Audio Amplifier turn-off control circuit

Abstract: A circuit comprising a transistor (10) which, when supply (V S ) is turned off, locks the output (9) of the audio amplifier (1) to the supply line (11), the potential of which is reduced gradually by a filter capacitor (25), so that the output voltage (V o ) follows the supply voltage (V S ) with the same slope, and is so controlled by the filter capacitor (25), thus eliminating undesired noise ("popping") at the output (9) caused by electric transients in the audio amplifier.

Description: The present invention relates to a circuit for turn-off control of audio amplifiers, in particular, single-ended, single-feed types. All audio amplifiers present a noise filtering capacitor between the supply line and ground. Due to the widely varying frequency and peak value of the noise at the supply, the filter capacitor presents a fairly high value, whereas the equivalent resistance of the overall amplifier circuit is small, thus resulting in a small discharge constant and in fairly fast discharge of the capacitor itself. When the audio amplifier is turned off, i.e. when the power supply is cut off, various high-speed electric transients therefore occur resulting in poorly controlled variations in the amplifier output and in an annoying "popping" noise on the loudspeakers driven by the amplifier. EP-A-0 281 117 describes a known solution to the problem of reducing pop noise during a voltage fall. According to this known solution, the amplifier output is connected to a capacitor which, when the circuit is switched off, is connected to ground through a switchable low resistance path, to gradually discharge therethrough. US-A-3,840,820 describes a muting circuit, wherein the amplifier input is connected to ground through a muting transistor which becomes operative when the amplifier is switched off, so as to avoid feeding of the audio signal to the amplifier. EP-A-0 368 528 describes a circuit implementing muting, stand-by and noise elimination functions in a single chip, without feedback elements for locking the output voltage to the supply voltage in presence of a supply voltage reduction. EP-A-0 069 943 describes a circuit wherein the inverting input of a differential stage is supplied in a differentiated manner dependant on the operative condition of the (television) circuit, and does not meet the problem of obtaining controlled transients on the output voltage when the supply voltage reduces due to power turn-off. It is an object of the present invention to provide a circuit designed to eliminate the above noise by guaranteeing a controlled transient output voltage of the audio amplifier when this is turned off. According to the present invention, there is provided an audio amplifier turn-off control circuit as claimed in Claim 1. The present invention thus provides for a circuit which, when the power supply is cut off, locks the output to the supply line, the potential of which is gradually reduced by virtue of the filter capacitor, so that the output follows the supply line, with the same slope, and is thus controlled by the filter capacitor. A preferred, non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which: Fig.1 shows a block diagram of the circuit according to the present invention; Fig.2 shows a time graph of a number of electric quantities (not to scale) in Fig.1; Fig.3 shows a circuit diagram of the circuit according to the present invention; Fig.4 shows a graph of the amplifier output during turn-off and in the presence of a signal. In Fig.1, the audio amplifier, indicated as a whole by 1, is represented by the cascade connection of two stages, more specifically, an input stage 2 having a high transconductance g m , and a gain stage 3 with gain -A v . The positive input IN+ of input stage 2 is supplied in known manner with the audio signal, while the negative input IN- is connected to output 9 of amplifier 1 via a feedback network 4 including a pair of resistors 5, 6 connected between the output and ground, and a capacitor 7 connected between the mid point of resistors 5, 6 and negative input IN-. A compensating capacitor 8 is provided between the input and output of gain stage 3. As shown in Fig. 1, a PNP type transistor 10 is provided for locking output 9 of amplifier 1 and supply line 11. More specifically, transistor 10 presents the collector connected to the input of gain stage 3, the emitter connected to output 9 via the interposition of a high-value resistor 12, and the base connected to the output of a control block 13 for generating a potential V B in relation to ground. The emitter of transistor 10, at which potential V D is present, is also groundable by a switch 15 having a control terminal 16 also controlled by control block 13. Control block 13 is in turn connected to supply line 11, set to potential V S in relation to ground, and to a voltage source 17 for generating a lower potential V A in relation to ground as compared with steady-state supply voltage V CC . More specifically, source 17 comprises a capacitor 18 and a resistor 19 parallel-connected between node 20 and ground, node 20 being connected to supply line 11 via a branch 22 formed by the series connection of at least one diode 23 and a resistor 24 (but possibly comprising a number of diodes and resistors). The Fig.1 circuit is completed by a filter capacitor 25 for filtering noise on supply line 11 and located between line 11 and ground; a resistor 26 representing the equivalent resistance of amplifier 1 as seen by filter capacitor 25 and in which capacitor 25 discharges when the power supply is cut off; and a switch 27 between supply line 11 and input 28 at which the voltage V CC supplied by an external power supply unit (not shown) is present. In the Fig.1 circuit, the time constant of the filter formed by components 25, 26 is smaller than that of filter 17 formed by components 18, 19, so that capacitor 25 discharges faster than capacitor 18, the voltage of which may thus be assumed more or less constant as regards the circuit according to the present invention. When activated, transistor 10, by which the output of audio amplifier 1 is locked to supply line 11, thus provides for local feedback between the amplifier output and the input of the gain stage. The transistor 10 and gain stage 3 structure may be represented schematically by an equivalent operational amplifier having a positive input corresponding to the base terminal of transistor 10, a negative input corresponding to the emitter of transistor 10 (with switch 15 open), and an output corresponding to output 9 of amplifier 1; and which, despite being closed in unit-gain manner by resistor 12, is stable. The stability of the equivalent operational amplifier is explainable by comparing it with audio amplifier 1. Both present gain stage 3 with compensating capacitor 8, but, whereas input stage 2 of amplifier 1 presents a high transconductance, the transconductance of the equivalent operational amplifier equals the inverse of the resistance of resistor 12 (1/R g , where R g is the resistance of resistor 12) which, by appropriately selecting R g , is much lower than g m . As a result, the loop gain of the operational amplifier is sufficiently lower as compared with audio amplifier 1 to achieve a lower closed-loop gain for a given degree of stability. Operation of the Fig.1 circuit will be described with reference also to Fig.2. Under normal operating conditions, voltage V S over line 11 presents steady-state value V CC (switch 27 closed); audio amplifier 1 is active; output V o presents the steady-state value; capacitor 25 is charged to V CC ; and capacitor 18 to V A . Control block 13 therefore maintains switch 15 closed, and supplies the base of transistor 10 with a positive potential V B in relation to ground, so that the emitter of transistor 10 is grounded; transistor 10 is off; and high-value resistor 12 has no effect on output potential. The turn-off phase commences when switch 27 is opened (instant t O ), at which point, capacitor 25 discharges rapidly into resistor 26, thus causing a corresponding reduction in V S ; and, at the same time, voltage V A at point 20 is reduced by virtue of capacitor 18 discharging into resistor 19, but at a slower rate than capacitor 25. Upon V S falling below V A (instant t 1 ), control block 13 switches and emits a signal for opening switch 15; switch 15 opens so that the emitter of transistor 10 is no longer grounded; and the emitter substantially switches to potential V o of output 9 of audio amplifier 1. Control block 13 also modifies the value of potential V B so that it follows voltage V S over supply line 11. Potential V B thus begins to fall, but, as V B > V D - V BE (where V BE is the base-emitter voltage drop of transistor 10, and, at this phase, V D ≃ V o ) still applies, transistor 10 remains off. Upon V B falling below V D - V BE (instant t 2 ), transistor 10 comes on and locks output 9 to voltage V B at its base and, consequently, to supply voltage V S , so that output 9 is grounded in controlled manner by the discharging of capacitor 25. Fig.3 shows a circuit embodiment, by way of example, of the Fig.1 blocks, in which can be seen stages 2 and 3 of audio amplifier 1; capacitors 25 and 18; resistors 12 and 26; supply line 11; switch 27; transistor 10; and branch 22 including diode 23 and resistor 24. In the example shown, resistor 19 is formed by the series connection of a resistor 30 (connected to capacitor 18 at node 46) and two pairs of diodes 31, 32; and block 13 comprises a differential stage 33 formed by two PNP type transistors 34, 35 having the emitters connected to each other and to a current source 36 also connected to supply line 11. The base of transistor 35 is connected to node 43 between resistor 24 and the anode of diode 23 (set to potential V E ), while the base of transistor 34 is connected to node 37 of a branch 38 formed by three diodes 39 series-connected between supply line 11 and node 37 (with the cathode facing node 37), and by two series-connected resistors 40, 41 between node 37 and ground. Node 42 between resistors 40, 41 is connected to the base of transistor 10; the collector of transistor 34 is grounded; the collector of transistor 35 is connected to the anode of a diode 44 having its cathode grounded and which, together with an NPN type transistor 45, forms switch 15; and transistor 45 presents its base connected to the anode of diode 44, its emitter grounded, and its collector connected to the emitter of transistor 10 at node 49. Fig.3 also shows, schematically, current sources 47 and 48 respectively relative to input stage 2 and gain stage 3, and controlled by the potential at node 50 between resistor 30 and the series connection of diodes 31, 32. Feedback network 4 and compensating capacitor 8 (connected as in Fig.1) are not shown in the Fig.3 circuit, which shows the connection, via resistor 52, of positive input IN+ of input stage 2 to a node 51 between diodes 31 and 32. Input IN+ of input stage 2 is supplied with the audio signal via a decoupling capacitor 53. The Fig.3 circuit operates as follows. Under normal operating conditions and in the absence of an audio signal, the value of output 9 of audio amplifier 1 is determined by the bias network (not shown) and normally equals half supply voltage V CC ( V o = V CC /2). Moreover, if R 1 and R 2 are the respective resistances of resistors 40 and 41; R 3 the (same) resistance of resistors 24 and 30; and V A , V B , V C , V D and V E the respective potentials of nodes 46, 42, 37, 49 and 43: V C = V CC - 3 V BE where V BE is the voltage drop of each diode 39; and V B = V C R 2 / (R 1 + R 2 ) V A = V CC - V BE - R 3 (V CC - 5V BE )/(2R 3 ) = V CC /2 + 3V BE /2 V E = V CC /2 + 5V BE /2 Under normal operating conditions, therefore, V E < V C , so that transistor 35 is on, and current I from source 36 flows into diode 44 by which it is mirrored to transistor 45. As a result, potential V D is fairly low, at any rate lower than V B + V BE , so that transistor 10 is off, and, by virtue of the low value of current I, normal operation is unaffected by the circuit. When switch 27 is opened, thus cutting off supply and gradually reducing voltage V S controlled by the discharging of capacitor 25, the turn-off control circuit is activated according to the following three phases: 1) V C > V E Potential V C dependent on V S falls gradually. Initially, however, as long as potential V C is greater than V E , the circuit remains in the foregoing condition. Potential V E and consequently V A also fall in this phase, but, due to the sizing of the components, at a slower rate as compared with V S . 2) V C < V E and V B > V D - V BE When this applies, transistor 35 is turned off, transistor 34 turned on, and consequently diode 44 and transistor 45 turned off. As the base-emitter voltage drop of transistor 10 prevents it from being turned on, potential V D follows output voltage V o of audio amplifier 1, so that the output voltage remains constant. 3) V C < V E and V B ≤ V D - V BE When this applies, transistor 10 comes on so as to ground output 9 (locked to the emitter of transistor 10) according to the fall in potential V B at the base of the transistor and consequently also in V S . The advantages of the circuit according to the present invention will be clear from the foregoing description. In particular, it provides for ensuring controlled reduction at all times of the output during turn-off, and so preventing undesired noise (popping) caused by transients occurring in the system. In the Fig.3 circuit, overall operation of the amplifier depends on the chain formed by branches 22, 19, and thus on potentials varying more slowly as compared with supply voltage V S . Dependence of the control circuit on the above chain enables the thresholds to be so calculated as to ground the output while input stage 2 and current sources 47, 48 of audio amplifier 1 are still active, thus maintaining feedback of the output. The first phase in the operation of the control circuit prevents the circuit from operating under steady-state conditions, in the event of temporary failure or a negligible fall in supply; while the second phase enables the emitter of transistor 10 to reach the output voltage value before voltage V B falls below the steady-state output voltage value minus V BE , thus preventing an undesired step transient and, consequently, popping of the amplifier output when switch 15 is opened. Current I of source 36 and resistance 12 may be so calculated that transistor 10 remains off even in the presence of a signal under normal operating conditions, for which purpose, the following must apply: V C > V E which undoubtedly applies by virtue of both V C and V E presenting a fixed direct current offset and being unaffected by the signal; V B > (V D ) DC + (V D ) AC - V BE where (V D ) DC and (V D ) AC are respectively the direct and alternating current offset of the potential at node 49, so that transistor 10 definitely remains off. When supply is cut off in the presence of a signal, the circuit operates in the same way as described above, except that the value of V D varies, so that transistor 10 alternates between phases wherein supply is locked and phases of inactivity, depending on the amplitude and frequency of the signal, the mean value of output 9, however, being grounded as shown in Fig.4. An important point to note is that, even in the presence of a signal, the control circuit becomes operative while the amplifier is still being fed back, so that the output is nevertheless related to the inputs and the potentials of the chain formed by branches 22, 19. The circuit according to the present invention is straightforward in design, reliable, and provides for troublefree manufacture and integration using current techniques. Moreover, the control circuit employs characteristics of the amplifier that are useful for other and/or existing functions, i.e. timing by the capacitors necessarily provided for in the application, and the references of the chain formed by branches 22, 19, which may also be employed for normal operation of the audio amplifier. To those skilled in the art it will be clear that changes may be made to the circuit as described and illustrated herein without, however, departing from the scope of the present invention as it is defined by the appended claims. In particular, voltage source 17 in Fig.1 may be implemented differently, either as a unit designed to maintain a constant voltage at its terminals, or a unit whose voltage falls alongside a fall in supply, providing the unit voltage falls more slowly than supply. Finally, though the solution shown, wherein the control feedback loop is located between the input and output of the gain stage only, is preferable in terms of stability, other solutions also including the input stage in the control loop are possible by connecting the feedback element (transistor 10) between the output and input of audio amplifier 1 as a whole.

Claims: A circuit for turn-off control of an audio amplifier (1) having an output terminal (9) with an output voltage (V o ) and supplied by a supply line (11) with a supply voltage (V S ), characterized in that it comprises a control stage (13) and a controlled switch (10), said control stage being connected to said supply line (11) and generating an activation signal for said controlled switch (10) in the event of a fall in said supply voltage (V S ), said controlled switch being located between said output terminal (9) and said supply line (11) and being controlled by said activation signal to lock said output voltage (V o ) to said supply voltage in presence of said supply voltage fall and so cause a reduction in said output voltage correlated to the reduction in said supply voltage. A circuit according to claim 1, characterized in that said control stage (13) comprises a voltage source (17) generating a reference voltage (V A ); and a comparator (13) having a first input connected to said voltage source, a second input connected to said supply line (11), and an output connected to a terminal for controlling said controlled switch (10), and which closes said controlled switch when said supply voltage (V S ) equals said reference voltage (V A ). A circuit as claimed in Claim 2, wherein a filter circuit (25, 26) having a first time constant is arranged between said supply line (11) and a reference potential line (ground); characterized in that said voltage source (17) comprises a capacitive element (18) and a resistive element (19) defining a second time constant greater than said first time constant. A circuit as claimed in Claim 3, characterized by the fact that said voltage source (17) is connected to said supply line (11) via a resistive branch (22) including voltage-controlled unipolar switches (23). A circuit as claimed in one of the foregoing Claims, characterized in that said controlled switch comprises a transistor (10). A circuit as claimed in Claim 5, characterized in that said transistor (10) presents a first terminal connected to said output (9) of said audio amplifier (1); a second terminal connected to an input of said audio amplifier; and a control terminal connected to said control stage (13); said first terminal of said transistor (10) being connected to a reference potential line (ground) via a switch element (15) controlled by said control stage (13). A circuit as claimed in Claim 6, characterized in that said control stage (13) presents a first output connected to said control terminal of said transistor (10), and a second output connected to a control terminal of said switch element (15). A circuit as claimed in any one of the foregoing Claims, characterized in that said comparator (13) comprises a differential circuit (33) having a first input connected to said supply line (11); a second input connected to said voltage source (17); and an output connected to a control terminal of a switch element (15) having a first terminal connected to a reference potential line (ground), and a second terminal connected to a first terminal of a transistor (10); said transistor (10) being connected by said first terminal to said output (9) of said audio amplifier (1) and by a control terminal to said supply line (11) via resistive elements (39, 40). A circuit as claimed in any one of the foregoing Claims from 5 to 8, wherein said audio amplifier (1) presents an input stage (2) having a high transconductance, and a gain stage (3) cascade-connected to each other; characterized in that said second terminal of said transistor (10) is connected to an input of said gain stage (3). An amplifier characterized by including a turn-off control circuit as defined in claim 1.

Inventors: FASSINA ANDREA, NEBULONI DANIELA, FASSINA, ANDREA, NEBULONI, DANIELA
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1992
Main classification: H03F1
IPC codes: H03G3, H03F1