06 amplificadores de potência

EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2017
Amplificadores de Potência
(Estágios de Saída)
Prof. Jader A. De Lima

η% - eficiência do amplificador
Pout – potência de saída do amplificador entregue à carga
Pdc – potência DC retirada da fonte de alimentação
Ex: amplificador
de audio

Output Stage Requirements:
• deliver a specified amount of signal power to a load
with acceptably low levels of signal distortion;
• high input impedance/low output impedance (why?);
• low quiescent power (when the input signal is zero
the power dissipation should be low).

Collector current waveforms for transistors operating in (a) class A, (b) class B,
Estágios de Sáida (Estágios de Potência)

(Continued) (c) class AB, and (d) class C amplifier stages.

Estágios de Sáida (Estágios de Potência)
Classe A - Seguidor de Fonte
( )oLm
0ioutin
out
V
r//Rg
1
1
1
v
v
A
+
==
=
∞→=
x
x
in
i
v
r
m
L
m
oL0vinout
g
1
//R
g
1
//r//Rr ≅==

• Class A amplifiers ( the transistor conducts for the entire
cycle of the input signal) are highly (power) inefficient.
• Large power dissipation occurs even for no signal input (standby).
• Why save power?
• Preserve natural resources/reduce pollution
• Extend battery life
• Reduce costs, improve reliability (power wasted
is dissipated in the active devices: temperature↑,
performance ↓, chance of failure↑ and larger
devices are required → cost ↑

Vin
VL
Vbe
- RL Is
Vcc - Vcesat
Vcc - Vcesat + Vbe
- RL Is + Vbe
Vin
VL
Vcc
-Vcc
Is
RL
Q1
~
Va
Vs
Rs
• Classe A (seguidor de emissor) com fonte de corrente

• class-A efficiency:






−= CEsat
CC
V
V
vo
2
max






−== CEsat
CC
LL
V
V
RR
vo
IQ
2
1max
min
CC
CEsatCC
V
VV 2
4
1
max
−
≤η
Ex: VCC = 3V e VCEsat = 0.3V → ηmax = 20.5%
< 25% !!
{
( )
CEsat
CC
L
CCL
CEsat
CC
VV
R
VR
VV
−
−
≤
2
2
2
1
2
maxη

• class-A exemple:

Vout x VG
- 277mA x 12.5 ohm

• class-A amplifier with inductive coupling
• small speaker of 3.2Ω (8Ω) needs only 100mW (500mW) to operate
• class-A amplifier may be adequate for output power of a few hundred mW
• using the transformer impedance reflexion, speaker load apperas (Np/Ns)2
larger
at the collector; Ex: if turns ratio is 10:1, a 3.2Ω-speaker appears as 320Ω load.

Classe B – Push-Pull
Transfer characteristic for the class B output stage

• Distorção de cruzamento (crossover distortion)

≈ 78.6%
• class-B efficiency:

• class-B amplifier with inductive coupling
• however, audio transformers are bulky and expensive

Classe AB – Push-Pull (eliminar distorção de cruzamento)
Class AB output stage. A bias voltage VBB is applied between the bases of QN and QP, giving rise to a bias current IQ .Thus, for
small vI, both transistors conduct and crossover distortion is almost completely eliminated.
• quiescent current

• D1 (D2) must match VBE curves of QN (QP)
in saturation current , area and temperature;
→ only good approach for integrated deisgn
• compensating biased diodes

ex:
Determinar o rendimento do estágio:
i) Ibias (resistores)
ii) IC_pk (transistor limite saturação)
iii) IC_av
iv) Idc
v) Pdc
vi) PL_max
vii) η

ex:

ex:
( ) ( ) A
R
VVV
I
L
CCCEsatCC
C 97.0
10
103.0205.0
max =
−−
=
−−
=

ex:
A
I
I
CMAX
AV 309.0
1416.3
97.0
===
π
( ) ( ) A
R
VVV
I
L
CCCEsatCC
C 97.0
10
103.0205.0
max =
−−
=
−−
=

ex:
AAmAIdc 311.0309.038.2 =+=
( ) ( ) A
R
VVV
I
L
CCCEsatCC
C 97.0
10
103.0205.0
max =
−−
=
−−
=
A
I
I
CMAX
AV 309.0
1416.3
97.0
===
π

ex:
WAVxPdc 22.6311.020 ==
( ) ( ) A
R
VVV
I
L
CCCEsatCC
C 97.0
10
103.0205.0
max =
−−
=
−−
=
A
I
I
CMAX
AV 309.0
1416.3
97.0
===
π
AAmAIdc 311.0309.038.2 =+=

( ) ( ) W
R
VVV
P
L
CCCEsatCC
L 70.4
10
103.020
2
15.0
2
1
22
max =
−−
=
−−
=
ex:
( ) ( ) A
R
VVV
I
L
CCCEsatCC
C 97.0
10
103.0205.0
max =
−−
=
−−
=
A
I
I
CMAX
AV 309.0
1416.3
97.0
===
π
AAmAIdc 311.0309.038.2 =+=
WAVxPdc 22.60311.020 ==

%5.75%100
22.6
70.4
%100
max
max === xx
P
P
dc
L
η
ex:
( ) ( ) A
R
VVV
I
L
CCCEsatCC
C 97.0
10
103.0205.0
max =
−−
=
−−
=
A
I
I
CMAX
AV 309.0
1416.3
97.0
===
π
AAmAIdc 311.0309.038.2 =+=
WAVxPdc 22.60311.020 ==
( ) ( ) W
R
VVV
P
L
CCCEsatCC
L 70.4
10
103.020
2
15.0
2
1
22
max =
−−
=
−−
=

• push-pull com multiplicador de VBE
VBB = VBE1 (1 + ( R2 / R1 ))

QPNPBEQQNPNBEQBB IVIVV @@ __ +=
- curvas dos BJTs devem ser consultadas para se determinar correto valor de VBB
R2/R1 definido
Projeto Multiplicador VBE
• passo #1

1
1_
2
max_max__
max__
21_max__max_3
max_max__max_33
R
V
I
R
VI
I
IIII
VVIRV
QBE
R
LNPN
o
NPN
NPNC
NPNB
RQCNPNBR
onpnBERCC
≅
==
++=
++=
ββ
• no máximo de excursão no semiciclo positivo tem-se:
R3 definido
para IB_Q1 << IR2
Obs:assume-se um valor inicial para IC_Q1 para se determinar VBE_Q1 a partir
Da curva característica IC x VBE de Q1
• passo #2

1
1_
1
max_max__
max__
11_max__max_4
max_max__max_44
R
V
I
R
VI
I
IIII
VVIRV
QBE
R
LPNP
o
PNP
PNPC
PNPB
RQEPNPBR
opnpBERCC
≅
=≅
++=
++=
ββ
• no máximo de excursão do semiciclo negativo tem-se:
R4 definido
• passo #3

• passo #4
• re-calcular valores de IR2 e VBE_Q1
• no caso de diferença importante, reiniciar a partir do passo #2
Homework
• Considerando npn Q2N2222 e pnp Q2N3906, projetar um estágio
classe-AB para IQ = 5mA, RL = 8Ω e Vo_max = 2.5V. Admitir fontes
simétricas, sendo VCC = 5V.

• capacitive coupling is not the preferred coupling mechanism for audio push-
pull stages (bulky caps!)
• common-emitter driver: In addition to providing a higher input resistance, the
buffer Q1 biases the output transistors Q2 and Q3
driver
(Av ~ R3/R4)
small

The compound-pnp configuration.
The Darlington configuration.

• overload protection

• overload protection
• short-circuit protection occurs by sensing
current threough R6
• VR6 = VBE_Q15
• When load current reaches a given limit, Q15
becomes forwardly-biased and diverts any
further base current of Q14
→ load current no longer increases

• thermal shutdown
• Q2 acts as a swicth and is normally off at
operating temperatures
• with temperature increase above a given threshold,
positive tempco of Zener and negative tempco of
VBE_Q1 increses Q1 current
→ VBE_Q2 increases and Q2 turns on

• power opamp
low-power
gain stage
current booster
• when Q5 turns on, it sources
additiona load current
• when Q6 turns on, it sinks
additiona load current
buffer

Class B circuit with an op amp connected in a negative-feedback
loop to reduce crossover distortion

• bridge amplifier
critical match

• 741

Class C (tuning amplifier)
• power devices conducts less than 180o

tank is driven by current pulses

rich in harmonics
(f, 2f, 3f, ..., nf)
only ressonance
frequency f
(like pure sinewave)
fundamental frequency f
Very-low impedance at harmonics
→ no gain

Series to Parallel Conversion for RL Circuits

• Coil Q > 50
• class-C amps have Q > 10 usually
(for overall circuit)
narrowband operation

- for QL_coil = 100, determine:
• resonance frequency: fr
• bandwidth: BW

class-A, B, AB
class-D
Class D

• power devices (normally MOSFETs) operate as switches (either fully ON or
OFF) → reducing their power losses (efficiency 90 – 95% is possible, as swicthes
have zero DC current when not switching and low VDS when conducting)
• input signal modulates a PWM carrier that drives the output switches
• commonly used in audio power amplifiers
PWM
~ lossless filter
high-side
low-side

Despite the complexity involved, a properly designed class D amplifier
offers the following benefits:
• Reduction in size and weight of the amplifier
• Reduced power waste as heat dissipation and hence smaller (or no)
heat sinks
• Reduction in cost due to smaller heat sink and compact circuitry
• Very high power conversion efficiency, usually better than 90% above
one quarter of the amplifier's maximum power, and around 50% at low
power levels

• The value of deadtime should be based on the device characteristics, ambient
operating conditions, parasitic parameters of switching devices and load conditions.
• Reduces the RMS output to a certain extent and increases THD.
No deadtime:
deadtime:

Using Feedback to Improve Performance

• Many class D amplifiers utilize negative feedback from the PWM
output back to the input of the device.
• A closed-loop approach:
• improves linearity
• allows better power-supply rejection.
• Open-loop amplifier inherently has minimal (if any) supply rejection.
• In closed-loop topology, as the output waveform is sensed and fed
back to the input of the amplifier, deviations in the supply rail are
detected at the output and corrected by the control loop.
• Drawback: control loop must be carefully designed and compensated
to ensure stability under all operating conditions

• Many Class D amplifiers are implemented as full-bridge output stage.
• A full bridge uses two half-bridge stages to drive the load differentially.
• The full-bridge configuration operates by alternating the conduction path through
the load. This allows bidirectional current to flow through the load without the need
of a negative supply or a DC-blocking capacitor.
Half Bridge vs. Full Bridge

• Half-bridge amplifier:
• output swings between VDD and ground and idles at 50% duty cycle
→ output has a DC offset equal to VDD/2
• efficiency >90% while delivering more than 14W per channel into 8Ω.
• Full-bridge amplifier:
• does not require DC-blocking capacitors on outputs when operating from a
single supply
→ offset appears on each side of the load, which means that zero DC current
flows at the output.
• can achieve twice the output signal as the load is driven differentially. → 4x
increase in maximum output power over a half-bridge amplifier operating from
the same supply (at cost of twice as many MOSFET switches)
• efficiency in the range of 80% to 88% with 8Ω loads

deadtime:

Class E

• current I is diverted through C1 when S1 is opened (see IC and IS)
• RFC: only DC current
• Theorectical zero overlap between VDS and IS → ideally 100% efficiency
• LC resonator ensures single tone at output
RFC

• C1: shunt cap to switch ( + device parasitics) – exact value for max efficiency
• L2 – C2 resonates below the operating frequency (↑Q → sinewave output current)
→ excessive inductive reactance → max efficiency at center frequency (not max power)
• ↑ L1 RF choke (only DC current)
high Qhigh L

• switch driven with 50%-duty cycle

Rise of Vds is delayed
until Is = 0
Vds returns to zero before Is increases

• efficiency as function of duty-cycle

Safe Operating Area (SOA)
• voltage and current conditions over which the device can be
expected to operate without self-damage
(only BJT´s)

Transistor Power Rating
• temperature at collector junction places a limit on allowable power
dissipation PD.
Ex: 2N3904 → Tj (max) = 150o
C
2N3710 → Tj (max) = 200o
C
• ambient temperature: heat produced in junction passes through the transistor
case (metal or plastic) and radiates to the surrounding air (ambient
temperature, usually around 25o
C)
• Derating Factor: data sheets often specify PD (max) @25o
C.
Ex: 2N1936 has PD (max) @25o
C = 4W.
• What happens if temperature is higher than 25o
C? → power rating must be
derated (reduced)

• Power Derating

• Heat Sinks
• increase transistor power rating
→ area of transistor case is increased

Ex: assuming the circuit below must operate from 0o
C to 50o
C, what is the
maximum power rating of the transistor?
• for TO-92 case, PD(max) = 625mW@25o
C
derating factor D = 5mW/o
C

• Failure mechanisms in ICs are accentuated by increased temperatures
(leakage in reverse biased diodes, electromigration, and hot-electron
trapping).
• To prevent failure, the die temperature must be kept within certain
ranges:
• commercial devices [0° to 70°C]
• military parts [–55° to 125°C]
• 40-pin DIP has a thermal resistance of 38 °C/W (natural) and 25 °C/W
(forced air convection).
→ DIP can dissipate 2 watts (natural) or 3 watts (forced), and still
keep the temperature difference between the die and the
environment below 75 °C
• PGA has thermal resistance from 15 ° to 30 °C/W.

Electromigration

Valvulated Amplifiers

Advantages of Valves
• Very linear (especially triodes) making it viable to use them in low distortion
linear circuits with little or no negative feedback
• Inherently suitable for high voltage circuits.
• Valves remained the only viable technology for very high power applications
such as radio and TV transmitters
• Electrically very robust, they can tolerate overloads for minutes, which would
•destroy BJT- and MOSFET-systems in milliseconds
• Withstand very high transient peak voltages without damage, suiting them to
certain military and industrial applications
• Softer clipping when overloading the circuit, which many audiophiles and
musicians subjectively believe gives a more pleasant sound.

Disadvantages of Valves
• Tubes require cathode heater. Heater power represents a significant heat loss
• They require higher voltages for the anodes compared to solid state amplifiers
of similar power rating.
• Tubes are significantly larger than equivalent solid-state devices
• High impedance and low current output is unsuitable for the direct drive of
many real-world loads, notably various forms of electric motors.
• Valves have a shorter working life than solid state parts due to various failure
mechanisms (such as heat, cathode poisoning, breakage, or internal short-circuits)
• Tubes are available in only a single polarity, whereas transistors are available
in complementary polarities (e.g., NPN/PNP), making possible many circuit
configurations that cannot be realized directly with valves.
• Valve circuits must avoid introduction of noise from ac heater supplies.

REFERÊNCIAS:
• Fundamentals of Microelectronics, B. Razavi, John Wiley
and Sons, 2006
• Microelectronic Circuits, A. Sedra and K. Smith, Oxford
university Press, 5th Edition, 2003
• Analysis and Design of Analog Circuits, Gary, Hurst,
Lewis and Meyer, 4th
Edition, 2001

06 amplificadores de potência

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