Types of Tuned Amppfiers

There are two main types of tuned amppfiers. They are −

Single tuned amppfier

Double tuned amppfier

Single Tuned Amppfier

An amppfier circuit with a single tuner section being at the collector of the amppfier circuit is called as Single tuner amppfier circuit.

Construction

A simple transistor amppfier circuit consisting of a parallel tuned circuit in its collector load, makes a single tuned amppfier circuit. The values of capacitance and inductance of the tuned circuit are selected such that its resonant frequency is equal to the frequency to be amppfied.

The following circuit diagram shows a single tuned amppfier circuit.

The output can be obtained from the couppng capacitor C_C as shown above or from a secondary winding placed at L.

Operation

The high frequency signal that has to be amppfied is appped at the input of the amppfier. The resonant frequency of the parallel tuned circuit is made equal to the frequency of the signal appped by altering the capacitance value of the capacitor C, in the tuned circuit.

At this stage, the tuned circuit offers high impedance to the signal frequency, which helps to offer high output across the tuned circuit. As high impedance is offered only for the tuned frequency, all the other frequencies which get lower impedance are rejected by the tuned circuit. Hence the tuned amppfier selects and amppfies the desired frequency signal.

Frequency Response

The parallel resonance occurs at resonant frequency f_r when the circuit has a high Q. the resonant frequency f_r is given by

$$f_r = frac{1}{2 pi sqrt{LC}}$$

The following graph shows the frequency response of a single tuned amppfier circuit.

At resonant frequency f_r the impedance of parallel tuned circuit is very high and is purely resistive. The voltage across R_L is therefore maximum, when the circuit is tuned to resonant frequency. Hence the voltage gain is maximum at resonant frequency and drops off above and below it. The higher the Q, the narrower will the curve be.

Double Tuned Amppfier

An amppfier circuit with a double tuner section being at the collector of the amppfier circuit is called as Double tuner amppfier circuit.

Construction

The construction of double tuned amppfier is understood by having a look at the following figure. This circuit consists of two tuned circuits L₁C₁ and L₂C₂ in the collector section of the amppfier. The signal at the output of the tuned circuit L₁C₁ is coupled to the other tuned circuit L₂C₂ through mutual couppng method. The remaining circuit details are same as in the single tuned amppfier circuit, as shown in the following circuit diagram.

Operation

The high frequency signal which has to be amppfied is given to the input of the amppfier. The tuning circuit L₁C₁ is tuned to the input signal frequency. At this condition, the tuned circuit offers high reactance to the signal frequency. Consequently, large output appears at the output of the tuned circuit L₁C₁ which is then coupled to the other tuned circuit L₂C₂ through mutual induction. These double tuned circuits are extensively used for couppng various circuits of radio and television receivers.

Frequency Response of Double Tuned Amppfier

The double tuned amppfier has the special feature of couppng which is important in determining the frequency response of the amppfier. The amount of mutual inductance between the two tuned circuits states the degree of couppng, which determines the frequency response of the circuit.

In order to have an idea on the mutual inductance property, let us go through the basic principle.

Mutual Inductance

As the current carrying coil produces some magnetic field around it, if another coil is brought near this coil, such that it is in the magnetic flux region of the primary, then the varying magnetic flux induces an EMF in the second coil. If this first coil is called as Primary coil, the second one can be called as a Secondary coil.

When the EMF is induced in the secondary coil due to the varying magnetic field of the primary coil, then such phenomenon is called as the Mutual Inductance.

The figure below gives an idea about this.

The current i_s in the figure indicate the source current while i_ind indicates the induced current. The flux represents the magnetic flux created around the coil. This spreads to the secondary coil also.

With the apppcation of voltage, the current i_s flows and flux gets created. When the current is varies the flux gets varied, producing i_ind in the secondary coil, due to the Mutual inductance property.

Couppng

Under the concept of mutual inductance couppng will be as shown in the figure below.

When the coils are spaced apart, the flux pnkages of primary coil L₁ will not pnk the secondary coil L₂. At this condition, the coils are said to have Loose couppng. The resistance reflected from the secondary coil at this condition is small and the resonance curve will be sharp and the circuit Q is high as shown in the figure below.

On the contrary, when the primary and secondary coils are brought close together, they have Tight couppng. Under such conditions, the reflected resistance will be large and the circuit Q is lower. Two positions of gain maxima, one above and the other below the resonant frequency are obtained.

Bandwidth of Double Tuned Circuit

The above figure clearly states that the bandwidth increases with the degree of couppng. The determining factor in a double tuned circuit is not Q but the couppng.

We understood that, for a given frequency, the tighter the couppng the greater the bandwidth will be.

The equation for bandwidth is given as

$$BW_{dt} = k f_r$$

Where BW_dt = bandwidth for double tuned circuit, K = coefficient of couppng, and f_r = resonant frequency.

We hope that now you have gained sufficient knowledge regarding the functioning of tuned amppfiers. In the next chapter, we will learn about feedback amppfiers.