GB2071945A - Apparatus for controlling the amplitude of an analog signal - Google Patents

Abstract

Apparatus for controlling the amplitude of an analog signal comprises a first and second impedance transformer (X,Y) and a resistance network (R4) connected between the output terminal of the first impedance transformer and the input terminal of the second impedance transformer. To control the transmission properties of the resistance network, two semi-conductor switches (S,T) are connected in series between points of the network (namely the output of X, and earth, respectively) carrying a signal voltage with respect to each other, both of which points are connected to the earth potential by means providing a low resistance (the output impedance of X is low). The junction of the switches is connected to the input of the second impedance transformer by means (R5) providing a low resistance. <IMAGE>

Description

SPECIFICATION Apparatus for controlling the amplitude of an analog signal This invention relates to apparatus for controlling the amplitude of an analog signal. More particularly, the invention relates to such apparatus of the general type comprising a first and a second impedance transformer and a resistance network arranged to transmit the analog signal from the output terminal of the first impedance transformer to the input terminal of the second impedance transformer, a control semiconductor switching means being provided for controlling the transmission properties of the resistance network and, consequently the amplitude of the analog signal appearing at the output terminal of the second impedance transformer.

The existing amplitude control devices of the kind above referred to are not entirely satisfactory. The transmission properties of the resistance network will vary with the magnitude of the signal supplied to the network, resulting in distorsion of the signal.

Moreover, comparatively strong switching transients ("spikes") will be present in the controlled signal appearing at the output end of the device.

The present invention, the general features of which are set out in the appended claims, has for its purpose to eliminate or substantially reduce the disadvantages above referred to.

The factors underlying the disadvantages above referred to shall now be explained in more detail, and embodiments of the invention shall be described, in the following description with reference to the drawings, in which Figure 1 is a diagrammatic representation of a field effect transistor, Figure2 is a diagrammatic representation of a field effect transistor with integrated driver circuit, Figure 3a is another diagrammatic representation of a field effect transistor with integral driver circuit, Figure 3b is a diagram showing an analog signal and the switching transients caused by the make and break operations of the transistor, Figure 4 shows a schematic circuit diagram of a usual type of amplitude control device of the prior art, Figure 5a is a schematic diagram of an embodiment of the present invention, Figure Sb is a schematic diagram illustrating a variant of the embodiment represented in Figure 5a.

The field effect transistor represented in Figure 1 may be of any of the types now available (J-FET, N-MOS, P-MOS, C-MOS). The diagram represents only the elements, or parameters, of the transistor the properties of which have to be taken into account in the present context. The resistance RDs of the transistor can assume either a low value RON or a high value ROFF, depending on whether the potential of the transistor gate is at one or the other of two control potentials supplied through suitable switching means. In present commercial transistor switches, the field effect transistor is frequently integrated with suitable eletronic control circuitry, for instance in the C-MOS series 4016. In this context, two properties of the switching means referred to are important.

1. The resistance RON of the field effect transistor in its conducting state varies with the transistor voltages. Therefore, the resistance RON will depend on the properties of the driver circuit which are dissimilar in different types of transistor switches with integrated driver circuit. Moreover, all transistor switches irrespective of type have the property that RON varies with the voltage of the analog in or out signal.

2. In the moment in which the switch-over occurs, disturbing pulses will be injected into the signal circuit by the gate voltage through the capacitances C05 and CGD. At resistive loads, said disturbing impulses will have the shape of "spikes", as shown in Figure 3.

The properties above described seriously impair the transmission quality in existing amplitude control apparatus of the kind in which the transistor switch is connected in a resistive network to control the transmission properties thereof. An important: application of apparatus of this kind is the so-called "electronic potentiometer" used to control the level of an audio signal.

Figure 4 represents the circuit of a well-known device of this kind. The device includes a transistor controlled switch which may be connected either as shown at "A" or as shown at "B". The properties of the circuit will be somewhat differently affected in said two positions of the switch. In the position "A", the variation of the resistance RON discussed in the passage "1" above does not appreciably affect the transmission properties of the circuit, as the variation in the analog voltage at the input and output of the switch is very small, RON being lowerthan R3. On the other hand, operation of the switch will superimpose large "switching spikes" on the analog signal.

In the position "B" of the switch, switching spikes will also be produced; moreover, the variations of RON will have a larger effect on the signal, as in this case the signal voltage supplied to the input and output of the switch is equal to the output signal of the circuit. In both cases, distortion of the analog signal will result.

The disadvantages of the circuit described will be substantially eliminated by the circuit of the present invention, an embodiment of which is shown in Figure 5a. A resistive network comprising resistors R4, R5 and two semiconductor switches S, T is connected between two impedance transformers X, Y, both of which have a high input resistance and low output resistance. Both of the impedance transformers may be operational amplifiers with negative voltage feed-back, as shown in Figure 5, with or without gain. The switches S, Tare controlled, by means well-known to those skilled in the art, in such a way as to be alternately conducting and insulating, one switch being conducting during the insulating interval of the other switch, and vice versa.When the switch S is closed, resistors R4 and Rg are connected in parallel, whereby a high signal amplitude is transmitted to the impedance transformer Y. When the switch T is closed, the resistor R5 is connected in a transversal branch between the output end of resistor R4 and earth, whereby a lower signal amplitude is transmitted to the impedance transformer Y. It will be noted that the input of switch S is permanently connected to the earth through the low output resistance ROUT of the impedance transformer X, and that the output of switch T is directly connected to the earth. As one or the other of the switches T, S is always closed, it follows therefrom that the junction of switches T, S is always coupled to earth through a low resistance. Consequently, the amplitude of the switching spikes injected into the signal circuit will always be low.When switch S is closed and switch T is open, the input and output of the switch S is subjected to a large amplitude of the analog signal, resulting in a large variation of the resistance RON of switch S. As, however, the switch S is connected to the high input resistance of the impedance transformer Y, and as the nonlinearily of the output signal is inversely proportional to the load, the distortion can be made as low as desired by the choice of a sufficiently high input resistance of the impedance transformer Y. When the switch S is open and the switch T is closed, the amplitude of the analog signal supplied to the switch T is low (RON must be less than R5; cf. Figure 4), resulting in a low variation of RON with the signal and consequently, a low distortion of the output signal.

A DC component present in the output signal of the impedance transformer X may produce undesirable effects, particularly if the apparatus forms part of an "electronic potentiometer". In order to prevent this, a coupling capacitor C may be inserted between the impedance transformer X and its output terminal, as shown in Figure 5b.

Claims (4)

1. Apparatus for controlling the amplitude of an analog signal comprising a first and a second impedance transformer having each an input terminal and an output terminal, the output resistance of each of said impedance transformers being lower than its input resistance, a resistance network connected between the output terminal of said first impedance transformer and the input terminal of said second impedance transformer, said resistance network comprising a) a first branch comprising resistor means providing a resistive coupling between the output terminal of said first impedance transformer and the input terminal of said second impedance transformer, b) a second branch comprising two series connected semiconductorswtiches, each of which is arranged to be conducting when the other switch is nonconducting, said switch branch being connected between two points of said resistance network carrying a signal voltage with respect to each other, each of said points being connected to zero potential with respect to the signal voltage through means providing a resistance lower than the resistance of each of the switches in the conducting state, c)a third branch comprising resistor means providing a resistive coupling between the junction of the switches and the input terminal of said second impedance transformer, the output resistance of said first impedance transformer not exceeding the resistance of each of said semiconductor switches in its conducting state, and the input resistance of said second impedance transformer exceeding the resistance provided by each of said first and third branches.

2. Apparatus as claimed in Claim 1 in which said second branch is connected between the output terminal of said first impedance of transformer and a point at zero potential.

3. Apparatus as claimed in Claim 1 in which a circuit element allowing the transmission of altering current only is provided between the first impedance transformer and its output terminal.

4. Apparatus for controlling the amplitude of an analog signal substantially as herein described with reference to Figures 5a and 5b of the drawings.