GB1581939A - Linearsed bridges including a semiconductor gas-detecting element - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO LINEARISED BRIDGES INCLUDING A SEMI-CONDUCTOR GAS-DETECTING ELEMENT (71) We, AUERGESELLSCHAFT GMBH, a German Body Corporate, of 24 Friedrich Krause-Ufer, 1000 BERLIN 65 (West), Federal Republic of Germany, do hereby declare the invention, for whidh we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a bridge system comprising a bridge circuit including a semiconductor gas-detecting element and a circuit arrangement for linearising the output signal voltage from the bridge with respect to t'he measured value, which circuit arrangement includes an amplifier circuit containing operational amplifiers.

Hereinafter such bridge system will be referred to as "of the kind described".

Gas-detecting and gas-measuring apparatus is known which operates with semiconductor gas-detecting elements. When acted on by certain gases or vapours, these gas-detecting elements alter their electrical conductivity due to the fact that they become heated. The resultant change in conductivity when the appropriate gas component is present is then measured. The sensitivity of such gas-detecting elements is generally very high. The function representing indication against gas concentration is non-linear: as concentration increases the change in resistance with concentration becomes smaller, so that the change in indication becomes smaller also and the indication of concentration becomes inaccurate.Particularly in the case of combustible gases and vapours in air, the pertinent range of concentrations which has to be detected may be of the order of magnitude of some volumes percent when it is desired to cover the range containing the "lower explosive limit (LEL) Our co-pending patent application no.

20782/77 discloses a gas-detection apparatus having a semiconductor gas-detecting element, in which the dependency of the output signal of the detecting element on gas concentration is made linear to a large extent by connecting a diode in parallel with the gas-detecting element in the circuit in the conductive direction relative to the applied voltage. A disadvantage of this circuit is that the operating range of this diode cannot be set independently of the supply voltage to the bridge.

It is an object of the invention to provide a circuit in which the output signal from the gas-detecting element can be linearised independently of the bridge voltage.

Accordingly the invention consists in a circuit arrangement of the kind described wherein a first resistor which has a nonlinear dependence on voltage is connected into the input circuit of the signal-processing amplifier, and the operatng point of said non-linear resistor is made adjustable by the provision of buffer amplifiers, the sensitivity of the gas-detectng element in the sensitive initial range being determinable by a resistor connected in parallel therewith.

Furthermore, said first voltage-dependent resistor may be a diode, particularly a silicon diode or alternatively a transistor whose collector/emitter path is utilised as a voltage-dependent resistor. To set an advantageous operating point for the diode it is also proposed to make the input resistors of the amplifier circuit of adjustable value.

The advantages achieved by the invention are in particular that it is possible to adjust for adequate linearity of the output signal from a semiconductor gas-detecting element over a wide range of concentrations up to the "lower explosive limit (LEL)", irrespective of the measuring head in the measuring apparatus, so that use can be made of the full range of sensitivity of the gas detecting element.

In order that the invention may be more clcarly understood, reference will now be made to the accompanying drawing which schematically shows one particular embodiment thereof by way of example.

Referring now to the drawing, in a bridge circuit I is arranged the gas-sensitive resistive component of a gas-detecting element 2 together with fixed bridge resistors 3, 4 and 6, 8. The supply voltage to the bridge is applied to points a and b. The bridge is balanced by means of a variable resistor 7. In parallel with the gas-detecting element 2 is a resistor 5 which sets the response range of the gas-detecting element 2 and which is matched to a required value.

The resulting bridge voltage c, d is fed to amplifiils 9 and 10. These amplifiers act in essence as impedance transformers between the relatively high-resistnce bridge output c, d and the succeeding amplifier circuit. From a low-resistance variable resistor 11, the resulting voltage e, f is fed via resistors 12, 13 to a buffer amplifier 14 which has a negative feed-back resistor 15.

In addition, a voltage from a voltage divider 18, 19 is fed via an amplifier 17 without any retroactive effect to the noninverting input g of amplifier 14 via a resistor 16. This voltage, which can be adjusted at resistor 19, provides a biassing voltage for a diode 21 which largely determines the operating point of the diode 21 and thus of the operational amplifier 20 to which it is connected can be fixed by means of variable resistors 23 and 25. By means of diode 22 and resistors 24 and 26, which may be variable resistors to allow exact balancing, it is possible to balance the behaviour of the amplifier circuit with temperature.

Positive voltages which are derived from the operating voltage are fed to points h and n. The output voltage m, p is fed to the remaining circuitry of the apparatus from point 27.

WHAT WE CLAIM IS:- 1. A bridge system of the kind described wherein a first resistor which has a nonlinear dependence on voltage is connected into the input circuit of one of the said operational amplifiers, the operating point of said non-linear resistor is made adjustable by the provision of buffer amplifiers, and the sensitivity of the gas-detecting element in the sensitive initial range is determined by a resistor connected in parallel therewith.

2. A circuit arrangement according to claim 1, wherein said voltage-dependent resistor is a diode.

3. A circuit arrangement according to claim 1 or 2, wherein said voltage-dependent resistor is a silicon diode.

4. A circuit arrangement according to claim 1, wherein said voltage-dependent resistor is a transistor.

5. A circuit arrangement according to any of claims 1 to 4, wherein said signal processing amplifier has resistors of adjustable value connected thereto.

6. A circuit arrangement according to any of claims 1 to 4, wherein a second a voltage-dependent resistor of the same kind as said first voltage-dependent resistor is connected to the non-inverting input of the signal-processing operational amplifier 7. A circuit arrangement substantially as hereinbefore described with reference to the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. detecting element. In order that the invention may be more clcarly understood, reference will now be made to the accompanying drawing which schematically shows one particular embodiment thereof by way of example. Referring now to the drawing, in a bridge circuit I is arranged the gas-sensitive resistive component of a gas-detecting element 2 together with fixed bridge resistors 3, 4 and 6, 8. The supply voltage to the bridge is applied to points a and b. The bridge is balanced by means of a variable resistor 7. In parallel with the gas-detecting element 2 is a resistor 5 which sets the response range of the gas-detecting element 2 and which is matched to a required value. The resulting bridge voltage c, d is fed to amplifiils 9 and 10. These amplifiers act in essence as impedance transformers between the relatively high-resistnce bridge output c, d and the succeeding amplifier circuit. From a low-resistance variable resistor 11, the resulting voltage e, f is fed via resistors 12, 13 to a buffer amplifier 14 which has a negative feed-back resistor 15. In addition, a voltage from a voltage divider 18, 19 is fed via an amplifier 17 without any retroactive effect to the noninverting input g of amplifier 14 via a resistor 16. This voltage, which can be adjusted at resistor 19, provides a biassing voltage for a diode 21 which largely determines the operating point of the diode 21 and thus of the operational amplifier 20 to which it is connected can be fixed by means of variable resistors 23 and 25. By means of diode 22 and resistors 24 and 26, which may be variable resistors to allow exact balancing, it is possible to balance the behaviour of the amplifier circuit with temperature. Positive voltages which are derived from the operating voltage are fed to points h and n. The output voltage m, p is fed to the remaining circuitry of the apparatus from point 27. WHAT WE CLAIM IS:-

1. A bridge system of the kind described wherein a first resistor which has a nonlinear dependence on voltage is connected into the input circuit of one of the said operational amplifiers, the operating point of said non-linear resistor is made adjustable by the provision of buffer amplifiers, and the sensitivity of the gas-detecting element in the sensitive initial range is determined by a resistor connected in parallel therewith.

2. A circuit arrangement according to claim 1, wherein said voltage-dependent resistor is a diode.

3. A circuit arrangement according to claim 1 or 2, wherein said voltage-dependent resistor is a silicon diode.

4. A circuit arrangement according to claim 1, wherein said voltage-dependent resistor is a transistor.

5. A circuit arrangement according to any of claims 1 to 4, wherein said signal processing amplifier has resistors of adjustable value connected thereto.

6. A circuit arrangement according to any of claims 1 to 4, wherein a second a voltage-dependent resistor of the same kind as said first voltage-dependent resistor is connected to the non-inverting input of the signal-processing operational amplifier

7. A circuit arrangement substantially as hereinbefore described with reference to the accompanying drawing.