US2453081A - Wide band amplifier - Google Patents

Description

INPUT 4 16 WA i E 2 Nov. 2 194. s. clszlKLAi WfDE BAND AMPLIFIER Filed Deg. 29, 1944 34' S/GIVAL 11 Tuenssr FREQUENCY r0 BE AMPZ/HED IN VEN TOR. 650/9 6' JZl/(AA/ ATTOENEV Patented Nov. 2, 1948 UNITED STAT ES PATENT OFFICE wine BAND AMPLIFIER George C. Sziklai, Princeton, N. 3., assignor to Radio Corporation of of Delaware America, a corporation Application December 29, 1944, Serial 6. 570,295

The present invention relates to amplifying systems, and more particularly to an improved wide band amplifier coupling network suitable for use as a video amplifier in televisionsystems and circuits.

In television systems and circuits it is necessary to amplify signal frequencies occupying a Wide range of the frequency spectrum from the very low frequencies up to frequencies of three or four megacycles. that the gain of an amplifier stage may be made more uniform over a given frequency range by employing filter circuits as an inter-sta e coupling means. It is also well knownthat the product of the band width and the amplification 1 ratio is dependent primarily on the transductance of the amplifier tube and of the capacities to ground. The product of the band width and=the amplification ratio is limited bythe ratio of the transductance Gm/c, in which 0 is the shunt capacitance. The efiect of the shunt capacitance decreases as the frequency increases. For example, the reactance of the shuntcapacitance, whether it is the inherent capacitance of the tube or a separate physical condenser, decreases at afairly uniform rate as theifrequency is increased. By using filter circuits, one may proinvention is to provide a simple and economical circuit means for efficient high frequency filter compensation.

Another and related object of the invention is to provide novel circuit means and to render: the alignment of these circuit means easy of accomplishment.

It is by now well known :1

3 claims. (01. 179-171) 21. An adjustable condenser3 4xserves as abridgingxcapacitor for the filter and has a valueof ap- A further object is to provide a filter circuit having a constant gain over the pass band, the gain being increased by adding the gain-band width area removed above the cut-off frequency to the area in the pass band. i

Other objects and advantages of the invention will, of course, become apparent and immediately suggest themselves to those skilled in the art I to which the invention is directed from a reading of the following specification in connection with the accompanying drawings in which:

Fig. 1 shows schematically a coupling circuit in accordance with the invention; and

connected to an output terminal ll. IZ of the tube I0 is supplied with signals from a terminal l4 whichmay be connected to the Fig. 2 shows curves serving toillustrate the advantageous features of the circuits of Figpl.

Referring for the present to Fig. 1 ofithe drawings, N] indicates a signal source, such as a vacuum tube amplifier, repeater or the like, the out- I put of which is to 'be coupled to a succeeding stage or translating device (not shown) which maybe The grid output terminal of a preceding stage of amplification orto asource ofimage signals, such as a camera tube or the like. In the event thatthe grid [2 is connected to a precedingstage of amplification, it willbe understood thata filter cou-. pling arrangement, such was the filter 2|", constituting the invention andto bedescribed hereinafter, will be substituted for the simple arjrangement shown which comprisesfia coupling condenser l6 and a grid resistor l8. Coupling between the outputof the signal source Ill, which,

'in the illustrative embodiment, is the plate. circuit IQ of the vacuum tube I0, is effected by a l circuit organization comprising the previously mentioned band pass filter 2 I. The filter is made upof series inductances 24 and26 andparallel capacitances 21, 28, and 3|.

The capacitances 21 and 28 are shown by dotted linesand represent the inherent input and output capacitances of the stages to be coupled. In the present instance, the capacitance 21 isithe anode-to-cathode'capacitance of the tube Ill, and ca'pacitanceiill is the input capacitance of the translating device or succeeding stage which is in communication with the terminal I l. The filter Zlisterminated by a resistor 33. l 3

The condenser 3! is a balancing capacitance having a value approximately equal to that of proximately The output connection! I is taken across the midshunt arm which comprises the condenser 28.

The filter 2l' is formed from a constant-K type ladder filter with the aid of a bridging capacitor,

and thus possesses a distinct advantage over the m-derived ladder filter which requires at least one additional inductor and is more difficult to adjust. Since filters of the constant K type have not provided suflicient gain over a sufiiciently large portion of the frequency spectrum to permit their most efiicient use, the relatively more ladder type m-derived filter without attendant disadvantages; The curves of Fig. 2 are obtained by plotting frequency f against gain indicated'as G on the ordinate scale. Curve 34 shows the gain obtained when a simple RC circuit is employed for coupling. An example of such a circult would be the plat load resistor shunted by the cathode-to-plate capacitance of a vacuum tube. of the frequency range is useful since the gain falls off rapidly. Also, there is no Well defined cut off region, and therefore, the area under the curve 23, while it is large, does not represent a useful gain-band width area.

An ideal response which may not be in practice is indicated by the, rectangle 36. Here,

.the'c utofls point is abrupt and all of the gainbandwidth area isavailable. The area under the curve 34 is approximately equal to the area under the curve 36, it being remembered that curve 34 stilllies above thezero axis at a remote, point, theoretically, at infinite frequency, From these considerations it appears that to solve the problem of increasing the available gain-band width area, it is'desirable toincreasethe gain obtained when a filter is employed by adding the gainband width area removed above the cut off frequency to the area in the pass band.

:Curve 38 is the curve which would be obtained by-employing a two-section constant K filter, for

example. This-may be of the type employing a resistor and a peaking coil and capacitances, such as the inherentinput and output capacitances of theinput and output circuits and/or the inherent capacities-of theinductive and resistive elements themselves. Very considerable improvement-in gain at the higher frequencies is to be observed. However, this curve represents, but to a lesser degree, a non-usable portion of the. curve i n-a region where attenuation is high at higher :frequencies. Considering Fig. .1 of'the drawings,

afilter constitutingonly the elements 24, 26, 21,

'28, and 3! with the output taken across the terminating resistor 33, would give a characteristic somewhat as shown by the curve 38. Some improvement would be obtained byconnecting the output connection across the .mid -shunt arm as shown. e

"In accordance Withthe invention, the capacitor-'34; the value'of whichwith respect to other circuit elements is discussed above, is bridged across. the inductances 24 and 26, the result being a characteristic indicated byreference character 39 By adding-this-capacitor it will be observed that a considerable portion of the gain-band width area is'removed above the cut off frequency and is made available in the pass hand. Moreover, the curve 39.more nearly approaches the ideal curve 36.

.The circuit just described is particularly useful when the capacitance 28 is greater than twice the capacitance 21, without the addition of capacitors; which is a common case.

attained I off efiect not only is retained but takes a more It will be noted that a very small part desirable form for video amplifier characteristics. Various modifications of the invention shown i and described herein by way of example are possible without departing from the spirit and scope of the described invention, and it is desired that any and all such modifications be considered within the purview of the present invention defined by the hereinafter appended claims.

Having now described the invention, What is claimed and desired to be secured by Letters Patent is the following: i .1. A video amplifier employing abridged-T network 'for' high frequency correction. comprising, a signal source serving as an input for said network, said network being of the low pass} type composed of inductors inseries bridgedibycae pacitors, a terminal resistorior said networkgan output connection to said network at the location of an intermediate shuntarm, and an adjustable balancing capacitance bridged across said inductances in series whereby to cause the out 01f of said network to bemore abrupt so -that a greater portion ofthe gain-band width area of theresponse curve of said network is avai-lablefor signal transfer. 1

2 A video amplifieremploying a bridged-T network for high frequencycorrection, the network comprising a pair of series inductances, shunt capacitances in said'network effectivelyat the inputand output ends of said inductances and at theirmid-connection, an output connection provided across the capacitance at the mid-com nection, and a capacitance bridging said inductances.,

GEORGEC. szIKnAL REFERENCES orTE D The'following references are of record iii-the file of this patent:

UNITED STATES PATENTS;

, Number Name 1 i'Date 1,896,468 Shoemaker Feb. '7, 1933 2,093,665 Tellegen l Sept;21, 1937 2,269,693 Schade Jan. 13, 1942 2,269,694 Schade e i- Jan. 13, 1942 2,315,784 Goodale Apr. 6,.1943

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