High impedance,high ratio current mirror

Abstract: The impedance of a current mirror circuit is incremented by cascoding the output transistor of the current mirror and the precision of the circuit is enhanced by employing a frequency compensated gain stage utilizing a field effect transistor to drive a current output transistor.

Description: The present invention relates to a circuit capable of generating, starting from a control current, another current that is n times larger than the control current. When designing electronic circuits there is often the need of implementing current mirrors having a large ratio between the mirrorred or output current and a reference or control current. Normally a current mirror is said to have a large ratio when it is in the order of ten. An additional requisite of current mirrors is to be very precise. A classical circuit of a current mirror is shown in Fig. 1 . When a particularly high precision is required, a modified circuit as the one depicted in Fig. 2. is often used. Such a modified circuit provides for a certain recovery of the base current by utilizing a third transistor, N3, for reducing an intrinsic error of the circuit. This error arises because in the control branch of the current mirror circuit comprising the diode-configured N1 transistor, among the current contributions there will be one due to the base currents of the two transistors of the mirror. This "offset" current produces an error proportional to the mirror's ratio plus a term given by 1/β. The additional transistor N3 permits a substantial recovery of the base currents of the transistors N1 and N2 and therefore a reduction of such an intrinsic asymmetry. This solution is not very effective in the case of circuits that must implement a relatively high mirror's ratio because the error remains large. A transconductance operational amplifier (OTA), provided with a feedback loop including a transistor P, as depicted in Fig. 3 , is often used in these cases. This circuit is capable of ensuring a high precision also in case of relatively large mirror's ratios. On the other hand, in certain applications, for example in telephone circuits and more in general where signal transmission lines are also used as power supply lines, i.e. where it is particularly important that the circuits possess a high Power-Supply-Rejection-Ratio (PSRR), the known current mirrors have an impedance, as measured between the supply nodes, which is not sufficiently high to make their behaviour relatively insensitive to the presence of AC signals on the supply line. Such a drawback of the known circuits becomes more marked in current mirrors having a relatively high mirror's ratio. Moreover, in some applications, the precision of a circuit made according to the known techniques, e.g. as depicted in Fig. 3 , is yet insufficient because of the finite gain of the OTA. An easily implementable current mirror circuit has now been devised, which is capable of ensuring a high degree of precision also in relatively large mirror's ratio circuits and has a high impedance as measured across the supply nodes. Basically, the current mirror circuit object of the present invention employs a field effect transistor for handling the current through a control terminal, e.g. a base terminal of a current output transistor, thus ensuring a high degree of precision of the current mirror. A frequency compensation of the gain stage is implemented by a feedback capacitance. The impedance of the circuit as measured across the supply nodes, is incremented by employing an additional transistor, functionally connected in the output branch of the current mirror circuit connected so as to form together with the output branch transistor of the basic current mirror circuit, a cascode-type circuit capable of increasing the output impedance of the current mirror. The output impedance of a current mirror circuit represents a most critical factor in determining a high impedance as measured across the supply nodes of the circuit, in view of the fact that it should be divided by the mirror's ratio. The high loop gain, as determined by the use of a gain stage, beside reducing the error, thus increasing the precision of the circuit, is synergistic to the attainment of a high impedance across the supply nodes of the current mirror circuit. In fact, because the control current of the current output transistor is driven by the field effect transistor of the gain stage, it is possible to reduce the current levels in the two branches of the current mirror without negatively affecting performance. Such a current level reduction, beside producing a sensible saving in power consumption and facilitating sizing of the components of the current mirror, further increases the impedance. The output impedance of the circuit, i.e. the output impedance of the transistor that is driven by the field effect transistor of the gain stage, increases. This represents a further advantage per se, because the output transistor of the current mirror often must drive relatively high currents and therefore should have a relatively low output impedance. The different aspects and advantages of the circuit of the invention, as compared to the known circuits, will become more evident through the following description of a preferred embodiment and by referring to the annexed drawings, wherein: Fig. 1. shows a basic circuit of a current mirror; Fig. 2 . shows a modified current mirror circuit, provided with a transistor for recovering the base current, according to a known technique, as mentioned above; Fig. 3. shows a current mirror implemented by the use of an OTA, provided with a PNP transistor feedback loop; Fig. 4 shows a basic diagram of a current mirror circuit made according to a preferred embodiment of the present invention. Notably all the current mirror circuits shown in the figures provide a mirroring ratio that is given by the following relation Ispec/Irif = R1/R2. With reference to Fig. 4, a current mirror circuit of the invention has a first control or reference branch, which comprises a biasing current generator N6, a first diode-connected transistor P1 and a first degenerating resistance R1, functionally connected between the transistor P1 and a supply node A of the circuit, according to a basic configuration. A second output branch of the current mirror comprises a biasing current generator N7, a second or output transistor P2 of the current mirror and a second degenerating resistance R2, functionally connected between the transistor P2 and the supply node A. According to such a basic current mirror configuration, to a certain control or reference current Irif, drawn from the connection node between the degeneration resistance R1 and the diode-connected transistor P1 of the reference branch of the mirror, corresponds a mirrored current Ispec delivered by the circuit through the output node, which is represented by the connection node between the degeneration resistance R2 and the output transistor P2 of the current mirror. According to the present invention, such a basic current mirror circuit is modified by adding a gain stage implemented with a field effect transistor M4, capable of driving a current output transistor P3, through which the mirrored current Ispec produced by the current mirror circuit is forced. The gain stage composed of the field effect transistor M4 is frequency compensated by means of a feedback capacitance Cc. The gain stage increases the loop gain of the circuit thus increasing the degree of precision beyond the precision that may be achieved when using a buffer, as done in the prior art. circuit depicted in Fig. 3 and which is limited by the finite gain of the buffer. The increased gain of the circuit also permits to reduce the current consumption because the driving current of the output transistor P3 is provided by the field effect transistor M4 and therefore the currents that flow through the two branches of the current mirror circuit (i.e. through P1, P2 and P5), may be freely designed to be relatively small, by suitably dimensioning the current generators N6 and N7, without negatively affecting the performance of the circuit. The impedance of the circuit, as measured between the supply node A and ground, is advantageously increased by employing a fifth transistor P5, functionally connected in the output branch of the current mirror in a way as to constitute together with the transistor P2 of the basic current mirror circuit a cascode circuit. In this case, a biasing resistance Rb must be introduced in the reference branch of the current mirror, as shown in the figure, to exclude the possibility that the transistor P2 of the current mirror might saturate, i.e. for ensuring the maintenance of a collector-emitter voltage (VCE) of the transistor P2 higher than the saturation voltage thereof. Substantially, the effect of the cascode circuit formed by the addition of the fifth transistor P5 is that of increasing the output impedance of the output transistor P2 of the current mirror thus determining a higher impedance of the circuit as seen from the supply node A. The cascoding of the output transistor P2 of the current mirror is particularly effective because the output impedance of this output transistor represents a determining factor for achieving a high impedance of the circuit as seen from the supply node. In fact, in this respect, the output impedance of the transistor of P2 should be divided by the mirror's ratio and in case of relatively high mirror's ratio it may result excessively low. Optionally, a further increase of the impedance of the circuit as measured across the supply nodes, may be obtained, as schematically depicted in Fig. 4 , by introducing two further degeneration resistances R3 and R4 between the ground node of the circuit and the biasing current generators N6 and N7, respectively. An even better result in terms of further increasing the circuit's impedance, may be obtained also by adding other cascode circuits in the two branches of the current mirror circuit. As depicted in the example of Fig. 4 , a frequency compensation capacitance Ccc is connected between the control node (base) of the current output transistor P3 and preferably the intermediate connection node between the pair of cascode-connected transistors P2 and P5, rather than to the gate of the gain stage transistor M4. This produces a more favourable impedance characteristic of the circuit versus frequency. Even though the circuit of the invention has been described in connection with a preferred embodiment, employing bipolar transistors of a certain type of conductivity with the exception of the M4 transistor of the gain stage that, for the example shown may be an n-channel MOS transistor, it will be evident to any skilled technician that a similar circuit may also be realized by employing transistors of an opposite type of conductivity and by inverting all the polarities. Moreover the circuit may also be made by employing field effect transistors in place of the bipolar transistors.

Claims: A current mirror circuit comprising a pair of biasing current generators (N6,N7) in the two branches of the mirror circuit, a first degeneration resistance (R1), functionally connected between a supply node (A) and a diode-connected transistor (P1) of a first or control branch of the mirror circuit, a second degeneration resistance (R2) functionally connected between said supply node and a second transistor (P2) of a second or output branch of the mirror circuit, the connection node between said first resistance and said diode-connected transistor constituting an input node of a control current (Irif) and the connection node between said second resistance and said second transistor constituting an output node of a mirrored current (Ispec) having a value equal to said control current multiplied by the ratio between the value of said first resistance and the value of said second resistance, characterized by comprising a current output transistor (P3) having a first terminal functionally connected to said output node, a control terminal and an output terminal; a gain stage comprising a field effect transistor (M4) having a gate functionally connected to said output branch of the current mirror, a source connected to said ground node and a drain connected to said control terminal of said current output transistor; a frequency compensation capacitance (Ccc) connected between said control terminal of said current output transistor and said output branch of the current mirror. A current mirror circuit as defined in claim 1, characterized by comprising another additional transistor (P5), functionally connected in said output branch of the current mirror circuit which comprises said second transistor and forming together with said second transistor a cascode circuit capable of increasing the impedance as measured across the supply nodes of the circuit, a control terminal of said additional transistor being connected to a terminal of a biasing resistance (Rb) connected in said control branch of the current mirror circuit and having a value sufficient to prevent saturation of said second transistor of the current mirror circuit. A current mirror circuit as defined in claim 2, wherein each of said biasing current generators is formed by a transistor (N6,N7) functionally connected in the respective branch of the current mirror and through a degeneration resistance (R3,R4) to said ground node of the circuit.

Inventors: CASTELLO RINALDO, TOMASINI LUCIANO, CASTELLO, RINALDO, TOMASINI, LUCIANO
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1992
Main classification: G05F3
IPC codes: G05F3, H03F3