High efficiency power audio amplifier comprising two amplifiers with a single feed

Abstract: The amplifier comprises a first and a second amplifier block (A1, A2) of opposite phase driven by a single input signal (Vin) and having its outputs (Vout1, Vout2) connected to the two terminals of a load (L). It also comprises circuit means (I1, I2) to disable one (A2) of the said amplifier blocks (A1, A2) when the absolute value of the input signal is less than a predetermined threshold level. A passive feedback system (R1, R2, R3, R4) capable of maintaining the amplifier gain constant is located between the aforesaid terminals of the load (L) and the inputs to the two amplifier blocks (A1, A2).

Description: This invention relates to a high efficiency power audio amplifier comprising two amplifiers with a single feed. Audio signals are frequently amplified using a pair of power amplifier blocks, normally of class A-B, caused to operated in a bridge configuration, or with the load governed by corresponding signals of opposite phase. The output curve from this type of amplifier for a sinusoidal signal starts from 0% in the absence of output power and reaches 70% under the conditions of maximum output power. GB-A-2 112 240 discloses an example of such an amplifier. It is also known that power amplifiers of the switch type in class D can be used to obtain greater output and thus lower consumption for equal power delivered. These have the advantage of very high efficiency, equal to approximately 95%, but often give rise to distortion problems, and are also rather complex with circuitry outside the device. As an alternative to these, amplifiers in class G can be used. In these amplifiers a normal final stage of class A-B is fed by two feed voltages which are switched depending on the amplitude of the output signal. For low power levels these provide a good increase in efficiency in comparison with a traditional amplifier. This applies in particular for musical input signals, which, substantially having an amplitude distribution of the gaussian type, allow the system to effectively double efficiency in comparison with a traditional class A-B amplifier. The disadvantage of these class G amplifiers lies in the fact that they require more than one feed circuit. As an alternative, as described in greater detail in Italian patent application no. 22228 A/89 lodged on the 31/10/89 in the name of the applicant, there is the possibility of simulating class G by using a pair cf class A-B amplifier blocks which are switched from single-ended to bridge and vice versa depending on the level of the input signal. This arrangement has the main disadvantage that in order to maintain constant gain when passing from single-ended to bridge a differential feedback amplifier which is particularly fast, and therefore costly and complex, is required. The use of such an amplifier simulating class G also provides for a large capacitance in series with the load, which eliminates the continuous component of the load in the case of single-ended operation. Its presence however introduces inevitable switching delays from single-ended to bridge and vice versa, caused by the time involved in charging and discharging. Obviously these delays have an adverse effect on the overall efficiency of the amplifier. The object of this invention is to provide a power audio amplifier simulating class G which has a high efficiency and in which the gain in the case of single-ended operation is equal to that in bridge operation and in which no intrinsic delays occur in the output wave front of the input signal when switching from single-ended to bridge. This object is accomplished in accordance with the invention by means of a power audio amplifier having a single feed which comprises a first and a second amplifier block of opposite phase driven by a single input signal and having their outputs connected to the two terminals of a load, and circuit means to disable one of the said amplifier blocks when the input signal is less in absolute terms than a predetermined threshold level, characterised in that a passive feedback system capable of maintaining the amplifier gain constant when operating with one or two amplifier blocks is placed between the said terminals of the load and the inputs to the first and second amplifier blocks. The amplifier according to the invention also preferably comprises a delay block capable of delaying switching of the amplifier from functioning on two amplifier blocks to one single amplifier block, or from bridge to single-ended. Finally provision is preferably made for a transconductance amplifier with an input connected to a common node between a series of two resistances placed in parallel with the load, the output of which is operationally connected to the said disabled amplifier block to balance the voltage on the load in bridge operation. In this way an appreciable power on the load can be obtained with a single relatively low feed voltage and above all with low dissipated power from the device when a musical signal is present. The efficiency of the amplifier is consequently high, and above all its gain is independent of the manner in which the amplifier operates (single-ended or bridge) and therefore independent of the level of the input signal. An amplifier without intrinsic delays is also obtained, and the switching within it takes place on the basis of each wave front of the input signal. The features of this invention as it is defined in the appended claims will become clearer from the embodiments illustrated by way of non-restrictive examples in the appended drawing, in which: Figure 1 illustrates a circuit diagram of a power amplifier according to the invention, Figures 2, 3 illustrate equivalent circuits to the amplifier in Figure 1 in the case of single-ended and bridge operation, Figures 4, 5, 6 illustrate three possible variant embodiments of the circuit in Figure 1 with a balanced load. With reference to Figure 1, two amplifier blocks A1 and A2 in class A-B with a single feed Vee and non-inverting and inverting inputs respectively connected together are driven by an input voltage Vin at their inverting and non-inverting inputs respectively, although connected together. Between the input for signal Vin and the inputs to amplifiers A1, A2 there is placed a series of a condenser C, decoupling buffer B1 and a resistance R1. The function of buffer B1 is to cancel out the effect of the series resistance of a non-ideal voltage generator Vin. The output of amplifier A1 is connected to a terminal Vout1 of a load L whose other terminal Vout2 is connected to the output of amplifier A2 via a first switch I1 and earthed through a second switch I2. The purpose of the abovementioned switches will be specified later. There are feedback resistances R2, R3 between terminal Vout1 of load L and the inverting input of amplifier A1 and between terminal Vout2 of load L and the inverting input of amplifier A2. The inverting input of amplifier A2 is also connected to earth through a resistance R4. The amplifier in Figure 1 also comprises a threshold detector block RS which comprises a first and a second amplifier block S1, S2 capable of receiving the input voltage Vin at its non-inverting and inverting inputs respectively, with reference voltages +Vref, -Vref respectively being present at the other inputs. In cascade with amplifiers S1, S2 there is an OR logic gate L2 whose output U1 indicates whether or not the absolute value of an input signal threshold Vin has been exceeded. In series with the threshold detecting block RS there is preferably a switching delay block T1 whose output directly controls switch I2 and switch I1 through an inverter I3. The function of block T1 is to delay switching of the amplifier from bridge to single-ended to avoid disturbances generated by the switching peaks. The operation of the amplifier in Figure 1 is as follows. As long as the absolute value of the input signal Vin remains below a predetermined threshold, logic output U1 of threshold detector block RS causes switch I2 to close and switch I1 to open. Amplifier A2 is thus in a tri-state condition, that is non-conduction with a non-defined output. Amplifier A1 drives and the equivalent circuit of the amplifier is that shown in Figure 2, corresponding to single-ended operation, where Rx indicates the earth resistance of switch I2 and RL indicates the load resistance. The load L is solely fed by voltage Vout1, while there is a voltage Vout2 of virtually zero on the other terminal. In this case it can be seen by simple reckoning, not shown, that the amplifier gain, or the ratio between the voltage at the terminals of load L and the input voltage Vin, is independent of resistance Rx and solely depends on the ratio between resistances R2 and R1. It is now assumed that the absolute value of the input signal Vin exceeds the predetermined switching threshold. Logic output U1 of threshold detector block RS then causes switch I2 to open and switch I1 to close. Under these conditions amplifier A2 will begin to conduct together with amplifier A1 and the equivalent circuit will be that shown in Figure 3 corresponding to the condition of bridge operation. In a wholly similar way to that described above it is possible by simple reckoning, not shown, to see that the amplifier gain solely depends on the ratio between resistances R2 and R1. This operating condition will persist until the input signal Vin falls below the predetermined threshold and causes switching back to the state of single-ended operation. This switching back may take place with some delay introduced by block T1 so as to avoid any distortion produced by repeated switching. In accordance with the variant illustrated in Figure 4, a trans-conductance amplifier TR is provided and on the non-inverting input of this there is a voltage taken from a common node C between two resistances R6, R7 in parallel with load L whose inverting input is connected to earth. The output from amplifier TR is connected through a switch F controlled by the output of inverter I3 to the inverting input of amplifier A2 which is connected to the non-inverting input of amplifier A1 via a resistance R5. In particular the function of transconductance amplifier TR is to balance the voltage on the load. Switch F in cascade therewith disconnects the output from amplifier TR if the amplifier operates in the single-ended condition, as in this case only the signal Vout1 should be present. As illustrated in Figure 5, as an alternative to that illustrated in Figure 4, the output of amplifier TR may be connected via switch F to the non-inverting input of amplifier A2, which is connected to the inverting input of amplifier A1 through a resistance R5. Finally, as illustrated in Figure 6, the output from amplifier TR may be connected to an intermediate node of amplifier A2. This circuit arrangement is obviously the simplest because it does not require a switch F to be present.

Claims: A high efficiency power audio amplifier with a single feed, comprising a first and a second amplifier block (A1, A2) of opposite phase driven by a single input signal (Vin) and having their outputs connected to two terminals (Vout1, Vout2) of a load (L) and circuit means (I1, I2, RS) to disable one (A2) of the said amplifier blocks (A1, A2) when the absolute value of the input signal is below a predetermined threshold level, characterised in that a passive feedback system (R1, R2, R3, R4) capable of maintaining the amplifier gain constant when one or two amplifier blocks (A1, A2) are in operation is located between the said terminals (Vout1, Vout2) of the load (L) and the inputs to the first and second amplifier blocks (A1, A2). An amplifier according to claim 1, characterised in that the said circuit means (I1, I2, R5) comprise a first switch (I1) placed between the output from the said disabled amplifier block (A2) and the load (2), a second switch (I2) placed between the load (L) and earth and a threshold detector block (RS) for the input signal (Vin) which controls the closure/opening of the said first switch (I1) and the opening/closure of the said second switch (I2) depending on whether or not the absolute threshold value of an input signal Vin has been exceeded. An audio amplifier according to claim 2, characterised in that it comprises a delay block (T1) placed between the output of the threshold detector block (RS) and the said switches (I1, I2) to delay switching of the amplifier from operation with two amplifier blocks (A1, A2) to operation with a single amplifier block (A1). An amplifier according to claim 1, characterised in that it comprises a transconductance amplifier (TR) whose non-inverting input is connected to a common node (C) between a set of two resistances (R6, R7) placed in parallel with the load (L) and whose output is functionally connected to the said amplifier block (A2) so as to balance the voltage on the load (L) when operating with two amplifier blocks (A1, A2).

Inventors: BOTTI EDOARDO, BRAMBILLA DAVIDE, BRASCA GUIDO, BOTTI, EDOARDO, BRAMBILLA, DAVIDE, BRASCA, GUIDO
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1991
Main classification: H03F3
IPC codes: H03G3, H03F1, H03F3