Positive Feedback Amplifier

Positive Feedback Amplifier

FEEDBACK AMPLIFIER FEED BACK: The process of applying a fraction of output energy of some device back to the input known as feedback. FEEDBACK AMPLIFIER: The amplifier using feed back process is called feedback amplifier. TYPES OF FEEDBACK AMPLIFIER: There are two types of feedback amplifier: a)Positive feedback amplifier b)Negative feedback amplifier POSITIVE FEEDBACK AMPLIFIER: >The amplifier in which feedback energy is applied in the same polarity or in same phase is called as positive feedback amplifier. increases the voltage gain of mplifier. his distortion is more. has increased instability. seldom used as an amplifier. Mostly used in oscillator. NEGATIVE FEEDBACK AMPLIFIER: >The amplifier in which feedback energy is applied in the opposite polarity or out of phase with input is called negative feedback amplifier. decreases the voltage gain. this distortion is less. is more stable. is mostly used as an amplifier. PRINCIPLE OF NEGATIVE FEEDBACK AMPLIFIER: >Block diagram of negative feedback amplifier: >A=Voltage gain without feedback. Af=A’oltage gain with feedback. Vi=lnput signal to be amplified.

Vo= Output signal of amplifier Vd=Actual input signal to the amplifier. Vf= Feedback signal applied to input side. m=feedback fraction >From block diagram it is clear that, m=VfNo i. e. Vf=mVo Again A=VoNd i. e. i. e. Vo=AVi-AmVo i. e. Vo+AmVo=AVl i. e. Af=l /(! +mA) OF NEGATIVE FEEDBACK AMPLIFIER: 1 -Resultant gain is independent of transistor parameters or supply voltage fluctuation. Hence gain is stable. 2-lt reduces non-linear distortion by a factor (1 +mA). 3-lt improves frequency response. 4-lt increases circuit stability. lt increases input impedance & decreases output impedance. INPUT IMPEDANCE & OUTPUT IMPEDANCE WITH FEEDBACK: >Figure shows the block diagram: >Given that: Zi=input impedance without feedback=Vd/i1 Zif=lnput impedance with feedback??”Wil Zo=Output impedance without feedback Zof=output impedance with feedback il ??”input current >From block diagram it is clear that Now Or VI/ It is clear that input impedance with feedback increases by a factor (1 +mA). Similarly, it can be derived that output impedance without feedback decreases by a factor 1 +mA). . e. +mA) EMITTER FOLLOWER: *It is a current amplifier that has no voltage gain. * It is a negative current feedback circuit. *It has high input impedance & low output impedance. *It is used for impedance matching. *It is also known as common collector amplifier. *Circuit diagram: *Circuit details: It consists of transistor Q, biasing resistor Rl& R2, emitter resistor RE, input coupling capacitor Cin, biasing supply +VCC, output coupling capacitor CC. Emitter resistance RE acts as the load & a. c. output voltage Vo is taken across RE.

Here biasing is provided by voltage divider method. lt has no collector resistance & no emitter bypass capacitor. *Operation: ac emitter current produces an output voltage across RE. This voltage opposes the input voltage, thus providing negative feedback. The output voltage Vo is equl to input voltage i. e. output voltage follows the input voltage. Hence the name is voltage follower. CHARACTERISTIC OF EMITTER FOLLOWER: *No voltage gain. Practically close to one. *It has high current gain & power gain. *Input & output ac voltages are in phase.