Chapter five
Tuned Amplifiers
Tuned Circuits
➢ Frequency selectivity of resonant circuits allows a radio to be tuned to
one of a set of transmitting stations.
➢ Tuning is usually undertaken by varying the capacitance of an
adjustable capacitor.
➢ Resonant circuits are also important for tuning and for transmitting
signals.
Tuned amplifiers
➢ To amplify the selective range of frequencies , the resistive load , Rc is
replaced by a tuned circuit.
➢ The tuned circuit is capable of amplifying a signal over a narrow band of
frequencies centered at fr.
1
fr =
2 LC
L
Zr =
CR
TYPES OF TUNED AMPLIFIERS
Single tuned amplifier
➢ one parallel tuned circuit is used as a load
➢ Limitation: Smaller Bandwidth , smaller gain bandwidth product, does
not provide flatten response.
Double tuned amplifier
➢ It provides high gain, high selectivity and required bandwidth.
➢ Used in IF in radio and TV receivers.
➢ It gives greater 3db bandwidth having steep sides and flat top . But
alignment of double tuned amplifier is difficult
Stagger tuned amplifier
➢ Two single tuned amplifier are connected in cascaded form.
➢ Resonant frequency are displaced.
➢ To have better flat , wideband charcteristics with a very sharp rejective,
narrow band characteristics.
Synchronously tuned amplifier
➢ To obtain a high over all gain , several identical stages of tuned
amplifiers can be used in cascade.
➢ All amplifiers stages are assumed to be identical and to be tuned to the
same frequency w0.
Q factor:
➢ Ratio of reactance to resistance.
➢ Measure of efficiency with which inductor can store the energy.
Unloaded Q of tuned circuit
➢ Ratio of stored energy to dissipated energy in a reactor or resonator.
Loaded Q
➢ It is determined by how tightly the resonator is coupled to its
terminations.
Instability of tuned circuits:
➢ Transistor are used at the frequencies nearer to their unity gain BW
to amplify narrow band of high frequencies.
➢ At this RF frequencies , inter capacitance between base and collector
becomes dominant.
➢ Some feedback signal manages to reach the input from output , so
circuit becomes unstable and generating the oscillation and can stop
working as amplifier.
Stabilization techniques:
1. Hazeltine neutralization
2. Neutrodyne neutralization
3. Neutralization using coil
4. Rice neutralization
CLASS C AMPLIFIERS
➢ Class C amplifiers cannot be used in audio circuitry because of their
high distortion.
➢ Class C amplifiers can be used as tuned rf amplifiers where the
undesired harmonic frequencies can be filtered out.
➢ A class C amplifier is more efficient than either a class A or class B
amplifier; its efficiency approaches 100%.
➢The input coupling capacitor, base resistor,
and base-emitter junction form a negative
clamper.
➢Because of the clamping action, only the
positive peaks of the input signal drive the
transistor, Q1, into conduction.
➢ The RBC time constant is made long with
respect to the period of the input waveform to
provide the proper clamping action.
Equivalent input circuit of Class C TUNED AMPLIFIER
▪At the resonant frequency, fr,
the impedance of the tuned LC
circuit is maximum.
▪ The tank impedance, Ztank, is
purely resistive at fr.
Application: used in radio
receivers ad mixer circuits.
