Miscellaneous active electrical nonlinear devices – circuits – and – Specific signal discriminating without subsequent control – By amplitude
Reexamination Certificate
2000-03-15
2001-08-14
Nu Ton, My-Trang (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific signal discriminating without subsequent control
By amplitude
Reexamination Certificate
active
06275076
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to sample-and-hold devices using complementary bipolar technology.
Sample-and-hold devices are used in many fields of electronics and especially in analog-digital converters and in analog signal processing systems.
The main function of a sample-and-hold device is to copy, starting from a given point in time t0 and for a very short period of time &Dgr;t, the level of a voltage Vin applied to an input of the sample-and-hold devices and to store it for a time that is long enough to perform a specified processing of this level at t0. The period of time &Dgr;t is understood to be very short as compared with the relative variations of the input voltage Vin.
The sample-and-hold device may take two different states corresponding to two different modes of operation.
In a first state, the sample-and-hold device is in “follower” mode and an output of the sample-and-hold device reproduces a faithful image of the input voltage Vin applied to its input.
In the second state, the sample-and-hold device is in the “held” mode and its output is isolated from its input.
A certain number of criteria of quality of the sample-and-hold device have been defined, certain criteria being related to one mode of operation or to the other.
For example, these criteria of quality include the following in follower mode:
the maximum level (full scale) of the voltage Vin that can be processed by the sample-and-hold device in relation to the supply voltage that is applied to it. Indeed, an increase in the level of the voltage Vin that can be processed leads to an improvement of the signal-to-noise ratio at output of the sample-and-hold device and to greater flexibility of use;
consumption: the dynamic performance characteristics are chiefly related to the currents biasing the different arms of the sample-and-hold device. Since the voltage is dictated (by the user), the designer seeks to minimize the current in order to reduce consumption;
linearity: in follower mode, it is important that the output voltage of the sample-and-hold device should faithfully reflect the input voltage Vin, and should do so along the full scale of voltage, namely from the maximum level to the minimum level of the voltage Vin;
the passband: expresses the ability of the sampler to follow a fast signal, and therefore to be used in applications that make heavy demands;
noise: when sampling is done, in addition to the signal Vin, the inherent noise of the sampler is held. If the noise has a spectrum that spreads beyond the spectrum of the signal, then the holding operation brings it to the baseband. This is spectral aliasing. It is therefore indispensable to limit the noise band when it is not indispensable;
harmonic distortion: this may be created by poor linearity as well as by reactive phenomena. In particular, the dynamic current flowing into the sampling capacitor is a cause of dynamic non-linearity and hence of distortion.
More particularly, in held mode, we may refer to:
input/output isolation;
drift: in general, the voltage sampled at the instant t0 is stored by means of a sampling capacitor. The voltage signal held by the sampling capacitor is liable to drift in time because of the presence of leakage currents and of the current consumed by the reading device of this capacitor. This drift should be low, independent of the level of the signal and compatible with the sampling period and a full scale of the signal;
releasing/holding speed: the speed of switching over from one state to another is a major parameter with regard to the use of the circuit in frequency. This requires the sample-and-hold device to receive a signal of maximum width from a clock controlling the change in state, the result of which is that what is actually made is a follow-and-hold device, and not a sample-and-hold device that is ideally controlled by a Dirac pulse.
An important characteristic of sample-and-hold devices is related to the precision with which the input voltage Vin is copied at the time of the holding.
Indeed, the input voltage Vin may be applied to the sampling capacitor through the emitter of a bipolar follower transistor. When the transistor is made non-conductive at the point in time t0, holding the voltage Vin at the terminals of the capacitor, the charge Qst stored in the transistor gets discharged partly by the base and partly by the emitter.
The charge stored by a bipolar transistor that is crossed by a collector current Ic with a transition time Tf characteristic of the technology is given by Qst=Ic.Tf. When the transistor is turned off, Ic becomes zero and the charge removed by the emitter is at the sampling capacitor after the turning off, giving rise to a voltage pedestal Vin.
SUMMARY OF THE INVENTION
The present invention provides improvements in the performance characteristics of the prior art sample-and-hold devices by proposing a sample-and-hold device comprising:
a follower input stage having one input receiving a voltage Vin to be sampled and at least one output,
at least one sampling circuit having a switching stage, the switching stage having at least one control input, one signal input connected to the output of the follower input stage, and one output. The switching stage is controlled by its control input by a digital command so that it is placed either in a first state called a “follower” state where its output follows the potential at its signal input or in a second state called an “isolated” state where its output is isolated from it signal input. The output of the switching stage is connected to the base of a first follower transistor whose emitter is connected to a terminal of an output sampling capacitor. The sampling circuit furthermore comprises a second transistor, having its emitter supplied by a current source and having its base connected to a potential copying that of the terminal of the output sampling capacitor, and a third transistor controlled by the digital command so as to be crossed by a current when the switching stage is in the <<isolated state>>, and so as to be off when the switching stage is in the <<follower>> state, the third transistor having its emitter connected to the base of the first transistor and its base connected to the emitter of the second transistor.
To copy the potential of the output sampling capacitor into the base of the second transistor, the sample-and-hold device preferably has an additional sampling capacitor and a fourth follower transistor having its base connected to the output of the switching stage and its emitter connected to a terminal of the additional sampling capacitor for the application, to this terminal, of a voltage copying the potential at the terminal of the output sampling capacitor, the base of the second transistor being connected to the base of the additional sampling capacitor.
In this embodiment, the sample-and-hold device has two capacitors linked to the sampling function: the output sampling capacitor connected solely to an amplifier for the reading of the sampled voltage, and the additional sampling capacitor having a value lower than that of the output sampling capacitor. The voltage of the terminals of the additional sampling capacitor is set up at greater speed than the voltage at the terminals of the output sampling capacitor, thus enabling the reverse potentials for turning the transistors off to be fixed at greater speed. The holding of the sample-and-hold device is thus anticipated, thus making it possible to obtain sharper sampling plateaus as well as better isolation.
In short, the fact of having, in addition to the output sampling capacitor, an additional sampling capacitor at the terminals of which the sampled signal is copied, makes it possible to avoid the injection, into the output sampling capacitor, of the disturbances due to the non-conduction of the transistors, which affect only the additional sampling capacitor.
In one embodiment, according to the invention, of the sample-and-hold device using complementary type technology, and in one and the same sampl
"Thomson-CSF"
Nu Ton My-Trang
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
LandOfFree
Sample-and-hold device using complementary bipolar technology does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Sample-and-hold device using complementary bipolar technology, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Sample-and-hold device using complementary bipolar technology will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2509282