ELEC 499B
Progress Report # 2
Group 9: D Class Amplifier
Alexandrous Dimopoulos
Calvin HopWo
Ian Wood
Abstract
A class D amplifier is an extremely power efficient audio amplifier. This efficiency is
due to the fact that the output signal is controlled using a pulse width modulated (PWM)
signal. The PWM signal is a signal that is either high or low with no intermediate values.
In other types of amplifiers an analog signal controls power transistors that in turn control
the output waveform. While the transistors are receiving the analog signal there is a
period where the voltage from the signal is not high enough to turn the transistor fully on
and not low enough to turn the transistor fully off. When a transistor is in a state between
fully on and fully off, it wastes energy.
The PWM signal eliminates this transistor state in between on and off, and raises the
efficiency of the entire circuit. However, the main problem in a class D amplifier is that
the output signal is distorted from the high speed switching of the transistors.
To solve the problem of efficiency and distortion a dual class amplifier is being created to
use a class B amplifier to give the output wave form, thus eliminating the distortion, and
to use a class D amplifier to supply the current and the majority of the power to the
output wave form, thus improving efficiency of the circuit. This makes an amplifier that
is power efficient and has less distortion than a normal Class D amplifier.
1.0 Introduction
As electronic audio devices have become progressively smaller, the energy and heat
dissipation requirements of traditional audio amplifiers have become very cumbersome.
Class D amplifiers are a potential solution for these devices. The PWM signal eliminates
the transistor state in between on and off, and raises the efficiency of the entire circuit.
Class D amps have close to 100 % power efficiency. This high efficiency makes it
possible to run the amplifier without a heat sink, which not only makes the device
cheaper to manufacture but also allows for its use in smaller electronic devices.
In a typical class D amp, an analog input signal is converted into a PWM signal by using
an analog comparator to take the difference between the signal and an arbitrary sawtooth
waveform. The PWM signal is then amplified by a pair of power MOSFET transistors –
one to source current during the positive portion of the PWM signal and one to drain
current on the negative portion of the PWM signal. The amplified PWM signal is then
converted to an amplified analog output by using a lowpass filter [1]. This process is
illustrated in Figure 1.
The traditional class D amp has several significant draw backs. Because of the switching
time in between when the source and drain transistors are activated the amplified output
signal typically is distorted to levels that are unacceptable in HI-FI applications. This
problem is inherent in the fact that the traditional D-Class amp has a non-linear power
stage [2].
The other drawback is that the low pass filter typically must be designed with a set output
load in mind. The impedance in audio speakers tends to vary. This of course changes the
resultant frequency response of the lowpass filter which can also degrade sound quality.
Figure 1: Block Diagram of Typical D-Class Amp [1]
To compensate for the dynamic load, an active filter or a high order low pass filter is
typically necessary. This naturally increases the design complexity and the part cost of
the amplifier.
Several solutions to these problems have been suggested and tested in literature [2-4].
Ixos Sound Design was founded to investigate these potential solutions and design an
implementation of the best solution using low cost and readily available components.
2.0 Revised Objectives
The objective of this project is to build a switch mode assisted class D Amplifier. This
type of amplifier has the high efficiency of typical class D amps, but utilizes a linear
input stage that eliminates cross over distortion. The goal is to have this amplifier
perform according to the following specifications.



Produce 20 Watts maximum output per channel
Frequency range 0 to 20 kHz
Ideally to have harmonic distortion under 0.1%
3.0 Progress
3.1 Research
The first step executed in the design process was to extensively research existing audio
amplifiers and class-D amp designs. The fundamental problem with typical class-D
designs lay in the non-linear nature of the power stage. Several solutions to linearize the
power stage were found.
One solution found in [2] was to linearize the power stage by using feedback loops. The
distortion error in the analog signal was compensated, in this solution, by use of a
negative feedback loop. The basis of this feedback loop is shown in figure 2.
Figure 2: Negative Feedback to Reduce Distortion [2]
A benefit of linearizing the power stage in this fashion is that the output filter is no longer
required [2]. This dramatically reduces the design complexity as well as saves on
component costs.
Although in theory this design would meet the desired audio quality requirements, the
feedback loops would require very precise component values in order to be effective.
Naturally the precision required would dramatically increase the cost of the components
used. Furthermore, successfully designing and implementing a feedback loop based
solution would be very time consuming – feedback would have to be extensively tested
and tuned to meet the application requirements. These problems ultimately made this
solution undesirable.
Another solution to linearize the power stage is a switch-mode assisted linear amplifier
[4,5]. In this solution, a linear amplifier is used in series with a D-class amp. The linear
amplifier (a class B for Ixos’ design) amplifies the signal by up to 2 W. When the output
of the linear stage is 2 W the D-Class amp is activated and amplifies the signal further.
For small signals (as well as the portion of the signal close to the switching transition) the
D-class portion is off, and the linearly amplified signal from the B-class amp is fed to the
load. This effectively removes the switching distortion inherent in typical class D designs
(as low level signals are linearly amplified by the B-class stage). The fundamental design
of this solution is shown in figure 3 [5].
Figure 3: Switch Mode Assisted Linear Amplifier [5]
Naturally, the presence of the linear portion reduces the power efficiency of the amplifier.
However, since the linear portion only produces a maximum of 2 W output power the
resultant power loss is not huge when compared to the overall power of the amplifier
(since most of the power is provided by the d-class section). 85 % power efficiency was
reported in [4], which is still dramatically better than traditional audio amplifiers.
Furthermore the relatively low power loss of the linear portion is not significant enough
to make a heat sink necessary [4].
This solution also has the advantage of not needing a lowpass filter to convert the output,
however generally component values can have realistic tolerances. The exceptions are the
resistors used in the current sensor at the output of the B-class stage. These resistors
values must have very low tolerances in order to drive the d-class stage as accurately as
possible. Furthermore, the design principles of linear amps are already well defined and
as such this solution would require less design time.
After weighing the advantages and disadvantages of each potential solution, it was
decided to use the switch mode amp.
In addition to modifying the design solution presented in [4] to use low cost, readily
available components, several modifications will be made to improve the B-class portion
of the amplifier.
3.2 Design
3.2.1 Linear Stage
The design of the B-class portion of the amplifier is complete. Since the design theory of
B-class amplifiers is very well established, the approach taken was to slightly modify
existing designs to make sure that all of the parts used were affordable and readily
available. Figure 4 shows the basic design from [4] that is being utilized.
Figure 4: Basis of B-Class stage Design [4]
Unfortunately several of the transistors used in this design were not available for
purchase from any major distributors. Research was done to locate common transistors
with comparable performance characteristics. Table 1 shows the part substitutions used.
Original
Part
BC556B
BC546A
Substituted
Part
2N5401
2N5551
Table 1: Part Substitutions
3.2.2 Modifications
In addition to the change in transistor models several modifications have been designed
to improve the performance of the linear stage.
The linear amplifier used by [4] utilizes a typical Lin three-stage topology with a class B
output stage. Although this amplifier should perform adequately, it was decided that
several modifications should be made. These modifications fall into two categories:
performance enhancement and protection.
The amplifier already includes several stability enhancement features such as a bias
generator to thermally track the OPS pre-driver transistors, and a Zobel network to
counteract speaker reactance. One feature that is not included though is two-pole
compensation, a method to extend linearity to higher frequencies.
At high frequencies, the Miller effect will cause enough of a phase shift to turn negative
feedback into a positive feedback, thus causing sustained oscillations. To avoid this, a
dominant capacitor is used in the VA stage to shorten the amplifier’s low-frequency
region, thus ensuring that overall gain will be below unity before a significant phase shift
occurs. This method was used by [4]. Performance can be improved by using an extra
capacitor, this is two-pole compensation. If two capacitors are used, a faster roll-off will
occur, thus a higher corner frequency can be used while still achieving the needed
attenuation.
Figure 5: VA stage with one-pole (left) and two-pole (right) compensation
Referring to Figure 5, for the two-pole compensation, RP is generally about 1 kΩ , C1 has
the same value as if it was used in single-pole compensation, and C2 is set to about 10
times C1 [6]. In this specific case, C1 has been set to 220pF, therefore C2 should be
2.2nF.
The VA stage can be further modified to provide over-current protection.
Figure 6: VA stage modified to include over-current protection
This modification is shown in Figure 6. R3, R4, and Q3 form a current limiter. When the
desired maximum collector current is drawn by Q2, a sufficient voltage drop occurs
across R4 such that a base current is applied to Q3 and it is driven into saturation. This
shunts base current away from Q1, Thus protecting Q2. Using a 2N5551 transistor for Q3,
47 Ω for R4, and 1 kΩ for R3, should limit IC for Q2 to about 20mA.
Two more modifications were considered, but will not be implemented. The design does
not have over-current and over-voltage protection for the OPS. This will not be
implemented since it should not be possible to reach device limits since the D-class stage
will begin assisting long before that point. An important missing feature is a speaker
protection circuit. Such a circuit protects speakers from the damaging effects of DC
offset, high frequency oscillations, and overheating. At this time, such a circuit cannot be
included since it requires access to the power supply’s power transformer outputs. Since
lab power supplies will be used, this cannot be done.
3.2.3 Simulation
In order to assist in design tweaks and modifications a simulation of the amplifier is
being developed in Multi-Sim. Most of the B-class stage has been modeled, however
several of the transistors that are to be used are not modeled in Multi-Sim. A subscription
based service is available that has updated transistor models. A more cost efficient
solution is being investigated.
3.2.4 Prototype Construction
Construction has begun on the prototype of the B-class portion of the amplifier. The parts
were all ordered and wired onto a bread board. The resistors used have power ratings of
up to 0.5 W, which generally is not a problem, however several resistors at the output of
the B-class stage require up to 1 W power ratings. These high power resistors must still
be obtained. Also, the sensor resistors must have lower tolerances than the 5 % of the
resistors currently used. This is absolutely critical, as resistance imprecision will result in
an inaccurate signal driving the D-class stage. The progress on the prototype bread board
is pictured in Figure 7.
Figure 7: Prototype of the B-class stage of the Amplifier
3.3 Testing Procedure
Testing procedures
Since the design template of the Class B amplifier and the Class D amplifier were taken
from [4], the results of the end product should be compared to his results.
The two main results that [4] shows are: Total Harmonic Distortion, and Power
Efficiency.
Harmonic distortion is distortion in the output signal caused by frequencies that are not
present in the input signal. Total harmonic distortion is the ratio of the intended output
signal to the total harmonic components added up geometrically. A low total harmonic
distortion means that the amplifier will have little noise in its output sound. The results
in [4] for Total Harmonic Distortion can be seen in Figure 8, while the total harmonic
distortion plus noise interference can be seen in Figure 9.
Figure 8: Total harmonic distortion plus noise interference (22Hz to 80 kHz) plotted
against RMS output power. (100 Hz, 1 kHz, 10kHz) [4]
Figure 9: Total harmonic distortion plus noise interference (22Hz to 80 kHz) plotted
against RMS output power. (1 W, 3 W, 20 W) [4]
The easiest way to test the total harmonic distortion (THD) of the amplifier would be to
input a sine wave at a certain frequency, and then observe and measure the output on a
spectrum analyzer. By comparing the intended output signal to the sum of all the other
harmonics the THD can be figured out..
The main point for merging a class B amplifier and a class D amplifier is the advantages
that the D class has in being power efficient. One of the goals of this project is to have
the amplifier perform on par with template designed in [4]. The results in [4] for Power
Efficiency can be seen in Figure 10.
Figure 10: Power Efficiency [4]
In addition to total harmonic distortion and power efficiency there are many other
characteristics that can be tested to give a more accurate description of the amplifiers
performance.
Input sensitivity: The signal level that is required to obtain full power at the amplifier’s
output. This can be tested by sending a generated input signal to the amplifier and then
seeing at what level the amp give maximum output power.
Crossover Distortion: Distortion caused by the output power devices when one switches
off, and the other switching on. Usually this is included in Total Harmonic Distortion.
Transient Intermodulation Distortion: Distortion that occurs when the input signal is
changed so fast that the output can not keep up. This is a problem because the feedback
systems cease to be as effective. However, this may not be a problem due to the fact that
not many feedback loops are used in IXOS’s design.
Frequency Response: Frequency versus amplitude distortion. To test this one can plug an
audio recording into the input that had sounds using low frequency and slowly step up the
frequency until the high range is achieved. The output can be observed for sound quality
RMS Output Power: The measured RMS output power of the signal. This is measured
using an 8 ohm load resistor.
Slew Rate: Slew Rate is the maximum rate of change (Volts per microsecond) of the
output of the amplifier. This can be measured using an oscilloscope attached to the
output of the device, while a input in generated through the use of a function generator.
3.4 Webpage Development
Ixos’ company website is currently being developed. A general template and site layout
have been designed. An attractive and professional logo has been created and is shown in
Figure 8.
Figure 8: Company Logo
The template for the website is shown in Figure 9. The final website will have sections
containing general information about the company, bios of each of the group members,
relevant documents, and detailed information about the amplifier design and
implementation.
Figure 9: Template for Website
4.0 Division of Work
The work described in the previous sections was distributed amongst the different group
members. A summary of the tasks undertaken and which group member was responsible
for that task is given in Table 2.
Task
Research
Replacement Part
Research
B-Class Prototype Wiring
Webpage Design
Part Acquisition
Group Member
Alex
Alex / Ian
Calvin
Ian
Calvin
Table 2: Division of Work Done So Far
5.0 Unresolved Issues
A number of issues remain that must be solved prior to the project’s completion. The
power MOSFETs and MOSFET drivers have been ordered but have not yet arrived.
These parts are critical to the completion of the D-class stage prototype.
The B-class stage requires several resistors with high power ratings. The techs here at
UVic are unable to obtain resistors with high enough power ratings, so Quail Electronics
will be contacted to see if they have any solutions. Also the low tolerance resistors for the
output of the B-class stage must also be obtained.
Ultimately, once the prototype has been constructed and tested, a PCB version of the
amplifier is desired. Several different PCB fabrication companies have been priced out,
however additional research must still be done.
6.0 Revised Timeline
The initial timeline presented in the first progress report has been updated to reflect the
issues and changes in the project so far. Table 2 shows the updated timeline.
Dates
March 7 - 13
Task
Aquire
Remaining
Parts
Simulations
Specifics
power resistors for OPS
sense resistors
OC and OV circuit components
D-stage simulations
protection circuits and tweaks on B-stage
update
webpage
PCB layout
March 14 - 20
complete
breadboard
circuit
update
webpage
PCB
March 21 - 28
PCB
initial layout contingent breadboard construction and testing
(parts availability)
complete construction
complete testing
complete layout
order PCB
construct circuit
test completed circuit
begin poster
March 29 - 31
prepare
presentation
complete poster
prepare presentation material
Table 2: Revised Timeline
References
[1]
B. Putzeys, “Digital audio’s final frontier,” IEEE Spectrum, pp. 34-41, March
2003. [2] Filterless Amp
[2]
C.C. Ho, J. Wu, and J. Kuo, “A Multi-Loop Voltage-Feedback Filterless Class-D
Switching Audio Amplifier using Unipolar Pulse-Width Modulation,” IEEE
Trans. Consumer Electronics, vol. 50, NO. 1, February 2004.
[3]
M. Berkhout, “An Integrated 200 W Class-D Audio Amplifier,” IEEE Journal of
Solid-State Circuits, vol. 38, NO. 7, July 2003.
[4]
G.R. Walker, “A class B switch-mode assisted linear amplifier,” IEEE Trans.
Power Electron., vol18, pp. 1278-1285, Nov. 2003.
[5]
R.A.R van der Zee, and E. van Tuijl, “A Power Efficient Audio Amplifier
Combining Switching and Linear Techniques,” IEEE Journal of Solid-State
Circuits, vol. 34, NO 7, July 1999.
[6]
G.R. Sloan, High-Power Audio Amplifier Construction Manual, Toronto, Canada:
McGraw-Hill, 1999.