Wide Bandwidth Feedback Amplifier

This invention relates to the field of wide bandwidth resistors, particularly as used in negative feedback amplifiers, and more particularly to thick film, deposited resistors in a conformation providing broad bandwidth along with high signal-to-noise ratio.

In an application such as a negative feedback amplifier where broad bandwidth is required, a fraction of the amplifier output is added to the input in phase opposition. When properly done, the amplification becomes substantially independent of the amplifier characteristics, the non-linear distortion produced in the amplifier is reduced, and the signal-to-noise ratio is greatly improved.

Negative feedback amplifiers as customarily used may be limited in bandwidth by the characteristics of the feedback resistor, since there is zero phase shift only in the frequency range for which the resistor remains purely resistive. With discrete resistors or known deposited resistors, high frequency rolloff is a limiting factor because of feedback capacitance and distributed capacity to ground.

In certain applications, it may be required to amplify very small currents coming from a high impedance source, such as 200 nanoamperes and 100 megohms. The feedback resistor provides a low input impedance for the amplifier so that the source capacitance will not limit the bandwidth. A high value resistor is required because of the small signal current.

It is an object of this invention to provide a broad bandwidth resistor having a high ohmic value for use at radio frequencies.

It is a particular object to provide such a high value resistor as for use in a negative feedback amplifier.

It is another particular object to provide such a resistor in deposited film form.

The above objects are achieved in accordance with the invention by forming two or more deposited film resistors having the same length, but different resistive values, on a substrate, very close together and in physically parallel conformation. The primary resistor, such as might couple a fraction of an amplifier's output back to its input, has a high resistance value and is made extra long to reduce to a minimum the feedback capacitance across the leads. The distributed capacitance, which would otherwise be to ground, is essentially eliminated by coupling the added low resistance value resistor or resistors to essentially the same potentials as the feedback resistor. In an amplifier, this could be done by connecting the "amplifier output" end of the feedback resistor to the adjacent end of the added resistor, and connecting the other end of the added resistor to a reference potential which is essentially equal to the potential at the "amplifier input". Thus, the distributed stray capacitance of the feedback resistor is almost entirely between points of equal potential and has no effect on the bandwidth. The stray capacitance of the low value resistor to ground also cannot effect the bandwidth.

In the drawing:

FIGS. 1A and 1B are plan views of the front and back respectively of a preferred embodiment.

FIG. 2 is a plan view of another preferred embodiment.

FIGS. 3A and 3B are plan views of still another preferred embodiment.

FIG. 4 is a cross-sectional view of the embodiment of FIG. 2, taken along the line 4--4.

FIG. 5 is an equivalent circuit of the prior art.

FIG. 6 is an equivalent circuit in accordance with the invention.

FIGS. 7A and 7B are waveforms indicating rise times of the circuits of the prior art and the invention respectively.

It is to be noted that all of the resistors to be described hereinafter may be formed according to any of the techniques known in the art of deposited resistors. In the simplest terms, this includes an insulating base, a deposition of strips of a material having a relatively high resistance, and a deposition of a highly conductive material for connection purposes. The base may be an alumina substrate. The resistive material may be a suspension of conductive metal particles in a glass or ceramic matrix which is deposited on the substrate by the silk screen method and fired at a high temperature. The conductive material may be gold or silver deposited by an electroless process. Interconnection leads are attached to the layers of conductive material. The above information is intended to be instructive only and is not intended to limit the scope of the invention to be described. It is also to be noted that the drawings are illustrative only and are not drawn to scale.

FIGS. 1A and 1B show one embodiment of the invention having a "guarded" resistor 10 on a first surface of the substrate 11 and a relatively wide "guard" resistor 12a on the opposite surface of the substrate (indicated by dashed lines in FIG. 1A for clarity). The meaning of "guard" and "guarded" as used in this description will be explained hereinafter in relation to FIG. 6.

FIG. 2 shows the resistor 10 and two guard resistors 12b on a single surface of the substrate 11. FIGS. 3A and 3B show the front and back of the substrate 11 in an embodiment having the resistor 10 and including all three guard resistors 12a and 12b.

In each of the above embodiments, all of the resistors are tied together at one end with a common conductive layer 13 which may be on only one side of the substrate as in FIG. 2 or wrapped around an edge of the substrate as in FIGS. 1 and 3. The common conductor 13 may be coupled by a lead 14 to an amplifier output as described hereinafter. The opposite end of the guarded resistor 10 is connected to a conductive layer 16 which may be coupled to an amplifier input by a lead 17. The opposite ends of the guard resistors 12a and/or 12b in all figures are connected to a conductive layer 18 which is coupled to ground or to a reference voltage by a lead 19.

FIG. 4 is a cross-sectional view of the embodiment of FIG. 2, illustrating a preferred conformation of the substrate 11, the resistors 10 and 12b, and the conductive layer 13. Typical dimensions for the width of the resistor 10 might be 0.05 inches (0.127 centimeters), the width of the resistor 12a might be 0.3 inches (0.76 centimeters), the width of the resistor 12b might be 0.1 inches (0.25 centimeters), the spacing between resistors might be 0.025 inches (0.06 centimeters), and the length of each might be 1.2 inches (3 centimeters).

FIG. 5 is a schematic diagram of the prior art showing a signal current source 20 such as an operational amplifier in equivalent circuit form, and having a low impedance or "equivalent ground" output. The signal source 20 inputs to an amplifier 22. The amplifier 22 is a negative feedback amplifier having forward gain of -A with no feedback and with a fraction B of the output returned to the input for stability and low signal to noise ratio over the frequency range where 1 - AB > 1. It can be shown that where AB is large, the gain of the amplifier 22 becomes independent of A and approaches the reciprocal of B. When the feedback voltage is developed by a purely resistive network, B has no phase angle and is, therefore, independent of frequency. Thus the amplifier will have negligible phase shift for all frequencies for which AB is large. In the prior art, the bandwidth limiting factors are a feedback capacitance (C.sub.f) 23 and a distributed capacity to ground (C.sub.d) 24. For example, if the feedback path is resistor 10, having a value of 600,000 ohms, a bandwidth extending to 10 MHz would require a value of C.sub.f .ltoreq. 1/2.pi.fR = 0.027pf. A single discrete 600,000 ohm carbon composition resistor, for example, might have a C.sub.f value of around 0.3pf and a C.sub.d of between 0.1 and 0.2pf. If the resistor is made longer to reduce C.sub.f, C.sub.d increases accordingly.

In FIG. 6, the signal source 20 inputs to the amplifier 22 and the feedback resistor 10 is coupled from the output to the input of the amplifier 22 as before, but the guard resistors 12a are also coupled to the output of the amplifier 22 with their opposite ends coupled to ground. As used in this description, "guarded" means protected from stray capacitance to ground, and "guarded resistor" indicates an element providing the protection. In order to provide this function, the resistors 10 and 12 must be placed very close together throughout their entire lengths, whether on the same side of the substrate or on opposite sides and their respective lengths must be substantially identical. All of the resistors should preferably vary linearly over their respective lengths, but it is required that each increment of length of the guard resistor comprise substantially the same percentage of its total resistance as the immediately opposite increment of length of each guard resistor comprises of its respective total resistance. When these restrictions are met, essentially all stray capacitance of the guarded resistor 10 is to the guard resistors 12a and/or 12b, as represented in FIG. 6 by capacitors 26. Since this capacitance is effectively between points of equal signal potential, there is no effect on the frequency response of the feedback circuit. Stray capacitance between the guard resistors 12b and ground is represented by capacitors 27. Since the resistors 12a and 12b are relatively low in resistance value, the capacitance to ground has no deleterious effect.

FIGS. 7A and 7B show voltage rise times through a feedback resistor of the prior art and the resistor of the present invention respectively, with identical square wave inputs and drawn to the same scale. It may be clearly seen that the prior art has a bandwidth limiting effect, and the resistor of the invention does not.

Thus, there has been provided, in accordance with the invention, a deposited resistor conformation providing a wide bandwidth characteristic for many applications. A negative feedback amplifier is one such application, and has been used as illustrative of the advantages and characteristics of the broadband resistor, but no limitation thereto should be inferred.