A muting circuit for audio amplifiers

Abstract: A muting circuit (1) for audio amplifiers (2) having first (3) and second (4) input stages, being supplied a supply voltage via a source (A1), with respective outputs (U3,U4) connected in parallel with each other to the input of an output stage (5) and having a signal input (IN) applied to the second stage (4), comprises means (10) of switching over the power supply between the first (3) and the second (4) stages operatively linked to comparator means (6,7) effective to compare the values of a reference voltage (Vd), associated with the pattern of the supply voltage with a predetermined time delay, to respective predetermined voltage values (Va,Vb) below the supply voltage.

Description: This invention relates to a muting circuit for an audio amplifier having a first and a second input stage supplied with a first supply voltage through a current source, with respective outputs connected in parallel with each other to the input of an output stage and having a signal input applied to the second input stage. As is known, with audio amplifiers, and particularly power audio amplifiers, it is preferable for the signals which appear at the input not to be passed to the output during the period immediately following the amplifier turning on. During that period, in fact, spurious signals are usually present at the input which originate from apparatus upstream of the amplifier and generate noise in the loudspeaker connected downstream from the amplifier. We will use the term "mute" and its derivatives hereinafter to denote a muted operational state in which the amplifier, while being kept alive, outputs none of the signals which appear at the input. In many circumstances, there exists a need to have the amplifier in a muted state even prior to its being turned off, or as the amplifier supply voltage Vc drops below a predetermined minimum value Vmin. The last-named ability would be specially welcome in car radio applications to suppress audio emission noise. From IEEE Transaction on Consumer Electronics, Vol. CE-32, No. 3, August 1986, page 651-657, New Yourk, US, it is known to mute the amplifier when power is turned ON or OFF and to switch OFF the power amplifier during muting and is known to generate control signals having different timings by means of the comparison of a voltage developed by charging or discharging of a capacitor in multiple comparators. From Patent Abstracts of Japan, Vol. 10, no. 87, (E-393) (2144), April 5, 1986, referring to JP-A-60-229507, it is known a circuit arrangement comprising a bipolar transistor in a common emitter configuration and having its base connected to a reference voltage determined by a Zener diode; this circuit arrangement is used to generate muting signals when a supply voltage is turned ON and OFF. The technical problem that underlies this invention is to provide a muting circuit for audio amplifiers which has such design and performance characteristics as to meet the demands specified above, while overcoming the limitations of the prior art circuits. This problem is solved by a circuit as defined in the appended claim 1. The features and advantages of the muting circuit according to the invention will become apparent from the following detailed description of an exemplary embodiment thereof, given by way of illustration and not of limitation with reference to the accompanying drawings. In the drawings: Figure 1 is a schematic block diagram of the muting circuit according to the invention; Figure 2 is a diagram illustrating in schematic form a detail of the circuit shown in Figure 1; and Figures 3 to 5 show in respective graphic forms the values versus time of voltages and currents present in the circuit according to the invention. With reference to the drawing views, generally and schematically indicated at 1 is a muting circuit for an audio amplifier 2. The amplifier 2 comprises first 3 and second 4 input stages having their respective outputs parallel connected to the input of an output stage 5. The output OUT of that stage 5 is intended for connection to an external loudspeaker, not shown because conventional. The amplifier 2 has a signal input IN applied directly to the second stage 4. The amplifier 2 is supplied with a first supply voltage having a pole Vc′ connected to the input stages 3 and 4 through a current source A1 and a switch-over means 10. The latter comprises a change-over switch having a movable contact between two fixed terminals A and B, respectively, each connected to a corresponding stage, 4 and 3. The switch 10 is controlled by the output of a logic gate 9 of the AND type having two inputs, each connected to a respective output of a voltage comparator. In particular, indicated at 6 and 7 are two voltage comparators having their respective inverting and non-inverting inputs connected to a source of a reference voltage Vd. The voltage Vd follows with a predetermined delay any change of a second supply voltage Vc applied to the circuit 1, as explained hereinafter. The reference voltage Vd is derived, for example, from a positive supply pole Vc via a resistive divider 8 which comprises a resistor pair, Rd1 and Rd2, connected serially to each other between that pole Vc and ground. A capacitor C is connected between the center tap of the divider 8 and ground to provide, in combination with the resistor Rd2, a parallel RC circuit 11. The voltage Vd is provided at the divider center. The circuit 1 further comprises a first voltage divider 17 formed of a pair of serially interconnected resistors R5 and R6 between the positive pole Vc and ground. The non-inverting input of the comparator 6 is connected to the junction point of resistors R5 and R6 for receiving a voltage Va. A second divider 19, comprising a second pair of resistors R7 and R8 interconnected in series, is connected between the pole Vc and the collector C5 of an npn-type transistor T5 having its emitter E5 grounded. The inverting input of the second comparator 7 is connected to the junction point of resistors R7 and R8 for receiving a voltage Vb. The base B5 of the transistor T5 is connected to the pole Vc via the series of a Zener diode Dz and a resistor R9. Under operational conditions, across the diode Dz there occurs a voltage drop Vz, while a voltage drop Vbe of 0.7 Volts occurs between the base B5 and the emitter E5 of the transistor T5. With specific reference to the example shown in Figure 2, the structure of the circuit 1 will be now described in greater detail. The comparator 6 comprises a pair of transistors T1 and T2 of the pnp type which have respective emitters E1 and E2 connected to each other, via corresponding resistors R1 and R2, and to the source A1 of current I. The base B1 of the transistor T1 is connected to the reference voltage pole Vd, and the base 82 of the other transistor is connected to the voltage pole Va. Likewise, the second comparator 7 comprises a transistor pair, T3 and T4, of the pnp type having their emitters E3 and E4 connected to each other, via respective resistors R3 and R4, and to the collector C1 of the transistor T1. The resistors R1, R2, R3 and R4 perform the sole function of restricting the flow of current from one transistor to another in each comparator. The base B3 of the transistor T3 is connected to the pole Vd, while the base B4 of the other transistor, T4, is connected to the voltage pole Vb. The collector C3 of the transistor T3 is connected directly to supply a current Ib to the first stage 3 of the amplifier 2. Also connected to that collector C3, and hence to the stage 3, is the collector C2 of the transistor T2 incorporated to the first comparator 6. The collector C4 of the transistor T4 is instead connected to the second stage 4 of the amplifier 2 to supply it with a current Ia. Now, while making specific reference to Figure 3, the timing is shown for various voltages of the circuit 1. Indicated at 12 is a graph of the supply voltage Vc, which rises from zero to the steady state value, and vice versa. On the voltage Vc exceeding a minimum value Vmin given by the sum of the breakdown voltage Vz of the Zener diode Dz plus the voltage drop Vbe between the base B5 and the emitter E5 of the transistor T5, this transistor will be conducting. If the values of the resistors R7 and R8 are sufficiently high, the voltage drop Vce between the collector and the emitter of the transistor T5 will almost nil. By suitably selecting the values for the resistors R5, R6 and R7, R8, the steady-state value of the voltage Va, as shown by the graph 13, is made larger than the steady-state value of the voltage Vb, illustrated by the graph 14. The reference voltage Vd will follow with a predetermined delay, as illustrated by the graph 15, the increase of the supply voltage Vc, to attain a steady state condition at a value which is lower than Va but higher than Vb. Shown in Figures 4 and 5 are the graphs 16 and 18 which illustrate versus time the values of the currents Ib and Ia flowing to the inputs of the stages 3 and 4, respectively, of the amplifier 2. Indicated at t1 and t2 are the times when a change from the on to the off state, and vice versa, of the amplifier 2 takes place. The operation of the inventive circuit will be now described. The current I from the source A1 is supplied, via the comparators 6 and 7, to the corresponding input stage, 3 and 4, of the amplifier 2. As the circuit 1 is turned on, the current I will flow through the transistors T1 and T3 to power the first stage 3 until (t1) the voltage Vd is held below the voltage value Vb. In this situation, the transistors T2 and T4 will not be conducting and the second stage 4 be off, thereby the signal applied to the input IN of the amplifier 2 will not be passed to the output OUT. Accordingly, the amplifier will be in a mute condition. On the voltage Vd exceeding the value Vb, at the time t1, the transistor T3 goes off and the transistor T4 on. The current I, now flowing through the transistors T1 and T4, will power the second stage 4, thereby the amplifier 2 is allowed to operate in its normal mode. On turning off (t2), the voltage Va will become lower than the voltage Vd and turn off the transistor T1 and enable the transistor T2 to become conductive to directly power the first stage 3 and restore the amplifier to its muted state. Thus, the circuit of this invention enables the amplifier to be muted over respective predetermined time periods, one immediately after turning on and the other preceding turning off. Furthermore, in all those cases when the supply voltage Vc drops or remains below the minimum value Vmin = Vz + Vbe, the transistor T5 will be non-conductive and on the base B4 of the transistor T4 there will be such a voltage value as to hold this transistor also non-conductive. In such situations, the amplifier 2 will be in a muted condition, thereby the circuit of this invention attains the added objective of muting the audio amplifier even in those instances when the supply voltage is lower than a predetermined minimum value. The circuit according to this invention solves, therefore, the technical problem in a simple and effective way with a reduced circuit overgrow.

Claims: A muting circuit (1) for an audio amplifier (2), the amplifier having a first (3) and a second (4) input stage supplied with a first supply voltage (Vc′) through a current source (A1), their respective outputs (U3,U4) being connected in parallel with each other to the input of an output stage (5), a signal input (IN) being applied to the second input stage (4), the circuit comprising a comparator means (6,7) which compares the value of a reference voltage (Vd, 15) to respective predetermined voltage values (Va, 13; Vb, 14) for controlling the switching over of said first power supply voltage between said first (3) and second (4) input stages; said reference voltage being derived from a second supply voltage (Vc, 12) and time delayed with respect thereto, and said respective predetermined voltages (Va, Vb) being derived from and lower than said second supply voltage; said first supply voltage being connected to the second input stage (4) when said reference voltage (Vd, 15) exceeds one (Vb, 14) of said respective predetermined voltages and said first power supply voltage being connected to said first input stage (3) when said reference voltage (Vd, 15) exceeds the other (Va, 13) of said respective predetermined voltages. A circuit according to Claim 1, characterized in that it comprises a means (10) for providing said switching over controlled by said comparator means (6,7) and in that said comparator means comprises first (6) and second (7) voltage comparators having respective outputs led to respective inputs of a logic AND gate (9) the output whereof is connected to drive said switch-over means (10). A circuit according to Claim 1, characterized in that it comprises first (6) and second (7) voltage comparators, first (17) and second (19) resistive dividers connected between the second power supply (Vc) and ground, and a source of said reference voltage (Vd), said comparators having respective pairs of input terminals, one input terminal of each pair being connected to the center tap of a corresponding divider (17, 19) and the other input terminal of both pairs being connected to said reference voltage source (Vd). A circuit according to Claim 1, characterized in that it comprises a resistive voltage divider (8) connected between the second (Vc) power supply and ground, and a capacitor (C) connected in parallel to a resistor (Rd2) of said divider (8) to provide a delay circuit (RC) in order to generate said reference voltage (Vd) at the center tap of said divider. A circuit according to Claim 2 or 3, characterized in that each comparator (6, 7) comprises a pair of transistors having their emitters connected to each other and to said current source (A1), with the collector of one of the transistors of each pair being connected directly to a corresponding one of said input stages (3, 4) of the amplifier (2). A circuit according to Claim 3, characterized in that it comprises a transistor (T5) having its emitter (E5) and collector (C5) connected between the second divider (19) and ground, and having its base (B5) connected to the second power supply (Vc) via the series of a Zener diode (Dz) and a resistor (R9).

Inventors: BOTTI EDOARDO, BOTTI, EDOARDO
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1989
Main classification: H03G3
IPC codes: H03F1, H03G3