Amplifiers – Modulator-demodulator-type amplifier
Reexamination Certificate
2000-07-24
2002-03-26
Pascal, Robert (Department: 2817)
Amplifiers
Modulator-demodulator-type amplifier
C330S20700P
Reexamination Certificate
active
06362683
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to techniques for alleviating distortion in switching amplifiers. More specifically, the present invention provides methods and apparatus for compensating for “break-before-make” distortion in digital switching amplifiers.
Digital power amplifiers are increasing in popularity due to their high power efficiency and signal fidelity. An example of an digital audio amplifier
100
which employs oversampling and noise-shaping techniques is shown in FIG.
1
. The input audio signal is oversampled and converted into 1-bit digital data. These data are used by power stage driver
102
to control power switches M
1
and M
2
which, in this example, comprise two nmos power transistors. To reduce the quantization noise introduced by sampling, amplifier
100
employs a noise-shaping loop filter
104
in a feedback loop which pushes the quantization noise out of the audio signal band. A low pass filter comprising inductor L and capacitor C
AP
filters out high frequency noise and recovers the amplified audio signal which drives speaker
106
.
Referring now to
FIGS. 1 and 2
a
, break-before-make (BBM) generator
108
receives the 1-bit switching signal Y from comparator
110
and generates two signals A and B which are 180 degrees out of phase with each other. A and B are level-shifted by power stage driver
102
to become A′ and B′ which are used to alternately turn on power transistors M
1
and M
2
. As is well known, if there is no dead time between pulses in A and B, i.e., time when both A and B are low, there is a possibility that both of transistors M
1
and M
2
may be turned on at the same time creating the potential for an undesirable and possibly catastrophic shoot-through current from the positive power supply VCC to the negative power supply VSS at each transition of Y (as shown in the waveform designated Ist). Such a condition may arise, for example, due to the delays for signals A and B through power stage driver
102
, as well as the rise and fall times of transistors M
1
and M
2
. At a minimum, such shoot-through current increases switching losses thereby reducing the amplifier's power efficiency. In the worst case, power transistors M
1
and M
2
may be damaged or destroyed.
To eliminate shoot-through current and avoid its deleterious effects, and as shown in
FIG. 2
b
, a period of dead time (also referred to herein as break-before-make (BBM) time) is introduced as between signals A and B such that there is a period of time between signal transitions during which both signals are low. This ensures that transistors M
1
and M
2
are never turned on at the same time even when there are delay mismatches between the rise and fall times of the transistors. Input data BBM<
2
:
0
> allows adjustment of BBM time to meet the design requirements of the particular amplifier as shown in Table I. Unfortunately, while the BBM generator eliminates shoot-through current, it produces a degenerative effect on amplifier performance by introducing harmonic distortion. The nature of this distortion is described below with reference to
FIGS. 3
a
,
3
b
, and
4
a
-
4
c.
Referring also to amplifier
100
of
FIG. 1
, when transistors M
1
and M
2
are off during the BBM period, the voltage at node C is determined by parasitic capacitance C
P
. Because inductor L resists instantaneous changes in current, when the output current of the amplifier is charging C
AP
, the voltage at node C is pushed to VCC (clipped by Schottky diode D
1
) during the BBM period. On the other hand, when the output current is discharging C
AP
, the voltage at node C is pulled down to VSS (clipped by Schottky diode D
2
).
TABLE I
BBM<2:0>
BBM time (ns)
000
0
001
40
010
80
011
120
100
160
101
200
110
240
111
280
FIG. 3
a
shows the current through inductor L when the input to amplifier
100
is grounded. Under this condition, the signal at node Y is a square wave and the resulting current through L is represented by a sawtooth waveform which changes polarity at each transition of the signal at node Y. By contrast,
FIG. 3
b
shows the switching pattern of the signal at node Y and the current through inductor L when the input to the amplifier is a sine wave. In the first half of the sine wave cycle, the switching pattern at node Y is modulated to have a relatively wide pulse width resulting in transistor M
1
being turned on more often than transistor M
2
. During this time, the inductor current is largely positive, i.e., charging CAP and directed into speaker
106
. During the second half of the sine wave cycle, the switching pattern at node Y is modulated to have a relatively narrow pulse width such that transistor M
2
is now on more often than transistor M
1
. This results in a largely negative inductor current, i.e., discharging C
AP
and flowing out of speaker
106
. Near the zero crossing of the sine wave, the switching pattern at node Y is similar to that shown in
FIG. 3
a
which causes the inductor current to switch polarity at each Y node signal transition.
With the description of
FIGS. 3
a
and
3
b
as background, the nature of the BBM distortion will now be described with reference to
FIGS. 4
a
-
4
c
.
FIG. 4
a
illustrates the case where the switching pattern at node Y is a square wave. This results in waveforms A and B with a predetermined BBM period as shown. The current through inductor L is also shown. Between t
1
and t
2
, i.e., the BBM period, both transistors M
1
and M
2
are off and the voltage at node C is pulled down to VSS because the inductor current is discharging C
P
. Between t
2
and t
3
M
2
is turned on, keeping the voltage at node C at VSS while the inductor current change polarity. From t
3
to t
4
, i.e., the next BBM period, M
2
is turned off again and the voltage at node C is pushed to VCC because the inductor current is now flowing in the other direction. Beyond t
5
, this switching pattern is repeated and it can be seen by comparing the signals at nodes Y and C that the BBM time has no effect on the output switching pattern when the input is a square wave.
FIG. 4
b
illustrates the case where the switching pattern at node Y has relatively wide pulse widths as described above with reference to the first half of the cycle of the sine wave of
FIG. 3
b.
As described above, this corresponds to an inductor current which is charging C
AP
and directed into speaker
106
. At time t
1
, M
2
is turned off and the voltage at node C is kept at VSS by the inductor current during the BBM period until M
1
is turned on at t
2
, at which point the voltage at node C is pulled up to VCC. When M
1
is turned off again at t
3
, the voltage at node C is again pulled down to VSS by the inductor current. After the next BBM period (t
3
-t
4
), M
2
is turned on and the voltage at node C is kept at VSS. By comparing the signals at node Y and C it can be seen that the output pulse width (at node C) is reduced from the input pulse width (at node Y) by a BBM period.
FIG. 4
c
illustrates the case where the switching pattern at node Y has relatively narrow pulse widths as described above with reference to the second half of the cycle of the sine wave of
FIG. 3
b.
As described above, this corresponds to an inductor current which is charging parasitic capacitor C
P
. At time t
1
, M
1
is turned off and the voltage at node C is kept at VCC during the BBM period until M
2
is turned on at t
2
, at which point the voltage at node C is pulled down to VSS. When M
2
is turned off again at t
3
, the voltage at node C is again pulled up to VCC by the inductor current. After the next BBM period (t
3
-t
4
), M
1
is turned on and the voltage at node C is kept at VCC. By comparing the signals at node Y and C it can be seen that the output pulse width (at node C) is increased relative to the input pulse width (at node Y) by a BBM period.
Thus, for example, for a sine wave input, the output switching pattern at node C introduces relatively little or no distortion at the zero crossings of the input signal. However, the pulse
Delano Cary L.
Miao Guoqing
Beyer Weaver & Thomas LLP
Choe Henry
Pascal Robert
Tripath Technology Inc.
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