frequency. Hence the RF amplifier provides image frequency rejection. Also, the gain provided by the RF amplifier will result in an improved signal-to-noise ratio in the output of the receiver. This is due to the fact that the incoming weak signal is raised to a higher level with the help of an RF amplifier before it is fed at the input of the mixer stage which contributes to most of the noise generated at the receiver.
  • However, if the incoming weak signal is fed directly to the frequency mixer, the signal-to-noise ratio at the output of the mixer stage is quite poor and hence any amount of subsequent application cannot improve the Signal to noise ratio. thus the one important function of the RF amplifier is to improve the Signal to Noise Ratio.
  • There are some cases also where an RF amplifier is not used in the receiver rather its use is uneconomical there. The best example of this kind of receiver is a domestic receiver used in a high signal strength area like a metropolitan city. Since in a metropolitan city like Delhi, several stations are situated and in such places, strength is obviously very high and thus there is no need for the use of an RF amplifier. In such cases, the tuned circuit connected to the antenna is the actual input circuit of the mixer.
  • However, a receiver having an RF amplifier is obviously superior in performance to a receiver without an RF amplifier.

Circuit Diagram of RF Amplifier

The below figure shows the circuit diagram of one stage RF amplifier using an NPN transistor. It is a small signal amplifier using parallel tuned circuits as the load impedance. This parallel output tuned circuit is tuned to the incoming desired signal frequency. The output from the receiving antenna is a transformer coupled to the base of the transistor.

The secondary coil of the input tuned circuit is tuned to the incoming desired signal frequency with the help of ganged tuning capacitor. In fact, the tuning capacitor i.e. variable air capacitors in the input side and the output side of the RF amplifier are ganged together.

In addition to this, small trimmer capacitors are connected in shunt with these tuning capacitors for the purpose of RF alignment.

A self-bias is provided with the help of resistors R1 and R2 and RE-CE assembly. A de-coupling network consisting of resistor Rb and capacitor Cb is placed in the collector supply lead.

circuit diagram of RF amplifier

The amplified R.F. signal developed across the collector tuned circuit is coupled through a step-down transformer providing the impedance matching between the high impedance of the R.F. Amplifier collector circuit and the low impedance of the base to emitter circuit of the following stage. Also, the collector is connected to a suitable point on the primary of the output transformer so that load impedance to the collector is optimum.

Frequently Asked Questions on RF Amplifier

  1. What is the use of an RF amplifier?

    Answer: A RF amplifier essentially is a tuned amplifier that enables the input signal of broadcast or transmitted information to control an output signal. The RF amplifier uses frequency-determining networks to convert the input signal into an output signal, providing the required response at a given frequency.

  2. What is a gain of an RF amplifier?

    Answer: RF amplifier gain is defined as the difference in power between the amplifier output signal and the input signal. It is assumed that both input and output impedances of the amplifier are the same as the characteristic impedance of the system.

  3. What is the function of the RF stage?

    Answer: (Radio Frequency stage) The frequency used to carry a radio signal (radio station, TV channel, cellphone channel, etc.). The RF signals are converted to an intermediate stage for demodulation.

  4. What is the RF stage in the superheterodyne receiver?

    Answer: RF tuning & amplification: This RF stage within the overall block diagram for the receiver provides initial tuning to remove the image signal. It also provides some amplification. There are many different approaches used within the RF circuit design for this block dependent on its application.

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Electronics Engineering(2014 pass out) Junior Telecom Officer(B.S.N.L.) Project Development, PCB designing Teaching of Electronics Subjects


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