Intrinsic offset recovery circuit particularly for amplifiers

Abstract: Intrinsic offset recovery circuit particularly for amplifiers, which comprises an input differential amplifier constituted by a first PNP transistor (1), by a second PNP transistor (2), by a third NPN transistor (3), by a fourth NPN transistor (4) and by a first constant-current source (8), and a unitary-gain output stage (10). The recovery circuit furthermore comprises, as connection between the input differential amplifier and the unitary-gain output stage (10), a gain stage which comprises a fifth NPN transistor (6) which is connected to the output of the input differential amplifier and is connected to a sixth NPN transistor (7) and to a seventh PNP transistor (11). The seventh transistor (11) is connected to the sixth transistor (7). The seventh transistor (11) and the sixth transistor (7) are connected to the unitary-gain output stage (10).

Description: The present invention relates to an intrinsic offset recovery circuit particularly for amplifiers. The use of differential amplifiers in operational amplifiers and in other types of circuits is currently extremely widespread. In the applications of these amplifiers in which good precision is required, said differential amplifiers must have a low offset. This means that the device, such as an operational amplifier, which includes the differential amplifier must be balanced, i.e. the voltage across the differential circuit must be zero. The offset can be due on one hand to mismatches of the various components which constitute the device and on the other hand to the type of circuit itself, which produces systematic offsets, also known as intrinsic offsets. Document EP-A-0 072 082 describes a differential amplifier circuit with precision active load as defined in the preamble of claim 1. The aim of the present invention is to eliminate the disadvantages described above in known types of operational amplifiers by providing an intrinsic offset recovery circuit particularly for amplifiers which eliminates intrinsic offsets. Within the scope of the above aim, an object of the present invention is to provide a recovery circuit which can be easily integrated. Not least object of the present invention is to provide a recovery circuit which is relatively easy to manufacture and at competitive costs. This aim, the objects mentioned and others which will become apparent hereinafter are achieved by an intrinsic offset recovery circuit particularly for amplifiers, according to the invention, which comprises an input differential amplifier constituted by a first PNP transistor and by a second PNP transistor, the emitters of said first and said second PNP transistor being connected to a first current source, said input differential amplifier further comprising a third NPN transistor and a fourth NPN transistor with respective bases connected with each other, the collectors of said third and fourth NPN transistors being connected to the collectors of said first and second PNP transistors respectively, the base of said fourth NPN transistor being connected to the collector of the same; a sixth NPN transistor whose collector is connected in series with a second current source; characterized in that it further comprises a fifth NPN transistor whose base is connected to the collector of said first PNP transistor and whose collector is connected in series with a third current source and whose emitter is connected to the base of said sixth NPN transistor and in series with a resistor; and a seventh PNP transistor whose base is connected to the collector of said fifth NPN transistor, whose emitter is connected to the collector of said sixth NPN transistor and whose collector is connected to the base of said sixth NPN transistor; said third current source being able to generate a current equal to the current generated by said first current source. Further characteristics and advantages of the invention will become apparent from the description of a preferred but not exclusive embodiment of an intrinsic offset recovery circuit particularly for amplifiers., according to the invention, illustrated only by way of non-limitative example in the accompanying drawings, wherein: figure 1 is a circuit diagram of a known operational amplifier; and figure 2 is a circuit diagram of an operational amplifier provided with a recovery circuit according to the invention. With reference to figure 1, a known operational amplifier comprises an input stage constituted by a differential amplifier. The differential amplifier comprises a first PNP transistor 1 and a second PNP transistor 2, whose base terminals are affected by the input signals V⁺ and V⁻, as well as a third NPN transistor 3 and a fourth transistor 4, whose emitter terminals are connected to one another, to a terminal of a resistor 5 and to the emitter terminal of a sixth NPN transistor 7. The collector terminals of the first transistor 1 and of the third transistor 3 are mutually connected, and so are the related collector terminals of the second transistor 2 and of the fourth transistor 4. The emitter terminals of the first transistor 1 and of the second transistor 2 are connected to one another and to a first constant-current source 8. The base terminals of the third transistor 3 and of the fourth transistor 4 are connected to one another and to the collectors of the second transistor 2 and of the fourth transistor 4. The output of the differential stage branches from the connection between the collectors of the first transistor 1 and of the third transistor 3, and is connected to the base terminal of a fifth NPN transistor 6. The emitter terminal of the fifth transistor 6 is connected, by means of the resistor 5, to the emitter terminals of the third transistor 3 and of the fourth transistor 4 and to the emitter terminal of the sixth transistor 7. The collector terminal of the fifth transistor 6 is connected to a second constant-current source 9, to the collector terminal of the sixth transistor 7 and to the input terminal of the unitary-gain output stage 10. The connection of the fifth transistor 6 and of the sixth transistor 7 is the well-known Darlington amplifier or connection. The collector currents of the third transistor 3 (I C3 ) and of the fourth transistor 4 (I C4 ) are identical, since the voltages between the base and the emitter in said transistors are the same. If the output differential voltage (V D = V⁺ - V⁻) must be zero when the amplifier is balanced, the collector currents of the first transistor 1 and of the second transistor 2 must be equal (I C1 = I C2 ). In order to obtain this result, the base current of the fifth transistor 6 (I B6 ) must be equal to twice the base current (I B4 ) of the fourth transistor 4, i.e. I B6 = 2I B4 . Assuming the third transistor 3, the fourth transistor 4 and the fifth transistor 6 are identical, the collector current of the fifth transistor 6 must be equal to twice the collector current of the fourth transistor 4, i.e. I C6 = I₈. The technical problem which the present invention aims to solve is that the collector current I C6 of the fifth transistor 6 varies considerably, especially with the temperature, since it depends on the base-emitter voltage V BE7 of the sixth transistor 7 and on the resistance value of the resistor 5, whereas I₈ cannot be made to vary in the same manner, since the current of the differential amplifier must generally be constant in order to avoid frequency instability problems. With reference to figure 2, an intrinsic offset recovery circuit particularly for amplifiers comprises an input differential amplifier, constituted by a first PNP transistor 1, by a second PNP transistor 2, by a third NPN transistor 3, by a fourth NPN transistor 4 and by a first constant-current source 8, whose reference numerals correspond to those given previously, and a unitary-gain output stage 10. The recovery circuit furthermore comprises, as connection between the input differential amplifier and the unitary-gain output stage 10, a gain stage which comprises a fifth NPN transistor 6, which has its base terminal connected to the output of the input differential amplifier and is connected to a sixth NPN transistor 7 and to a seventh PNP transistor 11. Said seventh transistor is connected to the sixth transistor 7. The seventh transistor 11 and the sixth transistor 7 are connected to the unitary-gain output stage 10. The emitter terminal of the seventh transistor 11 is connected to a second constant-current source 9, to the collector terminal of the sixth transistor 7 and to the unitary-gain output stage 10. The base terminal of the fifth transistor 6 is connected to the collector terminals of the first transistor 1 and of the third transistor 3. The emitter terminal of the fifth transistor 6 is connected, by means of a resistor 5, to the emitter terminals of the third transistor 3, of the fourth transistor 4 and of the sixth transistor 7, to the base terminal of the sixth transistor 7 and to the collector terminal of the seventh transistor 11. The collector terminal of the fifth transistor 6 is connected to the base terminal of the seventh transistor 11 and to a third constant-current source 12. In the invention, the fifth transistor 6 is driven by the current I₁₂ of the third source 12, ignoring the base current I B11 of the seventh transistor 11. The current I₁₂ must always be smaller than the current required in order to exactly have a base-emitter voltage V BE , and in particular the base-emitter voltage V BE7 of the sixth transistor 7, on the resistor 5, otherwise the sixth transistor 7 would be in its saturation operating area. The remaining amount of current required in order to exactly have the base-emitter voltage V BE7 on the resistor 5 and the base current I B7 are provided by the seventh transistor 11. If I₈ = I₁₂ in the circuit according to the invention, then I B6 = 2I B4 , i.e. the above described balance condition occurs, and the intrinsic offsets across the differential amplifier due to the different collector currents of the third transistor 3 and of the fourth transistor 4 are eliminated. Advantageously, the circuit according to the invention allows to provide an operational amplifier which has no intrinsic offsets, i.e. offsets not due to mismatches of the various components. The condition for achieving this purpose is that the relation I₈ = I₁₂ must be true, since it is relatively easy, especially in integrated circuits, to provide identical current sources. Conveniently, the collector current I C6 of the fifth transistor 6 is constant in all operating conditions, whereas on the contrary, in the circuit illustrated in figure 1 which relates to the known art, the collector current I C6 must also provide the base current I B7 to the sixth transistor 7, and this current may be very high: generally, in fact, I g >> I C6 , and the current furthermore varies in dynamic conditions together with the output signal. In applications, such as variable-transconductance amplifiers, in which a variable I₈ is required, zero offset is advantageously obtained in all operating conditions by varying I₁₂ in the same manner. The invention thus conceived is susceptible to numerous modifications and variations, all of which are within the scope of the inventive concept. All the details may furthermore be replaced with other technically equivalent elements. In practice, the materials employed, as well as the dimensions, may be any according to the requirements. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the scope of each element identified by way of example by such reference signs.

Claims: Intrinsic offset recovery circuit particularly for amplifiers, comprising an input differential amplifier constituted by a first PNP transistor (1) and by a second PNP transistor (2), the emitters of said first and said second PNP transistor (2) being connected to a first current source (8), said input differential amplifier further comprising a third NPN transistor (3) and a fourth NPN transistor (4) with respective bases connected with each other, the collectors of said third and fourth NPN transistors being connected to the collectors of said first and second PNP transistors respectively, the base of said fourth NPN transistor being connected to the collector of the same; a sixth NPN transistor (7) whose collector is connected in series with a second current source (9); characterized in that it further comprises a fifth NPN transistor (6) whose base is connected to the collector of said first PNP transistor (1) and whose collector is connected in series with a third current source (12) and whose emitter is connected to the base of said sixth NPN transistor (7) and in series with a resistor (5); a seventh PNP transistor (11) whose base is connected to the collector of said fifth NPN transistor (6), whose emitter is connected to the collector of said sixth NPN transistor (7) and whose collector is connected to the base of said sixth NPN transistor; said third current source (12) being able to generate a current equal to the current generated by said first current source (8). Circuit, according to claim 1, characterized in that the collector of said sixth NPN transistor (7) is connected to an input of a unitary-gain output stage (10).

Inventors: BRAMBILLA DAVIDE, STEFANI FRABRIZIO, BRAMBILLA, DAVIDE, STEFANI, FRABRIZIO
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1991
Main classification: H03F3
IPC codes: H03F3