# Feedback amplifier and tuned amplifier

### Feedback amplifier

Feedback factor varies with frequency, so negative feedback may change to positive and vice versa. OPAMP comparator doesn’t have any feedback. In common emitter amplifier the unbypassed emitter resistance is current series feedback. Voltage negative feedback is used in emitter follower. De-sensitivity factor is defined as the factor with which the feedback desensitizes the gain, It is also known as return difference. Advantages of negative feedback are increased input impedance, decrease in output impedance, linearity increases, stability increases, sensitivity decreases, frequency distortion decreases

Nyquist criteria is used to find out the feedback of an amplifier. If the nyquist plot of amplifiers encircles (-1, 0) in nyquist domain, it is positive feedback. If the nyquist plot of amplifiers doesn’t encircles (-1, 0) in nyquist domain, then it is negative feedback

### Tuned amplifier

A tuned amplifier is generally operated in class C operation. Tuned amplifier is used in RF applications. Frequencies above 200 kHz are called radio frequencies. At resonance power factor is 1. At resonant frequency voltage gain of a tuned amplifier is maximum, current and impedance is minimum. At series resonance, voltage across L is Equal to but opposite in phase to voltage across C. At resonance, the net reactive component circuit current is zero.In parallel resonance, the circuit impedance is L/CR (unit ohm). In LC circuit, if the input signal frequency is increased above resonant frequency then XL increases and XC decreases. In LC circuit, if the signal frequency is decreased below the resonant frequency, then XL decreases and XC increases

In series resonance, there is Voltage amplification. The Q of a tuned amplifier is greater than 10. If Q of an LC circuit increases, then bandwidth decreases. If L/C ratio of a parallel LC circuit is increased, the Q of the circuit increases. If the resistance of a tuned circuit is increased, the Q of the circuit decreases. The Q of a tuned circuit refers to the property of selectivity. At resonance, the phase angle between applied voltage and circuit is zero. At parallel resonance, the ratio L/C is very large. For frequencies below resonant frequency, a series LC circuit behaves as a capacitive load. For frequencies below the resonant frequency , a parallel  LC circuit behaves as an inductive load. For frequencies below the resonant frequency , a parallel  LC circuit behaves as an inductive load. If a high degree of selectivity is desired, then double-tuned circuit should have loosed coupling. In the double tuned circuit, if the mutual inductance between the two tuned circuits is decreased, the level of resonance curve raises. Double tuned circuits are used in IF stages of a radio receiver