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AE_Lecture 12_Tuned Amplifiers

Module by: Bijay_Kumar Sharma. E-mail the author

Summary: AE_lecture 12 deals with RF Amplifiers which always use tank circuit or parallel resonance circuit and have peak type frequency response.These find wide applications in Communication and Broadcast Receivers.

AE_LECTURE 12_Tuned Amplifier.


Tuned amplifiers are amplifiers as well as Band-Pass Filters.

RC-Active Filters are made of RC circuit and Op-Amp.

Figure 1
Figure 1 (Picture 1.png)

Figure 1. Circuit diagram of Low Pass Active Filter

This is a LOW PASS ACTIVE FILTER. But it operates at 100kHz and below because the frequency response of op-amp is very limited.

To operate in RF,VHF,UHF range we have to go for circuits using parallel resonance circuits as tank circuit made of L-C-R circuits.

In communication applications we require narrow band frequency selective amplifiers so that from among closely spaced broadcasting stations, a station of choice is tuned to.

In AM radio broadcast we may need to tune to 1 MHz with a B.W. of 20kHz.

Figure 2
Figure 2 (graphics1.png)

This means a Q=50 is required.

In FM Radio Broadcast which is in VHF range, we need to tune to 100MHz with a B.W.=200kHz,here Q=500.

These narrow band pass RF-VHF amplifiers are called Tuned Amplifiers.

The tuned amplifiers are Class A amplifiers but in tuned power amplifiers the conservation of power is at premium therefore we go for class C amplifiers. Class C amplifier gives an energy conversion efficiency of 99%.Therefore dissipation and hence heating of the components is reduced.

All communication and broadcast receivers require RF amplifiers and IF Amplifiers. These are all tuned amplifiers with a peak response and a narrow band-width.


Figure 3
Figure 3 (Picture 2.png)

Figure 2. Circuit Diagram of a Single Tuned Amplifier.

This is a CE BJT except that Rc is replaced by LC Tank Circuit. The equivalent circuit is:

Figure 4
Figure 4 (Picture 3.png)
Figure 3. The incremental circuit representation of the tank circuit.

All the losses of the tank circuit namely copper losses and dielectric losses are lumped in Rp. Since inductor is air cored hence no hysteresis and eddy current losses.

This gives a peak gain of:

AV(jωo) = -gmRp

Off resonance the gain rapidly falls. The rapid fall of the gain at off-resonance defines the selectivity or skirt selectivity of the tuned amplifier.

Figure 5
Figure 5 (Picture 4.png)

Figure 4. Definition of Skirt Selectivity from the frequency response curve of the tank circuit.

Skirt selectivity=

Figure 6
Figure 6 (graphics2.png)

Ideal tuned amplifier will have 0 dB or unity skirt selectivity but practically skirt selectivity is always greater than 1.

Figure 7
Figure 7 (graphics3.png)
Figure 8
Figure 8 (graphics4.png)
Figure 9
Figure 9 (graphics5.png)

Q of Resonance circuit= Q of inductor.

Figure 10
Figure 10 (graphics6.png)

Figure 11
Figure 11 (Picture 5.png)

Figure 5. The equivalent circuit representation of a tank circuit with losses represented by a series resistance in left hand diagram and by a shunt resistance in right hand diagram.

Figure 12
Figure 12 (graphics7.png)

For Q>>1, this equivalence is correct.


The problem(1) of the tutorials requires a Tank Circuit.

Figure 13
Figure 13 (Picture 6.png)

Figure 6. A tank circuit with 3.18µH in parallel with 8nF.

The inductance value is too small and it cannot be physically realized. So we use an Autotransformer to achieve the same.

Figure 14
Figure 14 (Picture 7.png)

Figure 7. Autotransformer Coupling for realizing small inductance of a given tuned circuit from a realistic real life inductor.

We require L=3.18μH and C=8 nF.

By use of Autotransformer if n=3

L’=(n)2L=9 X 3.18 μH=28.6μH

C’=C/n2=8 nF/9=0.9nF

L’ and C’ ae easily implementable.

Section 2.1.Use of autotransformer to reduce the loading of a subsequent IF Stage.

Figure 15
Figure 15 (Picture 8.png)

Figure 8. The deterioration of selectivity due to the loading caused by the reduced input impedance of the subsequent stage.

Rin will increase the B.W. and reduce the selectivity. Therefore tapped transformer is used.

Figure 16
Figure 16 (Picture 9.png)

Figure 9. The equivalent circuit for a tapped transformer coupled circuit.


Figure 17
Figure 17 (graphics8.png)
>>R1 ; Cin/n2<<C1 => Loading Avoided.


Figure 18
Figure 18 (Picture 10.png)

Figure 10. The circuit diagram of a double tuned r.f. amplifier.

In Figure 10 we see a tank circuit at the input as well as at the output. Because of the two tank circuits , these amplifiers are known as double-tuned amplifier. Double-tuned amplifiers are synchronous tuned and stagger tuned.

In synchronous tuned circuit, the two resonance frequencies are identical:


Figure 19
Figure 19 (Picture 11.png)

Figure 11. Frequency response of a synchronously tuned circuit.

Figure 20
Figure 20 (graphics9.png)

In practice it is difficult to achieve synchronous tuned amplifier. Since BJT is a non-unilateral device especially in CE configuration there is considerable interaction between input and output. If we tune input tank to f10 and we try to tune the output tank to same frequency f10 immediately input will get off-tuned. If we try to tune input then output will be off tuned. Therefore it is better to keep the two tanks off tuned by ∆f. This is called stagger tuned Amplifier.


Figure 21
Figure 21 (Picture 12.png)

Figure 12. Frequency response of a staggered tuned circuit.

Staggered tuned amplifier provides a better pass-band filter.. It has a wider BW maintaining a steep skirt. All IF Amplifiers in super-hetrodyne Receivers are double tuned amplifiers and are stagger tuned. There are several stages of IF Amplifiers for increased sensitivity. But to obtain the best results these multistage double tuned amplifiers need to be aligned. This is a strenuous process. By means of trimmer and padder this process is carried out.

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