Power Amplifiers Unit – 4.1

Classification of Power Amplifiers  Power amplifiers are classified based on the Q point  If the operating point is chosen at the middle of the load line, it is called Class A amplifier  If the operating point is chosen at the cut-off point it is called Class B amplifier  If the operating point is chosen beyond the cut-off point it is called Class C amplifier  It conducts for 360 0

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

Ic Vce Ib = 60μA Ib = 50μA Ib = 20μA Ib = 30μA Ib = 40μA 10mA 2mA 4mA 6mA 8mA 24 V0 V Q A B 10mA 6mA 4mA Class A

Class B amplifier  The Q point is chosen at the cut-off point  This will give swing only on one direction  Only one half of the input comes at the output  Hence Class B will give (amplitude) distorted output  It can handle large signals  Its efficiency is high  It conducts for 180 0

Ic Vce Ib = 60μA Ib = 50μA Ib = 20μA Ib = 30μA Ib = 40μA 10mA 2mA 4mA 6mA 8mA 24 V0 V Q 10mA 0mA Class B

Class C amplifier  The Q point is chosen at the beyond the cut-off point  This will give only a partial swing in one direction  Only a portion of the input comes at the output  Hence Class C will give (amplitude) severely distorted output  It can handle large signals  It conducts for less than 180 0

Ic Vce Ib = 60μA Ib = 50μA Ib = 20μA Ib = 30μA Ib = 40μA 10mA 2mA 4mA 6mA 8mA 24 V0 V Q` 10mA 0mA Class C

Class A Class B Class C

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

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

Impedance Matching  The speaker impedance is typically about 4 Ω  Hence there is a mismatch between the high Zo of the amplifier and the low impedance of the speaker  This will result in loss of gain  This can be avoided by connecting a transformer at the output stage  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 270 K5.6 K Re Rb1 Rb2 Ce Rc Vcc 270 K Re Rb1 Rb2 Ce

Drawback  The drawback of this circuit is that it cannot handle large signals  In a Class A amplifier, the operating point is chosen around the middle of the load line  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  Similarly, if the signal exceeds the saturation point, the output current cannot increase any further, even if the input signal increases

Ic Vce Ib = 60μA Ib = 50μA Ib = 20μA Ib = 30μA Ib = 40μA 10mA 2mA 4mA 6mA 8mA 24 V0 V Q A B Class A

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

Class B Push-Pull Vcc T1T1 T2T2 T3T3 TR 2 TR 1

Push-Pull Circuit  TR1 and TR2 are output transistors connected back to back, with their emitters grounded  The output transformer TR1 couples the push-pull output to the speaker  In the Push-Pull arrangement T1 conducts for one half of the signal & T2 conducts for the other half  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  The Driver Transformer TR2 gives 2 out of phase signals  During one half, the +ve half forward biases T1 while the –ve half reverse biases T2  Thus when T1 conducts, T2 is cut-off & vice- versa  This way both the transistors conduct alternately to give the full signal output

Class D Amplifier  During the +ve half cycle Q1 gets Forward Bias and it conducts  During the -ve half cycle Q2 gets Forward Bias and it conducts  Thus both the transistors conduct alternately  The amplifier works for  No distortion  100% efficiency

 During the first half T1 conducts  Ic flows from the centre-tapping through T1 to ground  This half is coupled to the speaker through TR1 Working of Push-Pull Circuit Vcc T1T1 T2T2 T3T3 TR 2 TR 1

 During the second half T2 conducts  Ic flows from the centre-tapping through T2 to ground  This half is coupled to the speaker through TR1 Working of Push-Pull Circuit Vcc T1T1 T2T2 T3T3 TR 2 TR 1

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

T1T1 T2T2 Vcc Complementary Symmetry Amplifier

 This circuit uses one NPN transistor & one PNP transistor at the output stage  During the +ve half, T1(NPN) base gets forward bias & it conducts while T2 (PNP) gets reverse biased and does not conduct  This gives one half of the signal at the speaker coupled to the emitter

Complementary Symmetry Amplifier  During the other half, T2 gets forward bias and conducts while T1 gets reverse biased and does not conduct  Thus T1 & T2 conduct alternately giving a distortionless output  This circuit does not require a phase shifter

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

Cross – over distortion Vbe -Vbe

Class AB amplifier  This circuit overcomes cross-over distortion  Biasing is done such that even if there is no input signal, a small current keeps the output transistor conducting  This circuit uses 2 diodes whose characteristics matches with that of the BE junction of the output transistors  Biasing resistors R1 & R2 are also identical values

T1T1 T2T2 Vcc Class AB amplifier R1 R2 D1 D2

Symmetrical components  Since R1 & D1 are identical to R2 & D2, the diode junction as well as the output point will be at half the supply voltage  Because of symmetry both T1 & T2 will conduct equally  Even when there is no input signal, there will be a current Icq = (I/2 Vcc – 0.6) / R1  This will keep the output transistors conducting

Elimination of cross-over distortion  Normally, during cross-over there will not be any output till the non-conducting transistor gets the minimum Vbe  This causes distortion  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  In addition, the two diodes also provide thermal stability  They prevent the output transistors going to Thermal Run Away  When the output current is high, heat dissipation is more  The increase in temperature produces more charge carrier in the BE junction of T1 & T2

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

 When the charge carriers increase in the B-E junction of T1 & T2, a similar increase takes place in D1 & D2, due to matching characteristics  This increase in the diode current, produces more drop across R1 & R2 and brings down the forward bias at the base of T1 & T2  Thus the 2 diodes prevent cross-over distortion as well as provide thermal stability

End of Unit – 4.1