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 3600

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

B10mA

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 1800

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 1800

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 K 5.6 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

T1

T2

T3

TR2

TR1

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 3600

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 CircuitVcc

T1

T2

T3

TR2

TR1

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 CircuitVcc

T1

T2

T3

TR2

TR1

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

T1

T2

Vcc Complementary Symmetry Amplifier

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

T1

T2

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