Download Class D amplifier

EXPERIMENT NO.II.11
CLASS D AMPLIFIER
I. OBJECTIVES
a) Understanding the operating principle of class D amplifier.
b) The knowledge of a pulse-width-modulation method.
c) The knowledge of a method to drive a push-pull stage with MOSFET with
avoidance of the “shoot through” phenomenon.
d) The knowledge of a method to reconstruct the amplified signal from the pulsewidth modulated signal
II. COMPONENTS AND INSTRUMENTATION
We use the experimental circuit presented in Fig.II.11.1. The input sinusoidal
voltage will be supplied by a signal generator. The signals from the circuit will be
measured using a dual-channel oscilloscope. The circuit is powered from a dual dc
regulated power supply adjusted at ± 9V.
III. PREPARATION
A class D amplifier is one in which the output transistors are operated as switches.
When a transistor is off, the current through it is zero. When it is on (extreme
conduction), the voltage across it is small, ideally zero. In each case the power
dissipation is very low. This increase the efficiency, thus requiring less power from
the power supply and smaller heat sinks for the amplifier. These are important
advantages in portable battery powered equipment.
First, the input signal vs is converted into a pulse-width-modulated (PWM) signal.
In order to achieve this, the input signal is compared with an (almost) triangular
waveform vt with a much higher frequency than that of the input signal. The
resulting PWM signal is amplified in the current (power) by means of the final
stage with two complementary MOS transistors. The amplified signal is
reconstructed to its initial shape using a low pass filter (usually LC), that remove
the switching frequency.
As one can see in Fig. II.11.1, the block diagram of the amplifier contains the
following blocks:
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 Triangular waveform generator: 555 timer, D1, D2, R1, R2, C1, C2;
 Pulse-width-modulator: LF412 op-amps used as simple comparators OAp şi
OAn, C5, C6, D3, D4, R3 – R8;
 Final stage: Tp, Tn R9, R10;
 Filter: L1,C7, L2, C8.
P1. Triangular waveform generator
 The astable multivibrator circuit composed by the 555 IC and afferent passive
components generates an almost triangular waveform in the point T by charging
up (through R1 and D1 from +VPS) and discharging (through R2 and D2 to the
ground) the C1capacitor. Even the risings up and falling down portions of the
signal are no linear ones (in fact they are exponential), we will call further this
signal triangular waveform.
 Prove that the triangular waveform varies between 3V and 6 V with the
frequency ft (the voltage across the diode in conduction is neglected):
1
f 
t
R  R C  ln 2
1
2 1


What is the value of the frequency?
 What are the amplitude and dc component of the triangular waveform in point
T?
 What does the signal in point T look like considering 26,7KHz frequency and
4.5V dc component?
P2. Pulse-width-modulator
In the Pulse-width –modulator block the input signal applied in the S point should
be compared with a triangular waveform with 0V dc level. The resulting rectangle
(width-modulated) signal is to be applied into the gates of the MOSFETs from the
final stage. In this case both transistors are simultaneously driven with the same
signal. The “shoot through” phenomenon that can appear leads to a reduction of the
efficiency and to a potential failure of the transistors. This occurs during the
transition when one device is being switched off and another one is being switched
on. During the transition, both devices are on and a large current pulse can flow
through the two.
The “shoot through” phenomenon can be eliminated by driving the gates of the
MOSFETs with asymmetrical square waveforms such that one device is switched
off before the other is switched on. One way to accomplish this is by driving the
gates of the MOSFETs with asymmetrical square waveforms. We will use two
comparators OAp and OAn, one for each MOSFET. A positive dc offset is added to
the triangle waveform input to the comparator which drives Tp transistor, while a
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negative dc offset is added to the triangle waveform input to the comparator which
drive Tn transistor. These offset voltages are provided by the R3 – R5, D3 group and
respectively, R6 – R8, D4 group.
 Consider a sine waveform with 1V amplitude and 2 KHz frequency applied to
the input in the S point.
 Plot the signals in the points S, Top and Ton.
 What does the signals in the points Mp and Mn look like? Are they
asynchronous rectangular signals width-modulated (variable duty-cycle)?
P3. Final stage
 Consider in the points Mp and Mn the signals found in the P2 paragraph.
 What is the role of R9 and R10? You may take into account the parasitic
capacities in the gates of the transistors.
 How does the Tp – Tn work: in active region (permanent conduction) or in
switching regime?
 Determine the waveforms for the currents through the Tn and Tp.
 Determine the waveforms for the drain to source voltages for Tn and Tp.
 Estimate the signal in the point U.
P4. Filter
The reconstruction of the amplified signal from the PWM signal is achieved with
two LC low pass cells.
 What is the resonance frequency for each cell?
 What is the attenuation outside the bandwidth, for each cell and for the full
filtering network?
We mention the values for the passive components were chosen so that the cut off
frequency at 3dB attenuation is the same with the input signal frequency (2 KHz).
In the frequency spectrum of the signal in point U the spectral component with the
higher amplitude is the one with the ft=26,7KHz (triangular signal frequency)
followed by the one with fs=2 KHz (sinusoidal signal frequency). After the first
filtering cell (in the point O1), in the frequency spectrum of the signal the spectral
component with the higher amplitude is the one with the fs=2KHz followed by the
ones with 2ft-fs and 2ft+fs frequency. To the circuit output (point O) the entire
superior spectral component has much less amplitudes than the component with 2
KHz frequency (amplified signal frequency), so the signal is reconstructed.
 Estimate the signals in the points O1 and O.
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IV. EXPLORATIONS AND RESULTS
E1. Triangular waveform generator
Exploration
 Supply the experimental board with symmetrical differential voltage  9V.
 Visualize with the oscilloscope and plot the signal in the point T.
 Measure the dc level, minimum and maximum values and frequency.
Results
The picture of the whole operation of the class D amplifier can be created by
observing the signals in the important points of the circuit. A very good idea is to
plot the signals in the points T, Top, Ton, Mp, Mn, U, O1, and O, one below the
other, time correlated. So let’s do it!
 What are the measured values of the frequency, amplitude and dc level of the
signal?
E2. Pulse-width –modulator
Exploration
 Apply in the point S a sine waveform with 1V amplitude and 2KHz (measure
the amplitude on the screen of the scope
 Visualize with the oscilloscope and plot the signals in the points Top, Ton.
 Visualize simultaneously with the oscilloscope and plot the signals in the points
Top, Mp, respectively Ton, Mn.
 Visualize simultaneously with the oscilloscope and plot the signals in the points
Mp, Mn.
Results
 What are the offsets value of the signals in the points Top and Ton?
 How can you explain the waveforms in the points Mp and Mn?
 What is the range of variation for the duty-cycle in each point?
 Compare the two signals from the point of view of the duration for high level
(9V), and respectively for the low level (-9V). What is the reason responsible
for the differences between them?
E3. Final stage
Exploration
 Visualize simultaneously with the oscilloscope and plot the signals in the points
Mp and U and afterwards in the points Mp and U.
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 How can you obtain the waveforms for the currents through R9 and R10?
Tip: Visualize simultaneously the potentials to the both ends of one resistor in
respect to the ground, than set the oscilloscope so that it shows you the difference
between the two signals. So you know now the voltage drop across the resistor and
also know the value of the resistor…
Results
 How is the signal in the point U compared with the signal in the point Mp
(inverting/non-inverting, greater, less or equal amplitude)?
 What is the maximum value of the current in the gate of each transistor? What
is the reason for the presence of this current?
 What does the waveforms of the drain to source voltages look like?
 Do the two transistor work in out of time regime? Can you justify the answer?
 What is the working frequency of the final stage? How is it by comparison with
the frequency of the amplified signal?
E4. Filter
Exploration
 Visualize simultaneously with the oscilloscope the signals in the points U and
O1 and afterwards in the points U and O.
 Plot the three signals one below the other.
Results
 How can you explain de waveform in the point O1? What are the spectral
components with greatest amplitudes? Who establish the signal amplitude?
 Compare the waveforms of the input (point S) and output (point O) signals by
the points of view of amplitude, phase shifting, frequency spectrum. Can you
assert that the output signal was “reconstructed”?
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+Vps=+9V
C4
VCC
TRIGGER
RESET OUTPUT
CONTROL
THRESHOLD
DISCHARGE
GND
+Vps=+9V
0
R4
0
3
D3
20k
R5
20k
C5
U2A
0
3
Top
47n
R2
1
2
2.2k
4.7k
+
OUT
-
LF412
1
Mp
IRF9533
R9
Tp
-Vps=-9V
4.7
4
C2
10n
D1
+Vps=+9V
8
2
4
5
6
7
R3
1k
47n
V+
R1
2.2k
4.7k
C3
100u
U5
V-
8
555B
C6
D2
T
vs
S
9m
9m
R10
1V
2KHz
C7
+Vps=+9V
U3A
0
3
Ton
+
8
10n
3.3n
C1
O1 L2
47n
0
OUT
R8
R6
-
D4
-Vps=-9V
D1N4148
0
Fig.II.11.1 Experimental circuit for class
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100n
220
0
-Vps=-9V
O
C8
RL1
Mn
LF412
1k
20k
1
4
20k
2
100n
IRF520
V-
0
R7
Tn
4.7
V+
0
U L1
RL
220
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