amplificadores de potência (estágios de saída)
TRANSCRIPT
Amplificadores de Potência(Estágios de Saída)
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
Prof. Jader A. De Lima
Ex: amplificadorde audio
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
η% - eficiência do amplificador
Pout – potência de saída do amplificador entregue à carga
Pdc – potência DC retirada da fonte de alimentação
Output Stage Requirements:
• deliver a specified amount of signal power to a load
with acceptably low levels of signal distortion;
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• high input impedance/low output impedance (why?);
• low quiescent power (when the input signal is zero
the power dissipation should be low).
Estágios de Sáida (Estágios de Potência)
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
Collector current waveforms for transistors operating in (a) class A, (b) class B,
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
(Continued) (c) class AB, and (d) class C amplifier stages.
Estágios de Sáida (Estágios de Potência)
Classe A - Seguidor de Fonte
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
( )oLm
0ioutin
outV
r//Rg
11
1
v
vA
+==
=
∞→=x
xin
i
vr
m
L
m
oL0vinout
g
1//R
g
1//r//Rr ≅==
Vin
VL
Vbe
Vcc - Vcesat
Vcc - Vcesat + Vbe
- RL Is + Vbe
Vin
VL
Vcc
IsRL
Q1
~
Va
Vs
Rs
• Classe A (seguidor de emissor) com fonte de corrente
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
- RL Is
-Vcc
• class-A efficiency:
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
( )CEsatCC VVvo −=2
1max
−==
L
CEsatCC
L R
VV
R
voIQ
2
1maxmin
( )CEsatCC
L
CCL
CEsatCC
VV
R
VR
VV
−−
=2
max
4
1η
CC
CEsatCC
V
VV −=
4
1maxη
Ex: VCC = 3V e VCEsat = 0.3V → ηηηηmax = 22.5%
< 25% !!
• Class A amplifiers ( the transistor conducts for the entirecycle of the input signal) are highly (power) inefficient.
• Large power dissipation occurs even for no signal input (standby).
• Why save power?
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• 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 ↑
• class-A amplifier with inductive coupling
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• 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
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
Transfer characteristic for the class B output stage
• Distorção de cruzamento (crossover distortion)
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• class-B efficiency:
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
≈ 78.6%
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• class-B amplifier with inductive coupling
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• however, audio transformers are bulky and expensive
Classe AB – Push-Pull (eliminar distorção de cruzamento)
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
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
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• D1 (D2) must match VBE curves of QN (QP)
in saturation current , area and temperature;
• compensating biased diodes
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
in saturation current , area and temperature;
→ only good approach for integrated deisgn
ex:Determinar o rendimento do estágio:
i) Ibias
ii) IC_pk (transistor lim saturação)
iii) IC_av
iv) Idc
v) Pdc
vi) PL_max
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
vi) PL_max
vii) η
ex:
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
( ) ( )W
R
VVP
L
CEsatCCL 85.4
10
3.020
8
1
8
122
max =−
=−
=
%8.75%1004.6
85.4%100
maxmax === xx
P
P
dc
Lη
• push-pull com multiplicador de VBE
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
VBB = VBE1 (1 + ( R2 / R1 ))
- curvas dos BJTs devem ser consultadas para se determinar correto valor de VBB
Projeto Multiplicador VBE
• passo #1
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
QPNPBEQQNPNBEQBB IVIVV @@ __ += R2/R1 definido
max_max__
21_max__max_3
max_max__max_33
VII
IIII
VVIRV
oNPNC
RQCNPNBR
onpnBERCC
==
++=
++=
• no máximo de excursão no semiciclo positivo tem-se:
• passo #2
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
1
1_2
max_max__max__
R
VI
R
VII
QBER
LNPN
o
NPN
NPNCNPNB
≅
==ββ
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
max_max__
11_max__max_4
max_max__max_44
VI
IIII
VVIRV
oPNPC
RQEPNPBR
opnpBERCC
++=
++=
• no máximo de excursão do semiciclo negativo tem-se:
• passo #3
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
1
1_1
max_max__max__
R
VI
R
VII
QBER
LPNP
o
PNP
PNPCPNPB
≅
=≅ββ
R4 definido
• passo #4
• re-calcular valores de IR2 e VBE_Q1
• no caso de diferença importante, reiniciar a partir do passo #2
Homework
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
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.
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• 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
small
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
driver(Av ~ R3/R4)
small
The Darlington configuration.
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
The compound-pnp configuration.
The Darlington configuration.
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• 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
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
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
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
→ VBE_Q2 increases and Q2 turns on
• power opamp
low-powergain stage
current booster
buffer
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
gain stage
• when Q5 turns on, it sources
additiona load current
• when Q6 turns on, it sinks
additiona load current
• bridge amplifier
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
critical match
Class C (tuning amplifier)
• power devices conducts less than 180o
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
tank is driven by current pulses
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
rich in harmonics(f, 2f, 3f, ..., nf)
fundamental frequency f
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
only ressonancefrequency f
(like pure sinewave)
Very-low impedance at harmonics → no gain
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• Coil Q > 50• class-C amps have Q > 10 usually
(for overall circuit)
narrowband operation
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
- determine:
• resonance frequency: fr
• bandwidth: BW
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
class-A, B, AB
Class D
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
class-D
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• 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
high-side
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
PWM
~ lossless filter
high-side
low-side
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
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
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
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
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
Using Feedback to Improve Performance
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• 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.
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• 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
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• 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
Half Bridge vs. Full Bridge
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• 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.
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• 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
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• 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
Class E
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
RFC
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• 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
high Qhigh L
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• 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)
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• switch driven with 50%-duty cycle
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
Rise of Vds is delayeduntil Is = 0
Vds returns to zero before Is increases
• efficiency as function of duty-cycle
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
Safe Operating Area (SOA)
• voltage and current conditions over which the device can be
expected to operate without self-damage
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
(only BJT´s)
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
Transistor Power Rating
• temperature at collector junction places a limit on allowable power
dissipation PD.
Ex: 2N3904 → Tj (max) = 150oC
2N3710 → Tj (max) = 200oC
• ambient temperature: heat produced in junction passes through the transistor
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• ambient temperature: heat produced in junction passes through the transistor
case (metal or plastic) and radiates to the surrounding air (ambient
temperature, usually around 25oC)
• Derating Factor: data sheets often specify PD (max) @25oC.
Ex: 2N1936 has PD (max) @25oC = 4W.
• What happens if temperature is higher than 25oC? → power rating must be
derated (reduced)
• Power Derating
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• Heat Sinks
• increase transistor power rating
→ area of transistor case is increased
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
Ex: assuming the circuit below must operate from 0oC to 50oC, what is the
maximum power rating of the transistor?
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• for TO-92 case, PD(max) = 625mW@25oCderating factor D = 5mW/oC
Ex: assuming the circuit below must operate from 0oC to 50oC, what is the
maximum power rating of the transistor?
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• for TO-92 case, PD(max) = 625mW@25oCderating factor D = 5mW/oC
• 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]
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• 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
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
REFERÊNCIAS:
• Fundamentals of Microelectronics, B. Razavi, John Wiley
and Sons, 2006
• Microelectronic Circuits, A. Sedra and K. Smith, Oxford
EEL 7303 – Circuitos Eletrônicos AnalógicosJader A. De Lima UFSC, 2013
• 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