Differential transistor pair current switch supplied by a low voltage VCC
The invention relates to current switches using a differential pair of transistors and being able to operate under a low supply voltage Vcc. According to the invention, provision is made for the current switch to include two differential pairs of two transistors each (T 1 , T 1 b ; T 2 , T 2 b ), cascaded together, the second pair (T 2 , T 2 b ) having complementary current outputs (H, Hb) that flip according to the states of the inputs (E, Eb). The first pair (T 1 , T 1 b ) is connected to a ground (GND) through a current source, supplying a current of value Io and comprising a transistor (Ts 1 ) biased by a voltage Vbias, and it is supplied by a voltage equal to N·Vbe+Vbias, where N is a whole number (preferably equal to 1) and Vbe is the base-emitter voltage of the transistor (Ts 1 ). The second pair (T 2 , T 2 b ) is connected to ground directly through a resistance (R 2 ). The invention can be applied to the on-off control of sample-and-hold circuits, multiplexers, fast low-voltage logic circuits, etc.
1. A current switch comprising:
two differential pairs of two transistors each, these pairs being arranged in cascade form, the second pair having complementary current outputs, one of which supplies current while the other is blocked and vice versa depending on the state of a differential signal applied between the bases of the two transistors of the first pair, the first pair being connected to a ground through a constant current source supplying a current of value Io, this source comprising a transistor, the base of which is held at a bias voltage Vbias,
wherein the second differential pair is connected to ground directly through a resistance, and the first differential pair is supplied by a voltage equal to N·Vbe+Vbias, where N is a whole number, and Vbe is the base-emitter voltage of the transistor of the current source supplying the current Io, and
wherein the first differential pair has voltage supplied by a supply transistor placed in series between a main supply voltage source Vcc and the differential pair, the supply transistor having its base controlled by a junction node between a low-voltage-drop current source connected to the supply voltage Vcc and a series-connected group, connected to ground, of a plurality of transistors mounted as diodes and a resistance, the voltage Vbias being taken at the base of one of the transistors mounted as a diode.
2. The current switch as claimed in claim 1 , wherein the low-voltage-drop current source is a bandgap type source supplying a temperature-regulated current.
3. The current switch as claimed in claim 1 , wherein N is equal to 1.
4. The current switch as claimed in claim 1 , wherein N is equal to 1.
5. The current switch as claimed in claim 2 , wherein N is equal to 1.
6. A current switch comprising:
two differential pairs of two transistors each, these pairs being arranged in cascade form, the second pair having complementary current outputs, one of which supplies current while the other is blocked and vice versa depending on the state of a differential signal applied between the bases of the two transistors of the first pair, the first pair being connected to a ground through a constant current source supplying a current of value Io, this source comprising a transistor, the base of which is held at a bias voltage Vbias;
wherein the second differential pair is connected to ground directly through a resistance, and the first differential pair is supplied by a voltage equal to N·Vbe+Vbias, where N is a whole number, and Vbe is the base-emitter voltage of the transistor of the current source supplying the current Io; and
wherein the first differential pair is supplied by the main supply voltage Vcc through a resistance connected on one side to the first differential pair and on the other side to Vcc, and in that means are provided for establishing in this resistance a current such that the voltage drop across its terminals is equal to Vcc−(Vbias+N·Vbe).
7. The current switch as claimed in claim 6 , the means for establishing a current in the resistance of the supply for the first differential pair also include means for establishing the bias voltage Vbias.