The instant invention relates to a differential current amplifier permitting to compare two currents with a very high accuracy and to supply an output voltage which is proportional to the difference between those currents.
It is well known that, when one tries to compare currents with a high accuracy by means of integrated circuits, one is limited by the offset voltage of the differential amplifiers. This offset voltage is the difference between the input voltages for a null voltage at the output.
In order to compensate for this offset voltage, amplifiers called auto-zero, operating according to a sequential mode, are conventionally used.
Such an amplifier-comparator operating in a sequential mode will be disclosed in relation with FIG. 1. This figure shows the two current sources to be compared I1, 12 connected between a low reference voltage VEE and respective switches S1 and S2 respectively closed during non-overlapping phases .PHI.1 and .PHI.2. The other terminals of the switches S1 and S2 are connected at a node N. The node N is connected with a high supply source VCC by a resistor R1. The node N is also connected with the first input (-) of a differential amplifier A1 through a capacitor C1. The second input of the differential amplifier A1 is connected with a voltage source of intermediate value, usually the ground when the high voltage VCC is positive and when the low voltage VEE is negative. The input and output of the amplifier A1 are looped through a switch S3, closed during the phase .PHI.1 during which the current source I1 is connected through the first switch S1.
This circuit operates as follows.
During a first phase, .PHI.1, the switches S1 and S3 are on and the switch S2 is off. The current I1 flows through the resistor R1 and the capacitor C1 is charged at the voltage VCC - R1I1 - Vos, where Vos is the offset voltage of the amplifier A1.
During the phase .PHI.2, the switches S1 and S3 are off and the switch S2 is on. The current I2 flows through the resistor R1 and the output voltage of the amplifier A1 is equal to : EQU V=G(A1) R1 [I1-I2]
where G(A1) is the open loop gain of the amplifier A1. Then, owing to the phase .PHI.1 called auto-zero phase, the offset voltage of amplifier A1 has been theoritically perfectly compensated for. However, an error inherent to the switching off of the switch S3 at the end of the phase .PHI.1 is still existing. Indeed, this switch is usually implemented by means of a MOS transistor and, at the switching off, a portion of the charges stored in the transistor channel is transferred on capacitor C1 and this injection of charges causes an error. If one wishes to be able to detect a minimum current Imin of about 1.5 microamperes, the injected charge has to be lower than Qmin=CRImin. With conventional values, C=10 pF, R=300 .OMEGA.and Imin=1.5 .mu.A, one obtains : EQU Qmin=4.5 10.sup.-15 coulombs
(this charge corresponds to about 28,000 times the charge of one electron). In practice, it is impossible that the amount of charges injected during the switching off of the switch S3 be lower than this value Qmin even by using circuitries that permit to minimize the injection of the charges.
Therefore, an object of the instant invention is to provide for a differential current amplifier compensating not only for the offset voltage of a differential amplifier, but also permitting to minimize the influence of the charge injection inherent to the switching off of a switch.