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Project 1 Report
George V. Rosenbaum
EE 4113
May 6, 2014
The University of Texas at San Antonio
1 - Abstract
The project was to build a simple audio amplifying system that performs the same basic
tasks of a commercial amplifier. The circuit had ±14 rails and a 2V input signal. This signal was
first passed into a bandpass filter that removes frequencies above and below the range of human
hearing. This was done to prevent any inaudible frequencies from drawing unnecessary current
to amplify a more complicated signal. This filter was actively operated, using op-amps in
addition to passive components. Once the unwanted frequencies were attenuated, it was sent to a
class AB push-pull MOSFET amplifier. This pushes a high amount of power through a small
power resistor, which acts as the replacement load for a speaker. A non-inverting feedback
system removed any existing crossover distortion and multiplied the 2 Vpp signal into 20 Vpp
before it entered the AB amp. A 10 kΩ potentiometer is used to passively divide the 2 Vpp signal
before it enters the amp, giving the user a form of volume control.
The device was first simulated in PSpice, particularly to design and simulate the pre-amp
filter configuration of choice. Once this design matches specification, the entire circuit was built
on a breadboard, and tested in two stages. When physical testing was completed, the PSpice
simulation was imported into a printed circuit board (PCB) editor, and a copper PCB was
fabricated. The components were then soldered into the board, and the circuit was re-tuned and
tested again.
2 - Introduction and Problem Description
2.1 - Introduction
In EE 4113, the students will learn circuit fabrication skills and be exposed to the
laboratory equipment and techniques used by real engineers. The specific version of the circuit
was designed and built by Abdullah Aldhalaan and George Rosenbaum.
2.2 - Problem Description
2.2.1 - Specifications of Project 1A
Part A of the project was to make the active filtering, bandpass filtering pre-amp stage.
For this project, single and dual 411-style op-amp 8 pin chips were available for use. The circuit
was protected by 100 uF electrolytic capacitors at the ±14 power source to filter out induction
voltage spikes when turned on and off. At least two 0.1 uF bypass capacitors filtered out noise in
each filtering stage. The passband range should be between 200 Hz and 18 kHz, with only 2 dB
of ripple for frequencies within the passband. The stopband attenuate frequencies below 40 Hz
and above 90 kHz, and should drop the amplitude of the signal below 22 dB of the maximum
gain in the passband.
This part required the design of an active bandpass filter that passes and stops the listed
frequencies. This can be done entirely by hand calculation. However, using circuit simulation
and filter software will make the process much easier. Once constructed, the circuit will be tested
by running a 2Vpp sine wave signal and recording the voltage changes from 1 Hz to above 90
kHz.
2.2.2 - Specifications of Project 1B
Part B of the project was to build and correctly tune a Class AB MOSFET based
amplifier. The amplifier must take a 2Vpp signal and amplify it to 20Vpp with no distortion to be
played on a speaker. The transistors used will be an IRF510 n-channel MOSFET and an IRF9510
p-channel MOSFET. Like part A, the circuit was protected by the same 100 uF electrolytic
capacitors. In addition, 0.1 and 0.01 uF bypass capacitors were used on the source terminals of
both transistors. A 10 kΩ potentiometer was used as a volume knob and a 411-style single op-
amp chip was used as a feedback loop, and was protected by 0.1 uF bypass capacitors. This
served as a negative feedback loop to reduce crossover distortion and boost the voltage to 20Vpp.
The speaker was replaced with a 20 Ω power transistor, which served as a dummy load.
This stage must be individually tested before it can be added to the filtered signal created
by the pre-amp. A 1600 kHz, 2Vp-p test signal was generated in place of our pre-amp output for
this stage of the project. The VGS voltages for both transistors were biased by two 100 kΩ
potentiometers, which must be manually tuned to control the stage the transistors operate in.
While tuning, the current drawn by the power transistors was carefully monitored to prevent
damaging them. They should operate between 120 mA and 200 mA, and anything above this is
cause for concern. Record the total harmonic distortion (THD) using the oscilloscope to ensure
that the output wave is not being distorted, ensuring this does go above 1%.
When the AB amplifier was properly tuned and connected to the volume knob and
feedback loop, it was combined with the pre-amp from part A. The entire circuit was tested as a
whole to demonstrate all the parts working together.
2.2.3 - Specifications of Project 1C
Once the full circuit was constructed on a protoboard, the PSpice design was imported
into a PCB editor. First, the PSpice model must be prepared for proper importing through
Cadence’s PCB software so it could be properly imported. This involves constructing virtual
voltage rails within the PSpice model, removing extraneous pins on parts, and creating PCB
footprints for each component. The voltage sources must be replaced with empty ports and the
input replaced with a BNC port.
With the fully prepared PSpice model the components were netlisted into the PCB editor.
The board size cannot be larger than 4.5” by 3.25”, with 20 mil signal traces and 50 mil power
traces. All traces must be routed on the bottom layer, with the top layer as an extra ground plane
to prevent isolating a section of ground on the bottom. The parts must be placed one by one
using the PCB editor, and must successfully connect all the traces together without overlapping.
When this is done, the artwork and drill files were made so that the designed copper trace
can be printed out. The board was inspected for any mistakes or loose copper shavings shorting
anything out. Then all the parts were removed from the protoboard and placed in the copper
board, using the testing methodology discussed in parts A and B. The parts were not soldered in
all at once, and each stage was tested by itself before it was added the rest of the circuit.
3 - Initial Design and PSpice Simulation
3.1 - Initial Design
Design is primarily necessary when the pre-amp stage of the circuit was created. There
were some minor design decisions for how the AB amplifier potentiometers were biased and
how much gain was set on the feedback loop. However, these were fairly simple choices that
didn’t require a lot of personal preference or creativity.
A simple two stage cascaded architecture was used for debugging simplicity. This was
done by passing an input signal through a highpass filter, whose output was then fed into a
lowpass filter. This way, each filter could be redesigned and retested independently. For further
simplicity, Bessel filters were used so the ripple calculation would only involve the drop in gain
at the pass and stop corner frequencies. The circuit design is shown in Figure 3.1 to demonstrate
what the design should look like.
F
igure 3.1 - Circuit Configuration for Part 1A
The circuit can be designed with either a good circuit textbook, or using analog filter
software: http://www.analog.com/designtools/en/filterwizard/#/type. Make sure that the
components to build the circuit are readily available.
3.2 - PSpice Simulation
Once the pre-amp was designed, it was taken and simulated on PSpice to ensure correct
pass and stop corners. The simulation and values for the circuit in the previous section are shown
in Figure 3.2 to show what our projected dB values will be at 200Hz and 18 kHz.
Figure 3.2 - PSpice AC Sweep between 1Hz and 1 MHz with Data Table
The maximum gain in this graph is -77.801dB. For proper attenuation, the 200 Hz and 18
kHz pass corner frequencies must be under 3dB of our maximum. But the cursors on the graph
show that the 200 Hz point is 3.484 dB from the maximum gain, and that the 18 kHz point is
4.383 dB away. In practice, a little overdesign is necessary for proper attenuation. When the final
circuit was built, the amount of signal attenuation at the pass frequency corners proved to be
within spec, despite what the initial simulation showed.
4 - Protoboard Circuit
4.1 - Part A: Pre-Amplifier
Building the pre-amp was relatively easy. Some care needed to be taken to ensure the
right op-amp chips are being used, but there is little to say about how it was fabricated.
Testing the pre-amp was somewhat tedious, but straightforward. The signal generator
was connected to the input of the bandpass, and the oscilloscope was connected to the output. A
range of different 2Vpp signals were generated between 40 Hz and 90 kHz, and the output peak-
to-peak amplitudes at all four corner frequencies were recorded to ensure the filter was passing
and stopping at the correct frequencies. The finished protoboarding is in Figure 4.1, to show the
entire one chip design.
Figure 4.1 - Part 1A in Protoboard
4.2 - Part B: Class AB Amplifier with Feedback
The wiring for the Class AB amplifier was a little more complicated because the 100 kΩ
biasing potentiometers had to be carefully arranged so each pin was on its own row of the
protoboard. They were supposed to be placed in the center of the board like an IC chip, but were
instead rotated slightly and stuck on individual sides of the board for symmetry. This caused the
potentiometers to fit poorly, and they would often pop out of the board. Doing this is not
recommended unless remembering which potentiometer controls which MOSFET is something
the user is likely to forget.
A few precautions had to be taken to prevent accidental damage to the MOSFET
transistors before adjusting the potentiometers. Voltage was applied to the potentiometers before
the transistors were even allowed in the board. The potentiometers were then adjusted until both
were at zero voltage. This was a necessary step to ensure the MOSFETS would not accidently
break from current overdraw. To monitor how much current the MOSFETs were drawing during
testing, the power supply was toggled to display drawn current. The current limits were carefully
taken off of the power supply, and a 1.6 kHz, 2Vpp signal was generated into the input.
Once these precautions were taken, the MOSFETs were finally added to the board. Heat
sinks were added to each transistor to drain heat away from the devices. The potentiometers were
then slowly adjusted to allow more VGS voltage into each MOSFET transistor. When the
MOSFETs drew around 150 mA at their rails, all visible traces of crossover distortion had been
eliminated.
The feedback loop was then added on to increase Vpp to 20V while eliminating any
remaining crossover distortion. Designing it was relatively simple. Two resistors were chosen so
bias the gain in the loop. The 10 kΩ was also added to the input, and the AB amplifier was tested
again to ensure the new maximum voltage of 20Vpp and voltage control below -5dB and 20dB.
Figure 4.1 shows the PSpice simulation of the circuit and Figure 4.2 shows what the circuit
looked like in the protoboard. Note how the MOSFETs and power resistor are spaced out in [4.2]
to prevent heat from getting trapped.
Figure 4.1 - Circuit Configuration for Part 1B
Figure 4.2 - Part 1B in Protoboard
5 - Printed Circuit Board Design
5.1 - PCB Editor
Before the finished circuit could b imported into the PCB editor, it was first be prepared
for the net listing process. All of the parts were given appropriate PCB footprints so the editing
software knew how much room to give each part. This was especially important for the
MOSFETs and the power load. Junctions must be added at the ±14 power rails, the GND rail,
and above the power load. A BNC socket must be added in place of the input voltage so it can be
attached to the function generator with a coaxial cable. When finished, the PSpice model will
look like Figure 5.1. See how in [5.1] the input was replaced with “BNC_5PIN,” and the ±14
voltage sources were removed for three “J” components, representing solder junctions.
Figure 5.1 - Complete PSpice Circuit with PCB Additions
With all the extra features added to the PSpice model, the circuit was imported into the
PCB editing program. The size of the board was limited to 4.5” by 3.25” and the parts were
added on. Care was taken when adding the parts so the routing algorithm knew how to draw the
traces. The initial PCB blueprint was shown in Figure 5.2 to demonstrate good part placement.
Figure 5.2 - Top PCB Diagram
The traces were added once all the parts were in place, using a routing tool that
automatically created connections between the components. This process took several tries,
constantly moving around parts to successfully route everything. Parts were arranged so traces
could flow underneath components. This was especially true for the op-amp chips, which have
many traces flowing to them, and easily cluttered the board when arranged badly. One good
technique involves rotating the op-amp chips so traces can flow underneath them to their
destination.
The final PCB trace is shown finished in Figure 5.3. An artwork file and drill file were
created so the board could be cut from copper PC board.
Figure 5.3 - Finished PCB Diagram with Traces
5.2 - Soldering
When the copper trace was made, the components were soldered to the board. Care was
taken to ensure no accidental shorts between traces to GND were made. All parts were tested
individually instead of soldering everything first, to make debugging easier. This involved
checking that the bandpass filter attenuated at the correct frequencies with no distortion and that
the potentiometers biased the MOSFETs to only draw 150 mA when a 16 kHz input signal is
applied. The finished product is shown in Figures 5.4 and 5.5.
Figure 5.4 - Top of Finished Board
Figure 5.5 - Bottom of Finished Board
5.3 - Final Testing
Once the circuit was successfully constructed on the PCB, final frequency tests were
conducted to show the circuit operation correctly at all frequencies. This involved recording the
peak-to-peak voltage at 1.6 kHz, and comparing it to the frequencies at 200 Hz and 18 kHz to
show the amount of attenuation. The frequency drop should not go below 79% of the input Vpp.
The three frequencies, and their AC voltage, are recorded in Figures 5.6, 5.7, and 5.8to check if
the ripple still passes specification.
Figure 5.6 - 19.4Vpp Signal at 200 Hz
Figure 5.7 - 22.6Vpp Signal at 1.6 kHz
Figure 5.8 - 19.8Vpp Signal at 18 kHz
Both corner peak-to-peak voltages were above 79% of 22.6V. This meant that after
soldering the entire board together, the bandpass filter still worked correctly.
6 - Conclusion
Not only was our team smaller than most, we were rather ‘unlucky’ to put it nicely. We
had to redesign our bandpass filter multiple times because it wouldn’t pass and stop at the right
frequencies. Trying to get the PCB traces to route correctly in the editor also proved very
difficult, as the algorithm wouldn’t create valid traces. When we finally got the board printed and
began soldering, the bandpass filter section would behave erratically for a very long time
because of a small floating ground within the traces. From this, we learned how to survive the
worst case scenario for testing and prototyping a circuit design.
The first lesson we learned is that starting over from scratch is a perfectly valid form of
debugging. If visually inspecting the wiring of a protoboard proves fruitless, try removing all the
parts and starting the process over. If the PCB editor will not make valid traces after several
attempts, take their footprints off and place them back on in a slightly different arrangement,
rotating the more connection heavy parts around. If the circuit is cheap enough and you’re given
the option by your overseer, you can even de-solder as many components as you can save, throw
away the faulty board, and print out another board to start over.
The second lesson is that soldering to a copper PCB requires more care than using a
protoboard. Not only is a steady hand and soldering practice required, but errors can occur even
if everything is soldered correctly. Inspect the board carefully, and use the probes to detect
abnormal signals between ground areas or solder joints. This may be the result of an isolated
ground plane or a copper shaving shorting the board. If possible, test every few connections you
make, or subdivide the testing methodology you used when constructing on the protoboard. For
example, instead of testing the bandpass filter all soldered together, try only soldering and testing
the highpass portion by itself before adding the lowpass filter.
Finally, learn to divide work well without worrying about dividing it evenly. In our case,
one team member ran through the testing procedure and did simulation, while the other team
member did the soldering or protoboard wiring. This is more difficult in large teams, where one
or two members may not get to do anything except watch everyone else work.
In spite of our difficulties, our team gained a lot of useful skills that will help us test and
debug our own future projects later in life.
7 - References
Figure 3.1 - Circuit Configuration for Part 1A Figure
Figure 3.2 - PSpice AC Sweep between 1Hz and 1 MHz with Data Table
Figure 4.1 - Part 1A in Protoboard
Figure 4.1 - Circuit Configuration for Part 1B
Figure 4.2 - Part 1B in Protoboard
Figure 5.1 - Complete PSpice Circuit with PCB Additions
Figure 5.2 - Top PCB Diagram
Figure 5.3 - Finished PCB Diagram with Traces
Figure 5.4 - Top of Finished Board
Figure 5.5 - Bottom of Finished Board
Figure 5.6 - 19.4Vpp Signal at 200 Hz
Figure 5.7 - 22.6Vpp Signal at 1.6 kHz
Figure 5.8 - 19.8Vpp Signal at 18 kHz
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