ecen326: electronic circuits fall...
TRANSCRIPT
Sam PalermoAnalog & Mixed-Signal Center
Texas A&M University
ECEN326: Electronic CircuitsFall 2017
Lecture 8: Output Stages and Power Amplifiers
Announcements
• Homework 8 due 12/6• Exam 3 12/11
• 8:00-10:00 • Closed book w/ one standard note sheet• 8.5”x11” front & back• Bring your calculator• Emphasis will be on feedback and output stages• Sample Exam 3 is posted on the website
• Reading• Razavi Chapter 14
2
Agenda
• General Output Stage Considerations
• Emitter Follower as a Power Amplifier
• Push-Pull Stage
• Improved Push-Pull Stage
• Large-Signal Considerations
• Heat Dissipation
• Efficiency and Power Amplifier Classes3
Why Special Output Stages or Power Amplifiers?
• Power amplifiers are necessary to efficiently drive a load with high power
• Examples• A cellphone may need 1W of power at the antenna• Audio systems require tens to hundreds of Watts
• As ordinary voltage/current amplifiers are not suited for efficiently supporting these power levels, specialized output stages are necessary
4
Power Amplifier Characteristics
• Can drive a small “heavy” load resistance• Example: Speaker resistance
can be 4-16
• Delivers large current levels• Often in the hundreds of mA
• Requires large voltage swings• Draws a large amount of
power from the supplies• Dissipates a large amount of
power, and thus can get hot5
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Power Amplifier Performance Metrics
• Linearity or Distortion• Audio amplifiers must have low distortion to
reproduce music with high fidelity
• Power Efficiency
• Maximum Voltage Rating• May require high power supplies• Transistors must have sufficiently high
breakdown voltages6
Supply fromDrawn Power Load toDeliveredPower
Agenda
• General Output Stage Considerations
• Emitter Follower as a Power Amplifier
• Push-Pull Stage
• Improved Push-Pull Stage
• Large-Signal Considerations
• Heat Dissipation
• Efficiency and Power Amplifier Classes7
Emitter Follower Large-Signal Behavior: Positive Half-Cycle
As Vin increases Vout also follows and Q1 provides more current No major issue with the input signal positive half-cycle
8CH 13 Output Stages and Power Amplifiers
Here VBE is assumed to be 0.8V due to the high current levels
Emitter Follower Large-Signal Behavior: Negative Half-Cycle
However as Vin decreases, Vout also decreases, shutting off Q1 and resulting in a constant Vout
The output signal clips at a minimum value of –I1RL
9CH 13 Output Stages and Power Amplifiers
Example: Emitter Follower
10CH 13 Output Stages and Power Amplifiers
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• At this current level, the VBE has dropped to 645mV• For voltage levels much below 400mV, the output
will saturate at –IlRL = -260mV
• What are the input and output voltages when IC1drops to 1% of I1?
Linearity of an Emitter Follower
As Vin decreases the output waveform will be clipped, introducing nonlinearity in I/O characteristics
Overall, the fixed I1 limits the output swing
11CH 13 Output Stages and Power Amplifiers
Agenda
• General Output Stage Considerations
• Emitter Follower as a Power Amplifier
• Push-Pull Stage
• Improved Push-Pull Stage
• Large-Signal Considerations
• Heat Dissipation
• Efficiency and Power Amplifier Classes12
Push-Pull Stage
As Vin increases, Q1 is on and pushes a current into RL. As Vin decreases, Q2 is on and pulls a current out of RL. For positive Vin, Q1 shifts the output down and for negative Vin,
Q2 shifts the output up.13CH 13 Output Stages and Power Amplifiers
Overall I/O Characteristics of Push-Pull Stage
However, for small Vin, there is a dead zone (both Q1 and Q2 are off) in the I/O characteristic, resulting in gross nonlinearity
The dead zone is from -|VBE2| ≤ Vin ≤ VBE114CH 13 Output Stages and Power Amplifiers
22
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Small-Signal Gain of Push-Pull Stage
The push-pull stage exhibits a gain that tends to unity when either Q1 or Q2 is on.
When Vin is very small, the gain drops to zero.15CH 13 Output Stages and Power Amplifiers
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Sinusoidal Response of Push-Pull Stage
For large Vin, the output follows the input with a fixed DC offset, however as Vin becomes small the output drops to zero and causes “Crossover Distortion.”
16CH 13 Output Stages and Power Amplifiers
Here VBE is assumed to be 0.6V for small currents near crossover
Agenda
• General Output Stage Considerations
• Emitter Follower as a Power Amplifier
• Push-Pull Stage
• Improved Push-Pull Stage
• Large-Signal Considerations
• Heat Dissipation
• Efficiency and Power Amplifier Classes17
Improved Push-Pull Stage
With a battery of VB inserted between the bases of Q1 and Q2, the dead zone is eliminated.
18CH 13 Output Stages and Power Amplifiers
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Implementation of VB
Since VB=VBE1+|VBE2|, a natural choice would be two series diodes I1 in figure (b) is used to bias the diodes and Q1.
19CH 13 Output Stages and Power Amplifiers
Example: Current Flow I
1 1 2in B BI I I I
Iin
20CH 13 Output Stages and Power Amplifiers
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21CH 13 Output Stages and Power Amplifiers
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Addition of CE Stage
A CE stage (Q4) is added to provide voltage gain from the input to the bases of Q1 and Q2
This push-pull circuit is often used in high-power output stages22CH 13 Output Stages and Power Amplifiers
Bias Point Analysis
VX=0Vout=0
23CH 13 Output Stages and Power Amplifiers
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For bias point analysis, the circuit can be simplified to the one on the right, which resembles a current mirror
For a well-defined IS ratio, D1 can be realized as a diode-connected transistor
Small-Signal Analysis - I
24CH 13 Output Stages and Power Amplifiers
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If β is low, the second term of the output resistance will rise, which will be problematic when driving a small resistance.
26CH 13 Output Stages and Power Amplifiers
• For the output resistance, we do need to consider the finite rO of Q3 & Q4
Agenda
• General Output Stage Considerations
• Emitter Follower as a Power Amplifier
• Push-Pull Stage
• Improved Push-Pull Stage
• Large-Signal Considerations
• Heat Dissipation
• Efficiency and Power Amplifier Classes27
Example: Biasing
28CH 13 Output Stages and Power Amplifiers
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Example: Biasing
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However, this design procedure neglects any output swing specification!
Problem of Base Current
195 µA of base current in Q1 can only support 19.5 mA of collector current, insufficient for high current operation (hundreds of mA).
30CH 13 Output Stages and Power Amplifiers
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Modification of the PNP Emitter Follower
31CH 13 Output Stages and Power Amplifiers
• Another output stage issue is that the Q2 PNP transistor typically have low current gain () and low fT
• Using a composite transistor consisting of an emitter follower PNP (Q3) and a common emitter NPN (Q2) allows for improved performance
Here Vin is the first stage output (VN)
Modified PNP Emitter Follower Gain & Output Resistance
32CH 13 Output Stages and Power Amplifiers
Small-Signal Model
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Modified PNP Emitter Follower Input Resistance
33CH 13 Output Stages and Power Amplifiers
Small-Signal Model
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Additional Bias Current
Q2 is often a large transistor to support the output current levels, resulting in large base capacitance
I1 is added to the base of Q2 to provide an additional bias current to Q3 so the capacitance at the base of Q2 can be charged/discharged quickly
34CH 13 Output Stages and Power Amplifiers
Example: Minimum Vin
Min Vin≈0Vout≈|VEB2|
Min Vin≈VBE2Vout≈|VEB3|+VBE2
35CH 13 Output Stages and Power Amplifiers
• There is a minimum Vin for which the effective emitter follower transistors remain in active mode
• Here we conservatively assume for active mode we need a min. VBC=0VSimple PNP Emitter Follower Modified PNP Emitter Follower
• The modified PNP emitter follower has a reduced input range by VBE2
HiFi Design
36CH 13 Output Stages and Power Amplifiers
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• As gm1 and gm2 vary dramatically during large signal operation, the circuit displays nonlinear distortion
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• Placing the output stage in a negative feedback loop allows for a more constant gain, and thus better linearity
Short-Circuit Protection
37CH 13 Output Stages and Power Amplifiers
• Excessive output stage current can result if the output is shorted to ground, damaging the circuit
• Qs and r are used to “steal” some base current away from Q1 when the output is accidentally shorted to ground, preventing short-circuit damage
• Drawbacks• r increases output resistance• Voltage drop across r reduces
maximum output swing
Agenda
• General Output Stage Considerations
• Emitter Follower as a Power Amplifier
• Push-Pull Stage
• Improved Push-Pull Stage
• Large-Signal Considerations
• Heat Dissipation
• Efficiency and Power Amplifier Classes38
Emitter Follower Power Rating
Maximum power dissipated across Q1 occurs in the absence of a signal.
39CH 13 Output Stages and Power Amplifiers
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Example: Current Source Power Dissipation
40CH 13 Output Stages and Power Amplifiers
• The power dissipated by the I1 current source is also important, as it will ultimately need to be realized at the transistor level also
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• Note that the I1 power is positive, as VEE is a negative supply
0
Push-Pull Stage Power Rating
41CH 13 Output Stages and Power Amplifiers
• Assuming a symmetric design, the power ratings of Q1 and Q2 are the same and can be computed by integrating over a half-period.
Push-Pull Stage Power Rating
42CH 13 Output Stages and Power Amplifiers
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Impossible since Vp cannot goabove supply (VCC)
43CH 13 Output Stages and Power Amplifiers
Heat Sink
Heat sink, provides large surface area to dissipate heat from the chip.
44CH 13 Output Stages and Power Amplifiers
Thermal Runaway
45CH 13 Output Stages and Power Amplifiers
• For a constant IC, VBE has a negative-to-absolute-temperature (NTAT) coefficient of approximately -2mV/K
• Conversely, if the push-pull output stage is biased with a constant voltage, the current will increase with temperature
• This increased current results in more heat (higher temperature), and thus more current
• This positive feedback loop is called thermal runaway and can result in transistor damage
Thermal Runaway Mitigation
46CH 13 Output Stages and Power Amplifiers
• Assuming that Q3 and Q4 have relatively constant bias currents with temperature (using good current mirror techniques), the diode voltages drop with temperature
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Using diode biasing prevents thermal runaway since the currents in Q1and Q2 will track those of D1 and D2 as long as their Is’s track with temperature.
Agenda
• General Output Stage Considerations
• Emitter Follower as a Power Amplifier
• Push-Pull Stage
• Improved Push-Pull Stage
• Large-Signal Considerations
• Heat Dissipation
• Efficiency and Power Amplifier Classes47
Power Amplifier Efficiency
• Power amplifier efficiency is critical because PAs draw a significant amount of power from the supply voltages
• Power amplifier efficiency is the ratio of the average power delivered to the load over the total power consumed from the supplies
48
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Emitter Follower Efficiency
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• The emitter follower output stage only has a peak efficiency of 25% as VPgoes to VCC
• Note, this can’t actually be achieved, as we need some VCE and voltage across I1
Emitter Follower Efficiency Example
50
• What if the emitter follower is designed to deliver Vp, but only operates with Vp=VCC/2?
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• Now the efficiency drops to only 6.7%
Push-Pull Stage Efficiency
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• The push-pull output stage has a peak efficiency of 78.5% as VP goes to VCC
• Note, this can’t actually be achieved, as we need some VCE across both Q1 and Q2
• Also, we are excluding the input bias stage
Total Push-Pull Stage Efficiency
52
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• Assume that I1=I2 and that they are chosen to allow a peak swing of VP
Class A Power Amplifiers
53
• Each transistor is on for the entire cycle• Emitter follower is a Class A power amplifier
• Display high linearity, but low efficiency (25% maximum)
Class B Power Amplifiers
54
• Each transistor conducts for half the cycle• Simple push-pull stage is a Class B power amp
• Display high efficiency, but low linearity• Bad crossover distortion
Class AB Power Amplifiers
55
• Each transistor conducts for slightly more than a half-cycle
• Improved push-pull stage is a Class AB power amp
• Compromise between Class A and B• Good distortion at slightly degraded efficiency
• Many other PA classes exist (C, D, E, …) and are often studied in grad-level RF circuit design classes