What is a Multistage Tuned RF amplifier | How to Construct a Multistage Tuned RF amplifier

An amplifier that uses multiple stages of tuned RF amplifier for amplification of an RF signal is called a multi-stage tuned RF amplifier. The individual single tuned RF amplifier stages are connected together in series by a coupling device such as a capacitor or a transformer. The output of the first amplifier stage is coupled to the input of the second stage. This type of joining together of two or more amplifier stages is called as cascading.

 

Types of Multistage Tuned RF Amplifier

There are several types of multistage tuned RF amplifiers and they are

1. Double tuned amplifiers: This is a transformer coupled amplifier. Here, both the primary as well as the secondary winding of the transformer is tuned. This type of amplifier has a wide bandwidth.
2. Stagger tuned amplifier: This is a tuned amplifier where two or more single tuned amplifier stages are tuned to a slightly different frequency. This amplifier has a wider bandwidth and it overlaps each other. It has a moderate gain.
3. Synchronous tuned amplifier: Here more than one single tuned amplifier is used and they are tuned to the same frequency. This amplifier has lower bandwidth and maximum gain.

Operation of a Multistage Tuned RF amplifier

The signal Vin that has to be amplified is given to the base of the first transistor Q1 through the capacitor C1. The transistor Q1 amplifies the signal and it is available at the primary winding of the transformer T1. The transformer T1 along with the parallel capacitor forms the LC tank circuit, which is tuned to the required frequency. The amplified signal from the collector is coupled to the input of the second transistor Q2 through the coupling capacitor C2.

The second stage provides further amplification of the signal and it is available at the primary winding of the transformer T2. The transformer T2 along with the parallel capacitor forms the LC tank circuit, which is tuned to the required frequency.

 

The amplified signal from the collector of transistor Q2 is available at the Vout through the coupling capacitor C3. The turns of the transformer can be adjusted so that the required coupling is maintained and thus maximum energy is transferred across the transformer windings.

The overall gain of a multistage tuned RF amplifier is determined by the formula

Gain of stage 1: Av1 = V2/V1

Gain of stage 2: Av2 = Vo/V2

Total gain of the stage is Av1 x Av2

Av = V2/V1 x Vo/V2 = Vo/V1
Av  = Vo/V1

Total gain of N number of amplifier stages is given by the formula
Av = Av1 x Av2 x AvN

Where Av = Overall gain
Av1 = Voltage gain of the 1st stage
Av2 = Voltage gain of the 2nd stage

Advantages of Multistage tuned RF amplifier

1. The voltage gain of a tuned RF amplifier is very high.
2. It has high impedance and high Q at its resonant frequency.
3. It has adjustable bandwidth depending on the type of amplifier that is used.
4. The current consumption of the amplifier is less.

Applications of Multistage Tuned RF amplifier

1. It is used as a multi-stage RF amplifier in communication receivers.
2. It is used as an IF amplifier in superheterodyne receivers.
3. It is used as an IF amplifier in television receivers and communication equipment.
4. It is used as an IF amplifier to convert higher frequency signals to a lower frequency in satellite receivers and communication equipment.

Watch the Video: https://youtu.be/HFm43ZZW5Hc