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SIXTH EDITION. MALVINO. Electronic. PRINCIPLES. Power Amplifiers. Chapter 11. Classes of operation. Class A means that the transistor collector current flows for 360 o of the ac cycle. Class B amplifiers have collector current for 180 o of the ac cycle.

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Presentation Transcript
slide1

SIXTH EDITION

MALVINO

Electronic

PRINCIPLES

classes of operation
Classes of operation
  • Class A means that the transistor collector current flows for 360o of the ac cycle.
  • Class B amplifiers have collector current for 180o of the ac cycle.
  • Class C means that current flows for less than 180o of the ac cycle.
  • Class D amplifiers switch on and off and spend no time in the linear mode.
slide4

IC

IC

A

B

t

t

IC

IC

ISAT

C

D

t

t

types of coupling
Types of coupling
  • Direct (dc amplifier)
  • Capacitive (ac amplifier)
  • Untuned transformer (ac amplifier)
  • Tuned transformer (narrowband ac amplifier)
slide6

dc

ac

Capacitive

Direct

ac

ac

Tuned

transformer

Transformer

slide7

vout

This amplifier has two load lines.

+VCC

RC

R1

RL

vin

R2

RE

slide8

100 mA

80 mA

60 mA

40 mA

20 mA

0 mA

VCC

VCE(cutoff) = VCC

The dc load line

IC(sat) =

RC + RE

14

12

10

IC in mA

8

6

Q

4

2

6

16

2

4

10

12

0

14

18

8

VCE in Volts

The ac load line has a higher slope.

large signals
Large Signals
  • When the Q point is at the center of the dc load line, the signal cannot use all of the ac load line without clipping.
  • MPP < VCC (max. peak-to-peak output)
  • MPP = 2MP
  • MP = ICQrcor VCEQ (whichever is smaller)
slide10

100 mA

80 mA

60 mA

40 mA

20 mA

0 mA

vpp2

MPP2

pout =

pout(max) =

8RL

8RL

14

12

10

IC in mA

8

Q

6

4

2

6

16

2

4

10

12

0

14

18

8

VCE in Volts

efficiency
Efficiency
  • The dc power supplied to an amplifier is Pdc = VCCIdc
  • Efficiency = h = pout/Pdc 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.
slide12

Push-pull Class B amplifier

Q1

T2

T1

+VCC

vin

Q2

On the positive half-cycle of the input, Q1 is on.

slide13

Q1

T2

T1

+VCC

vin

Q2

On the negative half-cycle of the input, Q2 is on.

class b operation
Class B operation
  • Each transistor conducts for half a cycle.
  • There is no bias so each transistor is at cutoff with no input signal.
  • The maximum efficiency is 78.5 percent.
  • Transformers are bulky and expensive.
  • The transformers can be eliminated.
slide15

1

fr =

2p

LC

A

Class C amplifier

Amax

+VCC

C

L

f

fr

RL

RB

vin

slide16

VCC

rc

Q

Class C amplifier dc and ac load lines

dc load line

IC

VCE

VCC

slide17

+ VP

0

0

-2 VP

- VP

Negative clamping at the base causes the

collector current to flow in brief pulses.

IC

+VCC

C

L

t

RL

RB

class c formulas
Class C formulas
  • BW = f2 - f1 = fr/Q
  • QL = XL/RS (inductor only)
  • RP = QLXL (equiv. parallel resistance)
  • rc = RP RL (ac resistance)
  • Q = rc/XL (overall circuit)
  • PD = MPP2/40rc
slide19

W

T

f

360o

Duty cycle and conduction angle

=

D =

f = 180o

f = 90o

slide20

h

100%

f

f

f

180o

180o

180o

MPP2

40rc

PD

The effects of conduction

angle in a Class C amplifier

Idc

derating factor
Derating factor
  • Power devices generate heat
  • PD(max) rating good up to 25 oC
  • Decrease in rating = DP
  • Derating factor = D (W/oC)
  • TA = ambient temperature (oC)

DP = D(TA - 25 oC)

  • PD(safe) = PD(max) - DP