Synthetic circuit component and amplifier applications

Synthetic circuit elements and amplifier applications for synthetic circuit elements are provided. The synthetic circuit elements disclosed herein may be configured to compensate for some or all of the parasitic capacitance normally associated with circuit elements disposed on a substrate providing a selectable impedance characteristic. Amplifier circuit constructed using such synthetic circuit elements exhibit improved performance characteristics such as improved recovery time, frequency response, and time domain response.

BACKGROUND OF THE INVENTION

This invention relates to synthetic circuit components and certain applications for synthetic circuit components that involve improving the frequency response of amplifier circuitry.

From a conceptual point of view, the simplest signal processing task is that of signal amplification. The need for amplification arises because certain “weak” signals, such as those produced by transducers, are frequently too small for reliable detection or processing. Amplifying weak signals increases signal amplitude while preserving the details of the subject waveform, making signal acquisition and additional signal processing feasible.

Currently, circuit designers may choose from a wide variety of amplifier configurations to obtain certain desired performance characteristics. For example, in a communications system, a voltage amplifier may be used to increase the magnitude of a data signal that has become attenuated from traveling across a lengthy transmission line. In a different application, such as a power supply, a current amplifier may be used that provides only a modest amount of voltage gain but substantial current gain. In yet other applications, amplifiers may be chosen for their frequency attributes or for their ability to perform some filtering or shaping of the frequency spectrum.

One characteristic of amplifier circuitry that is often of concern to system designers is frequency response. As a general principle, it is desirable to have the bandwidth of the amplifier be as large as possible so that it may be used in a wide range of applications.

In the past, input networks with large time constants have been utilized to improve the low frequency response of amplifier circuitry (i.e., reliably amplify low frequency signals). An example of a prior art circuit using this technique is shown in FIG.1. As shown, amplifier circuit100includes amplifier110, coupling capacitor120, resistor130and bias voltage140. Capacitor150represents the parasitic capacitance that often accompanies the input network of amplifier110. The values of resistor130and coupling capacitor120are typically selected to minimize jitter and amplitude deterioration experienced when amplifier110is required to maintain VOUTat a constant level over a relatively long period of time. This may occur, for example, when a digital communication system is required to produce a long series of logic high signals.

One deficiency of this approach, however, is that such input networks require relatively large components. In the example above, capacitor120may have a value of about 33 pF and resistor130may have a value of about 1 MΩ. Using components of this size with concomitant parasitic capacitance, tends to limit the bandwidth and increase recovery time of circuit100in addition to occupying a significant amount of die space when disposed on an integrated circuit.

Thus, in view of the foregoing, it would therefore be desirable to provide circuits and methods that improve the bandwidth and recovery time of an amplifier. It would also be desirable to provide circuits and methods for reducing the size of the components in an input network of the amplifier.

SUMMARY OF THE INVENTION

One object of the present invention is to provide circuits and methods that improve the bandwidth of an amplifier.

Another object of the present invention is to provide circuits and methods that improve the recovery time of an amplifier.

Another object of the present invention is to provide circuits and methods for reducing the size of the components in an input network of the amplifier.

Another object of the present invention is to provide novel synthetic circuit elements with adjustable impedance characteristics.

These and other objects are accomplished in accordance with the principles of the present invention by providing synthetic circuit elements with controllable impedance characteristics and amplifier applications therefor. The synthetic circuit elements compensate for parasitic capacitance normally associated with components disposed on a substrate. Amplifier circuits constructed using such synthetic circuit elements exhibit improved performance characteristics such as improved recovery time, frequency response, and time domain response.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2is a schematic diagram of an amplifier circuit200constructed in accordance with the principles of the present invention. Amplifier circuit200includes amplifier210, AC coupling capacitor220, synthetic circuit element230(and optionally synthetic circuit element232), coupling capacitor234, and bootstrap amplifier240. Capacitor250represents the parasitic capacitance that accompanies the input network of amplifier210. As illustrated by the dotted line inFIG. 2, VINmay be connected to amplifiers210and240, bypassing coupling capacitor220.

Although other arrangements are possible, amplifier circuit200is preferably disposed on a semiconductor die (not shown) and may be fabricated using field effect transistors (FETs), bipolar junction transistors (BJTs), a combination of the two, or any other integrated circuit technology, if desired. Moreover, synthetic circuit elements230and232may be formed from material present on a typical integrated circuit (discussed in more detail below).

Amplifier200has been improved as compared to amplifier100, by the addition of bootstrap amplifier240, synthetic circuit elements230and232, and coupling capacitor234. AsFIG. 2shows, the output of bootstrap amplifier240is coupled to synthetic circuit element230through transmission paths235and231(through coupling capacitor234). In some embodiments, it may also be desirable to couple synthetic circuit element232to bootstrap amplifier240(via transmission path236). Generally speaking, the purpose of bootstrap amplifier240and synthetic circuit components230and232is to provide and maintain a high impedance bias circuit for amplifier210in order to obtain an improved time domain response from amplifier200as compared to prior art systems. Further, as will be explained in more detail below, the resistive and frequency response characteristics of synthetic circuit elements230and232may be varied to improve certain performance characteristics of circuit200such as improved frequency response and fast settling time.

FIG. 3shows a cross-sectional view of one possible embodiment of synthetic circuit elements230and232constructed in accordance with the principles of the present invention. As can be seen, these elements are based on the design of a conventional integrated circuit-type polysilicon resistor. Moreover, synthetic circuit elements230and232act primarily as resistive circuit elements and in many cases function substantially similar to a conventional polysilicon resistor, but with certain enhanced characteristics. Synthetic circuit element230may include polysilicon resistor242, insulator243, N-well portions244, bootstrap contact245, N-well contacts246, and-P-type substrate247. Synthetic circuit element232includes substantially the same components and therefore uses prime versions of the some reference numbers used in element230to denote corresponding components (e.g.,242and242′).

Although two synthetic circuit elements are shown inFIGS. 2 and 3, it will be understood that any number by may be used if desired (i.e., one or more). Furthermore, because elements230and232operate in substantially the same way, the operation of synthetic circuit element230described in the following is also generally applicable to element232.

As shown inFIG. 3, the output of bootstrap amplifier240may be connected to various portions of the synthetic circuit elements such as at bootstrap contact245(through capacitor234). Amplifier240may also be coupled to one or more N-well contacts246and246′ depending on the needs of a user. In some embodiments, one or more N-well contacts246may be connected to ground rather than bootstrap amplifier240(not shown) to tailor the frequency response characteristics of amplifier210to meet specific needs. Synthetic circuit elements230and232connect to other circuits such as bias voltage239and amplifier circuit210at nodes237and238respectively through contact points248and248′.

Synthetic circuit element230operates by manipulating the amount of effective parasitic capacitance that is experienced by resistor242. Because a certain amount of parasitic capacitance exists at the junctions between resistors242and N-well portions244, the response of resistor242will change over frequency. For example, when a relatively low frequency signal is applied, the parasitic capacitance will tend to act as an open circuit, having little effect on performance i.e., element230acts like a pure resistor. However, as the frequency increases, some of the parasitic capacitance begins to behave like a short circuit, conducting an increasing amount of current, which attenuates any signal passing through resistor242. As the frequency continues to increase, greater portions of signal are dissipated, attenuating the signal even further. Thus, resistor242tends to function more and more like a low pass filter as signal frequency increases. This property can be problematic in circuit applications where frequency dependent signal attenuation is undesirable.

Synthetic circuit element230allows a user to partially or fully correct for this problem by varying the amount of parasitic capacitance experienced by resistor242. This is accomplished by coupling a drive signal, such as the one produced by amplifier240, to portions of element230. For example, coupling the drive signal to resistor242at contact245reduces the current flow from element230to element232and supplies current that charges the parasitic capacitance between resistor242and N-well portions244. As a result, the tendency of synthetic circuit element230to function as a low pass filter at high frequency is reduced (i.e., it tends to cancel the frequency effects of the parasitic capacitance).

The effective parasitic capacitance on resistor-242may be further reduced by coupling the drive signal produced by amplifier240to one or more conductive sections formed by N-well portions244(through contacts246). Specifically, this technique reduces the parasitic capacitance between P-type substrate247and resistor242. Applying the drive signal to both resistor242and N-well portions244provides an effective way to control the amount of parasitic capacitance experienced by resistor242.

Moreover, this capacitive compensation effect is directly proportional to the strength of the drive signal applied to resistor242. Generally speaking, when the gain of amplifier240is between zero and one, the greater the amplitude of the drive signal, the less parasitic capacitance experienced by resistor242. This is true until the net effective parasitic capacitance is fully canceled. For the case where amplifier240provides a negative gain, the parasitic capacitance experienced by resistor242increases. For the case where amplifier240provides a gain greater than or equal to one, resistor240experiences a “negative capacitance” and the potential for spontaneous oscillation increases.

Synthetic circuit element230may operate in essentially three different modes: under-compensated, perfectly-compensated, and over-compensated. Element230may be under-compensated by applying a drive signal that fails to fully cancel the effects of parasitic capacitance on resistor242. This may be accomplished in many ways. For example, a drive signal of insufficient magnitude may be applied to contacts245and246, or a signal of sufficient magnitude may be applied to only contact245or just to one contact246and not the other. Another technique may include phase shifting the drive signal so it is out of phase with the signal in resistor242. In the case where resistor242is under-compensated, it still has some parasitic capacitance; and therefore continues to exhibit some low-pass filter characteristics but with a higher cutout frequency than an uncompensated case. This configuration may be desirable, for example, when only partial reduction of parasitic capacitance is required (e.g., when constructing an attenuator).

Synthetic circuit element230may be perfectly-compensated by applying a drive signal that fully compensates for parasitic capacitance experienced by resistor242. This may be accomplished by providing a drive signal that is substantially in phase and has substantially the same magnitude as the signal passing through resistor242. In this case, the parasitic capacitive effects are substantially canceled so that element230acts a pure resistor (i.e., provides a constant resistance value that does not vary with frequency). This configuration may be desirable, for example, when constructing certain circuitry such as an attenuator.

Synthetic circuit element230may be over-compensated by applying a drive signal that over-compensates for parasitic capacitance. Overcompensation may be achieved whenever the drive signal applied to element230is substantially in phase with and has a greater magnitude than the signal passing through resistor portion242. In this case, the applied signal produces a negative impedance (capacitance) effect that causes the impedance of element230to increase somewhat or “peak” as frequency increases. In the over-compensated state, element230exhibits a frequency response similar to a large inductor. This configuration may be desirable, for example, when constructing certain circuitry such as a digital communications circuitry.

FIG. 4illustrates additional synthetic circuit elements254and256constructed in accordance with the principles of the present invention. Elements254and256are similar to elements230and232in many ways, and therefore the reference numbers for like elements remains the same. The primary difference between the two is the fabrication technique used to produce their respective resistor portions. In elements254and256, resistor252may be a diffused or ion-implanted type resistor with insulator243located on top rather than a polysilicon type resistor like resistor242with an insulator located underneath it. Otherwise, these specific implementations operate in substantially the same way. It will be understood, however, that synthetic circuit elements shown inFIGS. 3 and 4represent only two embodiments of a synthetic circuit element constructed according to the present invention, and that many other embodiments are possible. For example, portions of elements254and256may be configured as transistors (e.g., as FETs—section252would operate as a channel, portion243would operate as a gate insulator, and a gate terminal would be added) to perform substantially the functions described above (not shown). In this case, the well, gate, or both may receive a drive signal form a bootstrap amplifier to provide compensation.

FIG. 5is a top view of one embodiment of synthetic circuit element254illustrating how an N-well may be used to bootstrap the parasitic capacitance of a resistor. As shown, resistor252may be placed on top of N-well section244, which in this case, spans the entire area underneath resistor252. Node238is connected to external circuitry while the drive signal from amplifier240is connected to N-well contact246. Node231may be coupled to the drive signal, another synthetic circuit element, to ground or to another bias signal (not shown). The drive signal conducts through N-well well section244effectively charging the “bottom plate” of the parasitic capacitance, either reducing, canceling, or enhancing its effect.

The synthetic circuit elements described above may be used in a wide variety of electronic applications. One suitable application is the amplifier circuit shown in FIG.2. As shown, amplifier200is biased by synthetic circuit elements230and232and bias voltage239. In this particular application, which is suitable for use in a high speed communications system, it is desirable to over-compensate synthetic circuit elements230and232to provide a high impedance bias circuit whose impedance tends to peak at frequencies comparable to the time constant associated with elements230and232(also may be comparable with the time constant of coupling capacitor220). This arrangement has several advantages over prior art systems. One advantage is a relatively high cutin frequency (i.e., the frequency at which the amplifier begins to effectively amplify the input signal, sometimes referred to as the 3 dB point) which filters out low frequency noise and promotes fast settling times. Another advantage is a vastly improved time domain response that allows amplifier200to accurately reproduce long strings of logic low or logic high signals without sagging.

These advantages may be more fully understood by considering the waveforms shown in FIG.6. InFIG. 6, waveform260represents an input signal (VIN) applied to the input of amplifier200. Waveform265represents the output signal (VOUT) produced by amplifier200in response to input signal260. InFIG. 6, initial period270illustrates the case where waveform260has been stable at a logic high state for an indefinite period (although it will be understood that a logic low is also possible). During this period, coupling capacitor220becomes charged to a steady DC value. When waveform260begins to vary (e.g., when digital data transmission resumes) capacitor220must discharge during interval275(or must charge if held at a logic low), as shown inFIG. 6, before the incoming signal is reliably transmitted. Next, during interval280, a long continuous string of logic high bits is supplied by waveform260. Waveform portion290illustrates the virtually perfect replication of logic high bits for the length of the string provided by amplifier200compared to prior art systems waveform that tend to suffer from significant amplitude deterioration (i.e., sag) in this situation (depicted by dotted line285).

Furthermore, because synthetic circuit elements230and232may provide the needed impedance characteristics for optimum frequency response, the physical size of capacitor220and the synthetic circuit elements used in amplifier200may be reduced by a factor ten or more as compared to comparable components used in prior art systems. This saves valuable die space on an integrated circuit and reduces power consumption. In addition, the smaller capacitor220becomes, the greater the reduction of overall parasitic capacitance, which reduces capacitive loading on the circuitry producing VIN, increasing overall speed of circuit200. This feature is highly desirable in any type of digital transmission system, such as an optical communications system, where data is frequently interrupted. Thus, the degree of compensation provided to synthetic circuit elements in amplifier200may be chosen to obtain a desired recovery time.

Using the synthetic circuit elements described above it is possible to tailor the amount of parasitic capacitance experienced by a particular circuit element in order to obtain certain frequency-related performance characteristics. As described above, synthetic circuit elements may be under-compensated, perfectly-compensated, or over-compensated depending on the application. Furthermore, it is possible to vary compensation within these categories to obtain components that are almost perfectly-compensated, strongly over-compensated, etc.

One way in which the amount of capacitive compensation may be controlled is to vary the resistor area covered by an underlying N-well section. For example, rather than providing complete coverage as shown inFIG. 5, only certain portions of resistor242are covered by separate N-well sections244aand244b, with portion244c(the area not enclosed within a dotted line) having no underlying N-well coverage at all (shown in FIG.7). The shape and size of regions244a-cmay be tailored to increase or decrease the amount of parasitic capacitance compensation. In addition, the amount of parasitic capacitance experienced by resistor242will depend on the strength of the applied drive signal and the amount of resistor area without an underlying N-well portion. For example, the larger area244cbecomes the less compensation provided to resistor242. Furthermore, as can be seen inFIG. 7, N-well section244bis connected to ground. As a result, no capacitive compensation is provided to this section of resistor242.

A schematic diagram generally illustrating the equivalent circuit of the arrangement ofFIG. 7is shown in FIG.8. InFIG. 8, circuit portion291represents the portion of resistor242covered by N-well section244b, circuit portion292represents the uncovered section244c, and circuit portions293represents the section covered by N-well244a. In operation, synthetic circuit element232functions as a conventional resistor exhibiting a low pass filter characteristics over frequency. On the other hand, circuit portion293provides some capacitive compensation, while section292experiences some parasitic capacitance to the substrate (represented by the triangle S symbol).

Although preferred embodiments of the present invention have been disclosed with various circuits connected to other circuits, persons skilled in the art will appreciate that it may not be necessary for such connections to be direct and additional circuits may be interconnected between the shown connected circuits without departing from the spirit of the invention as shown. Persons skilled in the art also will appreciate that the present invention can be practiced by other than the described embodiments. For example, synthetic circuit elements based on an N-type substrate technology may be used rather than P-type. Furthermore, the amount of capacitive compensation provided may be adjustable by programmable switches (e.g., such as multiplexers, EPROMs, EEPROMs, pass transistors, transmission gates, antifuses, laser fuses, metal optional links, etc.) in an integrated circuit so an end-user may vary or select the degree of compensation to meet specific needs. This may be done for example, by dividing semiconductor wells into sections which may be programmably connected or disconnected by the programmable switches thereby altering the shape or size of the well region to obtain the desired compensation, etc. This may also be achieved using laser trimming or altering the DC bias of the well. Programmable switches may also be used to vary the gain of bootstrap amplifier240, to change the amount of capacitive compensation provided (an external network or component such as a resistor may also be used, if desired).

Moreover, it will be understood that the described embodiments are presented for purposes of illustration and not of limitation, and the present invention is limited only by the claims which follow. Persons skilled in the art will appreciate that the present invention can be practiced by other than the described embodiments, which are presented for purposes of illustration and not of limitation, and the present invention is limited only by the claims which follow.