Power amplifier

Abstract: A power amplifier having a standby switch according to the present invention comprises an amplifier circuit (108), a bias circuit (109) serving as a bias source of the amplifier circuit a detector circuit (104) for detecting a variation in a power source voltage to turn on or off the standby switch, a standby circuit (106) for extinguishing a waveform of an output voltage of the amplifier circuit the moment that the standby switch is turned off, and a delay circuit (107) for cutting off the bias source of the amplifier circuit after a lapse of a predetermined time period after the standby switch is turned off.

Description: The present invention relates to an integrated-circuit power amplifier of a bridge transformer less (BTL) type having a standby switch which is particularly used in audio systems. Recently, most of integrated-circuit power amplifiers used in audio systems have a BTL structure as shown in Fig. 1 in order to achieve high power. In the BTL structure, output voltages V₁ and V₂ of amplifier circuits 101 and 102 have phases opposite to each other and the same amplitude. Voltage V applied to load 103 such as a speaker is thus expressed as follows. V = V 1 - V 2 = V 1 - (- V 1 ) = 2V 1 where V₁ = -V₂. The amplitude in the voltage applied to load 103 is about twice as large as the ordinary amplitude. When the amplitudes of output voltages V₁ and V₂ of amplifier circuits 101 and 102 are increased in the same phase, no output waveform appears; accordingly no voltage is applied to load 103. The BTL type power amplifier usually includes a standby switch for not directly turning on or off a power source but turning on or off an internal transistor circuit and cutting off a circuit such as a bias source to equivalently turn on or off amplifier circuits 101 and 102. Therefore, no large-capacity relay or switch is required, and amplifier circuits 101 and 102 can be turned on or off by a small-power switch or a signal supplied from a microcomputer. Further, in the BTL type power amplifier, it is necessary to eliminate noise (i.e., pop) caused in the transition from the on-state to the standby state (in which the power source is on but the bias source is off and the amplifier circuits are equivalently in the off-state) of amplifier circuits 101 and 102 and to avoid a noise-remaining phenomenon in which the noise remains for a short period of time after the on-state has been changed to the standby state by the time constant of an external capacity. In the transition, therefore, the bias source is not turned off instantaneously but after the amplitudes of output voltages V₁ and V₂ of amplifier circuits 101 and 102 are quickly increased in the same phase so as to reach the power source voltage or ground voltage (or negative power source voltage) and then the output signals of the amplifier circuits are extinguished. The BTL type power amplifier having the above structure is used as an audio mute for eliminating noise caused when a tuner is detuned. If the power amplifier malfunctions, its on-state is changed to the standby state to prevent noise from generating from speakers. It is more effective when a system such as tuner and amplifier malfunctions earlier than the power amplifier itself by a sudden change of the power source voltage as in audio systems installed in automobiles. More specifically, when a power source (Vcc) line is monitored and its voltage is dropped to such an extent that the system cannot be normally operated, a standby signal is supplied to the power amplifier. If a microcomputer includes a low-voltage detecting function, a logic signal is supplied to the power amplifier to render it in the standby state. When the power source voltage is dropped even in the standby state, a battery line may be cut off by the main key switch of, for example, an audio system installed in an automobile. In this state, a circuit for maintaining the standby state cannot be normally operated and a pop may thus be generated in the output signals of the amplifier circuits. The detection of a decrease in a power source voltage using a power-source line monitor arranged at the power amplifier side is very difficult since a detected voltage and detection timing vary from system to system. If the bias source is instantaneously cut off in the transition from the on-state to the standby state, a pop may be generated by the influence of the external capacity. EP-A-0 299 665 discloses a power amplifier circuit having a stand-by state, comprising: an amplifier circuit; a bias circuit serving as a bias source of said amplifier circuit; a standby circuit by which two output signals of said power amplifier cancel each other with respect to the output load due to the fact that they are driven to approach the same value the moment the stand-by switch is turned off, wherein said stand-by switch cuts off said bias source from said amplifier circuit in accordance with signals output from a stand-by control circuit, and the amplifier further comprises a delay circuit for delaying the change of the amplifier means from the operation state to the standby state for a predetermined time. US-A-3 889 202 discusses the advantages of a muting circuitry for a power amplifier e.g. in the case when the voltage of the power source varies too much. According to this document a detector is used for detecting a variation in a power source voltage to turn on or off the muting circuitry. It is the object of the present invention to provide a power amplifier in which a variation (decrease) of the power source voltage can be detected and to prevent noise in the standby state and when the power source voltage is lowered. This object is solved by the power amplifier of claim 1. The subclaims 2 to 5 show further developments of the power amplifier of claim 1. The invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a view showing the principle of a conventional power amplifier having a BTL structure; Fig. 2 is a block diagram showing the principle of a power amplifier according to an embodiment of the present invention; and Fig. 3 is a circuit diagram specifically showing a delay circuit and a bias circuit of the power amplifier shown in Fig. 2. An embodiment of the present invention will be described with reference to the accompanying drawings. Fig. 2 is a block diagram showing the principle of a power amplifier according to an embodiment of the present invention. Power source line monitor circuit 104 is connected to a power source (Vcc) line and turns on or off standby switch 105 in accordance with variations in a power source voltage. Standby switch 105 is turned on or off by signals output from microcomputer 111. Reference voltage V₁ is applied to standby circuit 106 and delay circuit 107 by means of standby switch 105. Standby circuit 106 is connected to amplifier circuit 108. Delay circuit 107 is connected to bias circuit 109 serving as a bias source of amplifier circuit 108. Bias circuit 109 is connected to amplifier circuit 108 having a BTL structure. Load 110 such as a speaker is connected to an output of amplifier circuit 108. When power source line monitor circuit 104 detects that the power source voltage has been dropped by a predetermined value, standby switch 105 is turned off, irrespective of the signals output from microcomputer 111. The instant that standby switch 105 is turned off, standby circuit 106 increases the amplitude of an output voltage of amplifier circuit 108 in the same phase so as to reach a power source voltage or a ground voltage (or negative power source voltage) to extinguish an output waveform of amplifier circuit 108. At the same time, delay circuit 107 is operated to delay the signal by predetermined time by the time constant depending upon capacitance C₁ of capacitor 1, and then the bias source of amplifier circuit 108 is cut off. In the embodiment of the present invention, since delay circuit 107 is connected between standby switch 105 and bias circuit 109, no pop is generated on an output signal when the power source voltage is decreased after a transfer to the standby state. In other words, the time constant of delay circuit 107 has only to be preset so as to always cut off the bias source before standby circuit 106 for maintaining the standby state is disabled from normally operating. Fig. 3 illustrates arrangements of delay circuit 107 and bias circuit 109 of the power amplifier shown in Fig. 2. Reference voltage V₁ is applied to the bases of npn transistor Q₁ and Q₂ through standby switch 105. The emitters of transistors Q₁ and Q₂ are connected to ground (GND). The collector of transistor Q₁ is connected to the base of pnp transistor Q₃ through resistor 3. The emitter of transistor Q₃ is connected to a power source (Vcc) line. The collector of transistor Q₃ is connected to ground through resistors 4 and 5 and diode-connected transistors Q₄ and Q₅. Capacitor 2 is connected between a ground and a connecting point of resistors 4 and 5 and also connected to the base of npn transistor Q₆. The collector of transistor Q₆ is connected to the power source line. The emitter of transistor Q₆ is connected to ground through resistor 6 and diode-connected transistors Q₇ and Q₈. The base of npn transistor Q₉ is connected between resistor 6 and diode-connected transistors Q₇ and Q₈. The emitter of transistor Q₉ is connected to ground through resistor 7. The collector of transistor Q₉ is connected to the base and collector of pnp transistor Q₁₀. The emitter of transistor Q₁₀ is connected to the power source line. The base of transistor Q₁₀ is connected to the base of pnp transistor Q₁₁. The emitter of transistor Q₁₁ is connected to the power source line. The collector of transistor Q₁₁ is connected to the collector of transistor Q₂ and the base of npn transistor Q₁₂ and also connected to ground through capacitor 1. The emitter of transistor Q₁₂ is grounded. The collector of transistor Q₁₂ is connected between resistor 5 and diode-connected transistors Q₄ and Q₅ and also connected to the base of npn transistor Q₁₃. The emitter of transistor Q₁₃ is connected to ground through resistor R8. Referring to Figs. 2 and 3, an operation of the power amplifier will be described in detail. When amplifier circuit 108 is in an on-state, i.e., when the amplifier circuit generates an output signal, standby switch 105 is closed and reference voltage V₁ is applied to transistors Q₁ and Q₂. Transistors Q₁ and Q₂ are thus rendered in an on-state and transistor Q₃ is turned on. Transistors Q₄ to Q₁₁ and Q₁₃ are turned on, accordingly. Transistor Q₁₂ is however cut off since transistor Q₂ is turned on. This is because the voltage between the base and emitter of transistor Q₁₂ corresponds to a saturated voltage between the collector and emitter of transistor Q₂ and therefore it can be regarded as ≈ 0. If a power source voltage is decreased below a predetermined reference value, this decrease is detected by power source line monitor circuit 104. Power source line monitor circuit 104 causes an external trigger to turn off standby switch 105 and, in this time, transistors Q₁ to Q₃ are instantaneously turned off. However, transistors Q₄ to Q₁₁ and Q₁₃ are kept in an on-state for a while since the charges of capacitor 2 are maintained by the time constant of about C₂ (capacitance of capacitor 2) × R₅ (resistance of resistor 5). When transistor Q₁₁ is turned on, collector current I CQ11 is expressed by the following equation. I CQ11 = (V BEQ7 + V BEQ8 - V BEQ9 )/R 7 ≃ V BE /R 7 where R₇ is a resistance of resistor R₇, V BEQ7 , V BEQ8 and V BEQ9 are voltages between the bases and emitters of transistors Q₇, Q₈ and Q₉, and V BEQ7 = V BEQ8 = V BEQ9 = V BE . The collector current is thus a constant current. When transistor Q₂ is turned on, collector current I CQ11 is connected to ground through transistor Q₂. When transistor Q₂ is turned off, capacitor 1 starts to be charged. Transistor Q₁₂ can thus be turned on when electric charge q arrives at the time satisfying the following condition. q = C 1 · V BEQ12(ON) = I CQ11 · t t = {C 1 · V BEQ12(ON) }/I CQ11 where C₁ is a capacitance of capacitor 1, and V BEQ12(ON) is a base-to-emitter voltage for turning on transistor Q₁₂. Transistor Q₁₂ is thus turned on after a lapse of time t. If transistor Q₁₂ is turned on, transistor Q₁₃ is turned off and accordingly the bias source of the amplifier circuit is cut off. If delay time t is set so that the bias source can be cut off before a decrease in the power source voltage prevents standby circuit 106 from normally operating, no abnormal sound occurs. The following relationship is however required. t << C 2 × R 5 {C 1 · V BEQ12 (ON)}/I CQ11 << C 2 × R 5 With the arrangement described above, the bias source cannot be cut off instantaneously but after a lapse of predetermined delay time t after the waveform of an output signal is extinguished by standby circuit 105. An output signal of amplifier circuit 108 can thus be extinguished without any remaining sound or uncomfortable pop. Furthermore, after the bias source of amplifier circuit 108 is cut off and the other bias sources are then cut off, the whole system including the power amplifier is completely turned off. Delay time t is always set shorter than the time in which standby circuit 106 is disabled from normally operating when the power source voltage is decreased after the on-state is changed to the standby state. Even if standby circuit 106 is turned off by the decrease in the power source voltage, the bias source of amplifier circuit 108 is cut off. Therefore, amplifier circuit 108 does not malfunction, nor is any abnormal sound generated from amplifier circuit 108. In other words, a system for reducing an output voltage and preventing an abnormal sound from generating when an output is cut off, can be formed in a power amplifier, an audio system or the like having a standby switch. Reference signs in the claims are intended for better understanding and shall not limit the scope.

Claims: A power amplifier having a standby switch (105) comprising: an amplifier circuit (108); a bias circuit (109) serving as a bias source of said amplifier circuit; and a standby circuit (106) by which two output signals of said power amplifier cancel each other with respect to the output load due to the fact that they are driven to approach the same value the moment the standby switch is turned off, wherein said standby circuit (106) remains fully operational for a predetermined time period in case the power source voltage drops below the point where the power amplifier and the standby circuit can still be normally operated and said standby switch (105) cuts off said bias source (109) from said amplifier circuit (108) in accordance with signals output from a stand-by control circuit (111), characterized in , that a power line monitoring circuit as detector circuit (104) monitors decrease of a power source voltage supplied to said amplifier circuit and bias circuit, and, when said power line monitoring circuit detects that the power source voltage has been dropped by a predetermined value, said standby switch (105) is turned off, irrespective of the signals output from a microcomputer (111) as said stand-by control circuit; and a delay circuit (107) cuts off said bias source (109) from said amplifier circuit (108) after a lapse of a time period after the standby switch (105) is turned off, said time period being shorter than said predetermined time period during which the power amplifier and the standby circuit can still be normally operated. The power amplifier according to claim 1, characterized in that said amplifier circuit (108) comprises an amplifier circuit in bridge configuration having first and second output voltages. The power amplifier according to any of the preceding claims, characterized in that said standby circuit (106) maximizes, in phase, the two outputs of the amplifier circuit (108) in bridge configuration in towards the same value, thus minimizing the output signal. The power amplifier according to any of the preceding claims, characterized in that said detector circuit (104) switches said standby switch (105) so as to switch said power amplifier to a stand-by mode when the power source voltage falls below a predetermined value. The power amplifier according to any of the preceding claims, characterized in that said microcomputer (111) controls said standby switch (105) in a normal operating state.

Inventors: IMANISHI KAZUO C O INTELLECTUA, IMANISHI, KAZUO, IMANISHI, KAZUO, C/O INTELLECTUAL PROP. DIV.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1990
Main classification: H03F1
IPC codes: H01L21, H03F1, H01L27