US3757155A

US3757155A - Wide bandwidth deflection amplifier

Info

Publication number: US3757155A
Application number: US00209842A
Authority: US
Inventors: G Waehner
Original assignee: United Aircraft Corp
Current assignee: Raytheon Technologies Corp
Priority date: 1971-12-20
Filing date: 1971-12-20
Publication date: 1973-09-04
Anticipated expiration: 1990-09-04
Also published as: FR2165686A5; GB1407287A; CA980459A; JPS4871129A

Abstract

In a deflection amplifier, a sampling resistor is connected in series with a magnetic deflection yoke and develops a sampling voltage in response to the deflection current and parasitic current flowing therethrough. At low frequencies the sampling voltage causes a feedback current and a deflection current which is proportional to an input voltage. At high frequencies, a network responsive to the voltage provided to the series combination of the yoke and the sampling resistor causes the sampling voltage to be ignored and causes the feedback current and the deflection current to be proportional to the input voltage.

Description

ilnited States Patent Waeliner Sept.4, 11973 1 WlDE-BANDWIDTH DEFLECTION AMIPLKFHER [75] inventor:

22 Filed: 20, 1971 21 Appl. No.: 209,342

Glenn C. Waehner, Riverside, Conn.

[56] References Cited UNITED STATES PATENTS 9/1970 Rodal 315/27 TD 1/1967 Howden... 330/76 6/1972 Walters 330/107 2,752,433 White et a1. 330/103 Primary Examiner-Carl D, Quar forth Assistant Examiner-J. M. Potenza Att0rneyMelvin Pearson Williams [57] ABSTRACT 1n a deflection amplifier, a sampling resistor is connected in series with a magnetic deflection yoke and develops a sampling voltage in response to the deflection current and parasitic current flowing therethrough. At low frequencies the sampling voltage causes a feedback current and a deflection current which is proportional to an input voltage. At high frequencies, a network responsive to the voltage provided to the series combination of the yoke and the sampling resistor causes the sampling voltage to be ignored and causes the feedback current and the deflection current to be proportional to the input voltage.

2 Claims, 5 Drawing Figures PAn-tmiustr' m 3.? fr, 155

WIDE-BANDWIDTH DEFLECTION AMPLIFIER BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to amplifier circuits used for driving an inductive load, and more particularly to amplifiers for driving a magnetic deflection yoke in a CRT display.

2. Description of the Prior Art CRT displays employing magnetic deflection yokes commonly use one of two techniques to drive the yoke. A non-dissipative and inexpensive system currently in use is the resonant energy recovery circuit. This circuit has wide application where a raster is being generated, such as in commercial television, but has no application to display systems where deflection is derived from a stroke generator. A high degree of display linearity is not possible with a resonant energy recovery circuit, primarily because it is an open loop type of circuit.

A more flexible system is a magnetic deflection amplifier comprising an operational amplifier which is concurrently responsive to an input voltage and a sampling voltage. The sampling voltage is generated across a small sampling resistor by feeding the deflection current in the yoke through the sampling resistor. An input resistor and a feedback resistor cause the sampling voltage and the input voltage to be compared and the result of the comparison is used to cause the operational amplifier to provide a deflection current in proportion to the input voltage. Within the limitations imposed by the deflection amplifiers bandwidth, the waveform of the deflection current in the yoke is a faithful reproduction of the waveform of the input voltage (at the input of the deflection amplifier). For a stroke written display or a display where a very precise raster must be generated, this property is essential.

Since the sampling voltage is developed from the deflection current, the circuit properties of the yoke, which include its inductance and any parasitic circuit parameters associated with the yoke, are important in providing the desired deflection current. Among the predications of the present invention is the discovery that a parasitic capacitance (caused by the connection wiring of the yoke, or any other cause) across the yoke causes parasitic current at high frequencies to bypass the yoke and still pass through the sampling resistor, this is particularly important because the parasitic current may cause the sampling voltage to be a poor representation of the deflection current. A usual consequence of the presence of parasitic capacitance is an erroneous sampling voltage at high frequencies that causes an erroneous deflection current. Additionally bandwidth of the deflection amplifier is usually lower than the desired bandwidth because the frequency dependence of the effects of the parasitic capacitance cause a tendency towards instability at high frequencies. Production of a plurality of similar deflection amplifiers is difficult and costly because slight mechanical construction differences cause differences in the parasitic capacity associated with the yokes of individual deflection amplifiers; this results in a substantial unit to unit variation of the relative stability and high frequency performance between individual deflection amplifiers.

It is often desirable to have a deflection amplifier that is responsive to the voltage provided by a preemphasis network which attenuates the low frequency spectrum of its input voltage while leaving the high frequency spectrum unattenuated. The network thereby compensates for high frequency attenuation caused by the deflection amplifier and causes a flat response to the input voltage. These networks may have little or no effect upon the high frequency performance of a deflection amplifier because high frequency currents that are provided pass through the parasitic capacitor to the sampling resistor, thereby bypassing the yoke.

Of deflection amplifiers thus far devised, either their usage is limited to specialized application, they are of restricted bandwidth, or have a tendency towards instability.

SUMMARY OF THE INVENTION The object of the present invention is to provide an improved deflection amplifier that provides current to an inductive load in response to a deflection voltage.

According to the present invention, a network which is responsive to the voltage applied to the series combination of an inductive load and a sampling resistor has a transfer function related to the series combination of the inductance of an inductive load and the resistance of the sampling resistor; at high frequencies, the output of the network is coupled to a feedback resistor of an amplifier, providing a bypass for feedback current that results from a voltage developed across the sampling resistor and causing the amplifier to be responsive to the voltage provided by the network and substantially ignore the voltage developed across the sampling resistor.

The present invention increases the useful bandwidth of deflection amplifiers and causes the unit to unit performance of similar amplifiers to be more nearly alike than deflection amplifiers known in the prior art.

The present invention provides a deflection amplifier which is responsive to the effects of a preemphasis network, since the high frequency effects of parasitic capacitance are substantially ignored.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of a deflection amplifier known in the prior art;

FIG. 2 is a schematic block diagram of a preferred embodiment of the present invention;

' FIG. 3 is a schematic block diagram of a high frequency equivalent of the present invention;

FIG. 4 is a schematic diagram of a first network that has a desired transfer function for use in the embodiment of FIG. 2; and

FIG. 5 is a schematic diagram of a second network that has a desired transfer function.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. I, a deflection amplifier known in the prior art includes an operational amplifier 9 which provides the deflection current to an inductive load, such as a magnetic deflection yoke 10, in response to an input voltage applied to terminals 12. The deflection current flows through a sampling resistor 14 (on the order of one ohm) causing a sampling voltage (proportional to the deflection current) to be developed at a sampling junction 16. The non-inverting input 18 of the amplifier 9 is grounded, which causes a well known phenomena of a virtual ground to be impressed upon the inverting input 20. The sampling voltage causes a feedback current to flow through a feedback resistor 22 (of much greater resistance than the sampling resistor 14). As is well known to those skilled in the art, the current into the

inputs

18, 20 is negligible; an input resistor 24 provides the path for the feedback current. Negative feedback causes the feedback current to be proportional to the input voltage, which equals the voltage across the input resistor 24.

A parasitic capacitor 26 is representative of yoke parasitic capacity and wiring capacity that is associated with the wiring that connects the yoke 10. At high frequencies, a significant parasitic current flows through the parasitic capacitor 26 and through the sampling resistor 14 causing the sampling voltage to be a poor indication of the deflection current (current in the yoke therefore a component of the feedback current is conducted through the resistors 22, 24 that is not associated solely with the deflection current. By contributing to the feedback current, the parasitic capacitor 26 causes the deflection current to be different from the desired deflection current. In the absence of the parasitic capacitor 26, the sampling voltage is truly representative of the deflection current and the Laplace transform of the sampling voltage is given as:

The present invention provides a feedback current that would result from the sampling voltage that truly represents the deflection current.

According to the present invention, at low frequencies the feedback current is provided in response to the sampling voltage; at high frequencies a network provides a bypass for feedback current that results from the sampling voltage, thereby substantially ignoring the sampling voltage, and provides a feedback current in response to the voltage across the series combination of the sampling resistor 14 and the yoke 10. The present invention is predicated upon the knowledge that the effects of the parasitic capacitor 26 is negligible at low frequencies and at high frequencies the current in an inductor is related to the voltage impressed across the inductor, independent of any element in parallel therewith.

Referring now to FIG. 2, resistors 22a, 22b are each equal to one half the value of the resistor 22 (FIG. 1). A compensation junction 30 is formed at the juncture of the resistors 22a, 22b and a capacitor 32. As explained hereinbefore, the inverting input is at a virm/ ll) where V is the Laplace transform of the compensation voltage.

A network 34 has the desired transfer function that provides on a signal line 36 a voltage equal to one half of the sampling voltage in the absence of the parasitic capacitor 26. The capacitor 32 connects the line 36 to the junction 30 so that at high frequencies the compensation voltage is equal to the voltage provided on the line 36; the current through the resistor 22b, provided in response to the sampling voltage, is bypassed to ground. Hence, at high frequencies, the feedback current (through the resistors 22a, 24) is substantially representative of the deflection current. As is well known to those skilled in the art, the frequency above which the capacitor 32 substantially provides the compensation voltage is given as:

where w is the natural frequency above which the capacitor 32 substantially provides the compensation voltage;

R is the resistance of the resistor 22; and

C is the capacitance of the capacitor 32.

A high frequency equivalent of the present invention is shown in FIG. 3. The salient features therein are that the current in the yoke 10, the current through the parasitic capacitor 26 and the sampling voltage do not determine the compensation voltage. The transfer characteristics of the network 34 and the voltage at the output 28 substantially determine the compensation voltage and therefore the feedback current through the feedback resistor 22a and the input resistor 24.

Referring now to FIG. 4, the network 34 is comprised of resistors 38, 40 and a capacitor 42. Using well known network theory, the transfer function of this network is given as:

The transfer function of the network 34 is of the same form as the desired transfer function and is identical to it if the following relationships exist between the resistors 38, 4t), 14, the capacitor 42 and the yoke es 40 RA 2/RAC42 RM/LXO As is well known to those skilled in the art, an appropriate output impedence of the network 34 is obtained by providing a resistance R, very much less than the parallel combination of the resistances 22a, 22b.

Referring now to FIG. 5 a network 3430 consisting of an inductor 441, a resistor 46 and a resistor 48 may be used as an alternative to the network 34". The network 3 k; provides the desired transfer function when its components have a relationship to the resistor 14 and the yoke M) which is given as:

R48 R48 s 2RB/L44 id 10 The network 34a has an appropriate output impedence when the resistance of R is very less than the parallel combination of resistances 22a, 22b.

it should be understood that the present invention may be provided as an improvement to the invention disclosed in the applicants copending application, Wide-Band Magnetic Yoke Deflection System tiled Feb. 8, i971 and assigned Ser. No. 11,321.

Thus, there has been shown an improved deflection amplifier which provides the deflection current to a yoke, the current having a minimized degree of dependence upon the parasitic capacity across the yoke.

Although the invention has been shown and described with respect to a preferred embodiment thereof, it should be understood by those skilled in the art that various changes and omissions in the form and detail thereof may be made therein without departing from the spirit and the scope of the invention.

Having thus described a typical embodiment of my invention, that which I claim as new and desire to secure by Letters Patent of the United States is:

1. in a magnetic deflection amplifier of the type including an operational amplifier which is responsive to an input voltage applied to one end of an input resistor, the other end being connected to an input of said operational amplifier, the output of said operational amplifier providing deflection current and parasitic current to a load including an inductive load in parallel with a parasitic capacitor, and a sampling resistor in series with said load, the deflection current flowing through the inductive load, said load and said sampling resistor having a sampling junction therebetween and a sampling voltage being sustained across said sampling resistor, the improvement comprising:

feedback means, including a compensation junction, connected from said sampling junction to the input of said operational amplifier for providing a feedback current to said input resistor; and circuit means having an input connected for response to the voltage applied to the series combination of said load and said sampling resistor and having an output connected to said compensation junction, said circuit means having a frequency response characteristic such that at frequencies in excess of those frequencies at which the parasitic current becomes substantial with respect to the deflection current, it provides a current to said compensation junction which is substantially equal to the current which would be provided thereto by said sampling junction if the load were ideal and had no parasitic current, whereby at low frequencies the feedback current is provided substantially in response to the voltage at the sampling junction and at high frequencies a feedback current is provided substantially proportional to the voltage across the series combination of the yoke and the sampling resistor. 2. Apparatus according to claim ll wherein said feedback means includes a first feedback resistor connected from said sampling junction to said compensation junction and a second feedback resistor connected from said compensation to the input of said operational amplifier and said circuit means comprises:

a network having a transfer function given as:

where V is the Laplace transform of the voltage applied to the series combination of the inductor and the sampling resistor;

V is the Laplace transform of the voltage provided at the output of said network; S is the Laplace transform variable; R is the resistance of said sampling resistor; L is the inductance of said inductive load; R is the resistance of said second feedback resistor; R is the resistance of said first feedback resistor;

and a capacitor connected between the compensation junction and the output of said network.

Claims

1. In a magnetic deflection amplifier of the type including an operational amplifier which is responsive to an input voltage applied to one end of an input resistor, the other end being connected to an input of said operational amplifier, the output of said operational amplifier providing deflection current and parasitic current to a load including an inductive load in parallel with a parasitic capacitor, and a sampling resistor in series with said load, the deflection current flowing through the inductive load, said load and said sampling resistor having a sampling junction therebetween and a sampling voltage being sustained across said sampling resistor, the improvemEnt comprising: feedback means, including a compensation junction, connected from said sampling junction to the input of said operational amplifier for providing a feedback current to said input resistor; and circuit means having an input connected for response to the voltage applied to the series combination of said load and said sampling resistor and having an output connected to said compensation junction, said circuit means having a frequency response characteristic such that at frequencies in excess of those frequencies at which the parasitic current becomes substantial with respect to the deflection current, it provides a current to said compensation junction which is substantially equal to the current which would be provided thereto by said sampling junction if the load were ideal and had no parasitic current, whereby at low frequencies the feedback current is provided substantially in response to the voltage at the sampling junction and at high frequencies a feedback current is provided substantially proportional to the voltage across the series combination of the yoke and the sampling resistor.

2. Apparatus according to claim 1 wherein said feedback means includes a first feedback resistor connected from said sampling junction to said compensation junction and a second feedback resistor connected from said compensation to the input of said operational amplifier and said circuit means comprises: a network having a transfer function given as: