Patent Document

BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to a shock sound prevention circuit used in a BTL (Balanced Transformer-less) audio amplifier, specifically to a shock sound prevention circuit for an audio amplifier using two operational amplifiers which can be biased with the same reference voltage. 
   2. Description of the Related Art 
   Some of electronic cameras such as video camcorders and digital still cameras are equipped with liquid crystal display monitors. When equipped with the liquid crystal display monitors, they need audio functions and require speaker functions. For example, a speaker function tells a user with an audio sound that a power is turned on, when the user turns the power on. 
   Such a sound is called a beep sound, and is generated to last about 100 ms after turning the power on. Many of the electronic cameras use low power supply voltage. Thus, they often use BTL method, which can secure wide dynamic range even at the low power supply voltage. The BTL method provides each of both ends of the speaker with each of a pair of signals having a phase opposite to each other, making it possible to secure the wide dynamic range. 
   An audio amplifier according to the BTL method is shown in FIG.  2 . An audio signal is applied to an input terminal  1 , as shown in  FIG. 2. A  terminal  2  and a terminal  3  are pins of an IC (integrated circuit). A capacitance C 2  is an external part of the IC. The capacitance C 2  is a blocking capacitor. The audio signal is once outputted from the IC, and then returned into the IC after DC (direct current) component is removed. 
   The audio signal returned into the IC is amplified by a first operational amplifier  4 . A gain of the first operational amplifier  4  is determined according to a feedback ratio determined by a ratio between a resistance R 9 and a resistance R 10 . The audio signal is applied to a negative input terminal (−) of the first operational amplifier  4 , thus a reverse audio signal is obtained at an output terminal  5 . A capacitance C 1  for reference voltage is charged from the IC upon turning-on of a power supply. A reference voltage is generated from a reference voltage generation circuit  6 , as the capacitance C 1  is charged. The reference voltage is applied as a bias voltage to positive input terminals (+) of the first operational amplifier  4  and a second operational amplifier  7 . 
   The audio signal generated at the output terminal  5  of the first operational amplifier  4  is applied to a negative input terminal of the second operational amplifier  7 . Thus, an audio signal with a phase reversed once again is obtained at an output terminal  8 . In other words, an audio signal returned to a normal phase is obtained. The second operational amplifier operates similar to the first operational amplifier  4 , and its gain is determined by a ratio between a resistance R 11  and a resistance R 12 . 
   Therefore, a normal audio signal is obtained at the terminal  10  while the reverse audio signal is obtained at the terminal  9 . A speaker  11  is driven by the two audio signals to make a sound such as the beep sound. 
   However, an audio amplifier usually generates a shock sound when the power supply is turned on. On that account, there arises a problem that the shock sound disturbs the beep sound. 
   With this being the situation, this invention offers a shock sound prevention circuit in the BTL audio amplifier as shown in  FIG. 2 , which enables a stable operation 100 msec after turning-on of the power supply. Usually the audio amplifier shown in  FIG. 2  is integrated in an IC. Use of capacitance, which takes up a large pattern area on the IC chip, can be minimized with this invention. 
   SUMMARY OF THE INVENTION 
   This invention is directed to solve the problem addressed above, and offers a shock sound prevention circuit including a capacitance for reference voltage which is charged upon turning-on of a power supply and generates a reference voltage, a first operational amplifier to which an audio signal input and the reference voltage are applied, a second operational amplifier to which a reverse signal of the audio signal input and the reference voltage are applied, a first and a second buffers to which the reference voltage is applied and a level comparator which compares a voltage of the power supply with the reference voltage, wherein the first and the second buffers are turned on while the first and the second operational amplifiers are turned off according to an output signal of the level comparator as the voltage of the power supply varies, so that output signals of the first and the second buffers are led out instead of output signals of the first and the second operational amplifiers, and the capacitance for reference voltage is used for the level comparator, the first and the second operational amplifiers and the first and the second buffers. 
   And this invention offers a shock sound prevention circuit including a capacitance for reference voltage which is charged upon turning-on of a power supply and generates a reference voltage, a first operational amplifier to which an audio signal input and the reference voltage are applied, a second operational amplifier to which a reverse signal of the audio signal input and the reference voltage are applied, a first and a second buffers to which the reference voltage is applied and a level comparator which compares a voltage of the power supply with the reference voltage, wherein the first and the second buffers are turned on while the first and the second operational amplifiers are turned off according to an output signal of the level comparator as the voltage of the power supply varies, while the capacitance for reference voltage is used for the level comparator, the first and the second operational amplifiers and the first and the second buffers. 
   This invention also offers a shock sound prevention circuit including an input terminal applied an audio signal from which DC component is removed, a terminal for reference voltage a reference voltage at which rises according to turning-on of a power supply, a first operational amplifier to which the reference voltage from the terminal for reference voltage is applied as a bias voltage and amplifies the audio signal from the input terminal, a second operational amplifier to which the reference voltage from the terminal for reference voltage is applied as a bias voltage and amplifies a reverse signal of the audio signal applied to the first operational amplifier, a first and a second buffers to which the reference voltage from the terminal for reference voltage is applied, a first output terminal to which an output of the first operational amplifier and an output of the first buffer are connected together, a second output terminal to which an output of the second operational amplifier and an output of the second buffer are connected together, a power supply voltage level detector to detect a voltage of the power supply, a level comparator which compares an output voltage of the power supply voltage level detector with the reference voltage from the terminal for reference voltage and a control circuit which generates a control signal to turn on the first and the second buffers and turn off the first and the second operational amplifiers immediately after turning-on of the power supply and to turn off the first and the second buffers and turn on the first and the second operational amplifiers after a predetermined time according to an output signal of the level comparator, wherein the reference voltage from the terminal for reference voltage is used as the bias voltage for the first and the second operational amplifiers and the first and the second buffers, and a change in the reference voltage from the terminal for reference voltage is used to generate the predetermined time for the control circuit. 
   Furthermore, this invention offers a shock sound prevention circuit including an input terminal applied an audio signal from which DC component is removed, a terminal for reference voltage a reference voltage at which falls according to turning-off of a power supply, a first operational amplifier to which the reference voltage from the terminal for reference voltage is applied as a bias voltage and amplifies the audio signal from the input terminal, a second operational amplifier to which the reference voltage from the terminal for reference voltage is applied as a bias voltage and amplifies a reverse signal of the audio signal applied to the first operational amplifier, a first and a second buffers to which the reference voltage from the terminal for reference voltage is applied, a first output terminal to which an output of the first operational amplifier and an output of the first buffer are connected together, a second output terminal to which an output of the second operational amplifier and an output of the second buffer are connected together, a power supply voltage level detector to detect a voltage of the power supply, a level comparator which compares an output voltage of the power supply voltage level detector with the reference voltage from the terminal for reference voltage and a control circuit which generates a control signal to turn on the first and the second buffers and turn off the first and the second operational amplifiers immediately after turning-off of the power supply according to an output signal of the level comparator, wherein the reference voltage from the terminal for reference voltage is used as the bias voltage for the first and the second operational amplifiers and the first and the second buffers, and a change in the reference voltage from the terminal for reference voltage is used to generate the control signal of the control circuit. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a circuit diagram showing a shock sound prevention circuit according to an embodiment of this invention. 
       FIG. 2  is a circuit diagram showing a BTL audio amplifier. 
       FIG. 3  is a concrete circuit diagram showing a buffer, an operational amplifier and a control circuit according to the embodiment of this invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Next, an embodiment of this invention will be explained in detail, referring to the figures. As shown in  FIG. 1 , a reference voltage from a capacitance C 1  for reference voltage is inputted to a buffer  12 , which outputs a reference voltage of the same voltage as the input voltage. The reference voltage from the buffer  12  is applied as an input voltage to a first buffer  13  and a second buffer  14 . 
   An output of a first operational amplifier  4  and an output of the first buffer  13  are connected together to a terminal  9 . An output of a second operational amplifier  7  and an output of the second buffer  14  are connected together to a terminal  10 . Resistances R 6 , R 7  and R 8  divide a power supply voltage Vcc according to ratios of the resistances to generate reference voltages at a node A and at a node B. 
   A first level comparator  15  consists of transistors Q 4  and Q 5  and a resistance R 4 , compares the reference voltage from the capacitance C 1  for reference voltage with the reference voltage at the node B, and applies the results to bases of transistors Q 3  and Q 6 . A second level comparator  16  consists of transistors Q 9  and Q 10  and a resistance R 5 , compares the reference voltage from the capacitance C 1  for reference voltage with the reference voltage at the node A, and applies the result to a base of a transistor Q 8 . 
   A current mirror circuit  17  charges the capacitance C 1  for reference voltage with a current equal to a collector current of the transistor Q 3 , which is limited by a resistance R 3 . A control circuit  19  generates a control signal corresponding to turning-on of a transistor Q 7  or a transistor Q 8 . A switch  20  selects between a terminal a and a terminal b according to a control signal from the control circuit  19 . An operating current source  21  provides either of the first operational amplifier  4  and the second operational amplifier  7  or the first buffer  13  and the second buffer  14  with an operating current through the switch  20 . 
   Note that a common element is given the same symbol in  FIG. 1  as in FIG.  2 . 
   In the circuit shown in  FIG. 1 , the reference voltage from the buffer  12  is applied not only to the first operational amplifier  4  and the second operational amplifier  7 , but also to the first buffer  13  and the second buffer  14 . 
   And the first buffer  13  and the second buffer  14  are enabled while the first operational amplifier  4  and the second operational amplifier  7  are disabled immediately after turning-on of the power supply. Output voltages of the first buffer  13  and the second buffer  14  are exactly the same as an output voltage of the buffer  12 . 
   Hence, exactly the same DC voltages are generated at the terminal  9  and at the terminal  10 , thus no current flows through the speaker  11  and no sound is made. Because the output terminals of the first buffer  13  and the second buffer  14  have low impedances, and because both of the first operational amplifier  4  and the second operational amplifier  7  are disabled, they are not affected by the signal from the terminal  3  or noise due to rising power supply voltage. 
   This status is kept for duration of time that a shock sound occurs, during which the output voltage of the buffer  12  rises to a bias level Vref 1  which the first operational amplifier  4  and the second operational amplifier  7  need. Until then, the switch  20  is turned to the terminal a. Meantime, the first buffer  13  and the second buffer  14  charge the blocking capacitor C 2  to Vref 1  through the resistances R 9  and R 10 . 
   Because the operation described above makes DC voltages at the terminal  3 , at the terminal  9  and at the terminal  10  equal, no current flows through the speaker  11  when the switch  20  turns to the terminal b and the first operational amplifier  4  and the second operational amplifier  7  begin their operation. Since the DC voltages at the terminal  3 , at the terminal  9  and at the terminal  10  are equal, an AC (alternating current) audio signal from the terminal  2  is applied to the first operational amplifier  4  through the blocking capacitor C 2  and the resistance R 9 . An output signal of the first operational amplifier  4  is applied to the second operational amplifier  7  through the resistance R 11 , since the first buffer  13  is disabled. 
   Therefore, a normal audio signal is obtained at the terminal  10  while the reverse audio signal is obtained at the terminal  9 . A speaker  11  is driven by the two audio signals to make a sound such as the beep sound. 
   Next, how the reference voltage of the buffer  12  rises and how the switch  20  alternates between the terminal a and the terminal b will be explained. Now a voltage at a power supply line  25  in  FIG. 1  is assumed to be switched from a ground to Vcc. 
   Then, reference voltages are immediately generated at the node A and at the node B. The reference voltage at the node A is applied to a base of the transistor Q 10  and the reference voltage at the node B is applied to a base of the transistor Q 5 . Resistances R 1  and R 2  are set to have high value of resistance of about 50KΩ. 
   This is to remove ripples on the power supply line  25 . The reference voltage of the capacitance C 1  for reference voltage needs to be stable, because it is used as a bias voltage for the operational amplifiers. Consequently, a voltage at a terminal  26  is low at first. Therefore, the transistor Q 4  is turned on while the transistor Q 5  is turned off. When the transistor Q 4  is tuned on, the transistor Q 7  is turned on. 
   A voltage at a base of the transistor Q 9  is low because the voltage at the terminal  26  is low. Therefore, the transistor Q 9  is turned on while the transistor Q 10  is turned off. With the transistor Q 7  turned on, the control circuit  19  turns the switch  20  to the terminal a. 
   As a result, the first buffer  13  and the second buffer  14  are enabled while the first operational amplifier  4  and the second operational amplifier  7  are disabled immediately after turning-on of the power supply. 
   How long this status should last is determined according to timings of generation of the shock sound and the beep sound. Duration of this status is determined by a charging rate of the capacitance C 1  for reference voltage. That is, the status changes when the voltage charged to the capacitance C 1  for reference voltage exceeds the reference voltage at the node B. Since the resistance R 1  has the high value of resistance, charging only through the resistance R 1  takes a long time. 
   Thus, the charging rate to the capacitance C 1  for reference voltage is adjusted by the current mirror circuit  17  in the circuit shown in FIG.  1 . The transistor Q 3  is turned on at the same time the transistor Q 7  is turned on. When the transistor Q 3  is turned on, the current mirror circuit  17  is put in operation, and a current equal to a current flowing through the resistance R 3  flows into the capacitance C 1  for reference voltage. A conversion timing of the first level comparator  15  can be chosen freely, since the current flowing through the current mirror circuit  17  is determined by the resistance R 3 . 
   As described above, charging to the capacitance C 1  for reference voltage is made through the resistance R 1  and the current mirror circuit  17  to raise the voltage at the terminal  26 . When the voltage charged to the capacitance C 1  for reference voltage exceeds the reference voltage at the node B, the transistor Q 4  is turned off and the transistor Q 5  is turned on. When the transistor Q 5  is turned on, the transistor Q 6  is turned on and the reference voltage at the node B is turned to a low level. 
   This is made to reduce an inversion time of the first level comparator  15 . When the transistor Q 4  is turned off, the transistors Q 3  and Q 7  are turned off. When the transistor Q 3  is turned off, the charging made through the current mirror circuit  17  comes to a halt. With the transistor Q 7  turned off, the control circuit  19  turns the switch  20  to the terminal b. 
   As a result, the first buffer  13  and the second buffer  14  are disabled while the first operational amplifier  4  and the second operational amplifier  7  are enabled. At that time, the transistor Q 9  remains on and the transistor Q 10  remains off, since the reference voltage at the node A is set at high enough voltage compared with the voltage charged to the capacitance C 1  for reference voltage. 
   The capacitance C 1  for reference voltage is disposed primarily to apply the bias voltage to the first operational amplifier  4  and the second operational amplifier  7 . In the circuit shown in  FIG. 1 , the capacitance C 1  for reference voltage is also used to generate a predetermined duration (to keep the operational amplifiers disabled and the buffers enabled). The dual-purpose use of the capacitance C 1  eliminates the need for an additional capacitance for setting the duration, and reduces the number of elements, benefiting implementation of the circuit in an IC. 
   When the power supply is turned on, the shock sound does not occur because the first buffer  13  and second buffer  14  are enabled and the first operational amplifier  4  and the second operational amplifier  7  are disabled, as described above. 
   Next, turning off of the power supply is explained. When the power supply voltage falls to 0V, an abnormal waveform is applied to the terminal  2 . The abnormal waveform goes through the first operational amplifier  4  and the second operational amplifier  7  to the speaker where a shock sound occurs according to the abnormal waveform. Thus in the circuit shown in  FIG. 1 , utilizing the system to prevent the shock sound at the turning-on of the power supply, the first buffer  13  and the second buffer  14  are used to prevent the shock sound also when the power supply is turned off. 
   Now the voltage at the power supply line  25  in  FIG. 1  is assumed to be switched from Vcc to the ground. Then the voltages at the node A and at the node B fall immediately to the ground. When the voltage at the node A falls to the ground, the transistor Q 9  is turned off and the transistor Q 10  is turned on. The voltage at the terminal  26  is lowered gradually at low rate, since the capacitance C 1  for reference voltage is discharged through the resistance R 2 . 
   When the transistor Q 10  is turned on, the transistor Q 8  is turned on and the control circuit  19  turns the switch  20  to the terminal a. Voltages at the terminal  9  and at the terminal  10  are determined by a DC voltage of the buffer  12 . Therefore, the voltages at the terminal  9  and at the terminal  10  are not affected even the abnormal waveform is applied from the terminal  2 . 
   When the voltage at the node B falls to the ground, the transistor Q 4  is turned off and the transistor Q 5  is turned on. When the transistor Q 4  is turned off, the transistor Q 3  is turned off and the current mirror circuit  17  stops charging. 
   As a result, the shock sound at turning-off of the power supply also can be prevented according to the circuit shown in FIG.  1 . 
   An example of a concrete circuit for a case in which the first buffer  13 , the second buffer  14 , the first operational amplifier  4  and the second operational amplifier  7  in  FIG. 1  are driven by the control circuit  19  is shown in FIG.  3 . 
   The control circuit  109  in  FIG. 3  includes a voltage source  30  and a switch  31 , which flips to a-side when the buffer  32  is to be enabled and flips to b-side when the operational amplifier  33  is to be enabled. When the switch  31  flips to a-side, a voltage from the voltage source  30  is applied to transistors  34  and  35  to turn them on. When the transistors  34  and  35  are turned on, transistors  36  and  37  are turned on to put a differential amplifier  38  in operation. 
   The reference voltage VrefIN from the buffer  12  in  FIG. 1  is applied to an input terminal  39  of the differential amplifier  38 . Since the differential amplifier  38  makes 100% negative feedback operation, the same voltage as the voltage at the input terminal  39  is obtained at an output terminal  40 . Meantime, the operational amplifier  33  halts its operation since transistors  41  and  42  to supply an operating current are turned off. 
   When the switch  31  flips to b-side, the voltage from the voltage source  30  is applied to transistors  41  and  42  to turn them on. When the transistors  41  and  42  are turned on, a transistor  43  is turned on to put a differential amplifier  44  in operation. The reference voltage VrefIN from the buffer  12  in  FIG. 1  is applied to an input terminal  45  of the differential amplifier  44 . The differential amplifier  44  makes amplification with a gain defined by a ratio between resistances  46  and  47 . A signal from an input terminal  48  is amplified and led out to an output terminal  49 . 
   As described above, the buffer and the operational amplifier can be alternatively put in operation, according to the circuit in FIG.  3 . 
   According to this invention, the capacitance for reference voltage can also be used for setting the bias voltage for the first and the second buffers and the first and the second operational amplifiers, benefiting the integration of the circuit in an IC by reducing the number of capacitances. 
   And according to this invention, there arises no shock sound, since the first and the second buffers are turned on and the first and the second operational amplifiers are turned off immediately after turning-on of the power supply and the voltage charged to the capacitance for reference voltage is applied to the first and the second buffers to make the voltages at both ends of the speaker equal. Also according to this invention, only with addition of a level comparator, the same function used at the turning-on of the power supply is used to turn on the first and the second buffers and turn off the first and the second operational amplifiers to make the changes in the voltages at the both ends of the speaker exactly the same so that the speaker makes no shock sound at the turning-off of the power supply. 
   Furthermore, according to this invention, the duration of switchover from the buffers to the operational amplifiers can be adjusted, since there is a charge-boosting circuit to boost charging to the blocking capacitor according to the output signal of the level comparator.

Technology Category: 5