US2269693A - Wide range amplifier circuits - Google Patents

Description

Jan. 13, 1942. Q sc 2,269,693

WIDE RANGE AMPLIFIER CIRCUITS Filed Oct. 26; 1939 T COMPEN$ATION E RESONANCE FREQUENCY\ l 0 FREQUENCY 1 if); 2 f

g 3 /3 T0 our ur 1 l4 NETWORK 5 fl ll' rr' /6 *"J:,*" J:' 2 I 5 (Ti :1; 2 ,5 705001205 0F :9 VIDEO vozmae '5 I fi= 1 =12 INVENTOR. 0770 ll. sol/ADE ATTORNEY.

Patented Jan. 13, 1942 Otto H. Schade, West Caldwell,

Radio Corporation of America,

Delaware N. J assignor to a corporation of Application October 26, 1939, Serial No. 301,319 2 Claims. (Cl. 179-171) My present invention relate to wide range amplifiers, and more particularly to video amplifiers having a substantially flat'response over the operating frequency range.

One of the main objects of my present invention is to provide selective inverse feedback in a video amplifier thereby to secure complete compensation of deviations in gain or phase within a certain frequency range and without loss of normal gain.

Another important object of thisinvention is completely to compensate for response-varying effects caused by circuit elements in the plate circuit of a wide band amplifier; particularly designed circuit elements being inserted in the cathode circuit to effect the compensation.

Another object of my invention is to provide a video amplifier having a plate circuit constructed to impart a peak in the frequency-response characteristic thereof, and the cathode circuit including a degenerative network whose constants are chosen to compensate for said peak.

Still other objects of my invention are to improve the response characteristic of video amplifiers, and more especially to provide compensation networks of the type disclosed hereinafter which are readily and economically manufactured and assembled.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawing- Fig. 1 schematically illustrate the general circuit embodying the invention,

Fig. 2 shows a response characteristic of an amplifier employing the invention,

Fig. 3 shows a video amplifier network utilizing the invention.

Referring now to the accompanying drawing, the amplifier tube l of Fig. 1 has an input circuit connected to its cathode 2 and signal grid 3. The output circuit is connected between the plate 4 and the cathode, and includes the direct current source 5. The input voltage E1 is assumed applied between the input electrodes. The plate load impedance is considered as including a portion Zn, or desirable fraction thereof, and the portion Zx causing undesirable effects. The output voltage E0 may be considered as comprising the voltage En developed across n, and Ex developed across Zx. It can be shown that an impedance Zr, connected between cathode and ground, can be designed to compensate for the a video amplifier so that a age. The cathode undesirable effects causedby Zx in the plate circuit. The input voltage E1 is shown composed of rid to cathode voltage Eg and the voltage Ex developed across Zk.

In general, the ratio and phase relation of El to E remain unchanged when E1; and Ex furnish similar vector diagrams; in other words, the phase angle between Eg and Ek is equal to the phase angle between En and Ex at any one frequency. Further, if the following relation holds true:

the effects of an impedance Zk can be eliminated by inserting Zx in the plate circuit (as disclosed in my application Serial No. 301,320, filed October 26, 1939) and, conversely, the effects of an undesirable fraction Zx of the total plate impedance can be eliminated by inserting Z1; in the cathode circuit, Compensation over a frequency range requires that Equation 1 holds true over that range. If, therefore, the undesired effect in the plate circuit is aresonance effect, Zx is found to be the impedance'of a certain resonant circuit. Z1; is then designed as a similar resonant circuit having an equal resonance rise and an impedance at resonance which eventually resolves itself to:

I 1 a:(r) (2) ZMT) gm(lc) n Consider, now, a frequency-response characteristic as shown in Fig. 2. Frequencies are plotted against voltage, or gain, as ordinates. The solid curve is the voltage output without compensation, and rises to a peak near the cut-off frequency fc. It is desired to obtain the dotted curve; the shaded area must be eliminated. Assume that its shape is closely matched by the resonance curve of a single parallel tuned circuit. Its resonant voltage is then Em) as shown; the normal voltage desired being Eno). The impedance Zk(r) of the correction circuit has a positive value given by- Equation 2 and can be realized. It is not desirable to design the coupling network of dip occurs in the response characteristic, since in such case a negative value of Zk(r) would result and this would cause a loss of gain before compensation would be had.

In Fig. 3 there is shown a video amplifier network employing a correction circuit in the oathode lead of the video amplifier tube 5. This tube may be a pentode of the 1851 type and has its signal gridli connected to a source of video voltis connected to ground through resistor I which is shunted by condenser 8, and the latter condenser is in turn shunted by a coil 9. The plate ll of tube 5 is connected to the positive terminal of the direct current source I2 through a path which includes coil [3, coil [4 and resistor l5 arranged in series. The negative terminal of current source l2 may be established at ground potential, and screen grid I6 will be connected to a desired positive potential point on current source l2. The suppressor grid is connected to a point on the current source which is at a substantially lower potential than the screen grid connection point. The junction of coils l3 and I4 is connected to the signal grid of a following video amplifier tube 20, the usual coupling condenser 2| and grid leak resistor 22 being included in the coupling network to tube 20.

The coupling network between tubes 5 and 20 carries of one pi section and one-half section. Coil M has a value which is one-half the value of coil I3; the symbol Cp denotes the shunt plate capacity shown in dotted lines, and the symbol Cg denotes the shunt capacity due to the grid circuit of the following tube and is also shown in dotted lines. The shunt capacities C1 and C2, both shown dotted, are the respective distributed capacities of coils l3 and M.

The cut-off frequency of a low pass filter may be expressed as follows:

In the much used single half-section filter (peaking coil) the value C in Equation 3 is: 0:2 (C -l-C'g), while for the circuit shown in Fig. 3, C is equal to C The type 1851 tube has a capacity relation Cg equals two times Cp, which makes possible the use of the low pass filter shown in Fig. 3 for which this relation is required. The cut-oil frequency fc for Fig. 3 is, hence, theoretically three times higher for the same gain (same value of resistance of resistor !5) than for the normal circuit with a single peaking coil. Actually, the coil capacities C1 and 02 cause about 15% reduction of this ratio. This follows from the fact that the following relation exists:

Normally, the useful frequency range of this circuit for video purposes is approximately 0.7 ,fc, because it has a characteristic similar to that shown by the solid curve in Fig. 2. The peak near cut-off is undesirable, and is eliminated by a correction impedance network l-8-9 in the cathode circuit of tube Saccording to the discussion in connection with Fig. 1. Actual testing of an amplifier having seven circuits in cascade, and each circuit being of the type shown in Fig. 3, showed that one compensation network for the cut-off peak inserted in the cathode circuit of one of the tubes was sufficient for equalization of the entire video amplifier. The computed cutoff frequency of the amplifier was found to be 0:9 megacycles on 4 stages. Tests showed the end of the useful range to occur at 8 megacycles without causing transients. cuit thus extended the useful range to 0.89 fc.

The present invention is not limited to the specific circuit shown in Fig. 3, but may be used to correct other response characteristics or transients originating in any part of an amplifier circuit by inserting a selective degenerative net- The equalizing cirill work in the cathode circuit, or by inserting gain increasing circuit elements in the plate circuit according to the discussion in connection with Fig. 1. Because of the dependence of the correction impedance on gm, a voltage adjustment for the grid bias or screen-voltage of tube 5 should be provided for the correcting stage which permits one to vary gm. Re-adjustment of this control is necessary only to correct changes of gm during the life of the tubes, the correct setting being apparent by the absence of transients in the amplified picture signal.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. A wide range video amplifier capable of amplifying frequencies up to approximately 9 megacycles comprising an electron discharge tube of the type including input and output electrodes, a video input circuit connected to the input electrodes, a load circuit including resistive and reactive components connected between the output electrodes. said reactive component tending to produce an undesirable deviation in the frequency response characteristic at the high frequency end of the range, and a reactive network in the space current path of said tube, common to the input and output circuits of the tube, for compensating for said undesirable deviation, said reactive network comprising the shunt connection of a resistance, a capacitance and an inductance between the tube cathode and ground and being resonant to the high frequency end of the video range and having its impedance matched to that of the reactive component of the output load circuit at said end of the range, and said load circuit comprising a low-pass filter and a resistance in series.

2. A wide range video amplifier capable of amplifying frequencies up to approximately 9 megacycles comprising an electron discharge tube of the type including input and output electrodes, a video input circuit connected to the input electrodes, 2. load circuit including resistive and reactive components connected between the output electrodes, said reactive component tending to produce an undesirable deviation in the frequency response characteristic at the high frequency end of the range, a reactive network in the space current path of said tube, common to the input and output circuits of the tube, for compensating for said undesirable deviation, said reactive network comprising the shunt connection of a resistance, a capacitance and an inductance between the tube cathode and ground and being resonant to the high frequency end of the video range and having its impedance matched to that of the reactive component of the output load circuit at said end of the range, said load circuit comprising a pair of coils and a resistance in series, and a second amplifier tube having its input electrode connected to the common terminal between the pair of coils.

OTTO H. SCHADE.

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