audio_amplifier_fall2015_tas

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Audio Amplifier Configurations and Distortion

I. Introduction

In this lab project, an audio amplifier with different Class configurations was constructed, tested, and measured.

Class B open-loop, Class B closed-loop, and Class AB open-loop were the configurations that were analyzed. Each

Class configuration’s efficiency is determined by calculating the Total Harmonic Distortion (THD) from t he output

signal. In this lab, different components were soldered on a printed circuit board (PCB) to construct the audio

amplifier Class configurations. Distortion is a main concern that was analyzed by using total harmonic distortion

(THD) values obtained from the Fourier transform of the output signal for each audio amplifier Class configuration.

Theories will be tested through experimental work in order to compare experimental results with theoretical results.

Theoretical values will be obtained through simulations using Pspice software. Some important information to

understand is the functionality of how Bipolar Junction Transistors (BJT) work, specifically the NPN and PNP

transistors because these fundamental concepts help obtain the design equations for bias resistors and diodes in the

Class AB open-loop configuration.

II. Theory

A. Multistage Amplifier and BJTs

Electronic amplifiers typically have large output voltage swings at their outputs in order deliver significant

power. The Class configurations come from a two stage audio amplifier that are made up of stage one, A 1, and stage

two, A2. A1 represents the voltage gain needed and A2 represents the power gain stage; A2 is also sometimes referred

to as the buffer stage. Stage one will consist of an operational amplifier and stage two will be using the p ush-pull

configuration using bipolar junction transistors (BJT). A schematic of the multistage audio amplifier is shown in

Figure 2. It consists of a high voltage gain in the first stage, and a low power gain in the second stage. The power

gain acts like a buffer so it can collect low current from the input in order to finally distribute a large current at the

output. In this lab project the voltage gain, A1, and the power gain, A2, are separated in their own stage to construct a

two stage audio amplifier.

Figure 2. Two Stage Audio Amplifier

The gains are different in the stages of A1 and A2 because of loading effect. The second amplifier, A 2, acts like load

for the first amplifier, so depending on the input impedance, it can cause a loading effect if we reduce the loaded

gain of the first amplifier. A two stage amplifier is what is being constructed for this lab project. The first stage is

just and operational amplifier that will have a gain of, A 1=25. The second stage acts like a buffer and has a gain of,

A2=1, which is actually a push-pull emitter-follower configuration that uses an NPN and a PNP bipolar junction

transistor (BJT). A typical buffer amplifier configuration is shown in Figure 3 and can also be referred to as a

common collector configuration:

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Figure 3. Push-pull Amplifier Configuration

The idea of the emitter follower is that there is no AC signal applied at the collector, there’s an AC signal applied on

the base and the emitter which is the output. From an AC standpoint, it is AC coupled. Saturation region refers to a

region of operation where maximum collector current flows. There is a closed switch from collector to emitter.

Cutoff region refers to the region of operation near the voltage axis of the collector characteristics graph, where the

transistor acts like an open switch, only a very small leakage current flows in this configuration. Active mode or

region describes transistor operation in the region to the right of saturation and above cutoff, where a near-linear

relationship exists between terminal currents IB, IC, and IE. Bias refers to the specific dc voltage terminals and

current of the transistor to set a desired point of active-mode operation, called the quiescent point, or Q-point.

B. Total Harmonic Distortion (THD)

Fourier theory and Fourier transforms show that a sine wave that is distorted will contain the sine wave and

additionally components that are sine waves of harmonic frequencies. A popular measurement of the accuracy of the

distorted sine wave is its Total Harmonic Distortion (THD). The THD is the rms value of the harmonic components

of the output signal, excluding the fundamental, expressed as a percentage of the rms of the fundamental. Equation 1

is the fundamental expression for calculating the THD. Calculating the THD is important to know because

amplifiers are evaluated according to their THD on the output voltage, and also by the total power efficiency of a

system. When the THD is being measured, the harmonics are in the spectrum domain instead of a regular time

domain. The THD can determine if the output signal of a system is efficient or not which is extremely important

because the better the efficiency of a system or porotype, the higher the demands go for that certain device. Getting

the most efficient system is good engineering practice. Efficiency for THD is measured in percentage, but is kind of

backwards. Usually the greater the efficiency percentage is, the better the system would be, but for THD, the

smallest percentages are the most efficient amplifiers. So THD less than 1% is desired. The equation for determining

the THD is as follows:

𝑇𝐻𝐷 = √𝑉𝑜𝑟𝑚𝑠

2 −𝑉1𝑟𝑚𝑠2

𝑉1𝑟𝑚𝑠2 = √

∑ 𝑉𝑛𝑟𝑚𝑠2∞

𝑛=2

𝑉1𝑟𝑚𝑠2

𝑉𝑜𝑟𝑚𝑠2 = 𝑉1𝑟𝑚𝑠

2 + 𝑉2𝑟𝑚𝑠2 + ⋯ +𝑉𝑛𝑟𝑚𝑠

2 = 𝑉𝑜𝑟𝑚𝑠2 + ∑ 𝑉𝑛𝑟𝑚𝑠

2

𝑛=2

Where 𝑉1𝑟𝑚𝑠 is the amplitude of the rms value of the voltage output, 𝑉𝑜, at the fundamental harmonic frequency and

𝑉𝑛𝑟𝑚𝑠 is the nth harmonic frequency amplitude rms value. One of the easiest and fastest ways one could calculate the

NPN

PNP

(1a)

(1b)

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THD for several output voltages is to use Microsoft Excel by making a table and using the equations instead of

doing all the calculations by paper and pen.

C. Class Configurations

The Class B configuration consists of a complementary pair of transistors, NPN and PNP, connected in such a way

that both cannot conduct simultaneously as shown in Figures 3 and 4. In Class B configuration, resistor’s R1 and R2

are unknown so a design equation needs to be derived in order to get the correct values for the resistances. Since the

voltage gain is given as 25 for the first stage of the amplifier where the op amp is , the design equation used for

resistors R1 and R2 is found from the voltage gain equation shown in equation 3:

𝐴𝑣 =𝑉𝑜𝑢𝑡

𝑉𝑖𝑛

= 1 +𝑅1

𝑅2

Where Av is the voltage gain of 25. Replacing Av with the value of 25, the resistor values can be found which will be

shown in the results. The voltage output of the op amp from stage one, Va , is also the voltage input for both

transistors in stage two of the amplifier. When the voltage input, Va, for the transistors is zero, both transistors are

cut off and the output voltage, Vb, is zero. As Va, goes positive and exceeds 0.6V, the NPN transistor, TIP120,

conducts and operates in the active region as an emitter follower. In this case Vb follows Va and the NPN transistor

supplies the load current while the emitter-base junction pf the PNP transistor will be in reverse-biased by the -15V

of the NPN transistor. Therefore, the PNP transistor will be in the cut off region . Now if the input voltage Va goes

negative by more than 0.6V, the PNP transistor, TIP125, turns on and remains in the active region acting like and

emitter follower. Vb still follows Va, but in this case, the PNP transistor supplies the load current and the NPN will

be remain off in the cut off region. So basically there are to different equations for the output voltage Vb as follows

in equations 3 and 4:

𝑉𝑏 = 𝑉𝑎 − 𝑉𝐵𝐸𝑁

𝑉𝑏 = 𝑉𝑎 − 0.6𝑉

𝑉𝑏 = 𝑉𝑎 + 𝑉𝐸𝐵𝑃

𝑉𝑏 = 𝑉𝑎 + 0.6𝑉

Where 𝑉𝐵𝐸𝑁 is the base-emitter voltage for the NPN transistor when the input voltage exceeds 0.6V and 𝑉𝐸𝐵𝑃 is the

emitter-base voltage for the PNP transistor when the input voltage goes past -0.6V. Figures 3 and 4 are both Class B

but with different configuration. The difference is that Figure 3 is operating open -loop and Figure 4 is operating in

closed-loop. Closed-loop means that there is a feedback network in the circuit as shown obviously in Figure 4. The

feedback makes the signal leave the output of the of the transistors in this Darlington configuration, and then guides

the signal back to the input of the inverting part of the op amp which is also known as negative feedback. When this

feedback is applied in the circuit, Class B becomes more efficient operating in closed-loop than operating in open-

loop because this feedback technically reduces the amount of distortion the circuit has . There will be still some

visible distortion with negative feedback, but just not as much. So in general the crossover distortion of Class B

output stage can be reduced significantly by employing a high-gain op amp and overall negative feedback. The

±0.6V dead band is reduced to ±0.6V/ A1, where A1 is the gain of the op amp that was given as 25. During Class B

operation, there is only one transistor that is in the active region at any given time and provides current to the load,

so for an input of a sine-wave, the conduction angle is π.

(2)

(3)

(4)

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Figure 3. Class B Open-loop Audio Amplifier Configuration

Figure 4. Class B Closed-loop Audio Amplifier Configuration

For Class B configurations, the gain of this first stage can be adjusted by using trimmer R2. The second

stage is a Class B push-pull amplifier using transistors in a Darlington configuration. The Darlington configuration

utilizes two transistors feeding each other. Using two transistors, an NPN and a PNP for each half of the output

signal. One transistor conducts for the positive half of the waveform and another conducts for the negative half of

the signal. This has the effect of increasing the overall current gain at the expense of a larger base-emitter voltage

drop with 1.2V. The base emitter voltage, VBE, to be greater than the 0.6, which is required for a bipolar transistor to

start conducting. In the output a rather large capacitor is needed to obtain a cutoff frequency below 20Hz, which is

the lower end of the audible frequency range. Electrolytic capacitors are inserted in the circuit right before the output

in order to avoid reverse biasing of polarized capacitors. The resistors labeled RE_N and RE_P are known as

emitter resistors. Since diode drops are temperature dependent, the emitter resistors are inserted in the circuit to limit

the output current and to avoid thermal instability caused by thermal drifts , thus it stabilizes the system. The lower

part of the output waveform which is below 0.6V will not be reproduced accurately resulting in a distorted area of

the output waveform as one transistor turns off waiting for the other to turn back on. When this occurs, between the

switching, there is a small part of the output waveform at the zero voltage crossover point like is a dead band with

no gain, thus creating crossover distortion as mention before. Crossover distortion is a main concern in transient

circuits, so it is not good to have too much of it. Applying a negative feedback loop to the same circuit for Class B

will eliminate a lot of distortion, but there is even a more efficient way to reduce crossover distortion by biasing the

circuit and combining two Class configurations which is also explored in this lab project coming up in the

discussion of the Class AB open-loop configuration.

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The Class AB uses an open loop configuration and biasing as shown in Figure 5. Biasing a circuit is more

efficient at eliminating distortion than applying negative feedback. With the NPN and PNP power transistors now

biased in Figure 5, there should be more power and faster switching between the transisto rs occurring. Distortion is

always there, but it should not be visible. The loudspeaker containing 8Ω impedance needs signals that are generated

from a driving voltage having a magnitude of several volts that will produce the power to the output of the

loudspeaker. This is the configuration where the sound through the loudspeaker is tested . A loudspeaker with a

resistance of 8Ω was connected to the output, and an iPod was connected to the input with an auxiliary cord in order

to test the functionality of the amplifier. A 16 bit iPod usually has a frequency of about 44.1kHz.

Figure 5. Class AB Open-loop Audio Amplifier Configuration

The voltage drop across diodes D1 and D2 and across resistors RD1 and RD2 provides the required bias voltage. This

configuration operates open-loop, but the crossover distortion should be reduced so much that is it cant be visible.

One of the important values that need to be determined before performing simulation and experimentation is the

design values for resistors RBIAS1, RBIAS2, RD1, and RD2 in a given bias network that yields Ibias =10mA and Vbias=1V.

The design equation (4) for Rbias1 and Rbias2 will have the same value so the equation 5 is given as follows:

𝑅𝑏𝑖𝑎𝑠1 = 𝑅𝑏𝑖𝑎𝑠2 =𝑉𝐶𝐶 − ∆𝑉𝑏𝑖𝑎𝑠

𝐼𝑏𝑖𝑎𝑠

To get the design resistors for the diodes, equation 6 can be used to find the values which will be the same for both

resistors. The simulations performed in Pspice and the measurements taken from the oscilloscope will show how

much distortion has been reduced using the Class AB configuration.

𝑅𝐷1 = 𝑅𝐷2 =𝑉𝑏𝑖𝑎𝑠

𝐼𝑏𝑖𝑎𝑠

I. Experimental

Before starting the experiment, a list of components was given with a certain value, which actually helped out

with some calculations, such as the design values for the bias and diode resistors. A printed circuit board (PCB)

was used to build the audio amplifier. After the soldering of the components to the PCB was finished, the power

supply was setup to use dual polarity in order to power the op amp and transistors with +15V and -15V. The

function generator was also used to power the board with a sinusoidal voltage. The oscilloscope was used to

measure and analyze three different types of Class configurations . To build these classes a little work was

required to get the specific configuration that was needed. The potentiometer or trim pot that was given had to

be tweaked to a certain resistance where the voltage can be controlled for the output of the operational

amplifier. If viewed closely the schematic diagram shows where the jumpers should be placed and in which

class.

(5)

(6)

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II. Results

Figure 6. Class B Open Loop Input, Va, and output Vb Waveform Simulations

Figure 6 is a simulation performed in PSpice of Class B open loop operation after finding the design values R1 and

R2. The top waveform, V(1), is the input frequency which is a sine wave with a 200mVpk-pk. The middle waveform

V(2) from Pspice and is Va from the circuit diagram in Figure 3, which is the output from the op amp in stage one

and the input in stage two of Darlington Configuration. The bottom waveform, V(9), is the output voltage. There is

clearly crossover distortion visible.

Figure 7. Simulation Class B Open Loop Currents for the NPN and PNP Transistors

The waveform that is positive is the current from the NPN power transistor. The negative waveform is the current

from the PNP power transistor. As stated in the theory section, when the voltage goes past 0.6V, the NPN transistor

turns on which is positive and the PNP is off.. When the voltage exceeds -0.6V the PNP transistor turns on which is

negative and the NPN transistor turns off in the cutoff region. The dead band across the zero axis is when both

power transistors are in the cut off region.

Time

0s 0.1ms 0.2ms 0.3ms 0.4ms 0.5ms 0.6ms 0.7ms 0.8ms 0.9ms 1.0ms 1.1ms 1.2ms 1.3ms 1.4ms 1.5ms 1.6ms 1.7ms 1.8ms 1.9ms 2.0ms

V(9)

-400mV

0V

400mV

V(2)

-4.0V

0V

4.0V

V(1)

-100mV

0V

100mV

SEL>>

Time


I(REN) I(REP)

-300mA

-200mA

-100mA

-0mA

100mA

200mA

300mA

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Figure 8. Class B Closed Loop Va (top) and Vb (bottom) Waveform Simulations

In Figure 8, the top waveform is V(2) which is Va and the bottom waveform is the output voltage V(9) from Pspice

which is equivalent to Vb from the circuit in Figure 4. Applying feedback from the output of the second stage to the

inverting input of the op amp in the first stage reduced distortion significantly, but it is still visible. The power

transistors don’t stay in the dead band for as long as Class B open -loop were, which means the both transistors don’t

stay in the cutoff region as long.

Figure 9. Class B Closed Loop Transistors Current Waveform Simulations

After applying feedback to the Class B configuration, reduction in crossover distortion is noticeable. The positive

part of the waveform is the NPN power transistor and the negative part is the PNP transistor being . They are still

working the same but since the feedback made the input 25 times larger, the power transistors are turn on much

quicker.

Time


V(9)

-10V

-5V

0V

5V

SEL>>

V(2)

-10V

-5V

0V

5V

10V

Time


I(REN) I(REP)

-600mA

-400mA

-200mA

0A

200mA

400mA

600mA

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Figure 8. Class AB Open-loop Va (top) and Vout (bottom)

After the all the biasing resistors were entered in the Pspice code for each power transistor, there was no visible

distortion that one could see. V(2) which is Va in Figure 5 configuration and V(12) which is the output voltage are

almost identical to each other which means biasing a circuit is more efficient.

Figure 10. Class AB Open-loop Transistors Current Simulation

From the simulation of the NPN and PNP power transistors, one can see that crossover distortion has been

eliminated after biasing the Class AB. The currents of the transistors in this simulation are not touching which

means they both don’t go to cut off region simultaneously like the previous currents did in the other configurations.

This shows that at least one transistor is on all the time which is good because when they are both off that is when

the dead band occurs, crossover distortion has been significantly reduced. When the NPN transistor (positive

waveform) is on the PNP transistor (negative waveform) is at zero so its off and NPN is conducting positive voltage.

When NPN is off The PNP transistor is on conducting negative voltage as shown in Figure 10.

Time


V(12)

-4.0V

-2.0V

0V

2.0V

4.0V

SEL>>

V(2)

-10V

-5V

0V

5V

Time


I(REN) I(REP)

-300mA

-200mA

-100mA

-0mA

100mA

200mA

300mA

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Figure 11. Class B Open-loop Fourier spectrum of the Output Voltage

In Figure 12 Class B open loop experiences the highest THD percentage without even looking at the output file. Just

by looking at the spectrum, there are more harmonics than any other Class that was analyzed. The lower the THD,

the better the configuration will function because there is less distortion values. Less distortion means less

harmonics being produced.


This is a screen capture from the output file in Pspice showing the THD percentage for the Class B Open loop.

THD=39.95% which is not efficient

Frequency

0Hz 2KHz 4KHz 6KHz 8KHz 10KHz 12KHz 14KHz 16KHz 18KHz 20KHz 22KHz 24KHz 26KHz 28KHz 30KHz 32KHz

V(9)

0V

40mV

80mV

120mV

160mV

200mV

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Figure 13. Class B Closed-loop Fourier spectrum of the Output Voltage

In Figure 13 obviously has some distortion at first when operating with negative feedback, but applying the

feedback did reduce distortion that one can tell from the Fourier spectrum. Being compared to Figure 12, there are

less harmonics being produced, which is good and successful when feedback was to Class B multistage amplifier

configuration.


This is a screen capture from the output file in Pspice showing the THD percentage for the Class B Closed - loop.

THD=2% which is much lower than 39.95% because of the added negative feedback in order to reduce distortion.


Frequency


V(9)

0V

1.0V

2.0V

3.0V

4.0V

5.0V

Frequency


V(12)

0V

0.4V

0.8V

1.2V

1.6V

2.0V

2.4V

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In Figure 15, the simulation shows how significantly the distortion has been reduced on the output voltage. There

are no visible harmonics which is great because biasing a circuit rather than just giving it negative feedback is a

more efficient route to take if crossover distortion needs to get eliminated. There is going to still be some kind of

distortion but it has been reduced so much that it is not visible in this time scale.


Figure 16 shows the THD percentage in the output file simulated in Pspice. THD=0.70942%. Here is another proven

statement that biasing the second stage of the audio amplifier is the most efficient out of the three Classes analyzed.

Figure 17. Class B Open Loop Configuration of V in and Va

Channel one is the voltage, Va in the Class B open loop configuration. Channel two is the input voltage coming from

the function generator with 1kHz frequency. This screen capture shows that voltage. This capture shows that the

voltage Va still easily tracks the input but with a smaller voltage coming out the op amp.

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Figure 18. Voltages VA and VB being measured in Class B Open Loop

Channel one is the voltage, Va and Channel 2 is the output voltage,,Vb being compared with each other in the open

loop Class B configuration. The output voltage has crossover distortion as one can see as Ch.2 has the dead band

meaning both transistors are off. As mentioned earlier the power resistors are switching slowly because of the diode

drops of ±0.6V. When signal exceeds 0.6V, NPN transistor conducts and PNP cuts off. When signal pasts -0.6, NPN

turns off and PNP turns on the negative voltage seen in Figure 18.

Figure 19. Screen Capture of Crossover Distortion in Class B Open-loop

Channel one is the voltage Va from Figure 3 and Channel two is output voltage Vb of the second stage amplifier

where the power transistors are in Darlington Configuration. Crossover distortion is measured using the cursors in

this capture.

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Figure 20. Fourier Spectrum on Output Voltage from Class B Open-loop

Figure 20 shows a capture from the o-scope of the Fourier spectrum on the output voltage Vb with all of its

harmonics which is showing that there is major distortion being analyzed in this spectrum.

Figure 21. Voltages Va and Vb being measured in Class B Closed Loop

Figure 21 is a capture of voltages Va which is the channel one and the output voltage Vb which is channel two

coming from the power transistors. This is after negative feedback has been applied to the circuit. Applying

feedback to the inverting op amp in stage one reduced distortion, but it is still visible which means the transistors are

both still off during that dead band

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Figure 22. Fourier Spectrum on Output Voltage from Class B Closed-loop

Figure 22 shows the Fourier spectrum of the Class B with negative feedback to reduce distortion. Being compared to

Class B open loop, there is a noticeable change in the harmonics after negative feedback. There is less harmonics on

this spectrum.

Figure 23. Voltages VA and VB shows the Class AB Open Loop Configuration

Figure 23 is a screen capture of channel one, the Va voltage coming out of op amp and into the Darlington

configuration and channel two which is the output voltage Vb coming out of the power transistors. After

resistors biasing the circuit were added and the negative feedback removed, the crossover distortion has been

eliminated so its not visible at this time scale which means the power transistors are switching quickly and

efficiently.

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Figure 24. Fourier Spectrum on Output Voltage from Class AB Open-loop

After biasing the circuit with resistors and diodes, crossover distortion has been eliminated and the Fourier

spectrum shown in Figure 24 barely has that much harmonics making this a great way to reduce distortion.

Figure 25. Voltages Vin and Vb being measured in the Class AB Open Loop Mode

Figure 25 is showing the freequencies that an ipod produces when music is playing and crossover distortion has

been eliminated. Channel one is Vin and Channel two is Vb. Since the ipod produces frequencies as music plays

there was no function generator connected to the circuit. The output voltage is a faithful amplified replica of the

input voltage as shown in the screen capture.The output voltage is following the input almost perfectly while music

plays and frequency is generated throughout the circuit. The output signal wil always have a little delay compared to

the input in this transient circuit because of the all the other components in the circuit, mostly capacitors that stor the

energy so Vb will always have a a little delay because of the energy being stored in capacitors and not being fully

relaseased all at once like it stored it.

Table I shows the first ten harmonics for each Class configuration that was contructed and analyzed. The harmonics

are then converted into rms values in order to calculate the THD for each Class mode. After THD is calcuted

through Excel, it is then compared to the Pspice THD percentages that were simulated,thus getting a percent error

between the expermiental and theoritcal values from each of the three Class configuration s.

Table I of THD Values From First Ten Harmonics

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III. Conclusion

The lab project that was completed ended up going great as the theories that were discussed prior to

experimenting in which crossover distortion caused by the diode drops from the power tranisitors switching on

and off can be reduced and even visibly eliminated though distortion will always still be in amplifing circuits.

There were two methods that helped reduce distortion which consisted of applying negative feedback making

the gain 25 times larger in oerder to give more power to the transistors. When this was a pplied ome could see

how much THD has decreased. Then discovering that biasing the NPN and PNP pwer tranisitors in the

Darlington Configuration with biasing resistiors visibly elimanted distortion throughout the circuit and music

was played througha loud speaker demonstrating how smoothe the quality sounded after biasing was applied.

Biasing the right kind of amplifier configuartion is a powerful tool one can use, and could benfeit devices by

making them more efficient.

1st 2nd 3rd 4th 5th 6th 7th 8th 9th 10th n

-11.3 -42.9 -16.5 -38.5 -34.1 -46.5 -44.1 -51.7 -55.3 -56.2 dB Class B Open-loop Harmonics

0.27227 0.007161 0.149624 0.011885 0.019724 0.004732 0.006237 0.0026 0.001718 0.001549 RMS THD(PSPICE) 3.96E+01

0.074131 5.13E-05 0.022387 0.000141 0.000389 2.24E-05 3.89E-05 6.76E-06 2.95E-06 2.4E-06 Percent error 38.8080005

THD = 54.95409

Class B Closed-loop Harmonics

0.21 -33.7 -31.3 -46.1 -32.5 -48.9 -32.5 -48.1 -34.9 -49.7 dB THD(PSPICE) 2.01E+00

1.024472 0.020654 0.027227 0.004955 0.023714 0.003589 0.023714 0.003936 0.017989 0.003273 RMS Percent error 3.23E+01

1.049542 0.000427 0.000741 2.45E-05 0.000562 1.29E-05 0.000562 1.55E-05 0.000324 1.07E-05

THD = 2.657664

Class AB Open-loop Harmonics

4.23 -29.7 -43.7 -51.3 -52.9 -52.9 -51.3 -53.3 -54.9 -55.3 dB THD(PSPICE) 7.94E-01

1.627421 0.032734 0.006531 0.002723 0.002265 0.002265 0.002723 0.002163 0.001799 0.001718 RMS Percent error 4.95E+01

2.6485 0.001072 4.27E-05 7.41E-06 5.13E-06 5.13E-06 7.41E-06 4.68E-06 3.24E-06 2.95E-06

THD = 0.401328

audio_amplifier_fall2015_tas

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