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Review of basic of power amplifiers for analog electronics
In typical analog circuits (as in operational amplifiers and audio systems) the power
amplifier that drives the load must pay attention to the following aspects:
•
•
Power conversion efficiency h = PL / PS
defined as the ratio between the power
given to the load and the one taken by
the power supply (always less than
100%)
Supply PS
Power dissipation on the active device
•
Linearity of the output signal delivered
to the load
•
Frequency range of the output signal
input
signal
Pi
drivers
Power
circuit
load PL
the power gain between input and
output signals is not the main goal for
the power stage (except for RF power
amplifiers)
University Federico II
Dept of Electronics and Telecommunications
Paolo Spirito
Power Semiconductor Devices
1
Classes of operation of power amplifiers (and power circuits)
The operation of the power amplifiers is defined in different classes according to the
way the active devices in the circuit are operating during the period of input signal.
For the switching circuits we can define a
Class D operation where the device is made
to commutate between full conduction (on)
and interdiction (off) states (we will discuss it
later on)
University Federico II
Dept of Electronics and Telecommunications
current
Class B, where the device is conducting for
about one/half the time period of the input
signal waveform (two devices are required
to obtain a good output linearity)
time
time
current
Class A where the active device is
conducting during the entire time period of
input signal waveform
current
For analog amplifiers we define 2 main classes of operation:
time
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Power Semiconductor Devices
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Class A operation
In class A operation the device (here it is assumed a BJT) is biased at the middle
point – Q(Io, Vo) - of the load line, and the operating point is driven by the input
signal along the load line to a max current less or equal than Imax and min current
larger ore equal than 0.
The output power is max when the operating point reaches Imax (ideally, when the
Vcesat is neglected) and 0.
Class-A BJT output stage in
emitter follower configuration
Ic
Imax
Ip
Io
Q
Vp
VCC
Vce
Vo
University Federico II
Dept of Electronics and Telecommunications
Paolo Spirito
Power Semiconductor Devices
3
Class A power efficiency
Max power efficiency: assuming a linear operation up to the limit values one has:
Power absorbed from the supply : PS  VCC 
1
iC (t )dt  VCC  I 0

TT
VP  I P
( VP , I P peak values of the a.c component)
2
V
I
max a.c. peak values : VP  CC , I P  max  I O
2
2
V I
PLMAX  CC O
4
Power given to the load : PL 
The max power efficiency is then:
h MAX 
PLMAX 1
  25%
PS
4
The power absorbed by the supply is always constant and equal to Ps, so the
efficiency is linearly dependent on output power PL.
University Federico II
Dept of Electronics and Telecommunications
Paolo Spirito
Power Semiconductor Devices
4
Power balance in class A amplifiers
The power balance of the circuit is:
PS  PD  PL ( a.c.)  PL ( d .c.)
where:
PD is the power dissipatio n on the active device
PL ( a.c ) is the a.c. power delivered to the load
PL ( d .c ) is the d.c. power dissipated on the load
The max power dissipation on the device is obtained for zero a.c. power on the load, then:
PDMAX  PS  PL ( d .c.)  VCC I O 
VCC I O PS

2
2
The max power efficiency is:
h MAX 
PLMAX
 25%
PS
Let’s consider the meaning of these results:
• To obtain a (controlled ) power output of 50 W one need a supply power of at least
200 W
• the device must dissipate 100 W in the steady state to transfer a max power of 50 W
to the load!
University Federico II
Dept of Electronics and Telecommunications
Paolo Spirito
Power Semiconductor Devices
5
Class B operation
In class B operation the device (here it is assumed a BJT) is biased at zero current
point – Q(0, VCC) - of the load line. As a result the power dissipation in the quiescent
state is zero.
Two devices (and two power supplies) are needed to obtain an output signal analog to
the input one. The NPN device operate as an emitter follower for positive signal swing,
while the PNP device operates as an emitter follower for negative signal swing.
Ic
Class-B output stage with 2 BJT
in push-pull configuration
Ip
Io
Q
VCC
Vp
Vce
Vo
University Federico II
Dept of Electronics and Telecommunications
Paolo Spirito
Power Semiconductor Devices
6
Class B power efficiency
Max power efficiency:
assuming again a linear operation up to the limit values, and sine signals, one has:
For the power Ps absorbed from the supply,
assuming the following plot for the currents
given by the two supplies, one has:
2VCC T / 2
2VCC I MAX
PS 
I
sin

tdt

 MAX
T 0

IMAX
Q2
Q1
Q1
T
T/2
t
VP I P
(in that case, the max peak values are : VP  VCC , I P  I MAX )
2
V I
Then, the max output power on the load is:
PLMAX  CC MAX
2
PL 
The max power efficiency is then:
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Dept of Electronics and Telecommunications
h MAX 
POMAX VCC I MAX



  78.5%
PS
2 2VCC I MAX 4
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Power Semiconductor Devices
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Power balance in class B amplifiers
The power balance of the class B circuit is:
PS  PD  PL ( a.c.)
where:
PD is the power dissipatio n on both the active devices
PL ( a.c ) is the a.c. power delivered to the load(d.c. power is zero)
The power dissipation on the devices is null for for zero power on the load. To evaluate
the power dissipation (on both devices) as a function of the output signal, one has:
PD ( I P )  PS ( I P )  PL ( I P ) 
2VCC I P


I P2 RL
(a)
2
dPD ( I P )
2VCC 2 I P RL
2V
0

 0  I P*  CC
dI P

2
RL
PD ( I )  PDMAX
*
P
2
2VCC
 2
 RL
This is a second order function in IP, and the
max is located somewhere between 0 and
IMAX. It can be found as:
Substituting that value of IP* in (a) one has:
and we obtain the following
ratio between PDMAX and PLMAX
University Federico II
Dept of Electronics and Telecommunications
2
PDMAX 2VCC
2 RL
4
 2

 0.4
2
2
PLMAX  RL VCC 
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Power Semiconductor Devices
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Power balance in class B amplifiers
From the previous results on power efficiency and power dissipation it comes out that:
•
•
The max power conversion from power supply to load is 78%
the total power dissipation (on both device) is 40% of the max output power: then
each device must dissipate 20% of the max output power
Let’s consider the meaning of these results:
•
•
To obtain with class-B a power output of 100 W one need a supply power of at least
130 W
To transfer a max power of 100 W to the load, each device must be able to dissipate
20 W (at the IP* rated)
Conclusion: Class B is better than class A in power conversion, (we pay this with some
degradation in linearity), but this is still not sufficient if we need power conversion above
several kW.
For a 10 kW output power we need a power dissipation on each device of more than
2kW and this is not feasible with usual power packages, as we will see later on.
University Federico II
Dept of Electronics and Telecommunications
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Power Semiconductor Devices
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Example :
Do a SWCAD simulation of a simple Class B amplifier using
MOS devices
Compute Power efficiency
University Federico II
Dept of Electronics and Telecommunications
h MAX
POMAX

PS
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Power Semiconductor Devices
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Class D operation
To increase the power available at the output of a power circuit, one must decrease the
power dissipation of the active devices, that is limited by the package heat
dissipation (we will come back on that point later on).
The best way of reducing the power dissipation on the device is to let it operate in two
limit operating points:
a) OFF state, where the power dissipation is zero because the device current is null.
b) ON state, at the minimum voltage drop allowed by the operation of the device (often
indicated as saturation voltage)
This is the Class-D operation: the device operates as a switch, that is either open (OFF
state) or closed (ON state). In this way, the device, driven by input pulses capable to
bring it either in ON or OFF state, can operate at a power much less than the
available output power, thus increasing both the power output and the power
efficiency.
University Federico II
Dept of Electronics and Telecommunications
Paolo Spirito
Power Semiconductor Devices
11
Class D operation
With reference to a BJT device, the operating load line can trepass the max power locus
of the power dissipation, because in the ON state (point B) the dissipated power is
much less than the maximum power dissipation PDMAX, and in the OFF state (point A) is
almost zero (assuming negligible the leakage current)
University Federico II
Dept of Electronics and Telecommunications
Paolo Spirito
Power Semiconductor Devices
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Class D operation
However, one must pay attention on the time required by the device to switch between
ON and OFF states: we can define an average steady-state power dissipation PDS
and an average dynamic power dissipation PDd :
PDS: the average power dissipation
in the ON state (assuming
negligible the one in the OFF
state)
PDd: the average power dissipation
during the switching transition
between ON and OFF states
TON
PDS 
PDd 
TON
I ON  VMIN
T
D
T
T
1
i(t )v(t )dt

T DT
The power dissipation PD is the sum of the two components PDS and PDd indicated above.
University Federico II
Dept of Electronics and Telecommunications
Paolo Spirito
Power Semiconductor Devices
13
Class D amplifier
In class D amplifier, the information content of the signal can not modify the amplitude of
the pulses, because these latter are of constant amplitude, but it can be transferred to the
output by a modulation of the width of the pulses.
In other words, we need a Pulse Width Modulation (PWM) to drive the device and to
transfer this information to the (amplified) output, i.e. to the load.
The simplest PWM modulation technique is done by using a signal comparator to
compare the analog signal with a triangular waveform.
The output will be made of a pulse train having an amplitude equal to the supply voltage
of the comparator, and ON (OFF) duration defined by the time interval where the
triangular waveform is lower (higher) than the one of the modulation signal.
University Federico II
Dept of Electronics and Telecommunications
Paolo Spirito
Power Semiconductor Devices
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PWM Modulation of Class D amplifiers
An example of PWM modulation, made by a sinusoidal signal fS using a signal comparator
and a triangular waveform fM, is reported in the following plot.
University Federico II
Dept of Electronics and Telecommunications
Paolo Spirito
Power Semiconductor Devices
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Class D power block
Power
supply
signal frequency fS
carrier frequency fM
PWM
modulation
Class D
circuit
signal
demodulation
(filtering)
load
To reconstruct the output signal after the class D operation we need to demodulate
the signal by a low pass filter, that will cut off the carrier frequency fM, while leaving
unaltered the signal frequency fS .
The filter must be realized with only L, C components to minimize the power losses.
University Federico II
Dept of Electronics and Telecommunications
Paolo Spirito
Power Semiconductor Devices
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PWM
Modulation
The filter cut-off frequency fF must be:
fF<<fM
to suppress effectively the carrier frequency of the waveform
fS<<fF
to leave unaltered the signal frequency (up to the max frequency contained in
the signal waveform)
Then fS<<fM - This basic need to push up the operating frequency fM will require power
devices with high operating frequency and low switching times.
Low-pass LC filter with a
slope of 40 db/octave
40 dB/dec
|VO/VI|
(dB)
RL increas.
L
C
RL
fS
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Dept of Electronics and Telecommunications
n
fF
 (rad/s)
f
M
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Power Semiconductor Devices
f
17
Circuit simulation of a class D power Amplifier
A SWCAD analysis of a push-pull power amplifier operated in class D with a PWM
modulation with a voltage comparator and an LC filter at the output, and two
complementary Power MOS is done using the followingschematics:
University Federico II
Dept of Electronics and Telecommunications
Paolo Spirito
Power Semiconductor Devices
18
Power Circuits
The basic power circuits are:
DC/DC converters, that control the d.c. power
on the load, by variable control signals
unregulated
DC
DC Power
supply
regulated
DC
DC/DC
converter
load
control
unregulated
DC
DC/AC converters (Inverters), that generate
a regulated a.c power from a d.c. power
supply, and control the a.c. power delivered
DC Power
supply
regulated
AC
DC/AC
converter
load
control
unregulated
AC
AC/AC converters, that generate a
controlled a.c power (both in frequency and
amplitude) from the line a.c. power supply
AC line
regulated
AC
AC/AC
converter
load
control
University Federico II
Dept of Electronics and Telecommunications
Paolo Spirito
Power Semiconductor Devices
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DC/DC converters
Basic applications for these circuits are:
Regulated DC power supply
DC Motor drive
University Federico II
Dept of Electronics and Telecommunications
Paolo Spirito
Power Semiconductor Devices
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DC/DC converters
The DC/DC converters act as controlled d.c. voltage trasformers; the basic versions are:
• Step-down (or Buck) converter that gives an output voltage lower than the input one
• Step-up (or Boost) converter that gives an output voltage higher than the input one
There are also some combination of the two (like Buck-boost or Cuk) that allow an output
voltage both higher and lower than the input one, according to the control signal
A most general scheme is the bridge converter that allows both d.c. and a.c. output power
conversion.
The DC/DC converters usually have the DC input voltage generated from the a.c. line supply
through a rectifier circuit made by diodes connected in a full-wave bridge configuration
d.c. output
a.c. line
University Federico II
Dept of Electronics and Telecommunications
Paolo Spirito
Power Semiconductor Devices
21