Electricity: power supply or regulation systems – Self-regulating – Using a three or more terminal semiconductive device as the...
Reexamination Certificate
2000-12-12
2001-07-17
Berhane, Adolf Deneke (Department: 2838)
Electricity: power supply or regulation systems
Self-regulating
Using a three or more terminal semiconductive device as the...
Reexamination Certificate
active
06262568
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to a circuit for generating a potential, and more particularly relates to a common mode bias generator that can supply a required minimum amount of current without dissipating it unnecessarily.
As LSIs have been further miniaturized over the past few years, it has become more and more necessary for analog circuits for the LSIs to operate with even lower power dissipated or to further reduce their sizes. This is because the final value of an LSI product greatly depends on the performance of the analog circuits. Accordingly, the more stringent the requirements imposed on those analog circuits, the more pressing the need for reduction in power dissipation of the circuits. For example, particularly when high-speed transmission should be realized or a wide range of terminal potentials should be handled as in IEEE 1394-compliant systems, analog circuits such as drivers, operational amplifiers and comparators must operate to the limit of their abilities under the current circumstances.
More specifically, if an operational amplifier should supply a large current, then the size of output transistors on the last stage of the amplifier usually has to be increased. This is because if the size of the output transistors is not large enough, then the amplifier cannot supply current in the required amount and eventually the potential at the output node thereof adversely decreases. Thus, that is an inevitable choice to make even though the overall size of the amplifier needs to be reduced.
For example, suppose a TPBIAS circuit should provide an output potential at a certain level and supply a current of about−3 mA to about+25 mA. The circuit, complying with the IEEE 1394, generates a common mode bias voltage at a high-speed differential output node coupled to a so-called “twisted pair” cable. In that case, the sizes of transistors on the last stage of the circuit (e.g., PMOS transistors, in particular) should be increased to such an extent that the circuit can supply the maximum current of 25 mA.
However, as for a common mode bias generator such as that defined by the IEEE 1394, the amount of current to be supplied by the circuit changes incessantly. In other words, the circuit does not always have to supply the maximum amount of current. Accordingly, if the circuit has been designed to always supply the maximum amount of current, then the circuit will waste too much current in vain even while the circuit has to supply no currents (i.e., the amount of current to be supplied is 0 mA). This is because the last-stage transistors of the relatively large size also spend too much current even in such an idling state.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a potential generator that can always supply a required minimum amount of current without wasting it.
An inventive potential generator is so constructed as to generate a predetermined potential. The generator includes first operational amplifier, current supply circuit and current sink circuit. The first operational amplifier has a non-inverting input terminal, an inverting input terminal and an output node. A first reference potential is applied to the non-inverting input terminal of the first amplifier and a potential at the output node of the first amplifier is not only applied to the inverting input terminal of the first amplifier but also used as the output of the generator. If the potential at the output node of the first amplifier is lower than a predefined level, the current supply circuit supplies a current to the output node of the first amplifier. And if the potential at the output node of the first amplifier is higher than the predefined level, the current sink circuit drains a current from the output node of the first amplifier.
In the inventive generator, the first reference potential will be the potential at the output node of the first amplifier and will eventually be used as the output of the generator. Normally, only the first amplifier is used in the generator. However, if a current has been drained from the generator, then the amount of current to be output by the generator might exceed the maximum amount of current that the first amplifier can supply. Then, the potential at the output node of the first amplifier falls. And when the potential at the output node of the first amplifier gets lower than the predefined level, the current supply circuit supplies a current to the output node of the first amplifier. In this manner, if the amount of current to be output has increased, then the deficit is covered. On the other hand, if a current has been externally supplied to the generator, then the amount of current supplied to the generator might exceed the maximum amount of current that the first amplifier can drain. Then, the potential at the output node of the first amplifier rises. And when the potential at the output node of the first amplifier exceeds the predefined level, the current sink circuit drains a current from the output node of the first amplifier. In this manner, if the amount of current externally supplied to the generator has increased, then the surplus is cut down.
As can be seen, since the inventive generator includes the current supply and sink circuits, the size of transistors on the last stage of the first amplifier does not have to be increased according to the maximum amount of current to be supplied. Thus, the generator can be designed with the size of the transistors on the last stage of the first amplifier reduced so that the amount of current needed to operate the generator may be minimized during its normal operation. As a result, the current dissipated by the generator can be cut down.
In one embodiment of the present invention, the current supply circuit preferably includes second operational amplifier and first transistor, while the current sink circuit preferably includes third operational amplifier and second transistor. The second amplifier has a non-inverting input terminal, an inverting input terminal and an output node. The potential at the output node of the first amplifier is preferably applied to one of the non-inverting and inverting input terminals of the second amplifier. On the other hand, a second reference potential is preferably applied to the other input terminal of the second amplifier. The first transistor is preferably connected between a power supply node, which receives a supply voltage, and the output node of the first amplifier and preferably turns ON or OFF responsive to the output of the second amplifier. The third amplifier also has a non-inverting input terminal, an inverting input terminal and an output node. The potential at the output node of the first amplifier is preferably applied to one of the non-inverting and inverting input terminals of the third amplifier. On the other hand, a third reference potential is preferably applied to the other input terminal of the third amplifier. The second transistor is preferably connected between the output node of the first amplifier and a ground node and preferably turns ON or OFF responsive to the output of the third amplifier.
In the generator with this configuration, when the potential at the output node of the first amplifier gets lower than the second reference potential, the second amplifier provides an activating signal to the first transistor, thereby turning the first transistor ON. As a result, a current is supplied from the power supply node to the output node of the first amplifier, and the current to be output can have its deficit covered. On the other hand, when the potential at the output node of the first amplifier gets higher than the third reference potential, the third amplifier provides an activating signal to the second transistor, thereby turning the second transistor ON. As a result, a current is drained from the output node of the first amplifier to the ground node, and the current externally supplied to the generator can have its surplus cut down.
In this particular embodiment, th
Akamatsu Hironori
Hirata Takashi
Komatsu Yoshihide
Takahashi Satoshi
Terada Yutaka
Berhane Adolf Deneke
Matsushita Electric - Industrial Co., Ltd.
McDermott & Will & Emery
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