Published on August 9, 2014
UNIT 3 Amplifiers & Oscillators: UNIT 3 Amplifiers & Oscillators Presented by SATHISH.K(AP/ECE) Amplifier:- Device used to increase the signal strength. Oscillator:- Device used to produce the continuous on/off pulse.: Amplifier:- Device used to increase the signal strength. Oscillator:- D evice used to produce the continuous on/off pulse. RC(Resistor Capacitor) coupled Amplifier:: RC(Resistor Capacitor) coupled Amplifier: Cont…: Cont … Normal CE amplifier which comprises of the Resistor & Capacitor. It provides 180 degree phase shift at every stage. i.e., single stage provides 180 degree phase shift Capacitor Ce provides low impedance path & avoids loss due to negative feedback. The 2 nd stage of RC coupled amplifier reduces the gain. Frequency response: Frequency response PowerPoint Presentation: Low frequency range Mid frequency range High frequency range Gain BW product:: Gain BW product: Gain is inversely proportional to the BW. Advantages:: Advantages: Does not require bulky components Small Light Inexpensive Good frequency response Constant gain over audio frequency range Minimum non linear distortion, because no transformer is used. Disadvantage : Disadvantage Gain is small, due to loading effect. Noise depends on the climate changes. Poor impedance matching. PowerPoint Presentation: S.No . Voltage Amplifier Power Amplifier 1 Small signal amplifier Large signal amplifier 2 β = 100 β = 20 to 50 3 Power output is small Voltage output is small 4 Output impedance is high Output impedance is low 5 Collector current is small Collector current is large 6 Heat generation is very less. No cooling arrangements Heat generates. Heat sink is required 7 Usually RC coupling is used Invariable transformer coupling is used 8 Operation is restricted to linear portion Non linear operation is allowed 9 CE is mostly used with voltage divider bias circuit CE is mostly used, some times CC also used. Classification of power amplifier: Classification of power amplifier Classification depends on transistor biasing, Q points. Class A Class B Class C Class D Class AB Power Amplifiers: 12 Power Amplifiers Class A The amplifier conducts through the full 360 of the input. The Q-point is set near the middle of the load line. Class B The amplifier conducts through 180 of the input. The Q-point is set at the cutoff point. Power Amplifiers: 13 Power Amplifiers Class AB This is a compromise between the class A and B amplifiers. The amplifier conducts somewhere between 180 and 360 . The Q-point is located between the mid-point and cutoff. Class C The amplifier conducts less than 180 of the input. The Q-point is located below the cutoff level. Power Amplifiers: 14 Power Amplifiers Class D This is an amplifier that is biased especially for digital signals . Class A : Class A Conducts 360 degree of input signal. Q point is at center of the load line. Classification of power amplifier depends on load connected to the circuit. Direct coupled/ series fed class A amplifier(R L connected directly across collector) Transformer coupled class A amplifier(R L connected through transformer) Direct coupled/ series fed class A amplifier : Direct coupled/ series fed class A amplifier PowerPoint Presentation: % Efficiency ( η )= DC power P dc = V CC * I CQ AC power P ac = V rms * I rms V rms = V m /√2 V m = ( V max – V min )/2 I rms = I m /√ 2 I m = (I max – I min )/2 *100 PowerPoint Presentation: PowerPoint Presentation: Power dissipation P d = P dc – P ac Maximum power is dissipated when no input signal is applied to the input. P ac = 0. P dmax = V cc I CQ Advantages: : Advantages: Simple circuit Load connected directly so no need of transformer. Less number of components required Disadvantages : Disadvantages Maximum power is dissipated due to directly coupled R L Efficiency is poor Impedance matching is poor or no impedance matching. Transformer coupled class A amplifier: Transformer coupled class A amplifier PowerPoint Presentation: % Efficiency ( η )= DC power P dc = V CC * I CQ AC power P ac = V rms * I rms V rms = V m /√2 V m = ( V max – V min )/2 I rms = I m /√ 2 I m = (I max – I min )/2 *100 PowerPoint Presentation: PowerPoint Presentation: Power dissipation P d = P dc – P ac Maximum power is dissipated when no input signal is applied to the input. P ac = 0. P dmax = V cc I CQ Advantage:: Advantage: Efficiency is higher than direct coupled class A amplifier Impedance matching is possible. Disadvantage : Disadvantage Bulkier & costlier circuits due to transformer Circuit design is complicated Frequency response is poor Class B : Class B Each transistor Conducts only 180 degree of input signal. Q point in x-axis of the load line. 2 transistors are employed to remove distortion and improve efficiency. Classification depends on transistors used Push pull class B power amplifiers.[( npn+npn ) or ( pnp+pnp )] Complementary symmetry class B amplifiers. ( npn+pnp ) Class B push pull power amplifier: Class B push pull power amplifier Operation : Operation When V i =0 then V o =0. i.e., Q 1 & Q 2 are OFF During positive half cycle Q 1 = ON (F.B) I C = I C1 Q 2 = OFF (R.B ) I C2 = 0 During negative half cycle Q 2 = ON (F.B) I C = I C2 Q 1 = OFF (R.B ) I C1 = 0 PowerPoint Presentation: % Efficiency ( η )= DC power P dc = V CC * I DC AC power P ac = V rms * I rms = ( V m .I m )/2 V rms = V m /√2 I rms = I m /√ 2 *100 Advantages : Advantages Collector efficiency is high Delivers more output power No DC saturation Less distortion Eliminates even harmonics in output signal Disadvantages:: Disadvantages: It requires 2 transistors and two center tapped transformer. If the transistor is not matched, distortion increases. It introduces cross over distortion. Complementary symmetry push pull power amplifier: Complementary symmetry push pull power amplifier Combination of NPN and PNP transistors. Operation:: Operation: When V i =0, then V o =0. i.e.,Q 1 & Q 2 are OFF During positive half cycle Q 1 = ON (F.B) I C = I C1 Q 2 = OFF (R.B ) I C2 = 0 During negative half cycle Q 2 = ON (F.B) I C = I C2 Q 1 = OFF (R.B ) I C1 = 0 Advantages: : Advantages: Cross over distortion is reduced. No center tapped transformer is required. Transformer less so cost is reduced. Efficiency is high. Class C power amplifier:: Class C power amplifier: Generally class C is also called as tuned amplifier. Q point lies below the x- axis. It only conducts only less than 180 degree of the input waveform. Though Q- point lies below the x- axis, the efficiency is higher than class A & B amplifier. Tuning or resonant circuit is attached at the load and it is designed for Narrow band frequency. Circuit diagram & operation: Circuit diagram & operation Maximum output voltage due to usage of parallel resonant circuit. Its frequency is Where f 1, f 2 is half power frequencies. BW= f r /Q If Q is high means then BW is low. Class D power amplifier: Class D power amplifier It is designed to operate with digital or pulse type signal It will provide above 90% of efficiency. It always converts any type of input signal to pulse signal and at the output side that pulse is converted back to sine wave. Two push pull transistor switches are used in the circuit. Circuit diagram:: Circuit diagram: Operation:: Operation: When V i =0, then V o =0. i.e.,Q 1 & Q 2 are OFF During positive half cycle Q 1 = OFF (R.B) Q 2 = ON ( F .B ) During negative half cycle Q 1 = ON (F.B) Q 2 = OFF (R.B ) AMPLIFIER DISTORTION: : AMPLIFIER DISTORTION: If output of the power amplifier reproduces 360 degree of the input signal is the faithful amplification. If output is less than 360 degree means, then it is distortion. That is the output signal is non-sinusoidal. Causes of distortion: Causes of distortion Transistor is non- linear device. Which will cause distortion in the amplifier output, is called as non-linear or amplitude distortion or harmonic distortion. Input signal changes at the output side. This is due to the circuit elements and devices. Made changes in frequency is called frequency distortion. Amplifier distortion: Amplifier distortion Frequency distortion Amplitude distortion Phase distortion Intermodulation distortion Cross over distortion in Class B amplifier.