GB2377848A

GB2377848A - Wheatstone bridge microphone circuit

Info

Publication number: GB2377848A
Application number: GB0117743A
Authority: GB
Inventors: Andrew Michael Wilkes
Original assignee: NEC Technologies UK Ltd
Current assignee: NEC Technologies UK Ltd
Priority date: 2001-07-20
Filing date: 2001-07-20
Publication date: 2003-01-22
Anticipated expiration: 2021-07-20
Also published as: GB0117743D0; GB2377848B

Abstract

A transducer circuit is described in which an electret condenser microphone 5 is included in one arm of a bridge circuit and a noise signal appearing in the Wheatstone bridge circuit with a bias voltage 7 is balanced out as a result of the application of outputs from the bridge circuit to a differential amplifier 9. The circuit may contain a reactive element in one arm of the bridge to balance the impedance of the transducer 5.

Description

TRANSDUCER CIRCUIT This invention relates to a transducer circuit.

A transducer circuit in the form of a microphone circuit to be described below, by way of example in illustration of the present invention, minimises the effect of noise in a comparatively simple way. The circuit, in the example, employs an electret condenser microphone in one limb of a Wheatstone bridge, in which each arm of the bridge has substantially the same value. The transducer circuit may convert other signals than sound to electrical signals. For example, the transducer may be an optical coupler transducing light, or a strain gauge producing an electrical signal in response to a physical movement.

A circuit arrangement will now be described, by way of example, in the belief that it will enable the invention to be understood, with reference to the single figure of the accompanying drawing.

Referring to the Figure, there is shown a bridge circuit having a respective resistor of value R and labelled 1,2, 3 and 4 in each arm of the bridge. The resistor 4 is constituted by the inherent resistance value of a transducer in the form of an electret condenser microphone, which is indicated at 5, which includes a capacitive element and which provides a wanted signal Vs, as indicated at 6. A d. c. power supply 7 which is connected from a node formed by a junction a between the resistors 1 and 2

to a node formed by a junction b between the microphone 5 and the resistor 3 provides a d. c. bias current to the microphone 5. It is the case that noise, indicated at 8 by Vn, is liable to appear in the microphone circuit, generally having been introduced via the power supply line associated with the power supply 7, and this noise is applied, in the particular circuit being described, to a differential amplifier, such as that shown at 9 to provide an output. The amplifier 9 is connected from the junction c between the resistor 4 and the resistor 1 to the junction d between the resistor 3 and the resistor 2.

The bridge in this arrangement provides signal inputs to the differential amplifier 9 in which the noise Vn is attenuated, or nullified.

If necessary, a capacitor 11 having a value C may be introduced, as indicated by the dotted lines across the resistor 3, in order to balance the effect of the capacitance of the microphone 5. Furthermore, the resistors 2, 3 and 4 may be replaced by impedances of corresponding value to that of the microphone 5.

Considering the operation of the circuit shown in the Figure, a first potential divider connected across the bias voltage supply 7 between the nodes a and b is formed by the resistor 1 in series with the microphone 5, and the microphone 5 is referenced to zero volts. The microphone 5 is thus d. c. biased, and the microphone signal Vs is output from the first potential divider, plus any power supply noise Vn indicated at 8.

A second potential divider, which is also connected between the nodes a and b across the bias voltage supply 7, is formed by the resistors 2 and 3. As has been explained, the resistors 1,2 and 3 have the same value

of resistance as the nominal resistance value 4 of the microphone 5. The output from the second potential divider at the node d will include the power supply noise Vn with an equal amplitude and phase to that of the noise Vn which is included at the node c in the output from the first potential divider.

By the application of the outputs at c and d from the first and second potential dividers to the respective inputs of the differential amplifier 9, it is possible to obtain an output from the differential amplifier 9 from which the unwanted noise component Vn included in the two inputs has been cancelled. The output from the amplifier 9 is thus restricted to the wanted microphone signal Vs.

It will be understood that, in practice, it may not be possible to obtain the complete elimination of the unwanted noise, due to the difficulty in obtaining an exact match between the impedances of the microphone and those of the resistors 1,2 and 3. However, considerable degrees of attenuation of the noise Vn are possible.

It will be appreciated that, although a particular arrangement has been described, by way of example, in order to enable the invention to be understood, variations and modification thereof as well as other arrangements may be conceived within the scope of the appended claims.

For example, other forms of bridge may be employed, as may other forms of cancellation circuit than an amplifier having a differential input.

It would also be possible for the bridge to be inverted with the transducer represented by the microphone 5 and the balancing resistor or impedance 3 referenced to the positive supply rail a. Furthermore, any

impedance of the transducer 5 may be balanced by a reactive element in one arm of the bridge.

It will be noted that the transducer generator, represented by the microphone 5, is in one arm of the bridge rather than across the voltage supply 7. It will also be appreciated that, although in the particular arrangement described by way of example, a Wheatstone bridge is employed, other well known forms of bridge circuit may be employed.

Claims

1. A transducer circuit including a bridge circuit, the bridge circuit having first and second pairs of arms connected across first and second nodes, one of the first pair of arms including a transducer, first and second potential divider outputs being provided respectively at the junction between the first pair of arms and the junction between the second pair of arms, and a differential amplifier having first and second inputs, the first and second potential divider outputs being connected to the first and second inputs of the differential amplifier respectively.

2. A circuit as claimed in claim 1 in which the bridge circuit is a Wheatstone bridge.

3. A circuit as claimed in either claim 1 or claim 2 including a reactive element in one arm of the bridge to balance the impedance of the transducer.

4. A circuit as claimed in any one of the preceding claims in which the transducer is an electret condenser microphone.

5. A circuit as claimed in claim 1 including an arrangement substantially as described herein with reference to the single figure of the accompanying drawings.

5. A circuit as claimed in claim 1 including an arrangement substantially as described herein with reference to the single figure of the accompanying drawing.

Amendments to the claims have been filed as follows CLAIMS

1. A transducer circuit including a bridge circuit, the bridge circuit having first and second pairs of arms connected across first and second nodes, one of the first pair of arms including a transducer, d. c. supply means arranged to provide a d. c. bias current to the transducer, first and second potential divider outputs being provided respectively at the junction between the first pair of arms and the junction between the second pairs of arms, a differential amplifier having first and second inputs, wherein the first and second potential divider outputs are connected to the first and second inputs of the differential amplifier respectively, and wherein the d. c. supply means is connected across the said first and second nodes of the bridge circuit.