GB2043382A

GB2043382A - Amplifier and power supply circuit

Info

Publication number: GB2043382A
Application number: GB8004284A
Authority: GB
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1979-02-13
Filing date: 1980-02-08
Publication date: 1980-10-01
Also published as: SE8001041L; ES8102434A1; DE3003833A1; FR2449362A1; IT8019824A0; NL7901118A; ES488449A0; AU5537080A; IT1193375B; JPS55109006A

Abstract

A power supply circuit (1) feeds an amplifier stage (8) via output terminal (22) and in order to increase the efficiency of the arrangement the audio input (5) to the amplifier stage is also coupled to a control input (17) of the supply circuit via an amplitude detector (4) so that the supply circuit output voltage will follow the audio signal amplitude in a continuous manner, preferably being continuously maintained at a value just above the magnitude of the signal voltage swing at the output of the amplifier stage. A delay device (6) is included in the audio signal path to the amplifier stage to ensure that the audio signal does not reach the output of the amplifier stage before the supply voltage has been adjusted accordingly. <IMAGE>

Description

SPECIFICATION Amplifier circuit arrangement comprising a power supply circuit and an amplifier stage a power supply input of which is connected to the output of said power supply circuit The invention relates to an amplifier circuit arrangement comprising a power supply circuit and an amplifier stage a power supply input of which is connected to the output of said power supply circuit.

A known such amplifier arrangement is described in United States Patent Specification No. 3,961,280.

The power supply circuit in this known arrangement applies a supply voltage to the amplifier stage which, depending on the amplitude of an audio signal applied to an audio input of the amplifier stage, can assume two different values. In orderto achieve this the supply circuit is threshold-voltageresponsive and produces a supply voltage of a first constant value so long as the audio signal amplitude remains below this threshold voltage. If the amplitude of the audio signal exceeds the threshold voltage the supply voltage is brought to a second and higher constant value in a transient manner. The efficiency of this arrangement is optimized by presetting the threshold voltage to a value which is just above the statistically most likely amplitude value of the audio signal.

The invention has for an object to enable the efficiency of an arrangement of the type mentioned in the opening paragraph to be increased still further.

An arrangement according to the invention is characterized in that an audio input of the amplifier stage is coupled to a control input of the supply circuit via an amplitude detector, so that in operation the supply circuit will supply the amplifier stage with a supply voltage which varies continuously with the amplitude of an audio signal at said audio signal input and in the same sense.

Maximum efficiency can be obtained in such an arrangement by keeping the instantaneous difference between the continuously variable supply voltage and the magnitude of the audio signal swing at the output of the amplifier stage as small as possible, i.e. by performing forward control of the supply voltage in such manner that this voltage is continuously just greater than the magnitude of the swing of an audio signal at said audio signal input when multiplied by the gain from said audio signal input to the output of the amplifier stage. If this is done it is possible to obtain a higher efficiency than that obtained with the above-mentioned known arrangement wherein the supply voltage can only assume only two discrete values.

The audio input may be coupled to the amplifier stage via a delay circuit. Provision of such a delay circuit can prevent sudden transient amplitude increases of the audio signal from being followed too late by the necessary increase of the supply voltage, and hence can reduce the likelihood of the amplifier providing non-linear amplification under these conditions.

An embodiment of the invention will be described, by way of example, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is the circuit diagram of the embodiment; Figure 2 shows a possible construction for part of the embodiment of Figure 1 in detail; and Figure 3 shows voltage-time diagrams illustrating the operation of the embodiment of Figures 1 and 2.

Figure 1 shows an amplifier circuit arrangement comprising a power supply unit 1 having mains inputs 2 and 3 and a supply output 22, an amplitude detector4 which couples an audio input 5 to the supply unit 1, and a delay circuit 6, a pre-amplifier 7, an amplifier output stage 8 and a loudspeaker 9 in cascade, delay circuit 6 also being fed from the audio input 5.

The supply unit 1 comprises a diode bridge 11 to 14, which functions as a full-wave rectifier and to which the mains inputs 2 and 3 are coupled via a mains transformer 10. The anodes of the diodes 11 and 12 are connected to ground. The cathodes of the diodes 13 and 14 are connected to a supply input 15 of a voltage control device 16. At a supply output 22 the voltage control device 16 produces a controllable supply voltage for feeding the pre-amplifier 7 and the amplifier output stage 8. The voltage control device 16 has a control terminal 17 to which is coupled the output of the amplitude detector 4. The supply voltage at the supply output 22 of the voltage control device 16 increases when the control voltage at the control terminal 17 increases, and vice versa.

The control voltage at the control terminal 17 is formed in the amplitude detector4 by rectifying and smoothing an audio signal applied to the audio input 5.

This audio signal is amplified and reproduced by means of the preamplifier 7, the amplifier output stage 8 and the loudspeaker 9.

When an increase in the amplitude of the audio signal occurs the supply voltage at the supply output 22 also increases, making possible larger variations in the signal voltage at the output of the amplifier output stage 8. As the supply voltage across the amplifier output stage varies continuously in accordance with the ampitude of the audio signal to be amplified it is possible to optimise the efficiency of the amplifier by continuously making the difference between the supply voltage and the signal voltage swing at the output of the amplifier output stage 8 as small as possible.

In orderto preventthe audio signal applied to the audio input 5 from reaching the amplifier output stage before the supply voltage is brought to a (sufficiently high) value in response thereto, the audio signal is delayed for a certain period of time in the delay circuit 6 provided between the audio input 5 and the pre-amplifier7. This period of time is chosen taking into account the time delays occurring in the amplitude detector4 and the voltage control device 16.

Figure 2 shows in detail possible constructions for the voltage control device 6 and the amplitude detector 4, corresponding components to those of Figure 1 being given the same reference numerals in the two Figures.

The voltage control device 16 comprises a switching transistor 50, which functions as a switch and the emitter of which is coupled to the supply input 15 via a diode 53 which is connected in the pass direction of the switching transistor 50. The collector of transistor 50 is connected to the supply output 22 via a buffer capacitor 18 which is connected to ground.

Via the resistor 60 the base of the switching transis tor 50 is coupled to the collector of a transistor 52 which, together with a transistor 51, constitutes a voltage comparator. The collector of the transistor 52 constitutes the output of this voltage comparator.

The emitters of the two transistors 51, 52 are connected to ground via a current source circuit 56 to 59.

This current source circuit comprises a transistor 58, the collector of which is connected to the emitters of the transistors 51 and 52 and the emitter of which is connected to ground via an emitter resistor 59, and also a transistor 56 the collector-base junction of which is short-circuited. The base of this transistor 56 is connected to the base of the transistor 58. The collector of the transistor 56 is connected to the supply input 15 via a series combination of resistors 55 and 54. The emitter of transistor 56 is connected to ground via a resistor 57. The junction of the resistors 54 and 55 of the series combination is connected to the base of the transistor 51. The base of the transistor 52 is fed from the control terminal 17 via a matching resistor 61.

The control terminal 17 is fed from the audio input 5 via a rectifying diode 80 included in the amplitude detector 4. The cathode of this rectifying diode 80 is connected to the control terminal 17 and its anode is connected to the audio input 5. The cathode is also connected to a parallel arrangement of a buffer capacitor 81 and a resistor 82, which are both connected to ground.

The operation of the voltage control device 16 and the amplitude detector4 will be described with reference to Figure 3. In Figure 3 curve a, shows how the full-wave rectified mains voltage which is fed to the supply input 15 varies with time t and curve a2 shows the resulting voltage applied to the base of the transistor 51 via the voltage divider including resistors 54 and 55. Level b denotes a possible instantaneous value for the voltage at the base of the transistor 52 and curve q, C2, C3, C4 shows the variation of the output voltage at the supply output 22.

For simplicity, the variation of the audio signal at the audio input 5 and the time constant of the integrating network 81,82 are assumed to be such that the voltage at the base of the transistor 52 is substantially constant for at least 1 cycle of the mains supply voltage.

When the base voltage of the transistor 51 exceeds the base voltage b of the transistor 52 at instant t, the transistor 52 cuts off and, consequently, so does the switching transistor 50. The amplifier discharges the buffer capacitor 18 so that the voltage thereacross decreases exponentionally from point A thereafter in accordance with curve c,.

At the instant the base voltage of the transistor 51 becomes smaller than that of the transistor 52 so that the switching transistor 50 and the diode 53 become conductive. The buffer capacitor 18 is quickly charged and the supply voltage increases exponentionally from point B (curve c2). At the instant t3 the voltage at the supply input 15 decreases to below the supply voltage at the supply output 22 (point C on the curve and reverse-biasses the diode 53, so that the buffer capacitor 18 is prevented from discharging through the voltage divider resistors 54, 55 and the current source circuit 56-59.

The amplifier now again discharges the buffer capacitor 18 so that the supply voltage at output 22 decreases exponentionally from point C in accordance with curve C3. At the instant t4 the diode 53 becomes conductive again because the rectified mains voltage exceeds the supply voltage at output 22 (point D on the curve). In response thereto the buffer capacitor 18 is charged in accordance with curve C4. -~ At the instant t, the base voltage of the transistor 51 exceeds the base voltage of the transistor 52, so that the transistor 52 is cut off and, consequently, so is the switching transistor 50, and the amplifier discharges the buffer capacitor 18. This situation corresponds to the situation at instant 4.The supply voltage at terminal 22 then again decreases exponentionally from point Eon the curve, the curve corresponding to this being similar to the curve c,. In the same manner as that described above for the points B and C the supply voltage curve subsequently passes through discontinuities at the points FandG.

When the amplitude of the audio signal at the audio input 5 increases the control voltage at the control terminal 17 also increases and the voltage at the base of the transistor 52 increases to, for example, level b' in Figure 3. The level b' intersects the curve a2, which, as mentioned above, represents the base voltage of the transistor 51, at instants t,', t2,, 5' and t'. As a result thereof the switching transistor 50 is rendered non-conductive at a later instant in each half-cycle of the mains voltage, and this causes the buffer capacitor 18 to be charged to a higher voltage value (points A' and E').In addition, the switching transistor 50 is rendered conducting again at an earlier instant and is therefore non-conductive for shorter periods of time (t,'-t,' and 5'-t') so that the buffer capacitor 18 is also discharged for shorter periods of time.

The period of time (t3'-t4') in which diode 53 is cut off is now indeed longerthan in the preceding case (t3-t,), bust a suitable choice of the relevant circuit components enables the slightly larger voltage drop associated with the longer discharging period (t,'-') to be kept within acceptable limits.

The result of the base voltage of transistor 52 increasing to level b' is therefore that the supply voltage at the supply output 22 is increased relative to its value in the situation previously described, based on the level b.

An increase in the amplitude of the audio signal therefore results in an increase of the supply voltage at the supply output 22 and vice versa. In a practical construction the various circuit components had the following values: Resistors Values Capacitors Values 54 470 18 1000,u F 55 470 81 51LF 57 33 59 10 60 100 61 2K2 82 2K2 The diodes 53 and 80 were types BYX 22 and BAX 13 respectively, the transistor 50 was of the type BD 262 and the transistors 51, 52, 56 and 58 were of the type BC 107.

Claims

1. An amplifier circuit arrangement comprising a power supply circuit and an amplifier stage a power supply input of which is connected to the output of said power supply circuit characterized in that an audio input of the amplifier stage is coupled to a control input of the supply circuit via an amplitude detector so that in operation the supply circuit will supply the amplifier stage with a supply voltage which varies continuously with the amplitude of an audio signal at said audio signal input and in the same sense.

2. An arrangement as claimed in Claim 1, characterized in that the audio input is coupled to the amplifier stage via a delay circuit.

3. An arrangement as claimed in Claim 1 or 2, characterized in that the supply circuit has an input terminal for a mains voltage supply and comprises a voltage comparator to first and second inputs of which are coupled to said input terminal and said control input respectively, a buffer capacitor, and a switch connected between said input terminal and said buffer capacitor, an output of the voltage comparator being coupled to a control input of the switch.

4. An arrangement as claimed in Claim 3, characterized in that the voltage comparator comprises a long-tailed pair of transistors.

5. An amplifier circuit arrangement comprising a power supply circuit and an amplifier stage a power supply input of which is connected to the output of !said power supply circuit, substantially as described herein with reference to Figure 1 of the drawing or to Figures 1 and 2 of said drawing.