29/06/2011 - 1 ATLCE - F2 - © 2010 DDC

Politecnico di Torino - ICT School

Analog and Telecommunication Electronics

F2 - Power circuits

» Amplifier stage taxonomy» A, AB, B, C, D stages» Distortion» Power and efficiency

29/06/2011 - 2 ATLCE - F2 - © 2010 DDC

Lesson F2: Power stages

• Amplifier stage taxonomy– A, AB, B, C, D– CE/CS, CC/CD– Complementary final stages

• Power and efficiency

• Push-pull amplifiers– Complementary output stage– Crossover distortion– Output protection

• Learning material: tbd

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Introduction to power amplifiers

• All Amplifiers produce “power amplification”– If no power gain (e.g. a transformer), is not an amplifier

• Power amplifier:– Deliver/handle/control large amounts of power (> 1W)

• Two basic techniques– Linear power control– Switching power control

• Several circuits – Taxonomy for linear power amplifiers (stage types)– Circuits for switching regulators– Examples for Power Supply Units (PSU)

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Review of amplifier configurations

• Common Emitter / Common Source– Voltage and current gain– High Ro

• Common Collector / Common Drain– Only current gain– Low Ro– Power output stages

• Common Base / Common Gate– Only voltage gain– Low Ro– Used mainly in RF (cascode)

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Amplifier classes

• Class A: quiescent Ic > (max output current)/2– low distortion, but uses a great deal of power– used only if very low distortion is needed

• Class B: Ic = 0 for half period– distortion, but lower power consumption, – Used for higher efficiency (and symmetry)

• Class C: Ic = 0 for more than half period– high distortion, but higher efficiency – Need countermeasures (tuned circuit, feedback, …)

• Class D: Ic 0 or Imax (switch)– Switching power regulation– Highest efficiency

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Figure 14.1 Collector current waveforms for transistors operating in (a) class A, (b) class B, (c) class AB, and (d) class C amplifier stages

Classes and Collector currents

A AB

C

B

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Classes of amplifier: A

• Class A– active device conducts for complete cycle of input signal– poor efficiency

(normally lessthan 25%)

– low distortion

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Classes of amplifier: B

• Class B– Use transistors

to both source and sink load current

– Each active device conducts for half of the complete cycle of input signal

– Good efficiency

• Push-pull amplifier– Symmetric behavior for positive/negative output voltage– Vi must change 2 Vbe to switch active devices crossover distortion

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Classes of amplifier: AB

• Class AB– active devices

conducts for more than half but less than the completecycle of input signal

– Small current flow for Vi = 0

– efficiency & distortion depend on bias

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Classes of amplifier: C

• Class C– active devices conducts for less than half the

complete cycle ofinput signal

– high efficiency(close 100%)

– gross distortion– Good for RF

(paired with tuned circuit)

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Classes of amplifier: D

• Active devices are switches and are either ON or OFF– For an ideal switch either the current or the voltage is zero– No power dissipation (first approximation !)

• Real devices can make good switches

• Switching amplifiers or switch-mode amplifiers– Best choice for high power / medium frequency

(audio OK, no RF!)

• Efficiency is very high– Need filters for “analog” output

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Distortion in amplifiers

• Hard nonlinearities:– Saturation of V and I (hard limit)– Slew rate (dynamic saturation)– Crossover

• Soft nonlinearity– Large signal behavior in active region (exp, square, …)

• Any nonlinearity causes – Harmonics (distortion)

» For sine input, the output has several components (harmonics)– Gain change (gain depends on signal level)

» Usually compression: gain decreases as signal goes up

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Distortion and noise parameters

• THD: Total harmonic distortion– THD (%) = 100 * sqrt( F2^2 + F3^2 + ... + Fn^2 ) / F1– Fn = signal amplitude at the nth harmonic

» non-harmonic outputs are ignored by the THD calculation

• SFDR: Spurious Free Dynamic Range– SFRD = (fundamental)/(strongest spurious)

» Used for ADC and DAC

• SNR: Signal/Noise Ratio– SNR = (useful signal)/(noise)– Noise Figure F = SNRi/SNRo

• SINAD: Signal-to-Noise And Distortion ratio– SINAD = (signal+noise+distortion)/(noise+distortion)

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Handling or using nonlinearity

• Distortion can be counteracted by – Feedback– Filtering (removes harmonics)

• Distortion is exploited in – Compressing amplifiers– Sine generators– Nonlinear circuits (designed!)

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Amplifier efficiency

• Power dissipation waste heat produced– excess heat may require heat sinks, cooling fans, etc.

• To get high efficiency low dissipated power

supplythefromabsorbedpowerloadthetoelivereddpowerEfficiency

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Class AB efficiency

• Efficiency eta = Pload/Psupply

• Pload =

• Psupply =

• Max efficiency

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Class B efficiency

• Pload =

• Psupply =

• Max efficiency

• ..

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Class C efficiency

• Pload =

• Psupply =

• Max efficiency

• ..

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Class D efficiency

• Ideal switches do not have static power losses – ON: V = 0, OFF: I = 0

• Dynamic power required to charge/discharge capacitors– Losses caused by parasitic capacitors

• Psupply =

• Max efficiency

• ..

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How to deliver high power?

• Available voltage limited by power supply– High power high current low R load

• R loads: – Low output resistance to deliver a high current– CC (emitter/source-follower) often used– No voltage gain, low output resistance, current gain– Several circuit variations

• L or C loads: – Problem of overvoltage/overcurrent– Special protection circuits

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Basic CC/CD circuit

• Can provide a high current through the transistor, but asymmetric

– Less current through Re– Single-transistor circuits: good current source / poor current sink

(or reversal)

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Push-pull amplifiers

• Single transistor type + transformer– Old fashion, still used for RF

• Complementary transistors and DC coupling– Complementary class B and AB stages

• Benefits– Symmetric, current source/sink capability– Low Ro – Good efficiency

• Drawbacks– For AB, steady state current– Tradeoff efficiency/crossover distortion

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Figure 14.6 Transfer characteristic for the class B output stage in Fig. 14.5.

Transfer function in push-pull amplifiers

• The base voltage must change≈1 V to switch from Qn to Qp

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Feedback correction of distortion

• A feedback loop can remove crossover distortion– From Vi to Vo the circuit is a Voltage Follower

Figure 14.9 Class B circuit with an op amp connected in a negative-feedback loop to reduce crossover distortion.

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Figure 14.14 A class AB output stage utilizing diodes for biasing. If the junction area of the output devices, QN and QP, is n times that of the biasing devices D1 and D2, and a quiescent current IQ = nIBIAS flows in the output devices.

Class AB

• In steady state, both devices are slightly ON (as in Class Astages)

• No crossover distortion

• Higher power consumption

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Figure 14.15 A class AB output stage utilizing a VBE multiplier for biasing.

More control on bias point

• The difference of base voltages depends on the R2/R1 ratio(Vbe multiplier)

• Pd = Ie x Vce– Pdmax =

Vcc/2 x (Vcc/2 RL)= (Vcc^2)/(4 RL)

– If RL = 0 (short) unlimited Pd

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Protection of output stage

• The circuit includes current-limiting devices (Re)

• With short-circuit at output, current is limited by Re

– Pd = Ie x Vce– If RL = 0

limited Ie limited Pd– Voltage divider Re/RL– Power loss on Re

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Numeric example - 1

• Power supply +-12 V• Current limit

(with output to GND): 1 A– R =

• Max Pd on transitors

• Vo dynamic range

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Active output current limiter

• Voltage drop on Re1Ve1 = Re1 x Il

– If > 0,6V, Q5 turns ON– As Q5 is ON, base

current steered away from Q1

• Symmetric circuit for Q2/Re2

Figure 14.28 A class AB output stage with short-circuit protection. The protection circuit shown operates in the event of an output short circuit while vO is positive.

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Numeric example - 2

• Power supply +-12 V• Current limit

(with output to GND): 1 A– R =

• Max Pd on transitors

• Vo dynamic range

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• Double voltage swing across load

Figure 14.34 The bridge amplifier configuration.

Bridge amplifier

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• High input impedance symmetric circuit

Other bridge configuration

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Lesson F2: final test

• Describe the difference between class A, B, C amplifiers.

• Which are benefits and drawbacks of class C?

• CC/CD circuits are often used in final stages of power amplifiers. Explain the reason.

• Describe the benefits os complementary output stages

• Define crossover distortion, and describe technique to reduce it.

• Draw the diagram of a current limiter.

• Explain the difference between SNR and SINAD.

• How can we reduce distortion in power amplifiers?

• Define the efficiency of an amplifier.