Amplifiers – Modulator-demodulator-type amplifier
Reexamination Certificate
1999-09-22
2001-07-10
Mottola, Steven J. (Department: 2817)
Amplifiers
Modulator-demodulator-type amplifier
C330S20700P
Reexamination Certificate
active
06259317
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to an output stage for Class D amplifiers and the like, including a power supply that is floating with respect to ground, or any other reference except the supply pins.
2. Description of Prior Art
Class D amplifiers are desirable in audio power amplifiers and the like, because they are efficient, and can handle high power signals. The high efficiency allows for smaller power supplies, and smaller heat sinks.
Class D amplifiers typically use an output stage similar to the shown in
FIG. 1
(prior art). It includes a power supply block
101
which includes two individual power supplies
108
and
109
to supply positive and negative voltages V+ and V−. These voltages are typically equal in magnitude, but differ in polarity. Power supply block
101
is referenced to ground. The output is also referenced to ground. In general it is desirable to have the output referenced to ground, as this makes radio frequency interference easier to control, and is necessary in some installations. It is also desirable to achieve bipolar (plus and minus) output without the use of a coupling capacitor, especially if low frequency operation is needed, where a very large capacitor would be required.
Power supply
101
supplies a positive and negative voltage. Switch
102
is a single pole double throw (SPDT) switch, generally constructed from fast semiconductor devices, such as field effect transistors. Filter
103
is a low pass filter, designed to pass the audio band frequencies to load
105
(e.g. a speaker or other power device), while removing the switching frequencies. The switching frequency is typically 100 kHz-3 MHz. The duty cycle of the output is varied to produce any desirable output voltage.
When a low frequency, high amplitude signal is amplified, significant energy is passed between the two power supplies
108
and
109
.
FIG. 2
shows why this occurs. Assume that the power supply outputs V+ and V− are set to plus and minus 64 volts, and that the amplifier is connected to a 4 ohm load
105
. Assume that the input signal is requesting a +32 volt output. For simplicity, assume that the switch is perfect, and has no on resistance, infinite off resistance, and switches instantly. The switch waveform necessary to make a 32 volt output will have 102 in the positive position 75% of time, and in the negative position 25% of the time. This is shown as Vs.
FIG. 2
b
shows the current into the inductor of the low pass filter
103
(shown here as a simple LC section), labeled i
l
, and having an average value of 8 Amps. The current ramps up and down somewhat during each cycle, with the exact character of the wave shape determined by the design of low pass filter
103
. The power delivered to load
105
is 8 Amps*32 Volts, or 256 Watts. All of the analysis done here is slightly approximated, as the current wave shapes and their averages are modified by the characteristics of the filter, but the results of real world circuits are very close to these approximations, and the above example serves to illustrate the technical difficulties of building switched amplifier systems. In the illustrations, the current is shown as a linear ramp.
The waveform of
FIG. 2
c
shows the current delivered by the positive power supply, which delivers current for 75% of the time. While delivering the current, the average delivered is again 8 Amps, for an overall average of 6 Amps. The power delivered by the supply is therefore 6 Amps*64 Volts, or 384 Watts. The negative supply also delivers current in a positive sense. This is shown in
FIG. 2
d
. The average current is 2 Amps, for a delivered power of 2 Amps*−64 Volts, or −128 Watts.
FIG. 3
illustrates the power flow of the design of FIG.
1
. What is happening is that, of the 384 Watts drawn from the positive supply, 256 are delivered to the load, and the other 128 Watts are returned to the negative supply. The power supply must be designed with this in mind, and the supply must be able to deal with the transferred power, by either holding the current in capacitors for future use, dissipating it as heat, or transferring it to the other supply. The first solution requires large capacitors, especially if low frequencies are to be amplified. The second solution is functional, but works in contradiction to the design philosophy of a Class D design, which is to improve efficiency. The third solution requires additional circuitry, increasing cost and complexity.
FIG. 4
(prior art) shows one implementation of the SPDT switch, using power field effect transistors. Transistors
401
and
402
are FETs, diodes
403
and
404
are catch diodes to protect the transistors during the switching period, and circuits
405
and
406
are snubbing networks to control the wave shape during switching. Transformer
400
, wired with its outputs complimentary, ensures that exactly one of the transistors is on at any time, and provides the gate drive. In other known designs, the switches may be bipolar transistors, insulated gate bipolar transistors, or other devices. The drive may come from other transformer configurations, level shifters, or optical couplers. Those versed in the art can devise many different implementations of the switch unit.
A need remains in the art for an output stage for Class D amplifiers and the like, that places less demands on its power supply, does not require large de-coupling capacitors and works with a ground referenced output.
SUMMARY OF INVENTION
It is an object of the present invention to provide an output stage for Class D amplifiers and the like, that places less demands on its power supply, does not require large de-coupling capacitors and works with a ground referenced output.
A switching output stage according to the present invention includes a floating power supply providing a relative positive output voltage VP at a first line and a relative negative output voltage VM at a second line, a first single pole, double throw switch connected across the first line and the second line, a second single pole, double throw switch connected across the first line and the second line, a low pass filter connected to the output of the second switch, and means for connecting the output of the low pass filter to a load.
The first switch has a common to ground and switches between position A to the first line and position B the second line, and the second switch has a common to the low pass filter and switches between position C to the first line and position D to the second line. Thus, the input to the low pass filter can take on three values, VP, VM, and 0. The input VP to the filter is achieved by switching the first switch to VM and the second switch to VP, and the input VM to the filter is achieved by switching the first switch to VP and the second switch to VM. The output stage of claim
3
, wherein the input 0 to the filter is achieved in two ways, by either switching the first switch and the second switch to VP, or by switching the first switch and the second switch to VM.
The order of switch transition includes the following transitions:
AC−>(BC or AD)−>BD−>(BC or AD)−>AC. The order of switch transition also includes the following transitions: AC−>BD, and BD−>AC. 0 states AC and BD are selected alternately, one each in every other cycle.
The output stage also includes circuitry for sensing current connected to the first switch. This circuitry includes a resistor connected between the first switch and ground, and provides an output control signal based on the sensed current.
A pulse wave modulator (PWM) stage controls the first switch and the second switch, responsive to the output control signal.
An amplifier according to the present invention includes means for receiving two (or more) channels of audio signal, two PWMs for generating a series of pulses responsive to each received audio channel, an output stage as described above connected to each PWM, and means for connecting the outpu
Audio Logic, Inc.
Bales Jennifer L.
Macheledt Bales & Johnson LLP
Mottola Steven J.
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