Download ELEC 353: Analogue Electronics Practicals 2 & 3

ELEC 353: Analogue Electronics
Practicals 2 & 3
Power Amplifier Design and Simulation
This is a two-part lab. The first part involves the design of a Class-B power amplifier. This
amplifier is going to drive an 8 Ohm audio speaker and take input from an iPod. You have the
following main components:
• 741 op-amp
• TIP 32C PNP Power transistor
• TIP 31C NPN Power transistor
• ± 12 Volt power supply
1
Practical 1: Design
1.1
The Speaker
The load that the power amplifier will drive is nominally 8Ω. These are the most common
loudspeaker in hi-fi applications. The word ’nominal’ is used because loudspeakers have quite
different input impedance at different frequencies. This is especially true for loudspeakers with
more than one element (a tweeter and a sub-woofer for example). It is quite common for the
speakers to have a load impedance of less than 6 Ω at some parts of the spectrum. To allow for
this most audio amplifiers are designed to be capable of driving a 4 Ω load as well as 8 Ω loads.
1.2
The Input Signal
This amplifier will be designed to amplify the output from a standard digital audio player, like
an iPod. These devices typically output 1V RMS at maximum volume, and can drive a load
around 30 Ohms (good quality headphones).
1.3
SPICE Models
You should download spice-3 models for the TIP 32C transistor from the ON Semiconductor
website. Save these in a text file on your desktop. The LTSpice help has an example of defining
an NPN transistor with a .MODEL statement, in the section “Third-party models”.
1.4
Amplifier Model
Construct a SPICE model of your amplifier1 . Use an 8 Ohm resistor as the load, and follow the
schematic in Figure 1.
1.4.1
Basic Testing of your model
Choose an input signal that is a sine wave2 . Use transient analysis to verify that your amplifier
is working.
1
There is a 741 model in the file LM741.MOD, consult the LTSpice help for details on how to add it. This file
is a subcircuit.
2
v signal n+ n- AC 1 sin(0 2 1kHz 0 0)
1
Figure 1: Schematic of a basic audio power amplifier.
Characterize this amplifier’s gain and bandwidth. Find out the maximum power dissipated
in the transistors if the input voltage is limited to a 2V peak-to-peak sine wave (this is the
typical output of a modern digital audio device). How much power is consumed when there is
no input signal?
1.4.2
Input AC coupling
Modify your design to include a high-pass RC filter with a cutoff at 10Hz to protect the amplifier
from DC offsets on the input (AC coupling)
1.4.3
Volume Control
Provide a volume control (Hint: Use a 10k variable resistor and modify the feedback loop). You
might want to consider the basic circuit for a non-inverting op-amp amplifier as a starting point.
1.4.4
Check Transistor Load
Find a specification sheet for a TIP31C and a TIP32C and check that your design will not
damage the transistors. Check the power dissipated in each transistor is less than the maximum
allowed.
1.4.5
A better op-amp
Now test your design using an LT1115 high performance, high-fidelity audio operational amplifier. How does this change the frequency response of your amplifier.
1.5
Design Specifications
Now characterise the modified amplifier. You should include the gain, bandwidth and a plot
of the output waveform for a suitable input signal. What happens to the output as the input
amplitude is reduced to 10 mV or less?
1.5.1
Efficiency
The efficiency of an amplifier is important. It is the ratio of the power consumed by the power
supply to the power dissipated in the load. How does the efficiency change with input signal
amplitude? How well would your amplifier do with a 4 Ω load?
2
1.5.2
Power supply ripple rejection
How sensitive will the output of the amplifier be to ripple (an AC voltage) on the power supply?
1.5.3
Cross over distortion
As the output voltage crosses through zero one output transistor turns off as the other turns
on. This ‘crossover’ is never perfect. Display an output waveform that shows this distortion –
e.g.using a 1kHz input signal. Using the LT1115 op amp will reduce this distortion compared
to a 741. Which property of the op-amp leads to this distortion?
1.6
Design challenge
Modify your design so that it works from a single voltage supply, i.e. 0V to 12V.
1.7
Conclusion
By the end of this lab you should have designed an audio power amplifier and have a good
idea of how well it will perform. Your writeup should include a final design schematic for your
amplifier.
Figure 2: Mark Butler built this.
3
2
Practical 2: Power Amplifier Construction
Build the power amplifier that you designed in the previous section. First build it on the
breadboard to verify operation, then construct it using vero-board (This latter construction may
run over to the following practical, depending on your familiarity with building electronics.)
A picture of a completed amplifier from a previous year is shown in figure 2. Yours should
look similar, expect be more tidy and have a socket for the op-amp. You will also have an input
jack, and a wall power supply – assuming you take on the single rail challenge.
2.1
Volume Control
Make sure you put the potentiometer into your simulation. This is a 10k potentiometer, that
looks like two resistors whose value always adds up to 10k. Therefore your volume control system
should make use of this.
Check that the amplifier will produce acceptable results with the control at either end of the
scale (the resistance of one of the pair of resistors is zero at each end of the volume scale).
2.2
Parts List
• TIP 32C
• TIP 31C
• Op Amp
• DIP 8 socket
• piece of vero board
• 2x heat sinks
• 10k potentiometer
• screw terminals (speaker output)
• base board
• input 3.5 mm jack
• input power terminals (or jack)
2.3
Characterising your amplifier
You should now measure the properties of your built amplifier. You can do this using a signal
generator and the oscilloscope 3 . You should include some measure of both the frequency
response, and distortion.
2.4
What to hand in
Your amplifier should be handed in along with a specification sheet describing your amplifier
performance. Look up an amplifier spec sheet on the internet to see how it should be presented.
Your grade will be determined by how well your amplifier performs and how well it matches its
spec sheet.
3
The oscilloscopes have a spectrum function that you can use to estimate distortion.
4