BACKGROUND OF THE INVENTION

The present invention relates to a speaker driver and, more particularly, to a speaker driver used in a paging receiver or a pager operated by a battery.

FIG. 1 shows a general arrangement of a pager operated by a battery. This pager comprises a receiving circuit 205 for receiving a radio paging signal, an ID ROM 207 for storing a self-identification number (ID), a display 208 for displaying a destination telephone number or a message, a speaker 202 for generating an alert tone, a speaker driver 201 for driving the speaker 202, a decoder 206 for detecting the self-identification number in response to outputs from the receiving circuit 205 and the ID ROM 207 and supplying a display instruction to the display 208 while supplying an alert tone generation instruction to the speaker driver 201, and a DC/DC converter 209 for receiving power from a battery 203 having an internal resistor 204 and supplying a DC current to each part such as the decoder 206 excepting the speaker driver 201 and the speaker 202.

The speaker driver 201 for receiving power from the battery 203 parallelly with the DC/DC converter 209 is constituted by cascade connecting three transistors Q.sub.1, Q.sub.2 and Q.sub.3. The speaker 202 serves as a load of an output transistor Q3.

When the input transistor Q.sub.1 is ON/OFF-operated in response to an alert tone generation instruction from the decoder 206, the speaker 202 generates a sound corresponding to a repeating frequency of the ON/OFF operation.

Note that the alert tone generation instruction from the decoder 206 has a rectangular waveform having a frequency of several kHz, and a plurality of such instructions are output at predetermined intervals.

In the speaker driver 201 in FIG. 1, a voltage V.sub.B at a point 210 during a speaker drive mode, i.e., when an alert tone is generated, is varied in accordance with consumption of a current such as a speaker driving current and a voltage drop V.sub.DROP across the internal resistor 204 of the battery 203.

A current consumed when an alert tone is generated is large and is not significantly decreased even if a power source voltage is dropped. However, since the resistance of the internal resistor 204 of the battery 203 is increased as the battery capacitance is decreased, the voltage drop V.sub.DROP tends to be increased.

FIG. 2 shows the voltage V.sub.B. The level of the voltage V.sub.B is dropped in every alert tone generation period indicated by S. Finally, a voltage obtained by subtracting the voltage drop V.sub.DROP from the voltage V.sub.B is decreased to a level below a minimum operation voltage V.sub.MIN, which is a minimum value of the power source voltage required for assuring the operation of the DC/DC converter 209 or the receiving circuit 205.

In other words, in a conventional pager using the speaker driver 201, the battery must be replaced with a new one when the voltage V.sub.B =V.sub.MIN +V.sub.DROP. Theoretically, a battery can be used until its voltage value is decreased to the minimum operation voltage V.sub.MIN. In practice, however, a battery replacement time becomes earlier by a time corresponding to the voltage drop V.sub.DROP.

The conventional pager will be explained with reference to an example. The minimum operation voltage V.sub.MIN in a pager of this type is set at 1.0 V. Generally, the voltage drop V.sub.DROP is about 0.2 V. A battery replacement voltage becomes 1.2 V.

FIG. 3 shows a discharge characteristic of a SUM-3P battery. The terminal voltage becomes 1.2 V in about 41 hours, and 1.0 V in about 56 hours. The service life of the battery is shorter by about 15 hours than it should be.

In a pager of this type using a battery, the service life is an important parameter determining a commercial value of the apparatus, thereby posing a serious problem.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a speaker driver which is used in a pager operated by a battery and which can prolong a service life of the battery.

The speaker driver according to the present invention, operated by a battery, for driving a speaker to generate an alert tone in response to an externally input intermittent signal, comprises a reference voltage generator for generating a reference voltage, a transconductance amplifier for outputting a current corresponding to a differential voltage between a voltage of the battery and the reference voltage, an output control switch for ON/OFF-controlling the output current from the transconductance amplifier in accordance with the intermittent signal, and a DC amplifier for amplifying the ON/OFF-controlled output current as a driving current for the speaker.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an arrangement of a pager including a conventional speaker driver;

FIG. 2 is a timing chart showing a waveform of a power source voltage for explaining an operation of the speaker driver in

FIG. 3 is a graph for explaining a drawback of the conventional speaker driver;

FIG. 4 is a block diagram of a speaker driver according to the present invention; and

FIG. 5 is a timing chart showing a waveform of a power source voltage for explaining an operation of the speaker driver in FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will be described below with reference to the accompanying drawings.

FIG. 4 shows a speaker driver according to the embodiment of the present invention. The speaker driver comprises a reference voltage generator 101, a voltage/current converter or transconductance amplifier 102, an output control switch 104, and a DC amplifier 103.

In this embodiment, the transconductance amplifier 102, the output control switch 104, and the DC amplifier are integrated. A speaker 111 as a load is connected to the DC amplifier 103. A battery 113 having an internal resistor 112 serves as an operation power source.

The transconductance amplifier 102 comprises a resistor 123 having a resistance R, constant current sources 121 and 122 respectively having output current values I.sub.1 and I.sub.E, a differential transistor pair 124, and current mirrors 125, 126, and 127. A reference voltage V.sub.ref from the reference voltage generator 101 is applied to the base of one transistor of the differential transistor pair 124, whereas a voltage V.sub.if obtained by subtracting a voltage drop I.sub.1.R across the resistor 123 from a power source voltage V.sub.cc is applied to the base of the other transistor of the differential transistor pair 124.

The output terminals of the constant current sources 121 and 122 are commonly connected to the collector of a switching transistor 131 of the output control switch 104. An alert tone generation instruction, which is an externally input intermittent signal, is supplied from a decoder of the pager to the base of the switching transistor 131. The switching transistor 131 is operated in response to the alert tone generation instruction. As a result, an output current I.sub.01 supplied from the current mirror 127 to the DC amplifier 103 is ON/OFF-controlled in accordance with the alert tone generation instruction. In this case, the output current I.sub.01 is given by the following equation (1) according to general characteristics of the transconductance amplifier 102. ##EQU1## for V.sub.T =KT/q (K is a Boltzmann's constant, T is the absolute temperature, and q is the electric charge of an electron). Note that V.sub.id is a base-to-base voltage of the differential transistor pair 124 and is given by:

V.sub.id =V.sub.if -V.sub.ref =V.sub.cc -I.sub.1.R-V.sub.ref (2)

Therefore, equation (1) is rewritten as: ##EQU2##

The DC amplifier 103 comprises current mirrors 128, 129, and 130 respectively having emitter size ratios of 1:N.sub.4, 1:N.sub.5, and 1:N.sub.6. The DC amplifier amplifies the input current I.sub.01 to obtain a speaker driving current I.sub.02 for the speaker 111.

If a maximum capacity of the speaker driving current I.sub.02 is I.sub.02(MAX), it is given by: ##EQU3## for N=N.sub.4.N.sub.5.N.sub.6.

Therefore, if a minimum operation voltage of the pager is V.sub.MIN and V.sub.MIN =V.sub.ref +I.sub.1 R, equation (4) is rewritten as: ##EQU4## Then, the changes in I.sub.02(MAX) in accordance with the changes in V.sub.cc -V.sub.MIN can be represented by the following numerical values: ##EQU5##

In other words, as the power source voltage V.sub.cc is dropped, the maximum capacity I.sub.02(MAX) of the speaker driving current I.sub.02 is gradually decreased. If the power source voltage V.sub.cc is dropped to the minimum operation voltage V.sub.MIN, capacity I.sub.02(MAX) becomes zero.

As shown in FIG. 5, the speaker driver according to the present invention is operated such that the power source voltage V.sub.cc during the alert tone generation mode is always kept at the minimum operation voltage V.sub.MIN or more. The battery 113 can be used until its voltage is dropped close to the minimum operation voltage V.sub.MIN. Therefore, the service life of a battery can be greatly prolonged compared with that in the conventional speaker driver.

In the above-described embodiment, the integrated circuit arrangement is exemplified. However, a speaker driver having the same functions can be arranged by discrete parts.

As has been described above, according to the speaker driver of the present invention, since the speaker driving current is decreased as the power source voltage is dropped, an abnormal voltage drop during the alert tone generation period as in the conventional speaker driver can be prevented.

Therefore, in the pager using the speaker driver of the present invention, since the power source voltage during the alert tone generation period is not dropped below the minimum operation voltage of the pager, the service life of a battery can be prolonged, thereby greatly prolonging the battery replacement period, i.e., the usable period of the battery.