US2460907A - Cathode-coupled wide-band amplifier - Google Patents

Description

Patented Feb. 8, 1949 UNITED 9 9 7 a CAT'H0DEG0UPLEDWIDE-BAND AMPLIFIER i Alfred C- qhmefltt .E rvi l a-i e iemr is fiadiflfiqrljgration of American corporation of Dela a e Application December 28, 19.44, SerialNo, 579,176 6 iii-1.7

and low impedance circuits. However, in order to obtain the required gain with low impedance circuits, it is necessary'that high Gm (transconductance) tubes be used. For certain applications, such as the input stages of wide-band, high-gain receivers, triodes are more desirable 1 amslificeti O c were; whereas? of input signal voltages. Ph std-f a ures ch a eris i 0? y invar s t or h h art y 1.1 t e a s da c aim 1 i i t ti li wt er th t i er mze a an mq l i 2 ticn together with further objects and a,

. .a 's W111 b h Re eemed hr r b 1 irate y rio us modifications oi e} i eeeie li li t ar than hentodes because of their inherently lower I noise characteristics. A low impedancecircuit frequently used is the cathode output or cathode follower circuit. It has been proposed to use a circuit of this type in conjunction with a con vent al or u de h d me ifi r bu such a circuit, besides requiring the same number of circuit elements as two amplifier stages of cone t nal e ha h ret en us d i pentode tube as the grounded-cathode arnplifier.

In some cases this dual stage does not provide adequate stability, and also does not provide better noise characteristics than conventional circuits employing pentode tubes.

.;. have c r d hat by c m nin a athod out u a d ca hod p age us n a Sin twin-triode tube, a high-gain, wide-band'arne pli er Stage is obta ed hiQ mpares my??? a y Wi h a ipentode sta w reaped t in stability and economy, while it has far s noise characteristics. The new oincui have called the cathode-coupled i' WlHt-iihlijfld amplifier, provides greater flexibility than conventional amplifier circuits, and can be usedfor IF, yideo, comer-tenor detector s erv1 s. It is accordingly an object of the present invention to provide a wide band, stable amplifier having better signal-to-noise ratio inulti: grid tube amplifiers.

A u e Me o th inv nti i to pmr e an amplifier that has output to input shielding comparable to that of a screen grid or pentode tube but with relatively low-er input capacity:

A more specific object of the invention is to amide, air of aiho ourldtriosieomnlh A n T i wrmeeis an ai inn I1 1 1* 8 5 hi h s. d era i Q li th its. -1 arid lea e sonn t-r I}??? or IF tier minals t. cathode-coupled twin-. a I her n ee e-ha it 5. d t i f e'l m- Pl fi 9%? a i liitll stew ii .rer 11 which case the input terminalsl be connected to the output oi the converter stage.

The cathode 5 of section "I -1 is connected to input terminal 4 or ground through a radio frequency (RF) choke .1 n 8 9 reeiieeh connected for high irequencies to ground by way of condenser 9, the aglode b eing energized from a source of direct current represented by B+.

grid amplifier. tion, like the cathode of (the first section, is connected to the high potential endof coil 1 so that 3? .991} is th oi itput load impedance in e. or 21" the first l i i the dance'of the cathode circuit'of the form t v 0 sections.

has connected to it a load impedance, shown by way of example as a primary 13 of an output transformer T0, the secondary M of which feeds an RF (or IF) stage l which may be similar to the stage just described employing tube l. The condenser !6, shown dotted, since it may represent only the distributed capacity of winding l3, strays and tube capacities, serves to tune the output circuit of the stage to the center frequency of the band of frequencies to be amplified, which may, for example, be 4 to 6- maimegacycles) wide. Resistor l7 shunted across the transformer secondary It plus the plate resistand by-pass capacitor customarily required in a screen supply are eliminated. Since the plate currents in'the two triode-units swing in opposite ance of T2 serves to load the transformer to give the proper band width.

The cathode coupling coil 7 should have an impedance that is high compared tothe internal 1 input impedance of the grounded-grid stage '1:

over the frequency band to be amplified and serves to eliminate the apparent reactive (capacitive) component in the cathode circuit by parallel resonance with the distributed cathode to ground capacity l8, thereby improving the signal-to-noise ratio and .the gain, Because of the low cathode input impedance of T1 and T2 this resonant circuit has a very low Q and hence a very wide band. This makes the value of the choke uncritical. I have used one of approximately microhenries with very satisfactory results.

It is important for the proper operation of the circuits described herein that the grid bias and plate voltage be selected of such values that no limiting or clipping takes place. Although in Fig, 1, the grids of both sections operate at zero bias, the plate voltage is chosen of suitable value to avoid the limiting action mentioned.

The circuits of Figs. 2 and 3 are fundamentally similar to the circuit of Fig. 1 except that in Fig. 2 there is included in the cathode circuit, in series with the high frequency choke, the usual se1fbiasing resistance and shunt condenser network 19 for varying the gain of the amplifier, and in Fig. 3 the gain may be varied by changing the D. C. voltage on either, or both, of the grids by means of a voltage derived for example from an A. V. C. system, as is well known in the art.

With a type 6J6 tube I have used 70 volts on the plate with the grids at 0 and no cathode bias (Fig. 1), and about 125-150 volts on the plate with the grids at O, and about a 50-ohm cathode resistor unbypassed (Fig. 2).

The gain of the wide-band amplifier of the cathode-coupled twin-triode type herein described is equal to by reason of degeneration of the grid to cathode capacity, Gm being the transconductance of each tube section, C1 and Co the input and output capacities, respectively Tp the plate resistance, and Aw being equal to 21A The advantage of a lower capacity is that since the total capacity across the circuit is what limits the gain times directions, the total plate load tends to remain uniform, and subsequent similar stages have little influence on each other such as might result from varying'load on the plate supply.

While I have shown and described a preferred embodiment of my invention, it will be understood that various modifications and changes will occur to those skilled in the art without departing from the spirit and scope of this invention. I therefore contemplate by the appended claims to coverany such modifications as fall Within the true spirit and scope of my invention.

What I claim is:

1. Amide-band high-frequency amplifier comprisinga cathode-follower stage coupled to a cathode-input, grounded-grid stage, the coupling means between said stages being constituted by an inductance having an impedance at the frequencies to be amplified which is high compared to the internal input impedance of the groundedgrid stage.

2. A wide-band high-frequency amplifier comprising a cathode-follower stage coupled to a cathode-input, grounded-grid stage, the coupling means between said stages being constituted by a high frequency choke connected between the cathodes of both stages and ground, said choke resonating with the capacitance between the cathodes and ground over a wide range of frequencies.

3. A wide-band high-frequency amplifier comprising a cathode-follower stage coupled to a cathode-input, grounded-grid stage, the coupling means between said stages being constituted by an inductance which is common to the cathode circuits of both' stages, the value of said inductance being high compared to the input impedance of the grounded-grid stage at the frequencies to be amplified and being of the order of 20 microhenries.

4. A circuit for the amplification of a wide band of high frequencies comprising first and second electron discharge paths each constituted by a cathode, a grid and an anode, a source of high frequencies connected between the grid of the first discharge path and ground, a common reactive impedance connected between said cathodes and ground, a ground connection to the grid of the second discharge path, an output impedance connected between the anode of the second discharge path and a source of anode potential, and a high frequency ground connection to the anode of the first discharge path, the biases on the grids and the potentials on the anodes being chosenof such values that the circuit functions as'a linear amplifier over a wide range of input signal voltages;

5, Acircuit for the amplification of a wide band of high frequencies comprising first and second electron discharge paths each constituted by a cathode, a grid and an anode, a source of high frequencies connected between the grid of the first discharge path and ground, a common reactive"impedance connected between said oathodes and ground, a ground connection to the grid of the second discharge path, an output impedance connected between the anode of the second discharge path and a source of anode potential, V

and a high frequency ground connection to the anode of the first discharge path, said common cathode impedance having a value that is high compared to the input constituted by the grounded grid and its cathode, over the band of frequencies to be amplified.

6. A circuit for the amplification of a wide band of high frequencies comprising first and second electron discharge paths each constituted by a cathode, a grid and an anode, a source of high cathode impedance having a value that is high compared to the input constituted by the grounded grid and its cathode, over the band of frequencies to be amplified, and the biases on the grids and the potentials on the anodes being chosen of such values that the circuit functions as a linear amplifier over a wide range of input signal Vo1t-. ages.

ALFRED C. SCI-IROEDER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,068,112 Rust Jan. 19, 1937 2,162,878 Brailsford June 20, 1939 2,215,796 Rust et a1 Sept. 24, 1940 2,270,012 Shepard Jan. 13, 1942 2,276,565 Crosby Mar. 17, 1942 2,315,040 Bode Mar. 30, 1943 2,330,109

Brown Sept. 21, 1943

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