Description
Introduction
The Power Amplifier (PA) is one of the most important circuits in modern electronics. Critical
aspects of PA operation are its output power, efficiency and linearity. In audio systems in particular,
the linearity of the PA must be very high because our ears are very sensitive to sound distortion.
Class A PAs are linear but have poor efficiency. In contrast, class B PAs can have very high
efficiency but they operate under strong nonlinearities. The linearity of a class B PA can be significantly improved by employing a negative feedback around the power amplifier as shown in Figure
1. The opamp acts as an error amplifier which samples a fraction of the output voltage (Vo) and
amplifies the difference between it and the input voltage (Vi).
In this lab, you will simulate and test the Audio Power Amplifier with feedback shown in
Figure 1. This system consists of a differential pair, similar to the one in Lab 4, as well as a CMOS
PA (also called ”output stage”). You will build first a Class-A and then a Class-B output stage as
shown in Figure 2. Then, you will build the differential pair, as shown in Figure 3. Finally, you
will build the feedback loop around the differential pair and Class-B PA. For the differential pair,
you will use transistors ALD1101 and ALD1102 as in Lab 4, while for the CMOS PA, you will
use power transistors IRF510 and IRF9510. The power transistors have large Vbreakdown and are
able to sustain large currents and voltages (> 10A,<10V) at their terminals. An 8-Ω speaker is
connected to the output to convert voltage to sound.
Preparation
1. Run a 50-ms transient simulation for the Class-A output stage in Figure 2 (a) with 1-kHz
1-Vpp input. Show the input voltage and output voltage waveform. Set the maximum time
step of the transient simulation to 1 µs. Plot the output spectrum using the FFT function of
the simulator. Find the power consumption of the amplifier when the input signal amplitude
is 10mVpp and when it is 1Vpp.
2. Repeat step 1 for the Class-B output stage in Figure 2 (b). Use the value for R1, R2, R3,
and R4 determined in lab 3 such that both the p-MOSFET and the n-MOSFET are biased at
threshold (little or no current flowing through them).
3. Determine the gain of the amplifier in Figure 1. What happens to the volume that the speaker
would produce when the 1-kΩ feedback resistor value is varied?
4. The schematics in Figures 2 (a) and (b) use two supplies, 5V and -5V. This arrangement
centers the output at 0V and allows us to DC couple the output stage directly to the speaker.
Lab 5 Page 1 of 5
ECE331: Analog Electronics University of Toronto 2017
5V
-5V
5V
-5V
1k
200
Diff. pair CMOS PA
Vin
8 Speaker
Figure 1: Audio power amplifier with feedback loop
5V
Vo
-5V
IRF510
IRF9510
5k
Vi
5V
Vo
-5V
IRF510
IRF9510
5k
Vi
5V
5V-Vtp
-5V
-5V+Vtn
R1
R2
R3
R4
(a) CMOS Class-A PA (b) CMOS Class-B PA
10!F
10!F
Figure 2: CMOS output stages: a) Class A, b) Class B
Lab 5 Page 2 of 5
ECE331: Analog Electronics University of Toronto 2017
-5V -5V
5V 5V 5V
Vx Vo
Vip Vin
2mA
Figure 3: Differential Pair
There are cases where only a single supply is available and the speaker has to be AC coupled
through a series capacitor (Cs) since the output is no longer at 0V. Determine the value for
Cs for a cutoff frequency of 50 Hz or less (Hint: f3dB = 1/2πRLCs where RL represents the
load resistance).
Lab
The audio power amplifier simulated in the preparation is experimentally tested in lab. The minimum parts list for this lab is shown in Table 1.
The 8-Ω resistor will be used as a dummy load instead of the speaker to avoid unpleasant sound
during circuit debugging.
1. Set up the power supply to limit the output current to 400mA. This step is very important
for your safety because improper biasing of the power transistors (IRF510 and IRF9510)
can cause them to sink amperes of current. In such a condition, the power transistors
get extremely hot and they may even burn and/or pop. Set the maximum power supply
current as follows:
(a) Disconnect everything from the output port of the power supply except the connection
between the ’-’ and GND terminals.
Lab 5 Page 3 of 5
ECE331: Analog Electronics University of Toronto 2017
Table 1: Minimum parts list
Part Description Quantity
ALD1101 NMOS transistor pair 2
ALD1102 PMOS transistor pair 1
IRF510 NMOS power transistor 2
IRF9510 PMOS power transistor 2
– 8-Ω speaker 1
– 8-Ω resistor (1 W or more) 1
– 10-kΩ multi-turn potentiometer 1
(b) Turn the current limit to an arbitrary high value.
(c) Set the output voltage to 5 V.
(d) Turn the current limit to zero.
(e) Connect an 8-Ω resistor across the ’+’ and ’-’ terminals.
(f) Slowly crank up the current limit until the ammeter reading of the power supply reaches
400mA.
(g) Turn off the power supply and remove the short.
(h) Repeat (a)-(g) on the other output of the power supply but this time short the ’+’ and
GND in (a).
(i) Turn on the power supply. Do not touch the current limit and Voltage knobs from this
point onward.
If the current limiter trips during your experiment, do not crank up the current limit but
fix your circuit. If you are not sure about this step, ask your TA for assistance before
turning on the power supply.
2. Implement the Class-A output stage and an 8Ω load resistor. Do NOT connect the power
supplies before your circuit has been fully assembled as a precaution to getting your hands
burned from touching circuit components. Apply a 1kHz 1-Vpp sinusoid and observe the
output. Determine the power consumption when the input voltage is 10mVpp and 1 Vpp.
Keep the circuit as compact as possible to reduce parasitics. Show the measurements to your
TA.
3. Replace the 8Ω load resistor with the speaker. Connect your Class A output stage and apply
a sound waveform (from your cellphone or I-pod) at the input. Observe the sound quality.
4. Repeat step 2 and 3 with Class B PA.
Lab 5 Page 4 of 5
ECE331: Analog Electronics University of Toronto 2017
5. Build the linearized power amplifier with feedback in Figure 1 with the class B output stage
and set the voltage gain to 4 (by adjusting the 1-kΩ resistor). Replace the 8Ω load resistor
with the speaker. Connect your Class A output stage and apply a sound waveform (from
your cellphone or iPod) at the input. Observe the sound quality.
Lab 5 Page 5 of 5