Lecture #5 (Electronics)

2

Class C power amplifier design

• There are four important design parameters are of great importance for PA design in general

• These parameters are

– The output power

– Transistor power dissipation

– Maximum collector to emitter voltage VCEmax

– The maximum transistor output current Ip

3

Class C power amplifier design • The maximum collector current is given by

• Since • The collector current can be rewritten as

• The maximum current in terms of the output

current can be written as

• Note that the value of the collector voltage VCC can be written as

4

Class C power amplifier design • Now the maximum collector current can be

rewritten as

• A normalized peak collector current is defined as

5

Class C power amplifier design • A plot of the normalized peak current

versus the conduction angle is shown below

6

Class C power amplifier design • The power dissipated in the transistor is given by

• Note the value of Ip can be expressed as

• From we can conclude that

• If the value of Ip is substituted in the PT equation then

2)cos( 1IV

SinIV

PPP CCPCCOiT

cos1 M

p

II

2sin2

2 1

II p

7

Class C power amplifier design • The power dissipated in the transistor is

given by

• Or PT can be rewritten as

2)

2sin2

cossin(2 1

1

IVIVP CC

CCT

8


• A normalized plot of PT/PO versus theta is shown below

9

Class C power amplifier design example

Example: Design a class C amplifier that will deliver 5-W average power to a 50 Ω load at a frequency of 1 MHz using a transistor with a safe power dissipation rating of 0.5 W

Solution:

The average output power is given by

Or

VPRVCC OL 4.2255022

10

Class C power amplifier design example

Solution: Since the allowable power dissipation is The maximum conduction angle can be found

from the graph shown in previous slide or by solving the PT/PO equation

The value of the normalized current corresponds to this angle is refer to the figure in slide IM with ϴ

The peak collector current is given by

5.57

11


• An alternate design procedure for class C amplifiers is

– Select the power supply

– Select the transistor

– Determine the maximum output power without exceeding the transistor ratings

– The transistor then can be driven to its maximum allowed value of output current

– Determine the value of the load resistance that twill result in the maximum current according to

12


• Now the transistor power equation can be modified as

• The normalized transistor power dissipation is given by

• Where

cos1

)2sin2()cos(sin4)(

f

13


14

Class C power amplifier design example 2

• Example 2: Determine the maximum output power and the conduction angle of a class C amplifier using a transistor with maximum power dissipation rating of 4 W and a maximum output current of 1.5 A. The supply voltage is 48 V

Solution:

The normalized maximum transistor dissipation is given by

15


Solution:

The conduction angle for maximum normalized transistor power P΄T is found to be as

If we refer back (PT/PO vs θ ) plot we find that the value of PT/PO which corresponds to this angle is

The output power now can be found as

16


Solution:

Finally the value of the load resistance that results in this output power is given by

Summary

Class B

• A class B output stage can be far more efficient than a class A stage (78.5 % maximum efficiency compared with 25 %).

• It also requires twice as many output transistors…

• …and it isn’t very linear; cross-over distortion can be significant.

Class B • Class B amplifiers are used in low cost designs or

designs where sound quality is not that important.

• Class B amplifiers are significantly more efficient than class A amps.

• They suffer from bad distortion when the signal level is low (the distortion in this region of operation is called "crossover distortion").

Class B • Class B is used most often where economy of design

is needed.

• Before the advent of IC amplifiers, class B amplifiers were common in clock radio circuits, pocket transistor radios, or other applications where quality of sound is not that critical.

Power dissipation of the class B output stage

versus amplitude of the output sinusoid.

Class AB • Class AB is probably the most common amplifier

class currently used in home stereo and similar amplifiers.

• Class AB amps combine the good points of class A and B amps.

• They have the improved efficiency of class B amps and distortion performance that is a lot closer to that of a class A amp.

Eliminating crossover distortion in a transformer-coupled push-pull amplifier. The diode compensates for the base-emitter drop of the transistors and produces

class AB operation.

Load lines for a complementary symmetry push-pull amplifier. Only the load lines for the npn transistor are shown.

Single-ended push-pull amplifier.

A Darlington class AB push-pull amplifier.

A Class AB push-pull amplifier with correct output voltage.

Incorrect output waveforms for the amplifier in previous Figure.

Class AB

• With such amplifiers, distortion is worst when the signal is low, and generally lowest when the signal is just reaching the point of clipping.

• Class AB amps use pairs of transistors, both of them being biased slightly ON so that the crossover distortion (associated with Class B amps) is largely eliminated.

Class C • Class C amps are never used for audio circuits.

• They are commonly used in RF circuits.

• Class C amplifiers operate the output transistor in a state that results in tremendous distortion (it would be totally unsuitable for audio reproduction).

Basic class C amplifier operation (non inverting).

Basic class C operation.

Class C waveforms.

Tuned class C amplifier.

Tuned class C amplifier with clamper bias.

Class C

• However, the RF circuits where Class C amps are used, employ filtering so that the final signal is completely acceptable.

• Class C amps are quite efficient.

Power Transistor Limitations

POWER TRANSISTOR

Transistor limitations

• Maximum rated current,

• Maximum rated voltage,

• Maximum rated power.

The maximum rated power is related to the maximum

allowable temperature of the transistor.

– BJT

Large-area devices – the geometry and doping

concentration are different from those of small-signal

transistors

Examples of BJT rating:

Parameter

Small-signal

BJT

(2N2222A)

Power BJT

(2N3055)

Power BJT

(2N6078)

VCE (max) (V) 40 60 250

IC (max) (A) 0.8 15 7

PD (max) (W) 1.2 115 45

35 – 100 5 – 20 12 – 70

fT (MHz) 300 0.8 1

POWER TRANSISTOR

Current gain depends on IC and is smaller in power BJT.

The maximum rated collector current, IC(rated) may be

related to the following:

1. maximum current that the wires connecting the

semiconductor to the external terminals can handle

2. The collector current at which the gain falls below a

minimum specified value

3. current which leads to maximum power dissipation

when the transistor is in saturation.

– BJT POWER TRANSISTOR

Typical dc beta

characteristics

( hFE versus IC)

for 2N3055


The maximum voltage limitation:

• Avalanche breakdown in the reverse-biased base-

collector junction (involves gain and breakdown at the

p-n junction)

• Second breakdown – nonuniformities in current

density which inreases temperature in local regions in

semiconductor.


Avalanche Breakdown (Figure 1)

• In Figure 1, the breakdown voltage when the base

terminal is open-circuited (IB=0) is VCEO, approx. 130V

(Figure 1).

• All the curves tend to merge to the same collector-

emitter voltage, denoted as VCE(sus) once breakdown

has occurred.

• VCE(sus) is the voltage necessary to sustain the

transistor in breakdown.

• In Figure 1, VCE(sus) is approx. 115V



IC –VCE characteristics

showing breakdown

effect

Figure 1


BBECCEQ ivivp

The second term is usually small, hence;

CCEQ ivp

The average power over ONE CYCLE of the signal:

T

CCEQ dtivT

P0

1

The total instantaneous power dissipation in transistor


The average power dissipated in a BJT must be kept below

a specified maximum value to ensure that the temperature

of the device does not exceed the maximum allowable

value.

If collector current and collector-emitter voltage are dc

quantities, the maximum rated power, PT

CCET IVP

The power handling ability of a BJT is limited by two factors,

i.e. junction temperature, TJ and second breakdown. Safe

Operating Area (SOA) must be observed, i.e. do not exceed

BJT power dissipation.


The safe operating area (SOA) is bounded by IC(max); VCE(sus)

and maximum rated power curve, PT and the transistor’s

second breakdown characteristics curve (Figure 2)

SOA of a BJT

(linear scale)

Figure 2


SOA of a BJT

(log scale)

Figure 3


EXAMPLE

Determine the required ratings

(current, voltage and power) of

the BJT.


EXAMPLE – Solution

For the maximum

collector current; 0CEV

A 38

24max

L

CCC

R

VI

For the maximum collector-

emitter voltage; 0CI

V 24max CCCE VV



The load line equation

is;

The load line must lie

within the SOA

LCCCCE RIVV

The transistor power

dissipation;

LCCCCCLCCCCCET RIIVIRIVIVP 2



The maximum power occurs when

02 LCCC RIV

0C

T

dI

dP

i.e. when

or when A 5.1CI

At this point; V 12 LCCCCE RIVV

and; W18 CCET IVP

Differentiating



Thus the transistor ratings are;

W18

V 24

A 3

max

max

T

CE

C

P

V

I

In practice, to find a suitable transistor for a given

application, safety factors are normally used. The

transistor with

will be required.

W18 V, 24 A, 3 maxmax TCEC PVI


Physical structure;

• Large emitter area to

handle large current

densities

• Narrow emitter width to

minimize parasitic base

resistance

• May include small

resistors (ballast resistor)

in emitter leg to help

maintain equal currents

in each B–E junction.

Top

view Cross-

sectional

view

Revision in a different Form to previously covered material

Concept Preview

• Efficiency is most important in power amplifiers.

• Poor efficiency means that much of the input power is converted to heat.

• A class A amplifier conducts for the entire signal cycle and has the lowest efficiency.

• A class B amplifier conducts for only half of the signal cycle.

• A class C amplifier conducts for less than half of the signal cycle.

• A class D amplifier switches between cutoff and saturation.

Power Amplifier

PIN Efficiency =

Input signal

POUT

POUT

PIN

Output signal

HEAT = PIN - POUT High efficiency means less heat.

Efficiency

• The dc power supplied to an amplifier is

PIN = VCC x IDC

• Efficiency = POUT/PIN x 100%

• The maximum efficiency for Class A

amplifiers with a dc collector resistance

and a separate load resistance is 25%.

• Class A is usually not acceptable when

watts of power are required.

t

IC

t

IC

t

IC

t

IC

ISAT

A B

C D

The major classes of amplifier operation

Class and efficiency quiz

If POUT = 100 W and PIN = 200 W, the efficiency is _________. 50%

The efficiency of an ideal amplifier is __________. 100%

When efficiency is poor, too much of the input is converted to ________. heat

An amplifier that conducts for the entire cycle is operating Class _______. A

An amplifier that conducts for half the cycle is operating Class _______. B

Concept Review • Efficiency is most important in power amplifiers.

• Poor efficiency means that much of the input power is converted to heat.

• A class A amplifier conducts for the entire signal cycle and has the lowest efficiency.

• A class B amplifier conducts for only half of the signal cycle.

• A class C amplifier conducts for less than half of the signal cycle.

• A class D amplifier switches between cutoff and saturation.

Repeat Segment

Concept Preview

• Class A amplifiers operate at the center of the load line and have a large quiescent current flow.

• Class B amplifiers operate at cutoff and have no quiescent current flow.

• Class B amplifiers are usually operated in push-pull configurations.

• Class B amplifiers have crossover distortion.

• Class AB reduces crossover distortion.

• Bridge amplifiers provide four times the output power and eliminate the output coupling capacitor.

A large-signal amplifier can also be called a power amplifier.

This class A amplifier has a large quiescent collector current.

C

B E

VCC = 18 V

RL = 12 W RB = 1.2 kW

CC = 60

IB = VCC

RB

18 V

1.2 kW = = 15 mA

IC = x IB = 60 x 15 mA = 0.9 A

0 2 4 6 8 10 12 14 16 18

0.2 0.4 0.6 0.8

1.0 1.2

1.4

VCE in Volts

IC in A

5 mA

0 mA

25 mA

20 mA

15 mA

10 mA

ISAT = VCC

RL

18 V

12 W = = 1.5 A

Q

This is a Class A amplifier.

PC = VCE x IC = 7.2 V x 0.9 A = 6.48 W

0 2 4 6 8 10 12 14 16 18

0.2 0.4 0.6 0.8

1.0 1.2

1.4

VCE in Volts

IC in A

5 mA

0 mA

25 mA

20 mA

15 mA

10 mA Q

This is a Class B amplifier.

PC = VCE x IC = 18 V x 0 A = 0 W

Its quiescent power dissipation is zero.

0 2 4 6 8 10 12 14 16

0.2 0.4 0.6 0.8

1.0 1.2

1.4

5 mA

0 mA

25 mA

20 mA

15 mA

10 mA

The collector signal is too distorted for linear applications.

C

B

C

B E

E

+VCC

The complementary-symmetry Class B push-pull amplifier has acceptable

linearity for some applications.

NPN

PNP

NP

N

PN

P

Class B

C

B

C

B E

E

+VCC

Since the base-emitter junction potential is 0.7 V, there is some crossover distortion.

NPN

PNP

C

B

C

B E

E

+VCC

Crossover distortion is eliminated by applying some forward bias

to the transistors (class AB).

NPN

PNP

1.4 V

0 2 4 6 8 10 12 14 16 18

0.2 0.4 0.6 0.8

1.0 1.2

1.4

VCE in Volts

IC in A Q

The quiescent power dissipation is moderate for class AB.

The efficiency is much better than class A.

Cap. required

+VCC

RL

RL

+VCC

Single-ended amplifier

A bridge-tied load provides four times the output power for a given supply voltage and load resistance.

+VCC

2

Max.

Max. = VCC

Bridge amplifier

Max. = 2 x VCC

Max.

Class A, B, and AB quiz

Class A amplifiers are biased to operate near the ________ of the load line. center

Class B amplifiers have their Q-points at ____________. cutoff

The conduction angle for class B is _________. 180o

To reduce distortion, two class B transistors are arranged in _____________. push-pull

Class AB is a solution for __________ distortion. crossover

Concept Review • Class A amplifiers operate at the center of the load

line and have a large quiescent current flow.

• Class B amplifiers operate at cutoff and have no quiescent current flow.

• Class B amplifiers are usually operated in push-pull configurations.

• Class B amplifiers have crossover distortion.

• Class AB reduces crossover distortion.

• Bridge amplifiers provide four times the output power and eliminate the output coupling capacitor.

Repeat Segment

Concept Preview

• Class C amplifiers are biased beyond cutoff for a small conduction angle and high efficiency.

• Class C amplifiers used tuned tank circuits to reduce distortion in RF applications.

• Class C amplifiers cannot be used in wideband applications like audio.

• Class D amplifiers switch between cutoff and saturation for very high efficiency.

• Class D amplifiers operate at a relatively high switching frequency and often use PWM.

• Class D can be used in audio applications.

0 2 4 6 8 10 12 14 16 18

0.2 0.4 0.6 0.8

1.0 1.2

1.4 A

B

C

AB

The class of an amplifier is determined by the bias

which establishes the Q-point.

Class C is established by reverse biasing the base-emitter junction.

Conduction Angles & theoretical max. efficiencies:

• Class A = 360o 50 %*

• Class B = 180o 78.5 %

• Class AB 200o (between A & B)

• Class C 90o 100 %

*Class A amplifiers are seldom driven to maximum output and typically provide much less efficiency.

C

B E

VCC

RB CC

VBB

Class C amplifier

VBB reverse biases the base-emitter junction.

Tank circuit

The transistor is off for most of the input cycle

and the conduction angle is small.

VBB

0.7 V

0 A

VBE waveform

IC waveform

VCE waveform

Class C amplifier waveforms

(with tank circuit)

Low VCE when IC is flowing

C

B E

VCC

RB CC

Class C amplifier with signal bias

The base-emitter junction rectifies the input signal and charges CC.

Signal bias increases when the input signal increases in amplitude.

IB >> 0

Three transistor operating modes:

IB = 0 IB > 0

Cutoff Linear (PC > 0)

Saturation

PC = 0 in both of these modes

A switch-mode amplifier uses a rectangular input signal to drive the

transistor rapidly between cutoff and saturation. The efficiency is very high.

C

B E

RB

They are also called Class D

amplifiers.

If the switching frequency is a good deal higher than the signal frequency, a Class D amplifier is

capable of linear amplification. Pulse-width modulation and a low-pass filter are often used.

PWM Signal

Input Signal

PWM LPF

The low-pass filter rejects the switching frequency.

Class C and D quiz

Class C amplifiers use _______ circuits to restore sinusoidal signals. tank

The base-emitter junction in a class C amplifier is ________ biased. reverse

The theoretical maximum efficiency for class C is ___________. 100%

Class D amplifiers are also known as __________ amplifiers. switch-mode

Class D amplifiers employ a varying duty- cycle known as _________. PWM

Concept Review • Class C amplifiers are biased beyond cutoff for a

small conduction angle and high efficiency.

• Class C amplifiers used tuned tank circuits to reduce distortion in RF applications.

• Class C amplifiers cannot be used in wideband applications like audio.

• Class D amplifiers switch between cutoff and saturation for very high efficiency.

• Class D amplifiers operate at a relatively high switching frequency and often use PWM.

• Class D can be used in audio applications.

Repeat Segment

More Detailed analysis as A revision

95

Class-A,-B,-C operation modes

Vcc

0

2Vcc

0 2

t

i v

t

I

Iq

V

R

Vcc

0

i

vin

vin

Vin

Vb

t

i

2

Vp

Vcc

v

Class A

- input cosinusoidal voltage

tVVv cos inbin

tIIi cos q

tVVv cos cc

ccq0 VIP

VIP 0.5

2

1

2

1

qccq0 I

I

V

V

I

I

P

P

cc/ VV

1 / q II

.50

- output cosinusoidal current

- output cosinusoidal current

- DC output power

- fundamental output power

Transfer characteristic

Input voltage

Output current

Output voltage

- collector efficiency

- voltage peak factor

For ideal condition of zero saturation voltage when 1

- maximum collector efficiency in Class A

96

Vcc

2 0

2Vcc

t

i v

t

V R

Vcc

0

i

vin

vin

Vin

t

0 2

I

i

i1

= 90


2 , 0

, cos

q

t

ttIIi

cos 0 q IIi

I

Iq cos

cos cos tIi

cos 1 max IIi

-output current conduction angle 2 indicates its duty cycle

- input cosinusoidal voltage

tVVv cos inbin

For moment with zero current

For moment with maximum current

Class B


Output voltage

Output current

Input voltage

97


- quiescent current as function of half-conduction angle

where

cos q II

• when > 90 cos < 0 Iq > 0 - Class AB operation mode

• when = 90 cos = 0 Iq = 0 - Class B operation mode

• when < 90 cos > 0 Iq < 0 - Class C operation mode

3 cos 2 cos cos 3210 tItItIIi

0 0 cos cos 2

1

ItdtII

1 1 cos cos cos 1

ItdttII

,cos sin1

0

cos sin 1

1

2

1

2

1

0

1

0

1

0

1 I

I

P

P

1 When = 90 and .7850 4

- Fourier series

where - DC component

- fundamental component

- collector efficiency

- maximum collector

efficiency in Class B

- current coefficients

98


- dynamic characteristic of power amplifier or load line function within

- slope of load line

R

v

R

VIi

11

ccq

t

t 0

i

v

t

Imax

i

= 90

Vcc 2Vcc

Vsat

V

M N M' M'' N' N''

K

0

Iq

L

P

Vcos

I

RV

I

1

1

cos 1 tan

Output current

Input voltage


99

t 0

i

v

t

I

i

= 90

K

0

L

M P

Vcc 2Vcc

I


For increased input voltage amplitude:

Output current

Input voltage


Class B

MP – pinch-off region

• operation in saturation, active and pinch-off regions

KM – active region

KL- saturation region (depression in collector current waveform)

• load line represents broken line with three sections:

100


• collector current becomes asymmetrical for complex load impedance

0

i

v

a).

0

i

v

b).

t0

t1

t2

t2

t1

t0

asymmetrical load line

• for inductive load impedance, depression in collector current waveform is shifted to the left (a)

• for capacitive load impedance, depression in collector current waveform is shifted to the right (b)

Reason: different phase conditions for higher-order harmonics

101

Push-pull amplifiers

Push-pull operation helps to increase values of input and output impedances and to additionally suppress even harmonics

2 , 0

0, sin c

c1

Ii

2 , sin

0, 0

c

c2I

i

first transistor collector current

second transistor collector current

RL

T1 T2

Vb Vcc

ic1

ic2

iL

icc

n2

n1

n1

Ic

2

Ic

2

Ic

2

Ic

2

Ic0

ic1

ic2

icL icc

For 50% duty cycle of each device (ideal Class B) with driving sinusoidal voltage:

Being transformed through output transformer T2, total collector current:

sin cc2c1L Iiii

Current flowing in center tap of primary winding of transformer T2:

sin cc2c1cc Iiii

102

Push-pull amplifiers

sin sin L L cL VRIv

c

2

0

ccco 2

2

1 IdiI

ccc0

2VIP

cccout2

1VIP

%5.78 4

0

out

P

P

Ideally, even-order harmonics are canceled as they are in-phase and combined in center tap of primary winding of output transformer

RL

T1 T2

Vb Vcc

ic1

ic2

iL

icc

n2

n1

n1 To eliminate losses, it is necessary to connect bypass capacitance to this center point

As for 50% duty cycle, third- and higher-order odd harmonics do not exist, ideally sinusoidal signal will appear in load

Total DC collector current

For zero saturation resistance when collector voltage amplitude Vc = Vcc and equal turns of winding when VL = Vc, DC and fundamental output powers

Maximum theoretical collector efficiency that can be achieved in Class B operation

• Distortion in push-pull amplifiers

• Improved push-pull output stage arrangements

• Amplifier efficiency

– an important consideration in the design of power amplifiers is efficiency

– efficiency determines the power dissipated in the amplifier itself

– power dissipation is important because it determines the amount of waste heat produced

• excess heat may require heat sinks, cooling fans, etc.

supplythefrom absorbed power

load the indissipatedpowerEfficiency

Lecture #5 (Electronics)

Documents

maximum output power

p7 class c power amplifier

power supply

transistor power equation

average output power

maximum current

allowable power dissipation

class c amplifiers