Amplifiers – With periodic switching input-output
Patent
1982-07-29
1984-11-20
Mullins, James B.
Amplifiers
With periodic switching input-output
330 10, H03F 338
Patent
active
044841450
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention relates to a pulse width modulated power amplifier for amplifying the current and/or voltage level of an input signal and more particularly to an amplifier which can amplify input signals having a frequency range from 0 Hz (direct voltage) up to a maximum frequency related to the clock frequency generated in, or applied to the amplifier. Such an amplifier can be used as a power amplifier for signals, preferably within the audio band and therebelow, and as a transmitting step in an electronic line connection circuit in telephony and thereby provide the subscriber line with a speech modulated DC supply. (It can also be used as a ringing signal generator for telephone exchanges.)
BACKGROUND ART
Resistors have been previously used in supplying current to line circuits in telephony, these resistors being connected to a direct voltage source (usually-48 V), speech modulation being executed with the aid of a transformer (the speech transformer), as will be seen from the U.S. Pat. No. 4,056,689, for example. Linear amplifiers are used alternatively for the same purpose, a so-called class S amplifier described in "Circuits and Systems" IEEE, Volume 7, No. 10, December 1975.
DISCLOSURE OF THE INVENTION
In the current supply of a subscriber line via resistors, power is developed in the form of heat in the current supply resistor. The speech transformer must take care of the DC magnetization from the current supply, apart from the low frequency speech band signals, which results in it becoming unnecessarily large and heavy. This transformer is eliminated in line circuits with linear end stages, but the high power loss remains, thus the components unable to be fitted closer on the circuit board, even if their physical dimensions were to allow it. The introduction of a class D type amplifier eliminates the problem with the speech transformer, but as with the above mentioned case, the output voltage is here limited to the supply voltage, thus requiring a DC converter for ringing signal generation, and for current supply in some cases as well. The amplifier in accordance with the present invention is a variant of the converter described in the Swedish Patent application No. 8008633-3. The present amplifier contains, as is described in said patent application, a transformer to which controllable or steerable switching devices are connected on both battery and load sides. These switching devices are mutually connected and steered so that energy can be transferred from the voltage supply input ("battery side") to its output ("load side") as well as from the load side to the battery side. Contrary to the switching devices in the converter according to the above-mentioned patent application, the switching devices are two-way, so that the transport of energy takes place in two directions during each clock pulse interval. The energy transfer takes place in different ways depending on the signal polarity, which results in four different operational modes.
When the amplifier is in operation, the switching devices are energized in pairs, one on the battery side and one on the load side. This gives rise to four different combinations, one for each operational mode. The energized switches are opened and closed at a fixed switching frequency controlled by a clock signal. The pulse-pause relationship for the energized switches is controlled by a pulse width modulator, in turn controlled by an error voltage derived from the amplifier output voltage and a reference voltage. Energy is thus always transferred (at every instant) in the amplifier between the battery and load sides. Each clock period of the supplied clock signal is divided by the pulse width modulation into two time intervals during which the energy transport takes place in each direction. The output and input filters included in the amplifier have the task of smoothing the pulsing (mean value formation), so that the energy flow, i.e., the difference between energy fed to, and from battery and load does not vary appreciably in time with th
REFERENCES:
patent: 4192975 (1980-03-01), Brockmann
Mullins James B.
Telefonaktiebolaget L M Ericsson
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