Universal signal converter

Abstract: A signal converter comprising a first current mirror (10) having a first input terminal (11) adapted to receive a first input signal, having a first output terminal (12), and having a first common terminal (13) coupled to a first reference terminal (1), is adapted to receive a second input signal via the first input terminal (12). The adapted converter further comprises a second current mirror (20) having a second input terminal (21) coupled to a second reference terminal (2), a second output terminal (22) adapted to supply a first output signal, and a second common terminal (23) coupled to the first output terminal (12), a third current mirror (30) having a third input terminal (31) coupled to the second reference terminal (2), having a third output terminal (32) adapted to supply a second output signal, and having a third common terminal (33) coupled to the first input terminal (11), and a bias current source (40) coupled between the second and the third input terminal (21, 31) and the second reference terminal (2).

Description: The invention relates to a signal converter comprising a first reference terminal adapted to receive a first reference voltage and a first current mirror having a first input terminal adapted to receive a first input signal, having a first output terminal, and having a first common terminal coupled to the first reference terminal. Such a signal converter can be used universally in integrated semiconductor circuits, particularly for converting an input current into an output current. A known example of such a signal converter is a mirror arrangement comprising an input transistor having a control electrode coupled to the first input terminal and a having a main current path coupled between the first input terminal and the first common terminal, and an output transistor having a control electrode coupled to the control electrode of the input transistor and having a main current path coupled between the first output terminal and the first common terminal. The first input signal can be applied to the known signal converter as an (input) voltage or an (input) current. When the first input signal is applied as an (input) voltage between the first input terminal and the common terminal the (input) voltage generates an (input) current via the first input terminal and an (output) current via the first output terminal. When the first input signal is applied to the first input terminal as an (input) current the (input) current generates an (input) voltage between the first input terminal and the first common terminal and the generated (input) voltage generates an (output) current via the first output terminal. However, regardless of the first input signal ( i.e. either an (input) voltage or an (input) current) the known signal converter generates a single (output) current via the first output terminal, which single (output) current depends proportionally on the first input signal. A disadvantage of the known signal converter is that this signal converter is not capable of converting a single input signal into a differential output signal and a differential input signal into a differential output signal. The document 1992 International Symposium on Circuits and Systems, Vol.3, pages 1372-1375 "Wide Gm Adjustment Range, Highly linear OTA with Linear Programmable Current Mirrors" [RAMIREZ-ANGULO et al] discloses an operational transconductance amplifier comprising an arrangement of three current mirrors and a linear voltage to current converter. It is an object of the invention to provide a signal converter which effects a single-to-differential conversion or a differential-to-differential conversion depending on the input signal and which has a linear conversion characteristic which is independent of the type of conversion. A signal converter in accordance with the invention is characterized in that the first output terminal is adapted to receive a second input signal, and the signal converter further comprises a second reference terminal for receiving a second reference voltage, a second current mirror having a second input terminal coupled to the second reference terminal, a second output terminal adapted to supply a first output signal, and a second common terminal coupled to the first output terminal, a third current mirror having a third input terminal coupled to the second reference terminal, having a third output terminal adapted to supply a second output signal, and having a third common terminal coupled to the first input terminal, and a bias current source coupled between the second and the third input terminal and the second reference terminal. Since the second and the third current mirror and the bias current source have been added to the known converter the signal converter in accordance with the invention is capable of realizing a single-to-differential conversion and a differential-to-differential conversion. A simple adaptation of the signal converter in accordance with the invention also makes this converter suitable for realizing a single-to-single conversion and a differential-to-single conversion. said conversions will be explained with reference to the accompanying Figure. In this explanation it will also be outlined that the signal converter in accordance with the invention has a linear conversion characteristic regardless of the type of conversion. A further embodiment of a signal converter in accordance with the invention is characterized in that the first current mirror comprises a first input transistor having a control electrode coupled to the first input terminal and having a main current path coupled between the first input terminal and the first common terminal, and a first output transistor having a control electrode coupled to the control electrode of the first input transistor and having a main current path coupled between the first output terminal and the first common terminal, the second current mirror comprises a second input transistor having a control electrode coupled to the second input terminal and having a main current path coupled between the second input terminal and the second common terminal, and a second output transistor having a control electrode coupled to the control electrode of the second input transistor and having a main current path coupled between the second output terminal and the second common terminal, and the third current mirror comprises a third input transistor having a control electrode coupled to the third input terminal and having a main current path coupled between the third input terminal and the third common terminal, and a third output transistor having a control electrode coupled to the control electrode of the third input transistor and having a main current path coupled between the third output terminal and the third common terminal. The present signal converter in accordance with the invention is a simple embodiment, which may comprise transistors of a unipolar or of a bipolar type. These and other (more detailed) aspects of the invention will now be described and elucidated more elaborately with reference to the accompanying Figure, which Figure shows an embodiment of a signal converter in accordance with the invention. The accompanying Figure shows an embodiment of a signal converter in accordance with the invention. In accordance with the invention the signal converter shown comprises a first reference terminal 1, a second reference terminal 2, a first current mirror 10, a second current mirror 20, a third current mirror 30, and a bias current source 40. The current mirror 10 has a first input terminal 11, a first output terminal 12, and a first common terminal 13, which common terminal is coupled to the reference terminal 1. The current mirror 20 has a second input terminal 21, which input terminal is coupled to the reference terminal 2, a second output terminal 22, and a second common terminal 23, which common terminal is coupled to the output terminal 12. The current mirror 30 has a third input terminal 31, which input terminal is coupled to the reference terminal 2, a third output terminal 32, and a third common terminal 33, which common terminal is coupled to the input terminal 11. The bias current source 40 is coupled between the input terminals 21, 31 and the reference terminal 2. To illustrate a possible implementation of the signal converter in accordance with the invention the current mirrors 10, 20 and 30 in the present embodiment are implemented in a simple manner. A simple implementation of the current mirror 10 comprises a first input transistor 14 having a control electrode coupled to the input terminal 11 and having a main current path coupled between the input terminal 11 and the common terminal 13, and a first output transistor 15 having a control electrode coupled to the control electrode of the input transistor 14 and having a main current path coupled between the output terminal 12 and the common terminal 13. A simple implementation of the current mirror 20 comprises a second input transistor 24 having a control electrode coupled to the input terminal 21 and having a main current path coupled between the input terminal 21 and the common terminal 23, and a second output transistor 25 having a control electrode coupled to the control electrode of the input transistor 24 and having a main current path coupled between the output terminal 22 and the common terminal 23. A simple implementation of the current mirror 30 comprises a third input transistor 34 having a control electrode coupled to the input terminal 31 and having a main current path coupled between the input terminal 31 and the common terminal 33, and a third output transistor 35 having a control electrode coupled to the control electrode of the input transistor 34 and having a main current path coupled between the output terminal 32 and the common terminal. The aforementioned transistors (14, 15, 24, 25, 34, 35) may be of a unipolar or bipolar type and they may be of an N conductivity type or P conductivity type. When a first reference voltage is applied to the reference terminal 1 and a second reference voltage is applied to the reference terminal 2 the signal converter in accordance with the invention can be used for converting a single input signal into a single output signal, a single input signal into a differential output signal, a differential output signal into a signal input signal, and a differential input signal into a differential output signal. To illustrate the possible implementation of the signal converter in accordance with the invention the present converter is constructed to convert a differential input signal comprising a first input current and a second input current into a differential output signal comprising a first output current and a second output current, a positive voltage difference being provided between the reference terminals 1 and 2. The first input current is applied to the input terminal 11 by means of a signal current source 41 and the second input current is applied to the input terminal 12 by means of a signal current source 42, the first output current being available at the output terminal 22 and the second output current being available at the output terminal 32. Although the present converter is adapted to convert a differential input signal into a differential output signal a functional description of the signal converter in accordance with the invention will be given for a differential-to-differential conversion as well as for a single-to-differential conversion. In the case of the differential-to-differential conversion the signal converter may be constructed as shown in the accompanying Figure. In the relevant embodiment the bias current source 40 generates a bias current through the input transistors 14 and 34, in response to which current a voltage is generated across the respective transistors, the voltage across the input transistor 14 generating a bias current through the output transistor 15 and the voltage across the input transistor 34 generating a bias current through the output transistor 35. The bias current through the output transistor 15 causes a bias current to be generated through the input transistor 24, in response to which current a voltage is produced across the input transistor 24, the voltage across the input transistor 24 generating a bias current through the output transistor 25. Since the sum of the bias current through the transistors 14 and 34 and the bias current through the transistors 15 and 24 is determined by the bias current of the bias current source 40 all the bias currents are determined by the bias current of the bias current source 40. In a first case of a positive input current at the input terminal 11 and a negative input current at the output terminal 12 the current mirror 10 ensures that the current through the transistors 14 and 15 remains the same, whereas the current through the input transistor 34 decreases and the current through the input transistor 24 increases. As a result of this and also as a result of the respective current mirrors 30 and 20 the output current through the output transistor 35 decreases and the output current through the output transistor 25 increases. Since in a second case of a negative input current at the input terminal 11 and a positive output current at the output terminal 12 the signal converter provides a similar behaviour the present converter realizes a differential-to-differential conversion. In the case of a single-to-differential conversion the signal converter differs from the converter shown in that the signal current 42 is dispensed with. Although the bias currents in the resulting converter do not differ from the bias currents in the converter shown the input current causes a different signal behaviour. In a first case of a positive input current at the input terminal 11 the current through the input transistor 14 increases and the current through the output transistor 15 increases accordingly owing to the presence of the current mirror 10. As a result, the current through the input transistor 24 increases and the output current through the output transistor 25 increases accordingly owing to the presence of the current mirror 20. However, as the sum of the current through the transistors 15 and 24 and the current through the transistors 14 and 34 is determined by the bias current of the bias current source 40 the current through the input transistor 34 decreases and the output current through the output transistor 35 decreases accordingly owing to the presence of the current mirror 30. As in a second case of a negative input current at the input terminal 11 the signal converter exhibits a similar behaviour the present converter provides the single-to-differential conversion. The signal converter in accordance with the invention has a linear conversion characteristic regardless of the type of conversion. The relevant conversion characteristic results from the (substantially) symmetrical construction of the signal converter. When the signal converter in accordance with the invention is implemented by means of transistors of the unipolar type a maximum linearity is obtained, which maximum linearity results from the (absence of) control electrode currents. When the signal converter in accordance with the invention is constructed by means of current mirrors having a mirror factor the signal converter can provide gain or attenuation. A current mirror having a mirror factor is obtained when the respective input transistor and output transistor have different emitter or source areas. The invention is not limited to the embodiment shown herein. Within the scope of the invention defined by the appended claims, an unlimited number of modifications are conceivable to those skilled in the art. One possible modification is the provision of a connection between the second or the third output terminal and the second reference terminal, as a result of which connection a single-to-single conversion or a differential-to-single conversion is obtained. Another modification involves the application of an input signal in the form of an (input) voltage, which (input) voltage results in a current behaviour corresponding to that described above. A further modification concerns the take-off of an output signal in the form of an (output) voltage, which is possible when the second and the third output terminal are coupled to the second reference terminal by means of a respective impedance. As a result, the signal converter in accordance with the invention can be constructed to realize a current-to-current conversion, a current-to-voltage conversion, a (non-linear) voltage-to-current conversion, or a non-linear voltage-to-voltage conversion.

Claims: A signal converter comprising a first reference terminal (1) adapted to receive a first reference voltage and a first current mirror (10) having a first input terminal (11) adapted to receive a first input signal, having a first output terminal (12), and having a first common terminal (13) coupled to the first reference terminal (1), characterized in that the first output terminal (12) is adapted to receive a second input signal, and the signal converter further comprises a second reference terminal (2) for receiving a second reference voltage, a second current mirror (20) having a second input terminal (21) coupled to the second reference terminal (2), a second output terminal (22) adapted to supply a first output signal, and a second common terminal (23) coupled to the first output terminal (12), a third current mirror (30) having a third input terminal (31) coupled to the second reference terminal (2), having a third output terminal (32) adapted to supply a second output signal, and having a third common terminal (33) coupled to the first input terminal (11), and a bias current source (40) coupled between the second and the third input terminal (21, 31) and the second reference terminal (2). A signal converter as claimed in Claim 1, characterized in that the first current mirror (10) comprises a first input transistor (14) having a control electrode coupled to the first input terminal (11) and having a main current path coupled between the first input terminal (11) and the first common terminal (13), and a first output transistor (15) having a control electrode coupled to the control electrode of the first input transistor (14) and having a main current path coupled between the first output terminal (12) and the first common terminal (13), the second current mirror (20) comprises a second input transistor (24) having a control electrode coupled to the second input terminal (21) and having a main current path coupled between the second input terminal (21) and the second common terminal (23), and a second output transistor (25) having a control electrode coupled to the control electrode of the second input transistor (24) and having a main current path coupled between the second output terminal (22) and the second common terminal (23), and the third current mirror (30) comprises a third input transistor (34) having a control electrode coupled to the third input terminal (31) and having a main current path coupled between the third input terminal (31) and the third common terminal (33), and a third output transistor (35) having a control electrode coupled to the control electrode of the third input transistor (34) and having a main current path coupled between the third output terminal (32) and the third common terminal (33).

Inventors: RIJNS JOHANNES J F, RIJNS, JOHANNES J.F., RIJNS, JOHANNES J.F., C/O INT. OCTROOIBUREAU B.V.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1993
Main classification: H03F3
IPC codes: H03F3, H03G3, H03F1