Download Lab 3

LABORATORY 3
POWER AMPLIFIER
OBJECTIVES
1. To study Class B power amplifier circuits.
2. To observe crossover distortion present in Class B power amplifiers.
3. To simulate Class B and Class AB power amplifier circuits using MicroCap
software.
4. To design and test DC biasing and frequency response of a Class AB audio power
amplifier.
INFORMATION
1. Power Amplifier Class B
Class B amplification involves using a dual voltage power supply along with two power
transistors, an NPN, and its complementary PNP device. Such a circuit is shown in Figure
1 and its operation could be explained as following:
 In the absence of an input signal, neither transistor conducts; both transistors are
off.
 On the positive half of the input cycle, once the input signal is greater than 0.7 V,
Q1 will turn on and current flows as shown in Figure 1- a. Notice that the baseemitter voltage of Q1 causes Q2 to be held in the off state since Q2’s base-emitter
is reverse biased.
 As the input signal swings into the negative half of its cycle and exceeds 0.7V, Q2
is turned on and its base-emitter voltage reverse biases the base-emitter junction of
Q1, turning it off.
Vc c
I
IC
B
+
Vin
FG
_
I
C1
E
Vo
_
+
+
_
Q1
ON
RL
OFF
Q2
Vee
a) Positive half cycle operation
b) Class B output waveforms
Figure 1. Class B power amplifier operation
1
Typical output waveforms for both Q1 and Q2 BJTs and a Class B amplifier output are
shown in Figure 1-b. The time required for the input signal to move from zero volts to +0.7
V or to -0.7 V is the time during which conduction does not occur, consequently the output
sits at zero volts for this interval, producing what is called crossover distortion. Crossover
distortion takes its name from the dead-time distortion occurring when the input crosses
over from -0.7 V to +0.7 V or from +0.7 V to -0.7 V.
Class B has a very low (almost zero) Quiescent Current, and hence low standing power
dissipation and optimum power efficiency. However it should be clear that in practice
Class B may suffer from problems when handling low-level signals. In the absence of an
input signal, a Class B power amplifier should have zero volts dc on the output terminal
with respect to ground, if the transistors are well matched. Often, they are not well
matched, so the student should be aware that it is quite possible to have a dc voltage
present at the output. Some output loads, such as speakers, may be damaged by dc. If such
loads are to be used, they must be capacitively coupled to the output in order to block the
dc.
2. Power Amplifier Class AB
Crossover distortion could be eliminated in class AB power amplifiers by the addition of
the diode circuitry shown in Figure 2a.
Vc c
R1
C1
Q1
Vin
FG
ID
C2
D1
Vo
RL
D2
Q2
R2
Vee
a) Class AB circuit diagram
b) Class AB output waveforms
Figure 2. Class AB power amplifier circuit
Since the diodes in Figure 2-a are on all the time, both Q1 and Q2 are held at the edge of
the conduction mode by the diode voltages (A small but controlled Quiescent Current).
When the input goes either positive or negative, very little voltage is required to put Q1 or
Q2 into full conduction.
Typical output waveforms for both Q1 and Q2 BJTs and a Class AB amplifier output are
shown in Figure 2-b.
2
3. Transistors
You will be using the MJE800 NPN and the MJE700 PNP silicon Darlington pair power
transistors. These transistors are a set of complimentary pair silicon power transistors.
Two individual transistors connected in a Darlington configuration in each package will
provide a very large short circuit current gain β which is the product of the two β’s of each
internal transistor. For the transistors used here the manufacturer guarantees a minimum β
of 750. The transistor diagrams and package are shown in Figure 2.A and the data sheets
are attached in the appendix section of this manual.
C (2)
B (3)
C (2)
B (3)
Q1
R1
6k
Q2
R2
D
Q1
R1
6k
Q2
R2
150
150
E (1)
E (1)
MJE800
D
MJE700
a) MJE800 NPN and the MJE700 PNP diagrams
b) Package
Figure 3. MJE800 NPN and the MJE700 PNP diagrams and package
Note: Two resistors and a diode are integrated internally in the transistor device’s package
and one of the reasons for including these components is to prevent a thermal run-away
from occurring. These internal components are not shown on the circuit diagrams in
Figures 1 and 2 however they should be included in the device model in your circuit
simulation.
PRE-LABORATORY PREPARATION
The lab preparation must be completed before coming to the lab. Show it to your TA for
checking and grading (out of 15) at the beginning of the lab and get his/her signature.
1. Calculations
1. The purpose of this exercise is to design the output stage of an audio power amplifier
class AB that could be used with one or more of the earlier circuits to complete a power
amplifier. In your design set the dual DC power supply to ± 6VDC. The amplifier should
deliver approximately 500 mW of sinusoidal RMS audio power to an 8 Ω load, over the
standard audio range of 20 Hz to 20 kHz. In the laboratory, you will use an 8.2 Ω /5W load
resistor. It will make the lab a lot quieter! Include the basic power amplifier (Figure 1) and
the diode compensated circuit (Figure 4) in your pre-lab design and simulation. The
Figure 4 circuit must be designed at the edge of the cut-off region. Since we are using a
Darlington pairs instead of single NPN and PNP transistors, the diode compensation group
should contain three diodes instead of two, as it is shown in Figure 4. The class AB
3
amplifiers have a small IBIAS such that the DC quiescent operating point is just into the start
of the conducting region. This will prevent a certain amount of cross over distortion.
2. The class AB circuit must be designed at the edge of the cut-off region. For the circuit in
Figure 4.calculate the values of the resistors R1=R2 for a diode current of ID=5mA.
Figure 4. Real class AB power amplifier circuit.
3. For the circuit of Fig. 4 calculate the input power PDC, output power PAC and the
efficiency for an input signal Vin = 4Vp (using Equations 1 to 5). Assume
Vo = Vin. Enter the results in Table 4.
2. MicroCap simulations
2.1. The MicroCap-9.0 (Demo version) has limited library and doesn’t provide Darlington
transistor models. For these simulations, you will have to use MicroCap-9.0 (Professional
version) available only at SEB3108. For the simulations use the Darlington transistor
model of TIP140_FC (to simulate MJE800) and TIP145_FC (to simulate MJE700).
Determine the DC biasing voltages and currents with no ac signal for the practical class B
amplifier circuit in Figure 5.
Figure 5. Practical class B power amplifier
4
2.1.1. Fill the Table 1 with the simulated DC voltages when no AC input signal is
applied to the circuit.
2.1.2. An input and output waveforms for a sinusoidal input signal Vin=4Vp (peak) at
f=1kHz.
2.1.3. The output waveform for input voltage of Vin=8Vp (peak) at 1 kHz. A
comparison with the voltages observed in the lab should be made. Watch for any
distortion occurring in the output waveform.
2.2. Using calculated component values for resistors R1 and R2.determine the DC biasing
voltages and currents with no ac signal for the class AB amplifier circuit in Figure 4. You
should obtain the following information through the MicroCap simulations:
2.2.1. Fill up the Table 3 with the expected DC voltages when no AC input signal is
applied to the circuit
2.2.2. Print the input and output waveforms for a sinusoidal input signal Vin = 4Vp for
f =1 kHz.
2.2.3. Obtain the frequency response of class AB amplifier from 10 Hz to 100 kHz.
Print the Bode plots of the voltage gain and the phase frequency response of this amplifier
and bring these plots to the laboratory.
MicroCap simulations tips:
 To provide a power supply to the circuit use two “Battery” sources from the
MicroCap library. Connect them as Vcc and Vee voltage sources with common
ground and set them to a 6VDC.
 To obtain the values of all the bias currents and voltages on your schematic from
Analysis menu choose the Dynamic DC mode and click on Node Voltages and
Currents icons on the toolbar.
 For a sine wave signal source use a 1MHz Sinusoidal Source from the Micro–Cap
library. Set the AC Amplitude to A= 4(V) in the model description area of the signal
source. Note that A=4V corresponds to Vp=4V.
 Run “TRANSIENT ANALYSIS” to obtain an input and output waveforms.
 Run “AC ANALYSIS” to obtain the gain and phase frequency response plots for
this circuit for frequency range from 10 Hz to 100 kHz. Note: Set parameter P to
plot separate diagram for each curve.
EQUIPMENT
1.
2.
3.
4.
5.
6.
7.
Digital multimeter (Fluke 8010A, BK PRECISION 2831B).
Function Generator Wavetek FG3B.
Digital oscilloscope Tektronix TDS 210.
MJE800 NPN and MJE700 PNP Darlington transistors.
1N4148 diodes – 3.
C=47 F – 2; C=470uF – 1.
R=8.2  / 2W.
5
PROCEDURE
1. You are provided with two heat sinks, which should be attached to the transistors
during the lab exercise. The heat sink supposed to be electrically insulated from
the collector of the transistor, however it is always recommended to avoid any
contact of the heat sinks to the ground or to each other. Occasionally check the
temperature of the heat sink, if you cannot keep your finger of the heat sink for
more than twenty seconds the transistors may be too hot. Shut the power off and
check your circuit.
2. Connect the class B power amplifier shown in Figure 5 using MJE800 NPN and
MJE700 PNP Darlington transistors instead of single BJTs. Use the RL= 8.2 Ω
resistor to replace the loudspeaker’s load.
3. Use a dual voltage Power Supply and connect its POS terminal as Vcc, NEG
terminal as Vee and COM terminal as a common ground. Set the power supply
voltage to 6V DC. Measure the DC quiescent point values. Compare the voltages
and currents from simulation with the experimental data in a Table 1. If your results
are significantly different (more than 15%) from your simulated values, try to find
out and eliminate the reason for that discrepancy.
.
VCE
[V]
Q1
VBE
[V]
IC
[A]
VCE
[V]
Q2
VBE
[V]
IC
[A]
Simulation
Experiment
Table 1. Class B power amplifier DC biasing
4. Once you are satisfied that your circuit is biased correctly, then connect the signal
generator to the input. Set the signal generator to a frequency of 1 kHz. For the
input signal level of Vin = 3Vrms (~4Vp) sketch the output voltage across the 8.2 Ω
load on top of your MicroCap simulation plot. Compare the simulated and
experimental waveforms and explain the differences if any.
5. Increase the input sinusoidal voltage until you notice a clipping in the output
voltage. Record this value and compare with DC power supply voltages. For these
readings you can use the BK Precision meter to measure the AC input current (it
measures the RMS value), measure the input voltage after the digital meter (scope)
as it is shown in Figure 6.
6. For the input signal Vin= 2 Vrms and Vin=1.8Vrms calculate the input AC power Pin,
the output AC power Po, the DC input power from the DC supply PDC. Also
calculate the AC voltage gain AV [dB] (Equation (1)), the AC power gain [dB]
(Equation (2)) and the amplifier efficiency (Equation (5)) of the class B power
amplifier.
6
Vcc
R1
Q1
Iin
Vo
C1
A
FG
Io
Vin
R2
RL
CH2
Q2
C H1
CH1
C H2
Vee
Figure 6. Class B power amplifier measurements.
AV [dB]  20 log
Vo
Vin
Equation (1)
AP [dB]  10 log
Po
Pin
Equation (2)
PDC  VDC I DC  VDC
2
2 VOP
I o p  VDC

 RL
2
POAC
Equation (3)
2
V ( rms) VO ( P )
 O

RL
2 RL

Equation (4)
Po AC
PDC
Equation (5)
Record your measurements and calculations in Table 2. Determine if your amplifier is
capable of delivering 500 mW of audio power without distortion. If your circuit can
not deliver this power, do not lay the sole blame on the DC power supply, the
maximum current it can deliver is 200 mA.
AC input
measurements
Vin
Iin
Pin
[V]
[A]
[W]
AC output
measurements
Vo(p) PoAC
[V]
[W]
DC input
measurements
VDC
PDC
[V]
[W]
Calculations
AV
[dB]
Table 2. Class B power amplifier measurements.
7
AP
[dB]

7. Connect the class AB power amplifier in Figure 4. Use the calculated values of R1
and R2. Repeat the DC biasing measurements from point 2 and collect all data in
Table 3.
VCE
[V]
Q1
VBE
[V]
IC
[A]
VCE
[V]
Q2
VBE
[V]
IC
[A]
Simulation
Experiment
Table 3. Class AB power amplifier DC biasing
8. Repeat all measurements from points 3 and 4 and collect all data in Table 4.
Compare the results with the pre-lab calculations.
AC input
AC output
DC input
Calculations
measurements
measurements measurements
Vin
Iin
Pin
Vo(p) PoAC
VDC
PDC
AV
AP

[V]
[W]
[dB]
[dB]
[V]
[A]
[W]
[V]
[W]
3
2
1.8
Table 4. Class AB power amplifier AC measurements.
9. Determine the frequency response of the AC sinusoidal voltage gain of the
compensated amplifier over the range of 20 Hz to 30 kHz.
f[Hz]
20
50
100
200
500
800
1k
5k
10k
20k
30k
Vin [V]
Vo [V]
deg]











Av[dB]
Table 5. Frequency response of a class AB power amplifier.
10. Plot obtained voltage gain and phase data on top of your simulated Bode plots and
compare the results.
8
REPORT
Your Lab report is due one week later. Please submit it to your TA in the beginning of
your next lab session.
Note: You must copy/print the Signature and Marking Sheet from your manual
before coming to the lab session.
9
SIGNATURE AND MARKING SHEET – LAB 3
To be completed by TA during your lab session
Student Name:____________________
TA Name:___________________
Student # : _____________________
Check
boxes
Task
Max.
Marks
Pre-lab completed
15
Class B Amplifier Test completed
20
Class AB Amplifier Test completed
20
Overall Report Preparation
45
TOTAL MARKS
100
10
Granted
TA
Marks Signature