l09 power amplifier (class b and c)

8/12/2019 L09 Power Amplifier (Class B and C)

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Lecture 9Power Amplifiers

-Class B


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Class-B Amplifier :-

vin

vout

Av

Class-B operation :-

Common-collector class-B amplifier :-

+VCC

v

0

+0.7V

Transistorconducts

outvin

Transistor off

Class-B amplifier provides an output signal varying over one-halfthe input

signal cycle + zero phase shift.

?? where is the Q-point on the load line ???


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The dc bias point for class-B amplifier is therefore at 0 volt.

i.e. biased at cutoff :-

ICQ

= 0 and

V =VCEQ CE (off )

The advantage of a class-B amplifier is that the collector current is zero

when the input signal to the amplifier is zero.

Therefore the transistor dissipate no power in the quiescent condition,

i.e. more efficient !!

class-B amplifier was developed to improve on the low efficiencyrating of the class-A amplifier.

Obviously, the output is not a faithful reproduction of the input if only onehalf-cycle is present.

Therefore, a two-transistor configuration, is necessary to get a sufficiently

good reproduction of the input waveform.

This amplifier configuration is known as push-pull emitter follower (push-

pull amplifier)orcomplementary-symmetry amplifier.


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4Push-Pull Operation :-

Class-B push-pull amplifier circuit :-

+VCC

Q1= on

RL

Q2= on

-VCC

Q1

Q2v

in

The circuit configuration feature is the use of complementary transistors,

i.e. one of the transistors is a npnand the other is a pnp.

The term push-pull comes from the fact that two transistors in a class-B

amplifier conduct on alternating half-cyclesof the input.

The combined half-cycles then provide an output for a full 3600of operation.

Note :- Need dual-polarity power supplies.


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No Input :-

When the transistor is in its quiescent state (no input), bothtransistors are

biased at cutoff.

Positive Input :-

During the positive half-cycleof the input signal, Q1is biased above cutoff,

and conduction results through the transistorRL.

During this time, Q2is still biased at cutoff.

provide output on the positive-output half-cycle.

Negative Input :-

During the negative half-cycle of the input signal, Q1 is returned to the

cutoff state, and Q2is biased above cutoff.

As a result, conduction of Q2start to built while Q1 remains off.

provide output on the negative-output half-cycle.

The combined half-cycles then provide an output for a full 3600of operation.

It is important that the two transistors in a push-pull configuration be

carefully matched.

?? WHY ??


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6Crossover Distortion :-

Among the disadvantagesof a class-B amplifier is that the nonlinear cut-

offregion is included in the operation range.

Because of the biasing arrangement, class-B amplifiers are subject to a type

of distortion.

When VB= 0, the input signal voltage must exceed VBEbefore a transistor

conduct.

Therefore, there is a time interval between the positive and negative

alternations when neither transistor is conduction.The resulting distortion in the output waveform is quite common and is

called crossover distortion.

To prevent crossover distortion, both transistors will normally be biased at a

level that is slightly abovecutoff.

Biasing both transistors slightly above cut-off will allow the amplifier to

provide a linear output that contains no distortion.

Class-AB Amplifier


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7Class-AB Amplifier :-

To eliminate crossover distortion, both transistors in the push-pull

arrangement must be biased slightly above cut-off when there is no signal.

This can be done with, for example, a voltage-divider arrangement.

This variation of the class B push-pull amplifier is designated as class-AB.

(1) Voltage-Divider Bias :-

+VCC

RL

R1

R2

R3

R4

Q1

Q2

Q1= on

Q2= on

vin

Voltage-divider bias class-AB amplifier.

Note RLis capacitively coupled

(dual-polarity power supplies single-polarity power supply)


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However, difficult to maintain a stable bias point with this circuit due to

changes in VBEover temperature changes. (i.e. temp Q-point )

A more stable arrangement

(2) Diode Biasing Circuit:-

R1

D1

D2

+VCC

Q1

RL

Q2

R2

When the diode characteristics of D1 and D2 are closely matched to the

transconductance characteristics of the transistors, a stable bias can be

maintained over temperature.

This can be also be accomplished by using the base-emitter junction of two

additional transistors instead of D1and D2.

Although technically incorrect, class-AB amplifiers are often referred to

as class-B in common practice.


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9DC Operating Characteristics :-

+VCC

+

D1

D2

R2

Q1

Q2

R1

VCC

2

-+ +

VCC VCC

2 2

- -

DC equivalent circuit.

Assume :- (i) R1

=R2,

(ii) transconductance characteristic of the diodes and the

transistors are identical.

Q-point :-

VBE1Q VBE2Q , VCE1Q VCE2Q , IC1Q IC2Q , VCC 2VCEQ

Because both transistors are biased near cutoff :- ICQ

= 0


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10AC Operating Characteristics :-

VCE

VCC

VCEQ

IC(sat)

IC(mA)

aclo

ad

line

0V

Q1= on

Q2= on

VCEQ

=V

CC

2

vce

ic

RL

Q1

Q2

21 RR

AC equivalent circuit. AC load line.

Under maximum conditions :-

both transistors Q1 and Q2 are alternately driven from near cutoff to near

saturation

V VCC CCQ V Q 0

1 CC 2

2 2

and :-

v Vce (peak ) CEQ

VCCV

CEQ

2

vce (peak )

VCEQ VCCI = = =

C ( sat )

R R 2RL L L


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11In ac operation -

+VCC

RL

R1

R2

Q1

Q2

Q1= on

Q2= on

vin

D1

D2

vo

2CC

V

0

vL

0

CC

vin= +ve

When input vin is positiveand Q1is conducting, current is drawn from the

power supply and flows through Q1to the load.

vin= -ve

When Q1is cut-off by a negative input, nocurrent can flow from the supply.At those times, Q2 is conducting and capacitor CCdischarges through that

transistor.

Thus, current flows from the load, through CC, and through Q2 to ground

whenever the input is negative.

The RLCC time constant must be much great than the period of the lowest

signal frequency.

The lower cut-off frequency due to CC is given by f =CL

R C2

1


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12Power Calculations :-

(1) DC Input Power :-

The total (dc) input powercomes from the VCCsource :-

P (dc) = V Ii CC CC

I =I +ICC C ( ave ) 1

I I (I >>I )CC C ( ave ) C ( ave ) 1

P (dc) = V Ii CC C ( ave )

The total current drawn from the supply is the sum of the average Q1collector current and the current through the amplifier base circuit. The

average value of the current through the collector of Q1is given as -

I

1 IC ( sat )I =

T

I dt =C ( ave ) 0 C

t T

Ic(sat)

TT/2

Ic(ave)

c

i.e. just a standardIaveequation for the half-wave rectifier.

IC ( sat )Pi(dc) = V

CC

V ICC C ( sat )Pi (dc) =


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13(2) Maximum AC Output Power :-

IC(mA)

VCE

VCC

VCEQ

IC(sat)

acloadlin

e

0V

Q1= on

Q2= on

VCEQ

=V

CC

2

vce

ic

RL

Q1

Q2

21 RR

The class-B amplifier has the same (ac)output power characteristics as the

class-A amplifier :-

v2

o ( rms )P (ac) = i v =o c ( rms ) o ( rms )

RL

The maximum load poweris:

P (ac) = i vo max c (max)( rms ) o (max)( rms )

I VC ( sat ) CEQ

P (ac) =o max

2 2

IC ( sat ) VCCP (ac) =o max 2 2 2

I VC ( sat ) CC

P (ac) =o max

4

I V

PC ( sat ) CC

o(ac ) 4 = 100% = 100% = 100% = 79%

P( V I 4i dc ) CC C ( sat )

max 79%


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14

Power Amplifiers

-Class C


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Class-C Amplifier :-

V in VoutAv

Class-C amplifier operation (inverting).

Basic Operation :-

The transistor is biased with a negative VBE. Thus it will conduct only whenthe input signal is above a specified positive value.

i.e. transistor ON when Vin> VBB+ VBE


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A class-C amplifier load line, where VBEQis set to a negative value.

The power dissipation of the transistor in a class-C amplifier is low because

it is on for only a small percentage of the input cycle.

For example :-

If a sinusoid forms the input to a class-C amplifier, the output consists of

blipsat the frequency of the input.

Since this is a periodic signal, it contains a fundamental frequency

component plus higher-frequency harmonics.

If this signal is passed through an inductor-capacitor (LC) circuits tuned to

be resonant at the fundamental frequency, the output is approximately a

sinusoidal signal at the same frequency as the input.

This approach is often used if the signal to be amplified is either a pure

sinusoid or a moregeneral signal with a limited range of frequencies.

Class-C amplifiers are capable of providing large amounts of power,

They are often used for transmitter power stages, such as radio or

communications, where a tuned circuit is included to eliminate the higherharmonics in the output signal.

max> 98%


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17Tuned Class-C Amplifier :-

Because the collector voltage (output) is not a replica of the inputs, the

resistively loaded class-C amplifier is of no value in linear applications.

It is therefore necessary to use a class-C amplifier with a parallel resonant

circuit, as shown in Figure (a).

The resonant frequency of the tuned circuit is determined by the formula :-

1f =

2 LC

The tuned circuit in the output will provide a full cycleof output signal for

the fundamental or resonant frequencyof the tuned circuit of the output.

This type of operation is therefore limitedto use at one fixed frequency, as

occurs in a communications circuit, for example.

Operation of a class-C circuit is not intendedprimarily for large-signal or

power amplifiers.


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18Power Transistor Heat Sinking :-

Power transistor can dissipate many watts.

For example :- 2N3055, an inexpensive power transistor of great popularity,can dissipate as much as 115 wattsif properly mounted.

All power devices are packaged in cases that permit contact between a metal

surface and an external heat sink.

In most cases that metal surface of device is electrically connected to one

terminal (e.g. for power transistors the case is always connected to the

collector).

Insulator :- insulated from heat sink, as is usually necessary, especially if

several transistors are mounted on the same sink.

Chassis or heat sink :- provides additional surface area to conduct heat

away from the transistors more quickly to prevent overheating.


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The whole point of heat sinking is to keep the transistor junction (or the

junction of some other device) below some maximum specified operating

temperature.

For silicon transistors in metal packages the maximum junction temperature

is usually 2000C, whereas for transistors in plastic packages it is usually

1500C.


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2020


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21Summary:-

Power Amplifiers

Amplifier Efficiency

Amplifier Classification

Class-A Amplifier

Basic Operation Principle

DC Operating Characteristics

AC Operating Characteristics

AC Load Line

Amplifier CompliancePower Calculations

Maximum Efficiency

Class-B Amplifier


Push-Pull Emitter Follower

Crossover Distortion

Class-AB Amplifier

Voltage-Divider ConfigurationDiode Bias Configuration

DC Operating Characteristics

AC Operating Characteristics

Power Calculations

Maximum Efficiency

Class-C Amplifier


Tuned Class-C AmplifierBasic Operation Principle

Power Transistor Heat Sinking

l09 power amplifier (class b and c)

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