TRANSCONDUCTOR WITH CONTROLLABLE TRANSCONDUCTANCE FOR LOW SUPPLY VOLTAGES

Abstract: To control the transconductance of a transconductor, its voltage input terminal (Vin2) is connected to the base of a first and a second transistor (Q21, Q22) via respective capacitors (C21, C22). The interconnection points between the capacitors (C21, C22) and the respective base are connected to the respective control voltage terminal (Vreg2+, Vreg2-) of the transconductor via a first and a second resistor (R21, R22), respectively. The emitters of said first and second transistors (Q21, Q22) are interconnected, and the interconnection point is connected to a ground terminal via a third resistor (R23). The collector of the first transistor (Q21) is connected to the current output terminal (Iout2) of the transconductor, and the collector of the second transistor (Q22) is connected to the supply voltage terminal (Vcc2) of the transconductor.

Description: TRANSCONDUCTOR WITH CONTROLLABLE TRANSCONDUCTANCE FOR LOW SUPPLY VOLTAGES TECHNICAL FIELD The invention relates to a transconductor. BACKGROUND OF THE INVENTION Fig. 1 illustrates a known transconductor comprising a voltage input terminal Vinl, a current output terminal Ioutl, a supply voltage terminal Vccl, and two control voltage terminals Vregl + and Vregl-. A control voltage is to be applied between the control voltage terminals Vregl + and Vregl- to control the transconductance of the transconductor. The voltage input terminal Vinl is connected to the base of a transistor Ql 1, whose emitter is connected to a ground terminal via a resistor Rl 1 and whose collector is connected to an interconnection point between emitters of two transistors Q12 and Q13. The base of the transistor Q12 is connected to the control voltage terminal Vregl +, while the base of the transistor Q13 is connected to the control voltage terminal Vregl-. The collector of the transistor Q12 is connected to the current output terminal Ioutl, while the collector of the transistor Q13 is connected to the supply voltage terminal Vcc 1. By varying the voltage between the control voltage terminals Vregl + and Vregl-, the output current on the current output terminal Ioutl will vary correspondingly. The known transconductor shown in Fig. 1 cannot be used at low supply voltages applied between the supply voltage terminal Vccl and ground since it requires quite a large voltage range due to the fact that the transistors Ql 1, Q12 and Q13 are stacked on each other. SUMMARY OF THE INVENTION The object of the invention is to bring about a transconductor that enables power control also at low supply voltages. This is attained in the transconductor according to the invention in that it comprises only two transistors without stacking. Hereby, operation at a lower supply voltage is enabled. BRIEF DESCRIPTION OF THE DRAWING The invention will be described more in detail below with reference to the appended drawing on which Fig. 1, as described above, shows a known transconductor, and Fig. 2 shows an embodiment of a transconductor in accordance with the invention. DESCRIPTION OF THE INVENTION As the known transconductor in Fig. 1, the transconductor according to the invention, illustrated in Fig. 2, comprises a voltage input terminal Vin2, a current output terminal Iout2, a supply voltage terminal Vcc2, and two control voltage terminals Vreg2+ and Vreg2-, between which a control voltage is to be applied to control the transconduc- tance of the transconductor in Fig. 2. In accordance with the invention, the voltage input terminal Vin2 is connected to the base of two transistors Q21 and Q22 via respective capacitors C21 and C22. Also in accordance with the invention, the interconnection points between the capacitors C21, C22 and the respective base of the transistors Q21, Q22, are connected to the respective control voltage terminal Vreg2+, Vreg2- via resistors R21 and R22, respectively. The emitters of the transistors Q21, Q22 are interconnected, and the interconnection point between the emitters is connected to a ground terminal via a resistor R23. In the embodiment of the transconductor according to the invention shown in Fig. 2, the collector of the transistor Q21 is connected to the current output terminal Iout2, while the collector of the transistor Q22 is connected to the supply voltage terminal Vcc2. In that the transconductor according to the invention comprises only two transistors instead of three as in the known transconductor according to Fig. 1, the transconductor according to the invention can be used at lower supply voltages than the known transconductor. Moreover, the linearity will be higher in comparison with the known transconductor.

Claims: CLAIM A transconductor with controllable transconductance for low supply voltages, comprising a voltage input terminal (Vin2), a current output terminal (Iout2) connected to a collector of a first transistor (Q21), a supply voltage terminal (Vcc2) connected to a collector of a second transistor (Q22), and two control voltage terminals (Vreg2+, Vreg2-) which are connected to a base of the respective transistor (Q21, Q22) and between which a control voltage is to be applied to control the transconductance of the transconductor, characterized in - that the voltage input terminal (Vin2) is connected to the base of the respective transistor (Q21, Q22) via a first and a second capacitors (C21, C22), respectively, - that the control voltage terminals (Vreg2+, Vreg2-) are connected to the base of the respective transistor (Q21, Q22) via a first and a second resistor (R21, R22), and - that the emitters of said transistors (Q21, Q22) are interconnected and connected to a ground terminal via a third resistor (R23).

Inventors: SVALAENGE PAER, SVALAENGE, PAER
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET L M ERICSSON (PUBL)
Application year: 2000
Main classification: H03F3
IPC codes: H03F3, H03G1