Double Tuned Amplifier : Working and Its Applications

The double-tuned amplifier is one of the types of tuned amplifiers. The designing of this circuit can be done using two tuned circuits which are coupled inductively. The primary tuned circuit includes L1, C1 whereas the secondary circuit includes L2 C2. Here L1C1 and L2C2 are inductors and capacitors. In the collector terminals of the circuit, the coupling change in the tuned circuit will result in the change in the curve shape of frequency response. The adjustment of proper coupling among the two coils kin the double-tuned circuits, the necessary outcomes may be attained. This article discusses an overview of the Double Tuned Amplifier, construction, and applications.

What is a Double Tuned Amplifier?

This is one kind of tuned amplifier that uses the coupling of transformer among the two stages like inductances of both the windings. The tuning of these windings can be done separately across a capacitor.

For the transformer, there is a critical value of coefficient where the amplifier’s frequency response can be even maximally within the passband & the gain of this can be highest at the resonant frequency. The coupling can be used by the design which is greater than over coupling to get a level wider BW at the expenditure of a minute loss of gain within the middle of the passband.

The multiple stages of cascading in the amplifier can result in the bandwidth reduction in the entire amplifier. The BW of these stages includes 80% of the BW of the single stage. A substitute to these tuning neglects the bandwidth loss is called as staggered tuning. These amplifiers can be planned to a prearranged bandwidth that is superior to the BW of any single stage. But, this tuning needs several stages & includes less gain compare with double tuning.

Construction and Operation of Double Tuned Amplifier

The construction of this amplifier can be understood by the following circuit. This circuit can be built with two tuned circuits namely L1C1 & L2C2 within the collector segment of the amplifier.

The sign at the o/p of the primary tuned circuit like L1C1 can be coupled with the secondary tuned circuit like L2C2 throughout the common coupling technique. The other details of this circuit are similar to the single tuned amplifier.

Operation

The signal which has to amplify is a high-frequency signal and it is given to the i/p of the amplifier. The primary tuning circuit like L1C1 can be tuned toward the i/p signal frequency.

In this state, the tuned circuit gives high reactance toward the signal frequency. As a result, huge o/p becomes visible at the o/p of the primary tuned circuit then it is coupled with the secondary tuned circuit like L2C2 using mutual induction. These circuits are widely used to connect different circuits of TV and radio receivers.

Frequency Response

This amplifier includes a unique feature like coupling and it is significant in deciding the amplifier’s frequency response. The quantity of mutual inductance among the double-tuned circuits states the amount of coupling, which decides the circuit’s frequency response. To get an idea on the property of mutual inductance, one has to know the basic principle of mutual inductance.

Mutual Inductance

When the current-carrying coil generates some magnetic field approximately it, but one more coil is placed near to this coil, then it will be in the region of magnetic flux of the main, after that the changing magnetic flux makes an EMF within the secondary coil. If the first coil is named as a Primary coil, then the second coil can be named as a secondary coil. Once the EMF is induced within the secondary coil because of the changing magnetic field of the main coil then, this is named as the Mutual Inductance.

In the above figure, the source current and induced currents are specified with i_s & i_ind. The flux signifies the magnetic flux which is formed around the coil and it will increase the secondary coil.

By the voltage application, the current supplies & flux gets formed. When the flow of current changes, the flux gets changed and generates i_ind within the secondary coil because of the properties like mutual inductance.

Coupling

Based on the concept of mutual inductance, the coupling is shown in the following figure. As the two coils are spaced separately, the primary coil’s flux linkages will not link to the secondary coil. Here the two coils are represented with L1 & L2. At this state, these coils have loose coupling. The reflected resistance from the L2 coil on this state is minute & the resonance curve is sharp.

When the two coils are arranged together, then they have tight coupling. Below these forms, the reflected resistance will be huge & the circuit is lesser. The gain maxima two positions are obtained one above & the other under the resonant frequency.

Bandwidth

The bandwidth of this amplifier is shown in the above figure which states that the BW rises by the amount of coupling. In a double-tuned circuit, the determining factor is not Q other than the coupling. From this, we can conclude that for a known frequency when the coupling is tighter then the bandwidth will be greater.

The bandwidth equation is given as

BW_dt = kf_r

In the above equation

‘BW_dt’ is BW of a double-tuned circuit

‘K’ is a coupling coefficient

‘fr’ is a resonant frequency.

Advantages

The advantages of the Double Tuned Amplifierinclude the following.

• The main advantage of a double-tuned amplifier is an amplifier including a tuned circuit on the input & the output
• It has a narrow bandwidth.
• One more advantage of this circuit is impedance matching using the previous phase, etc.
• 3 dB BW is large
• It gives a frequency response including flatter sides.
• When the overall gain is increased then sensitivity will be increased. Here sensitivity is the capacity of receiving weak signals.
• Selectivity is improved.

Disadvantages

The disadvantages of a Double Tuned Amplifierinclude the following.

• These are not suitable for amplifying audio frequencies
• If the frequency band increases, then this design becomes complex
• The design uses tuning elements like capacitors & inductors, then the circuit is costly & bulky.

Applications of Double-tuned Amplifier

The applications of Double Tuned Amplifier include the following

• It is used in a superheterodyne receiver like an IF (intermediate frequency) amplifier.
• It is used in a satellite transponder like an intermediate frequency amplifier.
• These amplifiers are used within UHF radio relay systems.
• It is used in a spectrum analyzer like extremely narrow-band intermediate frequency amplifier
• These amplifiers are used like wideband tuned amplifiers intended for video amplification.
• These amplifiers are used like RF amplifiers within receivers.

Thus, this is all about Double Tuned Amplifier and can be defined as an amplifier which has a double-tuned section being at the collector of the amplifier. Here is a question for you, what is a tuned amplifier?