Power Amplifiers PartII[1]

8/3/2019 Power Amplifiers PartII[1]

1/15

The previous classes, A, B, and AB are considered linear

amplifier,

where the output signals amplitude and phase are linearly

related

to the

input signals amplitude and phase.

In the application where linearity is not an issue, and efficiency is critical,non-linear amplifier classes (C, D, E, or F) are used.

Class-C amplifier is

the one biased

so

that

the output current is

zero

for

more than one half of an input sinusoidal signal cycle. A tuned circuit or filter is

a necessary part of the class-C amplifier.

Class C Power Amplifier


2/15

VCC

RL

vo

RFC

CC

-VB

C Lvs

RL

R2

R1

The transistor is

biased

with

a negative

VBE

. Thus

it

will

conduct only when

the input signal is

above

a specified

positive value.

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


3/15

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

The output consists

ofblips at

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 more general signal with

a limited

range of frequencies.

The resonant frequency of the tuned circuit is

determined

by the formula


4/15

)cos(sinI

)t(i

)(IcosI)t(i

)wt(dI)wtsin(I

)wt(d)wt(i)t(i

sinII

otherwise

wtI)wtsin(I)t(i

PC

DPC

DP

CC

PD

DP

C

2

22

2

1

2

1

222

0

2

0

)sin(I

I

)wt(d)wtcos()wt(iII

P

Clfundamenta

222

1

1

1

-ID

Ip

2

-

iC

(t)

LLce iRv ac

load line

CECC VV


5/15

2

2

2

22

8

)sin()cos(sin

LPPCCD

LIND

RIIVP

PPP

)cos(sin

)(

PCC

CCCIN

IV

VtiP

)cos(sin

sin

4

22

IN

OUT

P

P

2

2

2

2

1

228

2

)sin(RI

P

RIP

LPL

LL


6/15

Performance Chart

PL

PD

100%

PIN 4

2

CCPV

I


7/15

Class D Power Amplifier

In a class D amplifier, the output filter blocks all harmonics;

i.e., the

harmonics see an open load. So a voltage square wave

is generated.

The current is in phase with the voltage applied to the filter, but the voltage

across the transistors is out of phase.

Therefore, there is a minimal overlap between current through the transistorsand voltage across the transistors. The sharper the edges, the lower the overlapBasic operation

Class

D amplifiers work by generating a square wave of which the low-

frequency portion of the spectrum is essentially the wanted output signal.

The high-frequency portion serves no purpose other than to make the wave-form binary so it can be amplified by switching the power devices

The term "digital amplifier" has gained currency to denote class D amplifierswith significant amounts of digital processing in them .


8/15

A passive low-pass filter removes the unwanted high-frequency components,

i.e., smooths

the pulses out and recovers the desired low-frequency signal.

The switching frequency is typically chosen to be ten or more times the highest

frequency of interest in the input signal.

This eases the requirements placed on the output filter

Theoretical power efficiency of class

D amplifiers is 100%. That is to say, all of

the power supplied to it is delivered to the load, none is turned to heat.

This is because an ideal switch in its on state will conduct all current but has

no voltage across it, hence no heat is dissipated.

And when it is off, it will have the full supply voltage standing across it, but no

current flows through it. Again, no heat is dissipated.

Real-life power MOSFETs are not ideal switches, but practical efficiencies well

over 90% are common. By contrast, linear amplifiers are always operated with

both current flowing through and voltage standing


9/15


10/15

Class D

)sin(

)sin(

)(

2

4

2

2

2

1

2

1

1

44

4

4

4

4

kk

A

TkTk

A

j

ee

Tk

A

dteA

T

a

AdtA

Ta

dtetxT

a

o

o

TjkTjk

o

T

T

tjk

k

T

To

T

tjk

k

oo

o

o

Fourier Transform of a square wave

)sin(

)sin(

)(

2

4

2

2

2

1

2

1

1

44

4

4

4

4

kk

A

TkTk

A

j

ee

Tk

A

dteA

T

a

AdtA

Ta

dtetxT

a

o

o

TjkTjk

o

T

T

tjk

k

T

To

T

tjk

k

oo

o

o

Fourier Transform of a square wave


11/15

Half-wave rectified signal

%100IN

OUT

P

P For ideal MOSFET

onL

L

IN

OUT

RR

R

P

P

For Non-ideal MOSFET

%100IN

OUT

P

P

L

L

p

L

R

VR

VP

2

21

2

82

L

IN

L

DD

DDIN

R

VP

R

Vii

iViVP

2

21

21

21

2111

8

4


12/15

The binary waveform is derived using Pule Width Modulation (PWM),

sometimes referred to as pulse frequency modulation

The most basic way of creating the PWM signal is to use a high speed

comparator that compares a high frequency triangular wave with the audio input.

This generates a series of pulses of which the duty cycle is directly

proportional with the instantaneous value of the audio signal.

The comparator then drives a MOS gate driver which in turn drives a pair of

high-power switches .(This produces an amplified replica of the comparator's

PWM signal).

The output filter removes the high-frequency switching components of the

PWM signal and recovers the audio information that the speaker can use.

DSP-based amplifiers can be used to generate a PWM signal directly from a

digital audio signal .

Signal modulation


13/15


14/15

Design Challenges

Two significant design challenges for MOSFET driver circuits in

class

D

amplifiers are keeping dead times and linear mode operation as short as

possible.

"Dead time" is the period during a switching transition when both output

MOSFETs are driven into Cut-Off Mode and both are "off".

During dead time, the current generates large high frequency current

waveform and causes EMI noises.


15/15

Dead times need to be as short as possible to maintain an accurate low-

distortion output signal, but dead times that are too short cause the

MOSFET that is switching on to start conducting before the MOSFET that is

switching off has stopped conducting.

The MOSFETs effectively short the output power supply through

themselves, a condition known as "shoot-through".

Meanwhile, the MOSFET drivers also need to drive the MOSFETs

between switching states as fast as possible to minimize the amount of time

a MOSFET is in Linear Mode, the state between Cut-Off Mode andSaturation Mode where the MOSFET is neither fully on nor fully off and

conducts current with a significant resistance, creating significant heat.

Driver failures that allow shoot-through and/or too much linear mode

operation result in excessive losses and sometimes catastrophic failure of

the MOSFETs

Documents

Power Amplifiers PartII[1]