AUDIO AMPLIFIER SWITCH-ON CONTROL CIRCUIT

Abstract: A capacitor (C₁) which is progressively chargeable in response to the switching on of the equipment, however brought about, is associated with an oscillating circuit (CB) with intervention threshold. As the load voltage of said capacitor (C₁) increases the oscillating circuit (CB) shifts from a first to a second operating state, generating a switch-on current for the audio amplifier (A). The latter can be placed in stand-by condition by discharging to earth said capacitor (C₁) by means of a suitable switch (IS).

Description: The present invention concerns an audio amplifier switch-on control circuit. It is known that in audio, TV and radio equipment, interference can be produced, upon switching on, at the audio amplifier outlet, that is in the loudspeaker, due to settling defects in the upstream circuits. Such disturbances are commonly called "switch-on noise". This inconvenience can be got round by using a so-called "muting" circuit, which controls the switch-on time of the audio amplifier so as to make it become dynamic only after the circuits upstream have duly settled. In other words, the audio amplifier is "muted", that is the input audio signal is prevented from reaching the loudspeaker, as long as the circuits upstream remain in a condition such as to generate switch-on noise. It is also known that in such equipment the "stand-by" function is sometimes provided, which means holding the equipment in a waiting condition with minimum absorption, which allows subsequent turning on again of the power part by nonpower telecontrol or switch. Switching on again after "stand-by" must naturally take place without noise production, that is with an appropriate delay in audio amplifier switch-an time. An audio-frequency power amplifier provided with a control circuit according to the preamble of claim 1 is disclosed in EP-A-0 043 707. The primary object of the present invention is to accomplish a new audio amplifier control circuit capable of appropriately delaying the audio amplifier switch-on time upon each new switching on of the equipment, however controlled. Another object of the present invention is to accomplish an audio amplifier control circuit also capable of causing switching off, and then correct switching on again, of the audio amplifier after a "stand-by" command. In view of these objects, the control circuit according to the invention is realized as set forth in claim 1. It appears evident that the time necessary for the load voltage of the capacitor to reach first the intervention threshold of the oscillating circuit and then carry the circuit into the second operating state represents the switch-on delay of the audio amplifier after the instant the equipment is switched on. This delay time can of course be selected so as to be certainly greater than the time necessary for the settling of the circuits which generate the audio signal. The bothersome switch-on noise is thus avoided. The control circuit according to the invention also makes possible turning off of the audio amplifier, with a resulting reduced current absorption, following a stand-by command. To accomplish this it is sufficient to short circuit and hence discharge to earth, by means of an appropriate switch, the capacitor which controls the oscillating circuit. By so doing, the oscillating circuit is caused to return to the initial state, taking away switching-on current from the audio amplifier and thereby causing it to be extinguished. Upon switching on again after stand-by, that is upon reopening of the aforesaid switch, the capacitor can also recharge itself and return, with the usual preset time delay, the oscillating circuit to the state of generation of the switch-on current for the audio amplifier. These and other characteristics of the present invention will be made evident by the following detailed description of two possible forms of practical accomplishment illustrated as examples in the annexed drawings, wherein:- Fig. 1 shows a control circuit according to the invention, associated with an audio amplifier with individual power supply; Fig. 2 shows in greater detail the control circuit of Fig. 1; Figs. 3-5 show curves representing voltage and current variation in time in the control circuit of Figs. 1 and 2; Fig. 6 shows a variant of the control circuit according to the invention, planned for use with an audio amplifier with double power supply; Fig. 7 shows a graph representing the variations in time of voltages and currents in the control circuit of Fig. 6. With reference to Fig. 1, A indicates an audio amplifier equipped with a positive input E, a negative input F and an output L for control of a loudspeaker AP. A feedback circuit between output L and input F includes a resistance R₅. A capacitor C₄ is inserted between the output L and the loudspeaker AP. The positive input E is connected by a resistance R₆ to a circuital node N connected to the main power supply V cc and to earth by a resistance R₃ having a capacitor C₂ in parallel. The input E is also connected by a capacitor C₅ to an audio signal input lN, to which arrives the audio signal processed by the upstream circuits, generally represented and indicated by GS. The negative input F is in turn connected to the power supply V cc through a current generator G₁ which generates a current I₁, and an earth through the series of a resistance R₄ and a capacitor C₃. Switching on of the amplifier A is provided for by a current generator G₂, which generates a switch-on current I₂. Current generators G₁ and G₂ are controlled by a circuital block B, which in turn is controlled by the voltage in a circuital node D connected to the power supply V cc through a resistance R₁ and an earth through a capacitor C₁. In parallel with the latter is indicated a stand-by switch IS. A more detailed illustration of the circuital block B, as well as the generators G₁ and G₂ controlled by it, is given in Fig. 2, in which it is seen that circuital node D is connected to the base, connected to earth by a zener diode Z₁, of an n-p-n transistor T₁ the emitter of which is connected to earth through the series of a resistance R₇ and a diode D₁ and the collector connected to the positive power supply V cc through a diode D₂. The collector of transistor T₁ is also connected to the base of a p-n-p transistor T₂, the emitter of which is connected to the power supply V cc and the collector connected to the base of an n-p-n transistor T₈, the collector of which is connected to the power supply V cc , the base connected to earth through a zener diode Z₂ and the emitter connected to earth through the series of two resistances R₈ and R₉ and a diode D₆. The base of transistor T₁ and a circuital node M intermediate between the two resistances R₈ and R₉ are connected, the first through a diode D₃ and the second directly, to the bases of their respective n-p-n transistors T₃ and T₇, which together form an oscillating circuit CB shiftable from one to the other of two operating states under the control of the voltages in nodes D and M. More precisely, the two transistors T₃ and T₇ have their respective emitters connected through equal resistances R₁₀ and R₁₁ to the collector of an n-p-n transistor T₆, the emitter of which is connected to earth and the base of which is connected to earth and of which the base is connected to a node intermediate between resistances R₉ and diode D₆. The collectors of transistors T₃ and T₇ are also connected to the positive power supply V cc through their respective diodes D₄ and D₅. To the collectors of transistors T₃ and T₇ are also connected the bases of their respective p-n-p transistors T₄ and T₅, the emitters of which are connected to the power supply V cc and perform the functions of the current generators G₂ and G₁ of Fig. 1, emitting the respective collector currents I₂ and I₁. The manner of operation of the control circuit of Figs. 1 and 2 can be understood with the help of the illustrative charts shown in Figs. 3-5. As shown in Fig. 3, from the moment of switching on of the equipment (t = 0) the positive power supply V cc causes through resistance R₁ the gradual charging of capacitor C₁ and the consequent increase in the charging voltage V D (voltage in circuital node D) in accordance with the exponential curve shape shown in said figure. As long as V D remains below an intervention threshold represented by a value V₁ equal to the sum of the voltages V BE (base-emitter voltage) of the transistor T₁ and the diode D₁ the control circuit remains cut off and with it the audio amplifier A. At the moment when V D = V₁ occurs (instant T₀ in Fig. 3), T₁ is placed in conduction and through the mirror D₂ , T₂ causes switching on of the zener Z₂ and the consequent conduction of transistor T₈. In node M is therefore established a steady voltage V M , which puts the oscillating circuit CB in a first operating state in which transistor T₃ is cut off and transistor T₇ conducts, causing through D₅, T₅ the generation of a substantially constant current I₁. The current I₂ is substantially nil, transistor T₄ being of course cut off. As capacitor C₁ continues to be charged, when the charging voltage V D (instant T₁ in Fig. 3) reaches a value V₂ equal to V M less the voltage V BE of transistor T₇ and the voltage drop on R₁₁, plus the voltages V BE of transistor T₃ and diode D₃, transistor T₃ goes gradually into conduction, subtracting current from transistor T₇ and also causing through D₄, T₄ the generation of a current I₂, which increases in the same way that I₁ decreases, keeping the relation I₁ + I₂ = constant. The two currents, whose curve shape is shown in graphs I₁ and I₂ of Fig. 3, will be equal when V D reaches a value V₃ equal to V M plus the V BE of diode D₃ (instant T₂ in Fig. 3). As V D continues to increase, transistor T₇ goes gradually into cutoff, while T₃ reaches maximum conduction, and the oscillating circuit CB is consequently shifted into a second operating state in which I₁ is nil and I₂ has reached the maximum value shown in Fig. 3 (instant T₃). The described variation of currents I₁ and I₂ under the control of tie circuital block B causes the following behaviour of the audio amplifier A in response to the switching on of the equipment. The current generator G₁ is connected to the inverting input F of the amplifier A and starting from instant T o charges with current I₁ gain capacitor C₃ up to a maximum value which remains substantially constant until the moment (instant T₃) when current I₁ is cancelled and capacitor C₃ consequently discharges progressively on resistance R₅, tending toward the value V cc/2 . The voltage curve (V F ) to the above inverting input is shown in Fig. 4. The current generator G₂, which is assigned to switching on of the amplifier A, remains in turn cut off until instant T₁ , consequently keeping the audio amplifier cut off. From instant T₁ it begins to deliver a progressively increasing current I₂, which gradually causes switching on of the amplifier, completing it at instant T₃. In the meantime however, as shown in Fig. 5, the inverting input F of the amplifier A was held at a voltage V F higher than that (V E ) created by the noninverting input E from the polarization group R₂ , R₃, R C (appropriately selected). Even with the amplifier switched on, the output voltage V L of the audio amplifier therefore remains nil and there is no signal at the loudspeaker AP. As shown in Fig. 5, voltage V L rises to a higher level only when voltage V F at the inverting input F falls to the level of voltage V E at the noninverting input E, that is at instant T₄. From that moment onward the audio amplifier begins to deliver current to the load, that is to the loudspeaker AP. The audio amplifier then becomes dynamic with a time delay T₄ after the instant the equipment is switched on. For all of this time the loudspeaker does not receive signals and hence does not produce sound. The upstream circuits, which generate the audio signal for the input lN, thus have time to settle. As already mentioned, the control circuit of Figs. 1 and 2 also provides for the possibility of placing the audio amplifier A, and the control circuit itself, in stand-by condition, that is in minimum current absorption condition. To achieve this it is sufficient to close the switch IS of Fig. 1, so as to bring about the discharge to earth of capacitor C₁. As can be, understood by observing Fig. 2, this causes the immediate cut-off of transistor T₁, and hence of transistor T₈, with the consequent return of the oscillating circuit CB to neutral position with both transistors T₃ and T₇ cut off and currents I₁ and I₂ of zero value. The audio amplifier A is thus cut off, as is the control circuit associated with it. Upon reopening of the switch 15, that is upon switching on again after stand-by, capacitor C₁ begins to recharge and the cycle described for delayed switching on again of the audio amplifier is repeated. Even in case of switching on again after stand-by, the transfer of the audio signals to the loudspeaker AP through the audio amplifier A is then delayed for the time necessary for the resettling of the audio signal generator circuits. The control circuit shown in Figs. 1 and 2 is suitable for the case of an audio amplifier with individual power supply. With two power supplies (+ V cc ; - V cc ) the control circuit may become simpler, and in particular generator G₁ and current I₁ can be eliminated. The control circuit can become as shown in Fig. 6 and the circuital block B can be simplified by directly connecting the collector of transistor T₇ to the power supply V cc and eliminating D₅ and T₅. As shown in Fig. 6 the inverting input F of the audio amplifier is connected to earth through a resistance R₁₂ and the noninverting input of the amplifier is connected directly to the input IN and to earth through a resistance R₁₃. The operational result is shown in Fig. 7, which shows how current I₂ begins to increase at instant T₁ and then reaches operating value at instant T₂. At this point the amplifier A switches on, applying to the loudspeaker AP an ouput voltage V L corresponding to the audio signal at the input IN.

Claims: Audio amplifier switch-on control circuit in an audio signal generating apparatus, including a capacitor (C₁) progressively chargeable in response to the switching on of the apparatus, an oscillating circuit (CB) with intervention threshold, which responds to the increase in load voltage of said capacitor (C₁) beyond a preset value so as to shift progressively from a first to a second operating state for the generation of a switch-on current for the audio amplifier (A), and two commutating devices (T₁, T₈) associated with said oscillating circuit (CB), the first of which is controlled by the load voltage of said capacitor (C₁) and the second of which is controlled by the state of said first commutating device (T₁), said second commutating device (T₈) acting on said oscillating circuit (CB) so as to place it in said first operating state after said load voltage reaches said preset value, characterized in that said commutating devices (T₁, T₈) consist of transistors with intervention threshold and said oscillating circuit (CB) includes two opposed transistors (T₃, T₇), the first of which is controlled by said second commutating device (T₈) for placing said oscillating circuit (CB) in said operating state and the second of which is controlled by said first commutating device (T₁) for the progressive shifting of the oscillating circuit (CB) from said first to said second operating state. Control circuit according to claim 1, characterized in that said second transistor (T₇) controls a first current generator (G₂) for generation of the audio amplifier switch-on current. Control circuit according to claim 2, characterized in that said first transistor (T₇) controls a second current generator (G₁) for the generation of a charging current for a control capacitor (C₃) controlling the inverting input of the audio amplifier (A). Control circuit according to claim 3, characterized in that said control capacitor (C₃) is selected so as to achieve a load voltage which prevails over the polarization voltage of the non-inverting input of the audio amplifier (A) for a time longer than the switch-on time of the audio amplifier (A). Control circuit according to claim 1, characterized in that it includes a stand-by switch (IS) placed in parallel with said capacitor (C₁).

Inventors: PALARA, SERGIO, TORAZZINA, ALDO
Applicants: SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1985
Main classification: H03F1
IPC codes: H03F1