Power Amplifiers Unit 4 1 Classification of Power

Power Amplifiers Unit – 4. 1

Classification of Power Amplifiers v Power amplifiers are classified based on the Q point v If the operating point is chosen at the middle of the load line, it is called Class A amplifier v If the operating point is chosen at the cut-off point it is called Class B amplifier v If the operating point is chosen beyond the cut-off point it is called Class C amplifier v It conducts for 3600

Class A amplifier v The Q point is chosen at the middle of load line v This will give equal swing on either direction v Both halves of the input comes at the output v Hence Class A will give (amplitude) distortionless output v It can handle only small signals v Its efficiency is less

Ic m. A 0 1 B A m Ib = 60μA 6 10 m. A A 4 m Ib = 50μA 8 m. A Q 6 m. A Ib = 40μA Ib = 30μA 4 m. A Ib = 20μA 2 m. A A 0 V 24 V Vce Class A

Class B amplifier v The Q point is chosen at the cut-off point v This will give swing only on one direction v Only one half of the input comes at the output v Hence Class B will give (amplitude) distorted output v It can handle large signals v Its efficiency is high v It conducts for 1800

Ic m. A 0 1 Ib = 60μA 10 m. A Ib = 50μA 8 m. A Ib = 40μA 6 m. A A Ib = 30μA 4 m. A 0 m Ib = 20μA 2 m. A Q 0 V Class B 24 V Vce

Class C amplifier v The Q point is chosen at the beyond the cut-off point v This will give only a partial swing in one direction v Only a portion of the input comes at the output v Hence Class C will give (amplitude) severely distorted output v It can handle large signals v It conducts for less than 1800

A Ic m 10 Ib = 60μA 10 m. A Ib = 50μA 8 m. A Ib = 40μA 6 m. A A Ib = 30μA 4 m. A 0 m Ib = 20μA 2 m. A Q` 0 V Class C 24 V Vce

Class A Class B Class C

Distortionless amplifier v Out of the 3 amplifiers, Class C is unsuitable as the distortion is very heavy v Class A is the best, as it gives distortionless output v But Class A cannot handle large signals as required by the Power Amplifier v Though Class B gives heavy distortion, it gives out one half of the signal perfectly v And Class B can handle large signals

Class A Audio Amplifier v As we have seen out of the 3 classifications, Class A is the best, as it does not give any distortion v Among the configurations, we know that CE is the best as it gives maximum power gain v A CE amplifier will have high output impedance v Unfortunately for an audio amplifier, the output device is the speaker which has a low impedance

Impedance Matching v The speaker impedance is typically about 4 Ω v Hence there is a mismatch between the high Zo of the amplifier and the low impedance of the speaker v This will result in loss of gain v This can be avoided by connecting a transformer at the output stage v The primary winding will match the high Zo of the amplifier while the secondary will match the low impedance of the speaker

Class A Audio Amplifier Vcc Rb 1 270 K Rb 2 Rb 1 Rc 5. 6 K Re 270 K Ce Rb 2 Re Ce

Drawback v The drawback of this circuit is that it cannot handle large signals v In a Class A amplifier, the operating point is chosen around the middle of the load line v If the signal exceeds the cut-off point, the output current stops and any signal with a lower amplitude will not come at the output v Similarly, if the signal exceeds the saturation point, the output current cannot increase any further, even if the input signal increases

Ic Class A B Ib = 60μA 10 m. A Ib = 50μA 8 m. A Q 6 m. A Ib = 40μA Ib = 30μA 4 m. A Ib = 20μA 2 m. A A 0 V 24 V Vce

Class B Push-Pull Amplifier v To avoid this we can use Class B which has a greater signal handling capacity v But Class B will give only one half of the signal v Hence we can use 2 Class B amplifiers v One for one half and one for the other half v This type of amplifier is called Push-Pull Amplifier

Vcc T 1 TR 2 T 3 T 2 Class B Push-Pull

Push-Pull Circuit v TR 1 and TR 2 are output transistors connected back to back, with their emitters grounded v The output transformer TR 1 couples the push-pull output to the speaker v In the Push-Pull arrangement T 1 conducts for one half of the signal & T 2 conducts for the other half v Both are biased in Class B and each gives one half of the signal & the combined output is coupled to the speaker

Push-Pull Circuit v The Driver Transformer TR 2 gives 2 out of phase signals v During one half, the +ve half forward biases T 1 while the –ve half reverse biases T 2 v Thus when T 1 conducts, T 2 is cut-off & viceversa v This way both the transistors conduct alternately to give the full signal output

Class D Amplifier During the +ve half cycle Q 1 gets Forward Bias and it conducts v During the -ve half cycle Q 2 gets Forward Bias and it conducts v Thus both the transistors conduct alternately v The amplifier works for 3600 v No distortion v 100% efficiency v

Working of Push-Pull Circuit Vcc v During the first half T 1 conducts v Ic flows from the centre-tapping through T 1 to ground v This half is coupled to the speaker through TR 1 TR 2 T 3 T 2 TR 1

Working of Push-Pull Circuit Vcc v During the second half T 2 conducts v Ic flows from the centre-tapping through T 2 to ground v This half is coupled to the speaker through TR 1 TR 2 T 3 T 2 TR 1

Drawbacks v Though this circuit functions well it has a few drawbacks v Transformer coupling affects the quality of output v Phase shifting circuit is a must v Both these drawbacks can be avoided if we use one pair of PNP and NPN transistors at the output

Vcc Complementary Symmetry Amplifier T 1 T 2

Complementary Symmetry Amplifier v This circuit uses one NPN transistor & one PNP transistor at the output stage v During the +ve half, T 1(NPN) base gets forward bias & it conducts while T 2 (PNP) gets reverse biased and does not conduct v This gives one half of the signal at the speaker coupled to the emitter

Complementary Symmetry Amplifier v During the other half, T 2 gets forward bias and conducts while T 1 gets reverse biased and does not conduct v Thus T 1 & T 2 conduct alternately giving a distortionless output v This circuit does not require a phase shifter

Cross – over distortion v Class B Push-Pull amplifier has one limitation v As the phase of the signal changes from +ve to –ve (or vice-versa) one transistor stops conducting while the other begins conducting v But the transistor cannot conduct instantaneously as it requires a minimum Vbe before it starts conducting v Thus as the signal crosses over zero, a distortion occurs v This is called Cross over distortion

Cross – over distortion Vbe -Vbe

Class AB amplifier v This circuit overcomes cross-over distortion v Biasing is done such that even if there is no input signal, a small current keeps the output transistor conducting v This circuit uses 2 diodes whose characteristics matches with that of the BE junction of the output transistors v Biasing resistors R 1 & R 2 are also identical values

Vcc Class AB amplifier R 1 T 1 D 2 R 2 T 2

Symmetrical components v Since R 1 & D 1 are identical to R 2 & D 2, the diode junction as well as the output point will be at half the supply voltage v Because of symmetry both T 1 & T 2 will conduct equally v Even when there is no input signal, there will be a current Icq = (I/2 Vcc – 0. 6) / R 1 v This will keep the output transistors conducting

Elimination of cross-over distortion v Normally, during cross-over there will not be any output till the non-conducting transistor gets the minimum Vbe v This causes distortion v This has been eliminated here, since the 0. 6 V across the diodes keep the transistors on and gives a continuous output signal without producing cross -over distortion

Thermal stability v In addition, the two diodes also provide thermal stability v They prevent the output transistors going to Thermal Run Away v When the output current is high, heat dissipation is more v The increase in temperature produces more charge carrier in the BE junction of T 1 & T 2

v This increases Ib & hence Ic v This in turn increases the power dissipation & hence the heat v This chain goes on till too much current flows and destroys the transistors v This is called Thermal Run Away v This is arrested by the diodes in the output circuit

v When the charge carriers increase in the B-E junction of T 1 & T 2, a similar increase takes place in D 1 & D 2, due to matching characteristics v This increase in the diode current, produces more drop across R 1 & R 2 and brings down the forward bias at the base of T 1 & T 2 v Thus the 2 diodes prevent cross-over distortion as well as provide thermal stability

End of Unit – 4. 1