IL99945A - High input impedance operational amplifier circuit - Google Patents

Description

A HIGH INPUT IMPEDANCE OPERATIOAL AMPLIFIER CIRCUIT A HIGH INPUT IMPEDANCE OPERATIONAL AMPLIFIER CIRCUIT ηπΐ-U nirjD rmu "?m rntu IIUD "PJUTD The present invention relates to high input impedance amplifier circuits.

The problem that the present invention solves is the deficiencies of state of the art high input impedance amplifier circuits.

The present invention solves this problem by providing a new high input impedance operational amplifier circuit, which has several advantages over state of the art high input impedance operational amplifier circuits.

The operational amplifier is a high input impedance component by itself, but state of the art high input impedance circuits are either complicated and costly, or else they degrade the possible performance of the operational amplifier.

Until now high input impedance operational amplifier circuits were implemented by connecting a very large ohmic value resistor from the non inverting input to ground, by not connecting a resistor at all to this input, or by resistance multiplication circuits. These methods require either very large value resistors, which are bulky and costly, or else they create problems with bias currents and offset voltage multiplication. (For information regarding operational amplifiers circuits and terms see, for example, National Semiconductor Corporation Linear Data Book, Volume 1,1988 edition, pages 2-254, 2-335, 2-336. See also Burr-Brown, Operational Amplifiers, Tobey, Graeme, Huelsman, 1971, pages 428-431,438,439, and also McGraw-Hill, Encyclopedia of Electronics and Computers, pages 28-29, and Analog Devices Linear Products Databook, 1988 edition, pages 2-94 to 2-98.) The present invention solves these problems by providing a high input impedance operational amplifier circuit which does not require very high valued resistors, has less components, and does not cause problems with bias current and offset voltage.

These and other advantages of the present invention will be more apparent from the following description and drawings which form an integral part of this disclosure.

In accordance with the present invention there is provided a high input impedance operational amplifier circuit , with unity gain, where the inverting and non inverting inputs of said operational amplifier are connected by a resistor , where said resistor is of relatively small value, where the input signal is connected to the non inverting input of said operational amplifier, and another resistor is connected between the non inverting input and the output of said operational amplifier, said circuit providing high input impedance, despite the fact that the resistor connected between the inverting and non inverting input of said operational amplifier is of relatively small value, the input impedance of said circuit being determined by the value of said relatively small valued resistor , the operational amplifier input offset voltage and input bias current , according to the following equation : Zin=Vin/(Ib+Vos R) Wherein: Zin is the input impedance of circuit, Vin is the input voltage to the circuit, lb is the input bias current of the operational amplifier, Vos is the input offset voltage of the operational amplifier, R is the value of the resistor connected between the inverting and non inverting inputs of the operational amplifier.

The innovation of the present invention is in the simplicity of the circuit, the high input impedance achieved with normal valued resistors, and the fact that bias and offset parameters are not degraded by the new circuit, which is not the case with state of the art circuits. Still another advantage of the present invention is that, by using a suitable guard on a printed circuit board, it provides a very high input impedance circuit, not degraded by leakage, while eliminating costly materials and production methods .

Certain preferred embodiments of the present invention will now be described and illustrated, but it will be understood that it is not intended to limit the invention to these particular embodiments ,and that these embodiments serve as examples only. It is intended to include all alternatives, equivalents and modifications as covered within the scope of the invention as defined by the appended claims.

LIST OF DRAWINGS Fig 1 : A unity gain, high input impedance, operational amplifier circuit.

Fig 2: A high input impedance operational amplifier circuit with an input protection resistor.

Fig 3: A high input impedance operational amplifier circuit with input protection diodes.

Referring to fig. 1 the new, high input impedance operational amplifier circuit is comprised of three main parts: an operational amplifier(l),a resistor connected between the operational amplifier inputs (2), and a resistor (3)connected between the inverting input and the output of the amplifier. The signal source (4) is connected to the non inverting input of the operational amplifier.

Fig. 2 shows the same circuit with the addition of two resistors: one is connected between the inverting input and ground (5),and the other resistor (6)is connected between the signal source (4) and the non inverting input of the operationaf amplifier.

Fig. 3 shows the circuit with input protection diodes(7) connected.

The mode of operation is as follows: the voltage across resistor (2) is the offset voltage of the operational amplifier (Vos).The worst case(biggest) input current of the circuit ( Iin(max)), is the sum of the bias current of the operational amplifier (Ib),and the current in resistor (2): Iin(max)=Ib+Vos/R, where R is the value of resistor (2) connected between the inputs of the operational amplifier.

For example ,an operational amplifier with lb of lOpA and Vos of 0.1 mV ,and a resistor (2) of 10 megaohms ,will result in a worst case input current of 20pA, which would otherwise require a resistor of approximately 500 gigaohms, or when using a resistance multiplication circuit with the same valued resistor, would give an offset voltage of 500 mV, while in the circuit shown the offset of the circuit is the offset voltage of the operational amplifier.

The dynamic input impedance of this new circuit is even higher ,as only the changes of lb with the input voltage changes are involved. The fact that the resistor used can be of a relatively small ohmic value, and hence is of a small physical size, enables the incorporation of a guard on the printed circuit board, eliminating the need for special board base materials, glass resistors, special standoffs, etc., and provides a high input impedance circuit which is not degraded by leakage currents.

The feedback resistor (3) can be eliminated by connecting the inverting input to the output of the operational amplifier ,further simplifying the circuit.

The values of lb, Vos and R can be selected to achieve the desired performance. For example, if the value of R is limited, selecting an operational amplifier with small lb and Vos will allow a smaller value for R(for the same input impedance), but it will be more expensive. Fig. 2 is another embodiment of the invention. An additional resistor (6) is connected between the signal source and the non inverting input of the operational amplifier. Its purpose is to protect the operational amplifier against excessive input current. The other resistor (5)which is shown in fig. 2 , enables gains other than unity. The gain being set by the ratio of the resistance of resistor (3) to the resistance of resistor(6)(provided the value of the gain setting resistors is small relative to the value of R ). If resistor (3) is Rf and resistor (6) is Ra, then the formula for the gain of the circuit, G. is G=l+(Rf7Ra), which is the same as conventional, non inverting circuits, with the added advantage of the very high input impedance, while using a relatively small valued resistor R.

Fig. 3 shows diodes (7) connected to protect the operational amplifier against over voltage at its input. The leakage current of the diodes must be very low, in the same order of magnitude as lb or lower, in order to avoid degrading the input impedance of the circuit. Other protection schemes are also possible.

In addition to its abovementioned advantages, the invention provides for a circuit with excellent electrostatic discharge ( ESD) protection, even when the circuit is unpowered, because all the terminals are terminated with relatively small valued resistors.

The embodiments and illustrations 'presented are to be considered illustrative and not restrictive so that the appended claims indicate the scope of the invention rather than the description, and all the changes and variations implied by the claims are therefore embraced within the claims.

Claims (7)

1. ) A high input impedance operational amplifier circuit , with unity gain, where the inverting and non inverting inputs of said operational amplifier are connected by a resistor , where said resistor is of relatively small value, where the input signal is connected to the non inverting input of said operational amplifier, and another resistor is connected between the non inverting input and the output of said operational amplifier, said circuit providing high input impedance, despite the fact that the resistor connected between the inverting and non inverting input of said operational amplifier is of relatively small value, the input impedance of said circuit being determined by the value of said relatively small valued resistor , the operational amplifier input offset voltage and input bias current , according to the following equation : Zin = Vin/(Ib+Vos R) Wherein. Zin is the input impedance of circuit, Vin is the input voltage to the circuit, lb is the input bias current of the operational amplifier, Vos is the input offset voltage'of the operational amplifier, R is the value of the resistor connected between the inverting and non inverting inputs of the operational amplifier.

2. ) A high input impedance operational amplifier circuit, as claimed in claim number 1, with the addition of a resistor connected between the inverting input of said operational amplifier and ground, providing gain according to the equation : G = 1+Rf/Ra wherein: G is the gain, Rf is the value of the resistor connected between the inverting input and the output of the operational amplifier, Ra is the value of the resistor connected between the inverting input of the operational amplifier and ground.

3. ) A high input impedance operational amplifier circuit, as claimed in previous claims, with the addition of a resistor connected between the signal source and the non inverting input of the operational amplifier, serving to limit the current flowing in the input of the operational amplifier in case of a fault condition

4. ) A high input impedance operational amplifier circuit as claimed in previous claims, with a guard around its inputs.

5. ) A high input impedance operational amplifier circuit as claimed in previous claims, with input protection diodes.

6. ) A high input impedance operational amplifier circuit as described with reference to the accompanying drawings.

7. ) A high input impedance operational amplifier circuit comprising of any combination of parts and methods as claimed in previous claims. CHEMTRONICS Ltd. ISRAEL RESHEF