US2170645A - High frequency amplifier - Google Patents

Description

Aug. 22, 1939. 1 c. PETERSON 2,170,545

HIGH FREQUENCY AMPLIFIER Filed Jan. 3, 1958 2 Sheets-Sheet yl.

/Nl/EA/ro@ L. C. PETERSON ATTORNEY Aug. 22, 1939.

l.. c. PETERSON 2,170,645

HIGH FREQUENCY AMPLIFIER Filed Jan. 5, 1958 2 Sheets-Sheet 2 /N'VE/VTOR L. C. PETERSON www ATTO/@MFV Patented Aug. 22, 1939 Unire STATES 'l 2,170,645. nien FREQUENCY, AMPLIFIER Liss (l. Peterson, Madison, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a .corporation vof New York Application January 3,1938, serial No. y182,955

6 Claims. (Cl.

The present invention relates to electrical wave amplifying circuits, especially for use at high frequencies where the eeot of parasitic impedanoe's may prove troublesome. i The general object of the invention is to provide effective neutralization of the inherent tube' f and circuit capacities and inductances which, when not neutralized, reduce the effective transmission through the amplifier circuit or produce singing tendency. v

The usual capacity neutralization of triodes or their equivalents proves inadequate as the frequencies become higher and higher principally because of the inductance of the tube leads which makes it impossible to locate the neutralizing capacity at the ideal position in the circuit.

The` present invention provides a type of neutralizing network which takes account of the tube and circuit capacities and the lead induct-L ances and gives a high degree of neutralization for the tube .and circuit. In the treatment 'to follow the electron transit time is assumed toloe negligibly small. While triodes are mentioned as examples, the invention is equally as applicablef, to pentodes and other types of tube with morej than one grid, it being assumed that any grids'4` other than the control grid 'are suitablybypassed to ground for alternating current. The ynature and objects ofthis invention lwill be more fully understood fromthe following de-r 3Q tamed description or Acertain embodiments that havev been chosen for illustration in the accomlf'*y panying drawings: j Fig. 1 shows v`an impedance diagram corre sponding to a triode with-a neutralizing imped Stance, on whichdthe calculations are based for a neutralized amplifier according'to the invention; and v l ,4 f Figs; 2 to 5, inclusive, showin diagramma form physicalernbodiments according to the infvention.

The diagram of Fig. 1 shows atriode with the principal parasitic or unavoidable impe'dances indicated that are signilicant at high frequencies. Starting at input. terminal a. there is a 4xslserics input lead impedance Zn in thengrid-lead, forexam'ple the impedanceV inside thetube between the vpin on thetube socket or ltube envelov and the eiective'portion of the'grid. The*` at this point a shunt impedance to ground "Z 5QThe internal grid-to-plate impedance is Z2. The

cathode lead impedance is Ziz and is the series impedance ofthe connecting leadtofthe" active?! cathode surface. The plate lead series impedanceV i 55Nbetween the anode andthe output terminal 'b 'l is Zn" and the shunt impedance to ground froml the plate is Zia. 1 r' The invention is based upon the'vdiscoverythat, I a single network can be' found which will-ine oq tralize all of these` parasitic impedancesg Assuming the four terminal network as in Fig. l with generalized impedances, applicant has found the following formula for the impedance oi a series network which when connected between the terminals a, and b through a 180 degree phase shifting arrangement, will neutralize the structure of Fig. 1 on the assumption of a negative In this expression Ro'=Ro-l-(,c-[-1)Z12 when Ro is the internal plate resistance and ,a the amplication constant.4

The derivation of this formula is not given herein since the derivation, while straightforward, is long and very involved. The general method of solution is indicated, however, as follows: First, the general expression is written down for the input current as a linear function of the output voltage and input voltage for the tube circuit without the neutralizing network. A similar general expression is then written for the neutralizing network. By combining these two expressions, the general linear expression is obtained for the total input current in terms of input and output voltages for the complete structure of vacuum tube circuit and neutralizing network. Since the complete circuit is assumed to be neutralized, the input current is to be independent of the output voltage. Therefore, we equateto zerothe coecient of the output voltagevinvthe general expression for the total input current. This leads directly to the expression for the. impedance ofthe neutralizing network.

Ihe problem now arises of nding a physically realizable ,networkl corresponding to the impedanc'effunction (1)' after the functional forms of the parasitic elements` have been assigned. We proceed here in steps and start with the following simple case.

Case' I-Z'ero grid and cathode lead impedances;-Since now Z12:0 and Z11=0, Equation 1 reduces to' w Let usfnoWff-urthervassume Z2 and 212 to be theimpedancesof pure capacities and Zn the impedance ofa pure inductance. Thus which impedance function corresponds to a series resonance circuit containing all the three kinds of elements, With the assumptions made this network neutralizes exactly at all frequencies and it is to be noted that the impedance Z12 does not enter. This particular case is illustrated in Fig. 2. The mutually coupled inductances l0 and ll are for the purpose of giving the 180 degree phase shift assumed.

Case 2.-We now assume an impedance Z12 in the cathode lead but retain the assumption of zero impedance in the grid lead. Equation 1 then gives ll Z0R/Z2Z11l-l Z2 1 i- Assuming the cathode impedance Z12 to be a pure inductance L12 and the remaining parasitic elements defined by (3), Equation 5 reduces to network upon Z12 and that the neutralization is exact under the stated assumptions.

Case 3.-We finally assume the grid to have a lead inductance L12 and a capacity C12' to ground, so that Z11/=PL11', Zul: (7)

siderable degree of accuracy. Such a procedure would probably result in a too complicated structure unsuitable in the high frequency range.

We arrive at a first approximation by the observation that the rst three terms of (8) correspond to a physical network and by the further observation that the small parasitic elements occur to a higher order than the first in the remaining terms. To a first approximation, therefore, we may take the first three terms of (8) and the physical configuration of this network is identical with that found for negligible grid lead inductance. The only modification required is in the value of the inductance of the neutralizing network. It is seen from the first term of (8) that to a first approximation the effect of the grid inductance and grid capacity is to introduce an inductance into the neutralizing network. This approximate neutralization arrangement is shown on Fig. 4.

For the sake of simplicity the foregoing discussion has dealt with the neutralization of a one tube circuit and it has been assumed that a means for reversing the phase was available, as for example, the transformer lll, Il. In practice it is difcult or impossible to build a transformer that will do this at very high frequencies, but resort may be had to a push-pull type of circuit for obtaining the necessary phase reversal. Such a circuit is illustrated in Fig. 5.

'In this figure the tubes l2 and I3 have their grids connected to input terminals lli, I4 across which is any suitable input impe-dance such as resistances I5 and l1 in series. The anodes are connected to output terminals l5, l5 across which are impedances shown as resistances I8 and I8 in series. Grid bias battery 2@ is connected between the cathodes and the junction of resistances I6 and l1. Plate battery 2l is connected between the cathodes and the junction point of resistances I8 and I9. Any other known or suitable means for providing grid bias and anode voltage may be used in place of the batteries shown. The parasitic impedances are shown in dotted lines and correspond to those indicated in the previous figures. The cross-neutralizing circuits are alike and each comprises a capacity 22 in shunt to a resistance 23 and both in series with an inductance 24 and a capacity 25 proportioned in accordance with the formulae above given and as indicated on Fig. 4. By comparison with Fig. 4 the corresponding elements will readily be identified.

It has been pointed out by F. B. Llewellyn in application Serial No.104,193, filed October 6, 1936, now Patent No. 2,118,931, that at sufciently high frequencies the inherent grid-to-plate impedance of a triode has an appreciable negative resistance component in series with the usually assumed capacity C2 in the above discussion. To indicate how this may be taken into account in practicing my present invention, the following example is given. As a special and simplified case, assume negligiby small inductances in the grid and cathode leads (Zn and Zia) The impedance between grid and plate is taken, in accordance with Llewellyns findings,

The neutralizing impedance then has the value Ln ,l C12/l Y Zn-ROCZ fz-i-PLn C2 To a rst approximation We have L11/' [Cm I2 y1 Zn-Rocz I2+pLn C2 R01-PEE and the corresponding network consists of a series connection of resistance, inductance and capacity. It is, however, only physically realizable if said device having inherent capacity between its grid and anode and the anode lead to said output circuit having effective series inductance, and a two-terminal network connected from a point of proper phase in the output circuit to the grid and consisting of capacity, inductance and resistance in series relation with one another, said network having its elements proportioned with respect to the other elements of the circuit to neutralize the said inherent capacity an effective inductance.

2. In combination with a space discharge device having a cathode, an anode and a control grid, input and output circuits connected tothe cathode and respectively to the grid and anode, said device having inherent capacity between its grid and anode, the anode lead to said output circuit having eiective series inductance and the lead from said cathode to said input and said output circuits having eiective series inductance, and a two-terminal network connected through a phase reversing device from the output circuit to the grid consisting of a parallel resistance and capacity in series relation with a capacity and an inductance, said network having its elements proportioned with respect to the elements of the circuit to` neutralize the said inherent capacity and effective inductances.

3. In a push-pull amplier having an input circuit connected to the grids and an output circuit connected to the plates, cross-neutralizing circuits connected from the plate of each tube to the grid of the other tube, each neutralizing circuit comprising in series an inductance, a capacity and a resistance shunted by a capacity, the values of said inductance, resistance and capacity elements being proportioned to make the amplier unilaterally conductive.

4. In an amplier, a discharge device having a cathode, an anode and a control grid, with an interelectrode capacity C2 between control grid and anode, an output terminal, a lead induc- C ll L11" 1+ C122 and a resistance' of value L11!! RC2 5. An amplifier according to claim 4 having also a cathode lead inductance L12 and a capacity of value in shunt to said resistance, the resulting neutralizing circuit serving to neutralize the effect on the transmission characteristic of GzLu and L12.

6. A push-pull space discharge circuit having the grids connected to respectively opposite input terminals and the anodes connected to respective output terminals, inherent grid-to-plate capacities and eiective inductance in the anode and cathode leads, cross-neutralizing circuits connected from each output terminal to the opposite input terminal, each neutralizing circuit comprising in series a capacity equivalent in value to the grid-to-plate capacity of the corresponding space discharge device, an inductance and a parallel-connected capacity and resistance, for neutralizing said grid-to-plate capacity and said .effective inductance in the anode and cathode leads.

LISS C. PETERSON.

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