GB2287147A

GB2287147A - Bridge circuit for driving a loudspeaker either from one end to earth or from both ends

Info

Publication number: GB2287147A
Application number: GB9403750A
Authority: GB
Inventors: Bernard Dugerdil
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 1994-02-26
Filing date: 1994-02-26
Publication date: 1995-09-06
Also published as: GB9403750D0

Abstract

Loudspeaker 40 may be driven either by amplifier 20 to earth via switch 35 or by amplifier 20 and amplifier 30 (supplied from 20) with switch 35 open. Power supply switch 25 is opened when 30 is not in use. The arrangement is such as to equalize efficiencies under two output levels. <IMAGE>

Description

LOAD DRIVING CIRCUIT FIELD OF THE INVENTION This invention relates to load driving circuits, and particularly, but not exclusively to load driving circuits for driving loudspeakers.

BACKGROUND OF THE INVENTION A loudspeaker driving circuit within a portable telecommunications device typically drives a single loudspeaker to generate all audible output signals.

Some devices generate more than one form of output signal. For example, in a mobile telephone an audio output signal and a ringing output signal are produced by the device. The loudspeaker driving circuit must drive the loudspeaker at different input powers if different volumes are desired for the two signals. The loudspeaker is driven with a lower input power for the audio output signal than for the ringing output signal, in order to generate the different volumes from the loudspeaker. However, a non-linear relationship exists between input and output powers: a twofold reduction in input power gives rise to a fourfold reduction in output power.

Thus, a problem with this arrangement is that the device is less power efficient when generating the audio signal output than when generating the ringing signal output.

As the device has a limited energy source (a battery), this problem shortens the operating period of the device.

This invention seeks to provide a load driving circuit in which the above mentioned disadvantages are mitigated.

SUMMARY OF THE INVENTION According to the present invention there is provided a load driving circuit for driving a load from at least one signal source, comprising an input terminal for receiving an input signal from the at least one signal source; first and second selectable signal paths for coupling the input terminal to the load; switching means for selecting one of the first and second signal paths to couple the input terminal to the load; wherein the load can be driven from the at least one signal source via the first or the second signal path to provide a first or a second output power level respectively, such that power efficiency of the load driving circuit is substantially the same for both the first and the second output power levels.

The load is preferably a loudspeaker and the first and second signal paths preferably further comprise first and second amplifier arrangements respectively.

Preferably the first signal path comprises a first amplifier arrangement and the second signal path comprises the first amplifier arrangement and a second amplifier arrangement coupled in a bridge configuration.

The at least one signal source preferably includes a first signal source and a second signal source such that the first and second signal paths selectively couple signals from the first and second signal sources to provide the first and second output power levels respectively.

The first signal source is preferably a speech signal source and the second signal source is preferably a ringing signal source of a telecommunications device.

Alternatively, the first and second signal paths are preferably selectively coupled to a single signal source of speech signals such that the first output power level is a low power level and the second output power level is a high power level.

In this way power consumption of a device incorporating the invention may be reduced, increasing the power efficiency and operating period.

BRIEF DESCRIPTION OF THE DRAWINGS An exemplary embodiment of the invention will now be described with reference to the single figure drawing which shows a preferred embodiment of a load driving circuit in accordance with the invention; DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Referring to the drawing, there is shown a load driving circuit incorporated in a power amplifier 5. The power amplifier 5 comprises an input terminal 10 for receiving an input signal and a supply voltage terminal 15, for receiving a supply voltage.

A first amplifier 20 has an input 21 coupled to the input terminal 10 and power supply connections 22 and 23 coupled to the supply terminal 15 and to ground respectively. An output terminal 24 of the amplifier 20 is coupled to a first terminal 41 of a loudspeaker 40, for providing an amplified output thereto. A second terminal 42 of the loudspeaker 40 is connected via a first switch 35 to ground.

A second amplifier 30 has an input 31 coupled to the output terminal 24 of the first amplifier 20 and power supply connections 32 and 33 coupled to the supply terminal 15 via a second switch 25 and to ground respectively. An output terminal 34 of the amplifier 30 is coupled to the second terminal 42 of the loudspeaker 40, for providing an amplified output thereto.

If the first switch 35 is closed and the second switch 25 is open, the second amplifier 30 does not receive supply voltage from the supply terminal 15 and is therefore 'off'. The loudspeaker 40 is coupled to transduce the amplified output from the amplifier 20 into audible signals, in a single ended configuration.

If the first switch 35 is open and the second switch 25 is closed, the second amplifier 30 receives supply voltage from the supply terminal 15 and is therefore 'on'. The second terminal 33 of the loudspeaker 40 is not coupled to ground, and therefore the loudspeaker 40 is coupled to transduce the amplified output from the combination of the amplifier 20 and the amplifier 30 into audible signals, in a bridged configuration.

In operation, with the first switch 35 closed and the second switch 25 open, the power amplifier 5 is set to the single ended configuration. The input signal received at the input terminal 10 is amplified by the amplifier 20. The resulting amplified voltage drives the loudspeaker 40, the associated current flowing to ground via the closed switch 35.

The expression for output power for this first configuration is given by:

Where: POUT1 = Output Power V1= Supply Voltage RL= Speaker Impedance In a second case, with the first switch 35 open and the second switch 25 closed, the power amplifier is set to the bridged configuration. The input signal received at the input terminal 10 is again amplified by the amplifier 20, but the amplified output from the amplifier 20 is fed to the input of the second amplifier 30, the amplified output of which is presented to the second terminal of the loudspeaker.

The expression for output power for this second configuration is given by:

Where: POUT2 = Output Power V2= Supply Voltage RL= Speaker Impedance By comparing the two equations above, it can be seen that for the same speaker impedance RL, the second configuration generates four times the output power of the first configuration. The second configuration also draws four times the supply power of the first configuration. Therefore a linear relationship between supply power and output power may be achieved for two power levels with the above arrangement.

This linearity is not possible with either of the above configurations when used alone, since the supply voltage (V1 or V2) which is adjusted in order to vary the input power, is a squared term in the output power expression for each configuration. Therefore an amplifier gain reduction of 6dB reduces the output power by a factor of 4 but only reduces the input power by a factor of 2.

Conversely, with the preferred embodiment herein described, for an amplifier gain reduction of 6dB, there is a reduction in both the output power and the input power consumption by a factor of 4 thereby maintaining the level of power consumption efficiency achieved with the higher amplifier gain.

This increased efficiency is particularly valuable if the power amplifier 5 is incorporated in a portable communications device having a limited energy source (a battery) and at least two audible signal sources, to be output at different audible levels. A mobile telephone typically has a ringing audible signal which is generated at a high power level and a speech audible signal, which is generated at a low power level. With the power amplifier 5 incorporated in a mobile telephone, the switches 25 and 35 would be controlled by a control element within the mobile telephone.

In such an arrangement, the first configuration would be used for the speech audible signals and the second configuration would be used for the ringing audible signals. The control element of the mobile telephone would switch between configurations depending upon the signal to be amplified. Since the mobile telephone would typically generate more speech audible signals than ringing audible signals, a significant extension to the operating period of the device would be achieved.

It will be appreciated by a person skilled in the art that alternate embodiments to the one described above may be achieved. For example, in a second alternative arrangement comprising a communications device incorporating the power amplifier 5, the first configuration would be used for amplifying speech audio signals to a first low volume and the second configuration would be used for amplifying speech audio signals to a second high volume. A user switch of the communications device would allow the user to switch between high and low volume configurations, the low volume configuration typically being used as a battery saving measure.

Furthermore, alternative amplification arrangements could be used instead of the amplifiers 20 and 30 described above.

Claims

1. A load driving circuit for driving a load from at least one signal source, comprising: an input terminal for receiving an input signal from the at least one signal source; first and second selectable signal paths for coupling the input terminal to the load; switching means for selecting one of the first and second signal paths to couple the input terminal to the load; wherein the load can be driven from the at least one signal source via the first or the second signal path to provide a first or a second output power level respectively, such that power efficiency of the load driving circuit is substantially the same for both the first and the second output power levels.

2. The load driving circuit of claim 1 wherein the load is a loudspeaker.

3. The load driving circuit of claim 1 or claim 2 wherein the first and second signal paths comprise first and second amplifier arrangements respectively.

4. The load driving circuit of claim 3 wherein the first signal path comprises a first amplifier arrangement and the second signal path comprises the first amplifier arrangement and a second amplifier arrangement coupled in a bridge configuration.

5. The load driving circuit of any preceding claim wherein the at least one signal source includes a first signal source and a second signal source such that the first and second signal paths selectively couple signals from the first and second signal sources to provide the first and second output power levels respectively.

6. The load driving circuit of claim 5 wherein the first signal source is a speech signal source and the second signal source is a ringing signal source of a telecommunications device.

7. The load driving circuit of one of claims 1 to 4 wherein the first and second signal paths are selectively coupled to a single signal source of speech signals such that the first output power level is a low power level and the second output power level is a high power level.

8. A load driving circuit substantially as hereinbefore described with reference to the accompanying drawing.