GB2043380A - Deriving a signal proportional to the difference between two currents - Google Patents
Deriving a signal proportional to the difference between two currents Download PDFInfo
- Publication number
- GB2043380A GB2043380A GB8003184A GB8003184A GB2043380A GB 2043380 A GB2043380 A GB 2043380A GB 8003184 A GB8003184 A GB 8003184A GB 8003184 A GB8003184 A GB 8003184A GB 2043380 A GB2043380 A GB 2043380A
- Authority
- GB
- United Kingdom
- Prior art keywords
- circuit
- input
- difference
- amplifier
- input amplifiers
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Withdrawn
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Classifications
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03F—AMPLIFIERS
- H03F3/00—Amplifiers with only discharge tubes or only semiconductor devices as amplifying elements
- H03F3/45—Differential amplifiers
- H03F3/45071—Differential amplifiers with semiconductor devices only
- H03F3/45479—Differential amplifiers with semiconductor devices only characterised by the way of common mode signal rejection
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- Engineering & Computer Science (AREA)
- Power Engineering (AREA)
- Amplifiers (AREA)
Abstract
Two input amplifiers (17 and 18) connected as current to voltage converters receive respective current signals, and their outputs are summed and integrated by a further amplifier (21) which provides a reference signal to the two input amplifiers so as to balance out common mode signals at the output terminals of the input amplifiers. The circuit also includes a difference amplifier (27) receiving inputs from two input amplifiers and having an adjustable offset nulling network (29 to 37). <IMAGE>
Description
SPECIFICATION
Circuit for generating an electrical signal proportional to the difference between two current signals
This invention relates to a circuit for generating an electrical signal proportional to the difference between two current signals.
One application of such a circuit is in a corona discharge type gas flow-meter intended for example, for incorporation in a road vehicle fuel injection and/or ignition control system. In such a meter a corona discharge is established between a discharge electrode and a split collector electrode. The current flowing from the discharge electrode is collected by the collector electrode which has two terminals such that the difference between the amount of current flowing to each terminal depends on the mass flow of gas through the space between the electrodes, the impedance of the discharge being maintained constant by means of a variable EHTsupply control with a feedback circuit.
Conventionally the current from the two terminals of the collecting electrode is applied to the inputs of two current-to-voltage converters. The outputs of the current-to-voltage converters are applied to a difference amplifier and a further adjustable amplifier is used to determine the gain of the circuit. Each converter and each amplifier in such an arrangement requires a separate offset nulling control which is complicated, but the main disadvantage of the conventional circuit is that, owing to the standing current in each inputterminal a large common mode voltage will appear at the outputs of the current to voltage converters. This common mode voltage has the effect of reducing the dynamic range of the amplifier.It might be offset by supplying a fixed reference signal to the inputs, but in this case atmospheric changes would cause the required reference voltage to change, because of variations in the total discharge current.
It is an object of the present invention to provide a circuit by means of which the problems are avoided.
According to the invention there is provided a circuit for producing an electrical signal proportional to the difference between two current signals, comprising a pair of input amplifiers operating as current-to-voltage converters and having the two current signals applied respectively thereto, means for applying a reference signal to the two input amplifiers and including a further amplifier connected to vary said reference signal in accordance with the sum of the output signals of the input amplifiers so as to cause said sum to approach zero, and a difference amplifier having its inputs connected to the outputs of the two input amplifiers.
Preferably the further amplifier has its inverting input connected by equal value resistors to the outputs of input amplifiers, its non-inverting input grounded and a feedback capacitor between its output and its inverting input so that the reference signal is proportional to the integral of the sum of the output signals of the input amplifiers.
Preferably, the difference amplifier includes an offset nulling circuit as hereinafter described.
An example of the invention is shown in the accompanying drawing which is diagram of the circuit as applied to a corona discharge type gas flow
meter.
The flow meter includes a duct 10, through which gas flows. Inside the duct is a discharge electrode 11 and a split collecting electrode 12a, 12b with terminals 13a and 13b. The voltage on the discharge electrode 11 is controlled by an EHT supply 14 which operates to maintain the discharge impedance constant. The EHT supply is controlled by a comparator
15 which receives an input representing the actual total discharge current and a feedback input via a resistor 16 connected to the output of the EHT supply.
The terminals 13a and 1 3b are connected to the inverting input terminals of two operational amplifiers 17 and 18 respectively. Each such amplifier has a feedback resistor 19,20 so that it acts as a currentto-voltage converter. The non-inverting inputs of amplifiers 17 and 18 are connected to the output of a further operational amplifier 21 which has its noninverting input grounded and its inverting input connected by two equal value resistors 22, 23 to the outputs ofthetwo amplifiers 17,18. Acapacitor24 connects the outputterminal of amplifier21 to its inverting input.
Amplifier 21 acts as an integrator, integrating the sum of the outputs of the amplifiers 17, 18. In equilibrium the reference signal applied to the noninverting inputs of the amplifiers 17, 18 will be such that the outputs of the amplifiers 17, 18 will be of equal magnitude, but opposite sign, i.e. their sum will be zero. Should the sum be other than zero, the error will be integrated out, changing the reference voltage until the sum is zero.
Equal value resistors 25,26 connect the outputs of amplifiers 17, to the inverting and non-inverting inputs of an operational amplifier 27 operating as a difference amplifier. A feedback resistor 28 connects the output of amplifier 27 to its inverting input and two resistors 29,30 in series connect the noninverting input to earth, the resistor 30 being of very low value compared with the resistor 29 and the sum of the values of resistors 29 and 30 being equal to the value of resistor 28. The junction of resistors 29 and 30 is connected to the slider of a potentiometer 31 the ends of which are connected by padding resistors 32,33 to positive and negative fixed voltage points established by zener diode/resistor combinations 34,35 and 36,37 respectively.Potentiometer 31 is adjusted to null offsets which occur in the difference amplifier or in the following gain stage (yet to be described).
The output of amplifier27 is connected directlyto the non-inverting input of an operational amplifier 38 in which feedback is provided by a variable resistance 39 connected between the output and the inverting input, a further resistor 40 connecting the inverting input to earth. The variable resistance 39 is adjusted to obtain the required gain.
As explained above, the use of an integrator for determining the reference voltage applied to the input amplifiers ensures that there are no common mode errors arising in the difference amplifier and also ensures maximum dynamic range. This and the simple offset adjustment circuit used makes it feasible for the circuit described to be mass producedonly two potentiometer need to be preset.
The comparator 15 in fact, receives its 1, + 12 input from the output of amplifier 21 as shown.
Claims (5)
1. A circuit for producing an electrical signal proportional to the difference between two current signals, comprising a pair of input amplifiers operating as current-to-voltage' converters and having the two current signals applied respectively thereto, means for applying a reference signal to the two input amplifiers and including a further amplifier connected to vary said reference signal in accordance with the sum of the output signals of the input amplifiers so as to cause said sum to approach zero, and a difference amplifier having its inputs connected to the outputs of the two input amplifiers.
2. A circuit as claimed in claim 1 in which said further amplifier has its inverting input connected by equal value resistors to the outputs of input amplifiers, its non-inverting input grounded and a feedback capacitor between its output and its inverting input so that the reference signal is proportional to the integral of the sum of the output signals of the input amplifiers.
3. A circuit as claimed in claim 1 or claim 2 in which the difference amplifier has an adjustable offset nulling circuit.
4. A circuit as claimed in claim 3 in which said offset nulling circuit includes a potentiometer connected between positive and negative fixed voltage points, the potentiometer slider being connected to the non-inverting input of the difference amplifier.
5. A circuit for producing an electrical signal proportional to the difference between two current signals substantially as hereinbefore described with reference to the accompanying drawing.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB8003184A GB2043380A (en) | 1979-02-24 | 1980-01-30 | Deriving a signal proportional to the difference between two currents |
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB7906601 | 1979-02-24 | ||
| GB8003184A GB2043380A (en) | 1979-02-24 | 1980-01-30 | Deriving a signal proportional to the difference between two currents |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| GB2043380A true GB2043380A (en) | 1980-10-01 |
Family
ID=26270688
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB8003184A Withdrawn GB2043380A (en) | 1979-02-24 | 1980-01-30 | Deriving a signal proportional to the difference between two currents |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2043380A (en) |
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4331037A (en) * | 1980-06-02 | 1982-05-25 | Tsi Incorporated | Fluid flow measuring apparatus |
| GB2122780A (en) * | 1982-04-26 | 1984-01-18 | Sharp Kk | Program modification system |
| US4441371A (en) * | 1981-04-04 | 1984-04-10 | Lucas Industries Limited | Gas flow meters |
| WO1996019866A1 (en) * | 1994-12-22 | 1996-06-27 | Whitlock William E | Differential audio line receiver |
-
1980
- 1980-01-30 GB GB8003184A patent/GB2043380A/en not_active Withdrawn
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4331037A (en) * | 1980-06-02 | 1982-05-25 | Tsi Incorporated | Fluid flow measuring apparatus |
| US4441371A (en) * | 1981-04-04 | 1984-04-10 | Lucas Industries Limited | Gas flow meters |
| GB2122780A (en) * | 1982-04-26 | 1984-01-18 | Sharp Kk | Program modification system |
| WO1996019866A1 (en) * | 1994-12-22 | 1996-06-27 | Whitlock William E | Differential audio line receiver |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| WAP | Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1) |