Tuned Amplifier

                                                              Chapter 1: Low Power Amplifiers

Sub point: 1.6 Tuned Amplifier

Tuned Amplifier

Need of tuned amplifier:

Tuned amplifiers are needed to pickup and amplify the specific frequency while rejecting all other frequencies.

General amplifiers are used to amplify overall frequencies where tuned amplifiers are used to select specified frequencies and for amplification. Following table gives difference between general amplifiers and tuned amplifiers.

TV and radio operates in the frequency rage say 500 KHz to few hundred MHz which is divided in many small frequencies that are assigned to different TV and radio stations. We need to select the desired frequency i.e. station and amplify it in order to receive it. Tuned amplifier does the same.

Parameter	General amplifiers	Tuned amplifiers
working principal	Amplify overall frequency	Amplify specific frequency
Load used	Resistive load is used	Tank circuit is used as load (before speaker)
Types	RC coupled, Transformer coupled, Direct coupled, power amplifiers	single tuned, double tuned, staggered tuned
Applications	Public address system, audio amplifier, power amplifier	TV receiver, radio receiver

 

Operating principle of tuned circuit:

Basics of tank circuit/tuned circuit

(Here after tuned circuit)

Tuned circuits are nothing but combination of inductor and capacitor. Variable (gang) capacitors are used to change the frequency to be tuned. Following are some combinations of tuned circuits.

 

 

A parallel tuned circuit consists of a variable capacitor and inductor that are connected in parallel. If ac quantities i.e. alternating voltage is applied parallel tuned circuit, the frequency of oscillations will be that of the applied voltage. However, if the frequency of applied voltage is equal to the natural or resonant frequency of LC circuit, then electrical resonance will occur. Under such conditions, the impedance of the tuned circuit becomes maximum and the line current is minimum.

In order to pick up and amplify the desired radio frequency signal, the resistive load in the audio amplifier is replaced by a tuned circuit (also called a parallel resonant circuit)as shown in the figure. The tuned circuit is capable of selecting as particular frequency and rejecting the others. Thus the use of tuned circuit in the transistor amplifier circuit, makes possible the selection and amplification of a particular desired radio frequency. Such an amplifier is called tuned voltage amplifier.

Thus an amplifier, which amplifies a specific frequency (or a narrow band frequencies), is

known as tuned amplifier. It is basically used for purposes like:

(i)  Selection of desired radio frequency signal.

(ii) Amplification of the selected signal to a suitable voltage level.

(i) Resonant Frequency(Fr): It is a frequency at which the inductive reactance is equal to the

capacitive reactance i.e. XL=XC. Resonant frequency is calculated by formula

Fr = 1/(2π√LC)

(Note: Both parallel and series resonant circuits uses same formula)

 

(ii) Q Factor: The quality factor or Q factor is a measure of the performance of a coil, capacitor inductor in terms of its losses and resonator bandwidth. Higher value of Q-factor provides the higher degree of selectivity in expense of bandwidth i.e. Higher the Q-factor, Higher the selectivity, but smaller bandwidth. On the other hand lower value of Q-factor provides poor selectivity but bandwidth is larger.

Q = 2πFrL/R

or

Q = Fr/BW

 

Where ,

Fr = Resonant frequency

L = value of circuit inductance.

R = Value of circuit resistance.

(iii) Bandwidth: Bandwidth is measure of its satisfactory working range.

BW = Fr/Q

Where ,

Fr = Resonant frequency

Q = Quality factor

•  Single Tuned Amplifier

 

      

OR

   

Figure shows circuit diagram of single tuned amplifier in which resistive load is replaced using tuned circuit which is made up of inductor and capacitor. The values of inductor and capacitor are selected in such a way that resonant frequency of tuned circuit is equal to the frequency to be amplified. The output of single tuned amplifier can be taken by using either coupling capacitor Cc or secondary coil as shown in figure. The signal to be amplified is given as input. Tuned circuit will offer high impedance to the signal frequency and hence there is large output across the tuned circuit. If there are many frequencies at the input, the only frequency belongs to the resonant frequency of tuned circuit will be amplified while all other frequencies are rejected.

•  Frequency response of single tuned amplifier

•  Double Tuned Amplifier

Figure shows circuit diagram of double tuned amplifier in which two tuned circuits are used (hence termed as double tuned amplifier). One tuned circuit is placed at collector and second tuned circuit at the output. Frequency of double tuned amplifier heavily depends upon coupling coefficient (K) i.e. mutual induction between tow tuned circuits. In case of loosely coupling tuned circuits resonant curve is sharp and for tightly coupled tuned circuits, reflected resistance is large.

·         Frequency response of doubled tuned amplifier

                             

 

 A plot of normalized voltage gain at resonance as a function of frequency for different values of K is shown:

1. The gain of double tuned amplifier is a function of K and is maximum at critical coupling ie K = Kc.

2. If actual K, coefficient of coupling is less than coefficient of critical coupling ie. K<Kc ,circuit is under coupled and the response resembles usual resonance curve except that the top of the curve is somewhat flat near resonance.

3. And when K>Kc circuit is over coupled. The over coupled circuit has two peaks in response characteristics one on each side of resonant frequency.

BW=K*Fr

where,

K = coupling coefficient

Fr = resonant frequency

·         Difference between single tuned and double tuned amplifier

Parameter	Single tuned amplifier	Double tuned amplifier
Operating principal	Parallel resonance	Parallel resonance
Selectivity	Very High	Moderate
Efficiency	High	Low
Bandwidth	Small	Moderate
Q-factor	High	High
Number of tank circuit	One	Two
Application	TRF receiver, TV receiver	IF amplifier in Radio receiver, TV receiver
Frequency response	Please Refer Figure	Please Refer Figure

 

•  Staggered Tuned Amplifier

Figure shows circuit diagram of staggered tuned amplifier in which different tuned circuits are connected in cascade. Each tuned circuit is tuned at slightly different frequency which is useful to obtain increased bandwidth.

  

·         Frequency response of staggered tuned amplifier

For more information check video lecture on tuned amplifier

(Note: this is zoom lecture, and the language of video in marathi and english)

 

 

Some numerical on tuned amplifier

Q. 1]    Calculate Resonant frequency of single tuned amplifier, if inductor L = 10mH and  

            Capacitor C = 4.7 μf of tank circuit.

 

Ans: Given,

            L = 10mH

            C = 4.7 μf

we know, Fr = 1/(2π√LC)

                        Fr = 1 /            (2π√(10mH * 4.7 μf))

                        Fr = 734.12 Hz

 

Q. 2]    Tuned circuit has resonant frequency of 1MHz, calculate value of Q-factor for 20KHz             bandwidth.

 

Ans: Given,

            Fr    = 1 MHz

            BW = 20 KHz

                        we know, BW = Fr / BW

                        BW = 1 MHz / 20 KHz

                        BW = 50