US2488314A - Amplifier with rising frequency response - Google Patents

Description

Nov. 15, 1949 R. c. MOORE 2,488,314

AMPLIFIER WITH RISING FREQUENCY RESPONSE Filed June 20, 1946 F/emum/cyr INVENTOR.

#055,4 7" c; MOO/3f Patented Nov. 15, 1949 AMPLIFIER WITH RISING FREQUENCY RESPONSE Robert C. Moore, Philadelphia, Pa., assignor to Philco Corporation, Philadelphia, Pa., a corporation of Pennsylvania Application June 20, 1946, Serial No. 678,014

4 Claims. 1

This invention relates to Vacuum tube amplifiers and particularly to a novel and improved amplifier having a signal transfer characteristic which rises with frequency throughout the higher frequency portion of its range. Such a device has particular application in the so-called head amplifier in a television camera system, where it serves to compensate the input impedance characteristic of the head amplifier fed by the usual camera tube. It is, of course, suited to any application where a response, substantially uniform for lower frequency components but peaked for higher frequency components, is desired.

Accordingly, a principal object of the invention is to provide an amplifier having substantial gain for lower signal frequency components and having a rising response for signal frequency components in the higher frequency portion of its range.

Another object is to provide an amplifier in which these characteristics are achieved by the use of separate signal paths; one path contributing to the output of the amplifier principally in one range of the frequency band, and the other contributing appreciably only throughout the remainder of the band.

Still another object is to provide an amplifier of this sort in which one of the paths includes a coupling between the plate of a first amplifier tube and the grid of a second; while the other path includes an impedance common to the cathode circuits of both tubes.

A further object of the invention is'to provide a two-path amplifier having the desired signal transfer characteristic, while avoiding regeneration at certain frequencies and oscillation which may result therefrom.

Another object of the invention is to provide a two-path amplifier in which use is made, in one of the paths, of a load inductor having fortuitous resistance, and in which means are provided for compensating the effect of said resistance so that said path will provide a substantially uniformly rising signal transfer characteristic.

Other objects and advantages of the invention will become apparent from a consideration of the following specification together with the drawings in which:

Figure 1 is a schematic diagram of a two-stage vacuum tube amplifier in accordance with the invention, and

Figure 2 is a graphic representation of the signal transfer characteristic of the amplifier as a whole, showing also the contributions of the separate paths.

Referring to Figure 1, input signal to the amplifier may be supplied through coupling condenser C1 to the grid of pentode vacuum tube T1. In the plate circuit of tube T1 is connected a load inductor L1, including its inherent fortuitous resistance represented by resistor R2. Signal developed across this load impedance is supplied through coupling condenser C2 and across grid leak resistor R2 to the control grid of a subsequent 1 pentocle T2. In addition to the signal transfer path between tubes T1 and T2 as just described, an additional path is provided comprising resistor R5, shunted by condenser C3 and common to the cathode circuits of both tubes T1 and T2.

Assuming the plate load impedance of tube T1 to be purely inductive, the transfer of signal from tube T1 to tube T2 by a coupling condenser C2 and grid leak resistor R3 would be substantially as represented by the sloping broken line a of Figure 2. This provides the desired rising signal transfer characteristic for higher signal frequency components. The resistor R5 in the cathode circuits of tubes T1 and T2, on the other hand, provides a signal transfer characteristic as represented by the broken line b in Figure 2. These two responses will be degrees out of phase and one will therefore complement the other to yield an over-all characteristic as represented at c in Figure 2, having the desired rising response for the' higher frequency components, while at the same time providing for a reasonable degree of amplification of the components in the lower frequency range. The condenser C3 in shunt with resistor R5 is provided to reduce the gain through the cathode path at the higher frequencies so as to prevent undesired regeneration at these frequencies which might tend to cause the amplifier to become oscillatory.

Furthermore, it is to be noted that if the plate load inductor L1 possesses any appreciable amount of fortuitous resistance, this will tend to increase the gain for lower frequencies through the path 1' rain the plate of tube T1 to the grid of tube T2. To avoid this, it is therefore desirable, in certain applications of the circuit (e. g. in a television head amplifier), either to minimize the fortuitous resistance R2 or to compensate for it, as may be done by proper adjustment of the coupling network C2, B3. To eliminate completely the effect of R2 and to obtain results equivalent to those which would be obtained with a purely inductive load, the values of C2 and R3 should be such as to satisfy the relationship 3 In other applications the Value of R2 may be used to control the shape of the low frequency response characteristic.

Thus, it is apparent that the resistor R5 is effective, primarily at lower frequencies, and the inductor L1 is efiective, primarily at higher frequencies, to produce the composite curvezrc, with itsflat contour at lowerfrequencies, moving, into steadily rising values at higher frequencies.

In a typical circuit according to Figure 1 foruse in a television system betweenthe camera tube and'the head amplifier, Where theinput'to" s entially'inductive-load in the plate circuit of said the head amplifier was equivalent to a parallel R'C circuit having a resistance of 220,000 ohms'anda:

capacitance of p4; farads; and: using twortypen SAC? tubes for T1 and T2, the values of the critical circuit elements were chosen as follows-:

L1 henries--. 18 R2 0hln' 0.5

C2 7 farad 0.007 R: o 4,700 farad 0 .1 F55 ohms 515 ing from the scopeof 'the invention as defined by.

theappended claims.-

1. Infan electrical." signal. amplifienfirst and second amplifier stages, each comprising a vacuumtubehavingat least'triode elements, an essentially inductiveload in .theplate circuit of saidv first tube and. coupling/from the plate of said first tube to the g-ridlof said second tube to provide a rising signal, transfer. characteristic bee tween. said stages with. increasing frequencies, andjan impedance common to the cathode circuits of both of: said tubes and adapted to provide relatively high signaLtransfer between said stages only. for relatively low signal frequency components, saiddmp'edance comprising a parallel-connected resistor and condenser.

V 2. Inan electricali'signall amplifier, first and second. amplifierstages, each comprising'a vacuum tube having at leasttriode elements, a load impedance in the plate circuit of said first tube, said load impedance comprising an inductance L1, and. an inherent resistance Hz, a condenser of capacitance C2 connectin the plate of said first tube to the grid of said secondtub'e and a gridleak resistor of resistance R3 connecting said. grid to a point of predeterminedpotential, the values of C2 and B: being. such that CzRs is. approxi mately' equal to Iii/R2, whereby said condenser and said grid-leak resistor cooperate with said load inductor to compensate the effectof said inherent resistance and to provide an essentially first tube, a coupling from the plate of said first tube to the grid of said second tube to provide a risingsignaltransfer characteristic between said stagesiwithincreasing frequencies, and an impedance including a resistor and capacitor comm'on to'th'e cathode circuits of both of said tubes and; adapted to provide relatively high signal transfer with a declining signal transfer characteristic-between said stages-only for-relatively low signal. frequency components, whereby a com posite signalv characteristic is secured withapproximately uniform alues for lower signal .fre: quencies and with increasing values for higher frequencies. I

4. A vacuum tube amplifier for, use in the head, amplifier of a television camerasystem .toprovide. a frequency response characteristic comprising substantiall uniform .responsethroughout a relatively low frequency range. and a response rising substantially uniformly with frequency for frequencies above: said. range, for compensatin the input impedance; characteristic presented by said. amplifier? to the. televisioncamera tube, said amplifier comprising first and second amplifierstageseach includinga vacuumtube having at least triode elements, .a load impedanceinthe plate circuit'of said first tube comprising an in ductance having negligible inherent resistance, a condnsericoupling thepIate-of. said first'tube to the grid. of said'seconclitub'e, a grid-leakresistor. connecting the .gridoflsaid second tube to apoint of. predeterminedl potential, and an impedance common. to the cathode circuits o'frbotli'oflsaid tubes, said last-named impedance includin a resistor. and a condenser. connected in parallel, said plate and cathodeload impedances cooperating to provide said frequency response characteristic ROBERT C. MOORE.-

REFERENC'ES CITED The-following references are-of record in the file of this patent:

2,313,098 Shepard, Jr Mar. 9, 1943

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