Amplifiers – With semiconductor amplifying device – Including atomic particle or radiant energy impinging on a...
Reexamination Certificate
1999-07-06
2001-03-13
Mottola, Steven J. (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including atomic particle or radiant energy impinging on a...
C330S261000, C330S292000
Reexamination Certificate
active
06201446
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to integrated circuit amplifiers having compensation for bandwidth stabilization over a wide range of operating supply voltages, and to integrated circuit infrared receivers embodying such amplifiers.
BACKGROUND OF THE INVENTION
With the increasing movement of solid state electronics from discrete component usage to integrated circuitry, such as ASICs (Application Specific Integrated Circuits), advantages have been gained in miniaturization, but certain limitations have been encountered as well. In prior art, amplification systems were built with discrete components such as capacitors and resistors being used in conjunction with high gain integrated circuit amplifiers to produce various amplifier configurations such as transconductance amplifiers. The intrinsic properties of the external components as a function of the operating voltage being used was seldom a problem, if the components were not used outside of their design voltage limitations.
The same is not necessarily the case for fully integrated circuit amplifiers where devices such as capacitors are integrated onto the same substrate as the amplifier. While the various methods of integrating capacitive devices is well known to those skilled in the art, it is also known that the parasitic capacitance of these devices to the common substrate varies with their dc-biasing. In a typical high-capacitance implementation, the capacitor is constructed by placing a polysilicon layer over an n-well region, whereby the polysilicon layer forms the top plate of the desired capacitor and the n-well forms the bottom plate of the desired capacitor. The n-well, however resides atop of the common p-type substrate that is shared with the rest of the integrated amplifier circuit. As a result,
a parasitic capacitance from the n-well back plate to (grounded) substrate exists similar to that of a reverse biased pn-junction. This results in a parasitic capacitance to ground that varies inversely with the dc voltage present on the back plate of the capacitor. Where such a capacitor is used as a high pass filter, for instance, its impedance would change as a consequence of changes in the biasing voltage on the back-plate of the capacitor, thereby changing the frequency response characteristics of the amplifier. For stability in capacitance it is desirable to stabilize the bias voltage on the device. Where there is sufficient power availability to support the losses of a voltage regulator, the capacitance could be stabilized by operating the amplifier from a regulated supply voltage. Unfortunately, for battery operated devices, only a small amount of power may be available, either because of battery size limitations, or other power usage requirements, and a voltage regulator may prove impractical by reducing battery operating time, or increasing the current loading. While switching regulators may be more efficient than other regulator types, they have a tendency of introducing electrical noise and are considered impractical for small integrated low-noise amplifiers. Accordingly it does not seem to be practical to control the bias voltage on the back-plate of the capacitor and another solution is required.
SUMMARY OF THE INVENTION
It has been found that bandwidth stabilization of an integrated amplifier using capacitors integrated onto the amplifier substrate may be achieved by providing the amplifier with open loop amplification characteristics (Ao) that compensate for changes in the capacitance of its capacitors.
Another aspect of the invention provides an integrated circuit amplifier, the open loop gain (Ao) of which is made responsive to the variation in supply voltage that causes the impedance variation in its associated components.
More specifically, where voltage dependent input impedances such as integrated circuit capacitors are used in the input of an amplifier, the bandwidth of such an amplifier can be stabilized by adapting the open-loop gain of the amplifier so that it is inversely dependent on said supply voltage.
Another aspect of the invention provides an integrated circuit transconductance amplifier with integrated capacitive input coupling in which the amplifier is compensated for supply voltage variation to provide controlled bandwidth by adapting the amplifier so that its open-loop gain varies inversely with the supply voltage.
In one embodiment, the invention provides an integrated circuit transconductance amplifier, powered by a supply voltage, the amplifier having input and output ports, and an integrated input hi-pass coupling capacitor biased indirectly by said supply voltage, the parasitic capacitance of said capacitor being dependent on said supply voltage, coupled to an input port of said amplifier. In one method of forming an integrated circuit capacitor, the reverse biased pn junction on the back-plate of the capacitor has a parasitic capacitance which increases with decreasing bias voltage.
The open-loop gain of said amplifier is adapted to depend inversely on said supply voltage, tracking the inverse dependance of the parasitic capacitance of said capacitor to the said supply voltage.
In one particular embodiment, the amplifier comprises at least one amplifying transistor connected to a loading impedance and a bias current element. In a differential amplifier, ie. one having two amplifying transistors, each amplifying transistor has its own loading impedance, but a single bias current element can be used to control bias current in either the single ended or differential configuration.
In order to control the open loop gain of the amplifier, the bias current of each amplifying transistor can be controlled and made responsive substantially inversely to supply voltage. This can be done by controlling the bias current element as a function of supply voltage.
One embodiment for achieving this is the configuration in which the bias current element comprises a transistor which is connected to a reference current source (diode connected transistor) to form a first current mirror circuit, wherein the current flowing through the bias current element transistor is proportional to (mirrors) the current flowing through the reference current source. The reference current source may be configured so that its current varies inversely with the supply voltage.
This can be achieved as follows: biasing the reference source by a fixed reference voltage and having it connected in parallel to a mirror transistor element of a second current mirror circuit, the second current mirror circuit also including a reference current source (diode connected transistor) biased by the supply voltage. The current from the second current mirror circuit acts subtractively to reduce the current in the first current mirror circuit. This has the effect of increasing the current to the amplifier transistor(s) and thereby increasing the amplifiers open-loop gain when the supply voltage decreases as less current is being drawn by the second mirror circuit when the supply voltage is reduced. This happens at the same time as the parasitic capacitance of the input capacitor increases due to the decreasing voltage. As a result the open loop gain of the amplifier is increased for supply voltage reduction and the bandwidth or high frequency cutoff of the transconductance amplifier can be stabilized with respect to supply voltage effects with the appropriate choice of circuit component values.
In a photosensitive receiver application of the above transconductance amplifier using a differential configuration, a reverse biased photo diode is connected across the inputs of the amplifier, so that current flows through the diode in proportion to the intensity of light falling on it. The high pass input capacitors exclude the DC and low frequency light variations allowing high frequency data signals to be amplified.
REFERENCES:
patent: 5225790 (1993-07-01), Noguchi et al.
patent: 5420542 (1995-05-01), Harvey
patent: 5432474 (1995-07-01), Lauffenburger et al.
International Business Machines - Corporation
Mottola Steven J.
Townsend Tiffany L.
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