High output impedance current mirror with superior output voltage compliance

[0001] This is a Continuation of application Ser. No. 10/237,914, filed Sep. 9, 2002 now U.S. Pat. No. 6,798,182.

[0002] 1. Field of the Invention

[0003] This invention relates to the field of electronic circuit design, and in particular to the design of a current mirror that provides a high output impedance and an accurate mirror of input current across a wide range of output voltages.

[0004] 2. Description of Related Art

[0005] Current mirrors are often used to provide a controlled current to a component without loading the source of the controlled current. An independent source generates a current at a given value; the current mirror provides an output current to a load, such that the output current corresponds to the value of the independently generated current. In this manner, the source of the desired current is isolated from the load that receives an equivalent current.

[0006] FIG. 1 illustrates an example circuit diagram of a basic current mirror 100. A transistor T1 is configured as a diode, by connecting its drain and gate, for communicating the independent source current, Iin, to ground. A second transistor T2 has its gate connected to the gate of T1, and has its source connected to the same potential as the source of T1. Thus, the gate-to-source voltages of each of the transistors T1 and T2 are equal, and, if the transistors T1 and T2 are operationally identical, the drain-to-source current through each will be the same. The current through T1 corresponds to the input current Iin; therefore, assuming that the source of the current lout is sufficient to provide at least this value of current, the output current, lout, will be equal to Iin. Note, however, that the characteristics of the load that is intended to draw the current lout can affect the operation of transistor T2, by affecting transistor T2's drain-to-source voltage, Vout. If the drain-to-source voltage Vout of transistor T2 does not equal the drain-to-source voltage Va of transistor T1, the current lout through transistor T2 will differ from the current Iin through transistor T1. If Vout is less than Va, then lout will be less than Iin. Similarly, if Vout is greater than Va, then lout will be greater than Iin. This is due to the limited output impedance of transistor T2.

[0007] Output voltage compliance is defined herein as the range of output voltages through which a current mirror will provide an output current lout that corresponds to the input current Iin. The current mirror 100 exhibits relatively poor output voltage compliance, because only when Vout is equal to Va will the output current lout equal the input current Iin, due in part to the limited output impedance of the transistor T2.

[0008] FIG. 2 illustrates an example circuit diagram of a current mirror 200 that provides greater output impedance, and thus a wider range of output voltage compliance than the current mirror 100 of FIG. 1. In the current mirror 200, a differential amplifier Al and transistor T3 are configured to assure that the drain to source voltages Va, Vb of the input T1 and output T2 transistors are equal. The amplifier A1 and transistor T3 control the drain-to-source impedance of transistor T3, such that a controlled output current lout (=Iin) is provided independent of the output voltage Vout, when Vout is greater than Vb. Because the gate-to-source voltage and the drain-to-source voltage of each of the transistors T1 and T2 are assured to be equal, the output current lout is assured to be equal to the input current Iin, when the voltage Vout is greater than Vb. In the current mirror 200, the output impedance and voltage compliance is improved, compared to the current mirror 100, because in current mirror 200, the output current lout will equal the input current Iin whenever Vout is greater than Vb, which is set equal to Va. In this case, the voltage compliance is limited to the lower value of Va, which is generally determined by the source of the input current Iin.

[0009] FIG. 3 illustrates an example circuit diagram of a current mirror 300 that is operable to lower ranges of output voltages than the current mirror 200, as taught by U.S. Pat. No. 5,612,614, issued 18 Mar. 1997 to Barrett et al., and included by reference herein. In current mirror 300, transistors T1 and T4 are configured having a common channel and two gates, thereby forming a composite transistor. This composite transistor T1–T4 is diode-connected, by coupling the gates of each transistor T1, T4, to the drain of T4, thereby forming a two-input diode device that has an intermediate node between the gates that provides the drain voltage Va of transistor T1. By dividing the input source voltage Vc between the transistors T1 and T4, the voltage Va at the drain of transistor T1 is lower than the input source voltage Vc. The relative sizes/transconductances of transistors T1 and T4 determine the value of Va relative to Vc. Because the diode arrangement requires that the transconductance of transistor T4 be substantially higher than the transconductance of transistor T1, the value of Va relative to Vc is limited.

[0010] It is an object of this invention to provide a current mirror having a large output voltage compliance. It is a further object of this invention to provide a current mirror that dynamically adjusts for differences between an input source voltage and an output load voltage, so as to provide a large output voltage compliance.

[0011] These objects and others are achieved by providing a current mirror that divides an input source voltage dynamically, to provide a controlled voltage that corresponds to an output load voltage. The correspondence between this controlled voltage and the output load voltage determines the correspondence between the output current and the input current. By dynamically adjusting the controlled voltage, the correspondence to the output load voltage can be maintained to very low voltage levels. Preferably, the output load voltage is also dynamically divided to provide a comparison voltage for comparing to the controlled voltage when the output load voltage is high, thereby providing the appropriate output current at high voltage levels.

[0018] Throughout the drawings, the same reference numerals indicate similar or corresponding features or functions.

[0019] FIG. 4 illustrates an example circuit diagram of a current mirror 400 in accordance with a first aspect of this invention. The current mirror 400 includes the conventional transistor pair T1, T2 having a common gate potential and common source potential. As discussed above, equal current will flow through transistors T1 and T2, provided that their respective drain-to-source voltages Va, Vb are equal.

[0020] A differential amplifier A2 and transistor T5 are configured to assure that the drain-to-source voltages Va, Vb of transistors T1, T2, are equal. As contrast to the conventional current mirrors 200, 300 of FIGS. 2 and 3, however, the amplifier A2 and transistor T5 are configured to adjust the drain-to-source voltage Va on the input transistor T1 to match the output voltage Vb, whereas current mirrors 200, 300 adjust the drain-to-source voltage Vb on the output transistor T2 to match the input source voltage Va.

[0021] As illustrated, the transistor T5 is connected in series with the input transistor T1. The conductance of the transistor T5 is determined by the amplifier A2. Transistors T5–T1 form a voltage divider of the input source voltage Vc. If the voltage at Va is larger than Vb, the conductance of transistor T5 is decreased, thereby introducing a larger drain-to-source voltage drop across T5 and a corresponding decrease in the voltage Va. In like manner, if the voltage at Va is smaller than Vb, the conductance of T5 is increased, reducing the voltage drop across T5, and thereby increasing the voltage Va. That is, the drain-to-source conductance of T5 is adjusted to assure that the input voltage Va corresponds to the output voltage Vb.

[0022] The current mirror 400 of FIG. 4 is able to track to very low output voltage levels, and to levels substantially as high as Vc. In accordance with a second aspect of this invention, FIG. 5 illustrates a current mirror 500 that is configured to track to output voltage levels above Vc.

[0023] In current mirror 500, a second differential amplifier A3 is configured to control a transistor T6, based on a comparison of voltages Vc and Vb. If Vout is much less than Vc, Vb must likewise be much less than Vc, and the amplifier A3 drives the transistor T6 to an “on” state, effectively coupling Vb directly to Vout. In this state, with Vout=Vb, the operation of mirror 500 substantially corresponds to the operation of the mirror 400, detailed above.

[0024] As Vout increases, and Vb approaches Vc, however, the amplifier A3 limits the conductance of transistor T6, thereby introducing a voltage drop across transistor T6, reducing the voltage Vb to a voltage less than Vout. As the output load voltage Vout continues to increase, beyond Vc, Vb attempts to increase with Vout, but the amplifier A3 limits the conductance of transistor T6 further, thereby keeping Vb equal to Vc. In this manner, Vb is maintained equal to Vc, Va is controlled by amplifier A2 to match Vc, and therefore the current lout through transistor T2 is maintained equal to the current Iin through transistor T1.

[0025] Thus, the current mirror 500 provides tracking to both very low levels of Vout and to very high levels of Vout, by operating the transistor T6 as a closed switch for low-level tracking, and as a variable conductance device, for high-level tracking.

[0026] The foregoing merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are thus within its spirit and scope. For example, NMOS transistors are illustrated in each of the figures, although the principles presented in this disclosure are applicable to other transistor types, including bipolar, PMOS, BiCMOS, and so on. Replacing transistors T5 or T6 with PMOS devices, for example, merely requires a change of the sense of the corresponding amplifiers A2 and A3. In like manner, the figures illustrate a fairly primitive form of current mirrors comprising a single input stage and output stage, for ease of understanding. One of ordinary skill in the art will recognize that existing techniques for improving the performance of a current mirror, or providing additional capabilities, can be included in the mirrors 400, 500 while still realizing the wide range of voltage compliance that these mirrors provide. For example, each of the mirrors 400, 500 may be configured as variable-current-gain devices, as compared to the 1:1 mirror gain illustrated, or configured to provide improved noise immunity, or improved temperature independence, and so on. These and other system configuration and optimization features will be evident to one of ordinary skill in the art in view of this disclosure, and are included within the scope of the following claims.