US3162802A - Dietrich - Google Patents

Description

Dec. 22, 1964 B. D|E\TRICH 3,162,802

VOLTAGE DIVIDER Filed Jan. 25, 1965 F a G 2 INVENTOR.

BERNHARD DIETRICH AT OR NEY United States Patent 3,162,802 VOLTAGE DIVEDER Bernhard Dietrich, l rciburg im Breisgau, Germany, as

This invention relates to voltage dividers generally and, in particular, to high-resistance continuously-adjustable potentiometers and like devices.

It is well known in the art that purely resistive voltage dividers of high impedance exhibit substantial nonlinearity in response at high frequencies, i.e., in excess of 100 kc. due to the effect of distributed capacitance In practice, therefore, it is necessary to design voltage dividers intended for high frequency applications with low resistance because substantially linear response can be achieved only when the time constants (the product of the resistance element and its distributed capacitance) are much lower than the reciprocal of the operating frequency. A more detailed discussion of this problem is undertaken hereinbelow.

One approach to the elimination or suppression of frequency effects in high-frequency, high-impedance voltage dividers is to connect capacitors of fixed values in parallel with the distributed capacitance. This, however, is not a satisfactory solution to the problem because it impairs the frequency range of the device by increasing the time constant. As a result the wave form of the divided voltage would vary in accordance with the setting of the divider. To avoid the increase in time constant it would, of course, be possible to lower the value of the resistive impedance element but then the result is a low-resistance network.

Another possibility to avoid limiting the frequency range of the voltage divider is to lump the added shunt capacitors in an adjustable rotary condenser having its shaft mechanically coupled to a rotary potentiometer which provides the voltage dividing resistance. The disadvantage of this arrangement is that it requires the toleration of relatively large values of distributed capacitance because adjustment to achieve substantially complete compensation would entail reduction of the resistive impedance to the undesired low values. Furthermore, this system would raise the need for extensive shielding.

The fundamental object of the present invention is to overcome or mitigate at least one of the problems of the prior art as outlined above.

A more specific object is the provision of a high re sistance voltage divider having substantially linear response over a wide range of high frequencies.

Another object is the provision of a voltage divider as characterized in the immediately foregoing object which does not require an adjustable capacitor nor extensive shielding.

For the fulfillment of these and further objects, the present invention contemplates a voltage-divider network which comprises an adjustable resistance having in parallel therewith voltage-dependent capacitance elements such as semi-conductor diodes. Means are provided for applying a D.C. potential to the capacitance elements and for adjusting the potential conjointly with adjustment of the adjustable resistance in such a manner as to maintain substantially constant the product of the resistance and capacitance.

Additional objects of the invention, its advantages, scope, and the manner in which it may be practiced will be more fully apparent to persons conversant with the art from the following description of an exemplary embodiment taken in conjunction with the subjoined claims and annexed drawings in which like parts are designated by like reference characters throughout the several views and FIGURE 1 is a schematic circuit diagram of a conventional voltage divider network which will be referred to for explanatory purposes in the following description; and

FIGURE 2 is a schematic circuit of a voltage divider network embodying the present invention.

As previously mentioned high-resistance voltage dividers exhibit non-linear response at high frequency due to distributed capacitance effects. In the prior art type of divider shown in FIGURE 1, the distributed capacitance represented by broken line capacitors C C shunting the resistance element R of a voltage divider 10'. As illustrated, voltage divider 1% takes the form of a rotary potentiometer having an adjustable tap 12 which divides resistance R into segments R and R in accordance with its angular. position 0. For the purposes of example and ease of description, the network according to the present invention will also be described as embodying rotary potentiometers; however, it will be understood that potentiometers having rectilinearly slidable taps as well as other voltage-divider components can be employed.

Continuing with FIGURE 1, a pair of input terminals A, B are connected to respective ends of resistance R; the output voltage is derived between a common terminal B and a terminal A connected to tap 12.

At low frequencies, the division ratio R /R is a function of the angle of rotation 6 of the adjustment tap 12, the function F is determined by the physical structure of the potentiometer. if a high frequency A.-C. voltage is applied to terminals A, B, the division ratio becomes equal to inwhich F is an indefinite function due to distributed capacitance and related to both the frequency, f, of the input voltage and the angular displacement 9 of tap 12.

As previously explained, one approach to the elimination or reduction of frequency dependence involves shunting the distributed capacitance C C with capacitors C C FIGURE 1. However, this is undesirable because it increases the time constants of the network to In accordance withthe present invention, a pair of voltage-dependent capacitance elements such as semi-conductor diodes D D are connected in parallel with the resistance element R of potentiometer it) as shown in FIGURE 2, Specifically, diodes D D are arranged in series, with the cathode of the former connected to the anode of the latter. The cathode "of D is connected to one end of resistance element R the other end of which is coupled by a capacitor C to the anode of D The fundamental principle of the present invention is the suppression of frequency eifects by varying the capacity of diodes D D in accordance with adjustment of R R so that the RC products or time constants R C and 12 C (where C and C are the capacitances of the diodes D and D respectively) remain substantially constant over the entire range of adjustment of potentiometer lid.

To this end diodes D and D are reverse-biased from a suitable source, E, of D.-C. potential applied through a dropping resistor R to the anode of diode 1),. For adjustment of the bias potential a voltage divider such as rotary potentiometer id is provided, having a resistance element R and an adjustable tap 12. Resistance element R shunts diodes D D and has one end connected to bias potential source B through R The other 3 end of R is connected through a resistance R; to adjustment tap 12 which in turn is connected to a point 14 between diodes D D The adjustment tap of voltage divider is also coupled to point 14 by means of blocking capacitor C As indicated by broken line 16, the respective tape of potentiometers 10 and 1d are mechanically linked for conjoint adjustment. In the case of rotary potentiomcters, as shown in the illustrated embodiment, taps 12 and 12 can be mounted on a common shaft.

, As the capacitance of reverse-biased diodes D D varies inversely with the square root of the biasing potential, voltage'divider 10 should have a quadratic voltage-division characteristic in order to maintain uniform values of time constants R C and R C over the range of adjustment of divider 19. Of course, in the event that voltage-dependent capacitance elements other than semi-conductor diodes are used, having an exponential characteristic other than quadratic, voltage-divider lib would need to be adapted accordingly.

In the illustrated embodiment, the quadratic characteristic of voltage-divider 1% is approximated by means of fixed resistance R in parallel with R.

Capactitors C and C serve to isolate voltage divider 10 and the A.-C. input terminals A, B from the D.-C. bias source E.

By Way of example, in one reduction to practice, the FIGURE 2 circuit had the following specific values of resistance and capacitance:

R=O.1 megohm R'=l.0 megohm R =0.5 megohm C6,

The circuit described exhibits substantially linear response at 200 kc. for voltage division ratios as high as 3.5: 1; a comparable conventional circuit at this frequency is limited to a ratio of 1.2: 1.

While there has been described what at present is believed to be the preferred embodiment of this invention,

i it Will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is aimed, therefore, to cover in the appended claims all such changes and modifications as fall Within the true sipirit and scope of the invention.

What is claimed and desired to be secured by United States Letters Patent is:

1. A voltage-divider network comprising: a resistance having an adjustable tap; a pair of semi-conductor diodes arranged in series with the cathode of one connected to the anode of the other, the cathode of said other diode being connected to one end of said resistance; a capacitor coupling the anode of said one diode to the other end of said resistance; a second capacitor coupling said adjustable tap to a point of common potential between said diodes; a second resistance having a second adjustable tap, said second resistance having one end connected to the anode of said one diode; means electrically connecting said second adjustable tap to the other end of said second resistance and to said point of common potential; and means mechanically linking said adjustable taps for conjoint movement.

2. A voltage divider according to claim 1 including a fixed resistance connected between said second adjustable tap and said other end of the second resistance.

3. A voltage divider according to claim 2 including input terminal means for impressing an A.-C. voltage across said first-mentioned resistance; output terminal means for deriving an A.-C. voltage between the adjustable tap and one end of said first resistance; and means for applying to said one end of the second resistance, a D.-C. voltage for reverse-biasing said diodes.

References Cited in the file of this patent UNITED STATES PATENTS 2,712,625 Blitz July 5, 1955 3,095,533 Keizer June 25, 1963 3,109,995 Wargo Nov. 5, 1963 3,110,004 Pope Nov. 5, 1963

Claims (1)

1. A VOLTAGE-DIVIDER NETWORK COMPRISING: A RESISTANCE HAVING AN ADJUSTABLE TAP; A PAIR OF SEMI-CONDUCTOR DIODES ARRANGED IN SERIES WITH THE CATHODE OF ONE CONNECTED TO THE ANODE OF THE OTHER, THE CATHODE OF SAID OTHER DIODE BEING CONNECTED TO ONE END OF SAID RESISTANCE; A CAPACITOR COUPLING THE ANODE OF SAID ONE DIODE TO THE OTHER END OF SAID RESISTANCE; A SECOND CAPACITOR COUPLING SAID ADJUSTABLE TAP TO A POINT OF COMMON POTENTIAL BETWEEN SAID DIODES; A SECOND RESISTANCE HAVING A SECOND ADJUSTABLE TAP, SAID SECOND RESISTANCE HAVING ONE END CONNECTED TO THE ANODE OF SAID ONE DIODE; MEANS ELECTRICALLY CONNECTING SAID SECOND ADJUSTABLE TAP TO THE OTHER END OF SAID SECOND RESISTANCE AND TO SAID POINT OF COMMON POTENTIAL; AND MEANS MECHANICALLY LINKING SAID ADJUSTABLE TAPS FOR CONJOINT MOVEMENT.

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