Power amplifiers

There is little or no voltage gain but substantial current gain

iii vip

0vip oo

io vv

io ii io pp

Requirements for power amplifiers

low output resistance (can deliver output

signal without loss of gain )

good linearity (non distorted output signal)

%1THD

Hi-Fi amplifiers

(very) high efficiency (prolonges batteries life)

1

24

23

22

rms

rmsrmsrms

V

VVVTHD

rms – root mean square

rms voltage effective voltage

Amplifier Classes

• according with the conduction time of an amplifying transistor

in a period of a sinusoidal input signal

4/19

Amplifier Classes

Power transfer

odPSi pppp

PSi pp

PS

o

p

p

BJT in permanent conduction (aF) - remembering

vBE

vBE

0.6V, ( )

0.6V, ( )BE F

BEv f

v a

T o f

T

0.6V, (

0.6V, ( )

)BE F

BEv T o

v a

f

T

f

npn pnp

iE =iC+iB Always valid

In the active region (aF) iC=βiB

CCCCE iiiii

11

1

iE=(β+1)iB ≈βiB

β = 100 – as a rough guide

iE≈iC

iE ≈ iC = βiB

I BE Ov v v

O I BEv v v

I BEE

E

v vi

R

0.6V, ( )BE Fv T a

CE PS O PS E Ev V v V R i

BJT in permanent conduction (aF) - remembering

Class B amplifier

each transistor

conducts current for a

half of every period

The circuit

complementary pair

push-pull configuration

Operation

CCCECCo VVVv satmax

)V6.0;V6.0(Iv T1 – (off), T2 – (off)

V6.0Iv T1 – (on), T2 – (off)

V6.01 IBEIO vvvv

0Oi OC ii 1 02 Ci

V6.0Iv T1 –(off), T2 –(on)

V6.02 IBEIO vvvv

0Oi OC ii 2 01 Ci

CCsatCECCo VVVv 1min

Voltage transfer

characteristic

crossover

distortions

Waveforms

Assume the signal magnitude sufficiently high to

neglect the crossover distortions.

CCo VV ˆtVtv oo sinˆ)(

0)(,0

0)(,)(

)(1

tv

tvR

tv

ti

o

o

L

o

C

)()(1 tvVtv oCCCE

Waveforms. Powers

)()()( 111 tvtitp CECT

L

oo

R

tvtp

)()(

2

PS

O

P

P

dtR

tVV

TdttiV

TP

L

o

T

CCC

T

CCPS

sinˆ1)(

12

0

1

0

dttTR

VV

TP

T

L

oCCPS

2

0

2sin

ˆ1

L

oCCPS

R

VVP

ˆ1

CCo VV ˆ

L

CCPS

R

VP

2

max

ˆ2

If

L

CCO

R

VP

2

2

max CCO VV ˆIf

L

oCCPSPSPS

R

VVPPP

ˆ2

L

OOrmsOrmsO

R

VIVP

2

ˆ 2

CC

O

OCC

L

L

O

PS

O

V

V

VV

R

R

V

P

P ˆ

4ˆ22

ˆ 2

CCO VV ˆIf %5,784

max

Powers Efficiency

The amplifying transistors

OPST PPP 2

1 L

o

L

oCCOPST

R

V

R

VVPPP

4

ˆˆ1

2

12

What is the maximum average power dissipated by a

transistor depending on the output voltage magnitude?

0dt

dPT CCCCo VVV 64,02ˆ

L

CC

L

CCT

R

V

R

VP

22

2max 1,01

L

CC

L

CC

L

CCT

R

V

R

V

R

VP

222 ˆ068.0

4

ˆ1

CCO VV ˆ

If

maximum average powerCCo VV 64,0ˆ

L

CC

L

CCT

R

V

R

Vp

22

max 25,04

1 maximum instantaneous power CCo VV ˆ

L

CCC

R

Vi max CCCE Vv 2max

OPTIONAL

Class AB

- Crossover distortion, specific to class B amplifier can be

virtually eliminated

Class AB Amplifier. The Basic Circuit

• biasing the output transistors at a

small output current

Vbias ≈ 0.6V

T

Bias

V

V

SeII

vI - positive tvVVtvtv IBEnBiasIO

vI - negative tvVVtvtv IBEpBiasIO

Voltage Transfer Characteristic

Solutions to

generate VBias

???

Biasing Using Diodes

R should allow all the current in the diodes

and in the bases of the transistors, even for

maximum output current

T is the driver transistor

from previous amplifier

stage

Biasing Using the VBE Multiplier

1

2

11 BEBB VR

RV

OPTIONAL

Short-Circuit Protection

EnEn

BEo

RR

Vi

V6,02lim

•Effective in ensuring device

safety

•Disadvantage: under normal

operation about 0.5V might

appear across each protection

resistor. The voltage swing will

be reduced by that much, in each

direction

•Negative feedback, protection of

the output transistor against

thermal runaway

Protection against the effect of

short-circuiting the output

Illustration

mA61,0

6,06,06,0

BiasI

P adjusts the biasing

voltage between

VBE and 3.5VBE

C - in the bases of Tn

and Tp it is the same

variable input voltage

iOlim=0,6A

OPTIONAL

Use of Compound Transistors with Higher Current Gain

•Necessary for high output currents

The Darlington npn configuration

The compound pnp configuration preferred in the IC

Problem

a) What is the expression vO(vi) for vi[-10V;10V]? Plot the vO(vi) VTC.

What is the maximum theoretical average efficiency?

b) Plot vO(t) for: (1) vi(t)= 0.2sint [V]; (2) vi(t)= 10sint [V].

c) For vi(t)= 10sint [V], vO(t) can be well approximated by a sinewave

(neglecting crossover distortions). Compute: the average output power,

the average power consumption, and the average efficiency of the stage.

Plot the collector-emitter voltage and collector current for T1 and T2.

d) Propose a solution to eliminate crossover distortion.

Ω5001 LR

Ω502 LR

± VCC = ± 15V