Speaker driving circuit

A speaker driving circuit for use with an audio amplifier made selectively ineffective comprising switching means for providing intermittent current to the speaker when said means is energized.

BACKGROUND AND SUMMARY OF THE INVENTION 
This invention relates to a speaker driving circuit and particularly to a 
circuit for providing, from an audio speaker, sounds by which operations 
of a key-board, various switches and the like are confirmed, for example, 
in a PLL synthesized radio receiver with the key-board for data entry. 
In such receiver, when a key push button on the key-board is depressed, a 
sound is generated from the audio speaker equipped on the receiver 
housing, whereby the depression can be confirmed. 
Prior art circuits for providing such operation confirming sounds have used 
an audio amplifier which is in active state, and, therefor during the 
operation mode of muting, squelching or the like wherein the audio 
amplifier is made ineffective, the operation confirming sounds could not 
be generated. 
The main object of the invention is to provide the operation confirming 
sounds even though the audio amplifier is ineffective. Other objects of 
the invention will be clear from the detailed explanation of the invention 
.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIG. 1 showing one of the typical prior art circuit for 
providing the operation confirming sound in response to the operation of 
the key-board, switch or the like, a conventional audio amplifier 10 
driving a loud speaker 12 through a D.C. rejecting capacitor 14 receives 
an audio signal at an input 16 which may be an output of a detector or 
discriminator circuit of the PLL synthesized radio receiver. The amplifier 
10 also receives the operation confirming signal at an input 18. The 
operation confirming signal which may be generated in a muting or squelch 
circuit of the receiver in response to the actuation of the push button on 
the key-board or the switch of the receiver is in the form of the burst of 
a sine or square waveform. The input 16 is connected to the input 18, and, 
therefor, whenever the operation confirming signal is received at the 
input 18, the audio amplifier 10 amplifies the operation confirming signal 
so that it drives the speaker 12. 
The other typical prior art circuit shown in FIG. 2 uses feedback 
technique. This includes a positive feedback network 20 which is well 
known in the art. The feedback network 20 is connected between the input 
and output of the amplifier 10 through a electronic switch 22 normally 
opened. When the switch 22 receives at its input 24 the operation 
confirming signal which is one logic level, for example, to close the 
switch 22, the positive feedback network 20 becomes effective so that the 
amplifier acts as an oscillator. 
As is clear from the prior art circuit, the operation confirming sound from 
the speaker 12 can be provided only when the amplifier 10 is effective. 
Therefore, with these circuit constructions, it is impossible to create 
the operation confirming sound at the time when the operation of the audio 
amplifier is interrupted to save power consumption, for example, during 
muting or squelching of the radio receiver. 
This invention overcomes the above prior art disadvantages. FIG. 3 is one 
embodiment of the invention, which comprises an audio power amplifier 30 
adapted to receive an audio signal at an input 32. The amplifier 30 
amplifies this audio signal so that the output from the amplifier 30 
drives a loud speaker 34 through a DC rejecting capacitor 36. One side of 
the speaker 34 is connected to the ground. The amplifier 30 also receives 
at its second input 38 an amplifier operation inhibiting signal, such as a 
muting or squelch signal from the radio receiver set. During this 
amplifier operation inhibiting signal applied to the input 38, the 
amplifier is made ineffective in the conventional way, so that even though 
there is the audio signal at the first input 32 the amplifier does not 
produce its output to the speaker 34. The amplifier is energized by B+ 
power supply at a terminal 40. It is known in the prior art to construct 
such amplifier arrangement with a conventional IC. 
In accordance with the principle of this invention, a switching means such 
as a transistor 42 is provided for applying switch current to the speaker 
34 in response to the operation confirming signal. Specially, in FIG. 3 a 
PNP transistor 42 is connected to the circuit in such a manner that its 
emitter is coupled to the B+ terminal and the collector is coupled to the 
ungrounded side of the speaker 34 through a diode 44 and a resistor 46. 
The anode of the diode 44 is connected to the collector of the transistor 
42 and the cathode of the diode is connected to the junction of the 
capacitor 36 and the ungrounded side of the speaker 34 through the 
resistor 46. The base of the transistor 42 receives the operation 
confirming signal at the terminal 48 through a resistor 50. The operation 
confirming signal is in the form of a sine wave or square wave for example 
and generated in response to the pushing of a key on the key-board or the 
actuation of a switch bottom of the radio set. During the ground and minus 
levels of the waveform, the transistor 42 is intermittently turned on. 
This produces intermittent current I passing from the B+ terminal 40 
through the conducted transistor 42, the diode 44, the resistor 46 and the 
voice coil of the speaker 34 to the ground. Please note that this current 
can flow even when the amplifier 30 is made ineffective. The intermittent 
current creates the operation confirming sound from the speaker 34. The 
diode is provided for a mere protection purpose, and therefore it is not 
an essential part for the invention. The resistor 46 limits the amount of 
the current I. 
FIG. 4 is another embodiment of the invention wherein a NPN transistor 42' 
is used in place of the PNP transistor 42 in FIG. 3 embodiment. 
Corresponding elements in FIG. 4 to those in FIG. 3 have respective dashed 
reference numerals. The one end of the voice coil of the speaker 34 is 
connected to the B+ power supply terminal 40' and the other end is 
connected to the collector of the NPN transistor 42 through the current 
limiting resistor 46' and the protection diode 44', of which emitter is 
grounded. The base of the transistor 42' is adapted to receive the 
operation confirming signal at the input 48' in the form of the sine or 
square wave which makes the transistor 42' to turn on intermittently to 
flow the intermittent current into the speaker voice coil.