Electronic circuit

Details Electronic circuit Electronic circuit

2000-12-20

2001-12-25

Pascal, Robert (Department: 2817)

Amplifiers

With periodic switching input-output

C330S069000, C330S150000

Reexamination Certificate

active

06333673

ABSTRACT:

This application claims priority under 35 U.S.C. §§119 and/or 365 to 9930388.5 filed in United Kingdom on Dec. 22, 1999; the entire content of which is hereby incorporated by reference.
TECHNICAL FIELD OF THE INVENTION
This invention relates to an electronic circuit, and in particular to a low voltage differential amplifier incorporating switched capacitors.
BACKGROUND OF THE INVENTION
A standard, fully differential, operational amplifier has specific requirements as regards the bias voltage which must be supplied thereto, in order for it to operate effectively. Specifically, the amplifier usually includes at its input a single pair of differential transistors. If PMOS input devices are used, then, in order to provide suitable bias voltages to these transistors at low supply voltages, the input reference voltage level needs to be nearer to the negative supply voltage than to the positive supply. This restricts the voltage swing which can be handled by the input stage, and hence restricts the maximum available dynamic range at the output of the operational amplifier. Likewise, if NMOS input devices are used, the input reference voltage level needs to be nearer to the positive supply voltage than to the negative supply.
U.S. Pat. No. 5,565,813 discloses a low voltage differential amplifier, in which the voltage on a capacitance circuit, which is switched in and out of the circuit, is used to provide a bias voltage.
SUMMARY OF THE INVENTION
The prior art has the problem discussed above that, at low supply voltages, the maximum available dynamic range at the output of the operational amplifier is restricted.
One possible solution to this problem is to use a voltage boosting circuit such as a charge pump, to provide a supply voltage to the amplifier circuit which is higher than the actual supply voltage level. With a higher amplifier supply voltage, the input reference voltage level can be set close to half way between the negative supply voltage and the positive supply, while still providing suitable bias voltages to the input stage transistors. This maximises the dynamic range of the circuit. However, this solution has the disadvantage that it wastes power to boost the supply voltage in this way.
According to the invention, a switched capacitor circuit provides the bias voltages for a differential amplifier circuit, supplied from two reference voltages. More specifically, a first reference level is used to bias the amplifier input stage, and a second reference level is used to set the centre of the dynamic excursion range of the amplifier output stage. Thus, the first reference level can be set to a level determined by the biassing requirement of the amplifier input The second reference level can be set to one half of the supply voltage.


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