US2356446A - Amplifier circuit - Google Patents

Description

and serve to separate Patentecl Aug. 22 1944 UNl-T ED STAIE'S PATE-N'I' OFFICE Mark E. Campbell, Mounta.in Lakes, N. J assignor to Bel! Telephone Laboratories, Incorporated, New York, N. Y., a corporatlon of New York Application April 16, 1942, Serial No. 439,214 3 Claims. (01. 179- 71) The present invention relates to coupling circuits for couplin a vacuum tube amplifier to a terminal inputor output circuit to secure efiicient transfer of waves of a broad band of frequencies. A typical, though not limiting, use for such a circuit is instanced by a multiplex carrier telephne system having a terminal amplifier common to all of the channels and included between the channel terminals and the line.

The general object of the invention is to secure lncreased efiiciency and improved broad band performance in a coupling circuit of the general type mentio'ned.

A related object is te noise ratio.

The nature and objects of the invention will appear more fully from the following detailed description with reference to the attached-draw ings in which:.

Fig. 1 is a block schematic diagrain of an overa11 system to which the invention is especially applicable;

Fig. 2 shows a. prior art type repeater and input circuit therefor;

Fig; 3 shows graphs to be referred to in the description;

Figs. 4 to 9, inclusive, are schematic circuit improve the signal-to- The rarticular embodiment of the invention that is to be specifically disclosed herein is ina broad band system of the type indicated in -outline in Fig. 1, in which the carrier terminal I at the transmitting station comprises a plurality of carrier telephone ending in selective filters 2 facing the multiplex line 1. A television terminal is also indicated at 3 which may be used simultaneously with the telephone channels or alternatively with respect to the telephone channels. Broad band filters 4 the two types of transmission.

' Whether both types of transmission are used appear as in Fig. 2 representing the input cou- The terminating resistance is 11.

diagrams of coupling circuits in ccordance wi h transmitting channels the ranges employed for or several filters 2, 4 or 5, as the case may be.

These filters are in pract ice designed to work into a resistive circuit and it'is necessary for best performance to provide an input coupling into the amplifier 6 thatcpresents a resistive impedan ce into which these filters.may work of the same value at all frequencies in the transmissipn band.

Amplifier 6 is shown wcrang into a coaxial line 1 having intermediate repeaterS 8 spaced along it and leading eventually to a common output; amplifier 9 which may be similar in design to amplifier 6. A similar problem arises here also, of providing a resistive impedance termination facing the receiving end filters I0, II or the individual channel filtersin the carrier telephone terminal IZ. The television receiving terminal is at I3,

It is desirable touse a transformer in the coupling circuit in order to transform the impedances to obtain a match between the nominally '72-ohm circuit represented by the terminal circuits and the grid or plate impedance of the amplifier, which may be, e. g. 7,000 ohms. If a transformer alone were used together with the usual terminating resistance; the network would pling case. This figure shows the equivalent impedance dia- -gram where the ideal transformer is shown at I 4, the leakage inductance at I 6 and the shunt.

capacity at I8. Such a. circuit has the configuration of a low-pass filter and would give a very poor impedanceas shown bythe curve 20 of Fig. 3. The resistance is seen to fall off gradually over the transmission range so that if very much of the avalable band were used, the resistance would vary widely over the band. It is noted that in this figure the amplifier 6 is shown as a three-Stage stabilzed feedback amplifier of the series feedback type, the feedback impedance being at I9. The cathodes are at ground potenti al and the capacity to ground, 2! of the input coupling circuit appears effectively in shunt to the feedback impedance. The gu'idto cathode capacity is shown at 23 which in this caseis also the grid to ground capacity.

In accordance with one feature of the invention, the 'coupling circuit is made to have a more nearly constant resistance over the band by building out the circuit to the form shown in Fig. 4, which can be broken down into five parts A, B, C, D and E. The A part is the ideal transformer, B'is a half sectinn of an M-derived includine the remainder of the shunt capacity l8b. E is the terminating resistance 1. 'I'his It not only is a age so far as the network itself the dispositiorr of the parasitic capacities is more gether and through a feedback network has a characteristic of the type given by the dotted curve 22 of Fig. 3.

In general, the distributed capacity of the transformer is limiting and hence to keep the filter elements IG and I8 in the proper relationship I6 is padded out.

While this network has a more nearly uniform,

high resistance characteristic over a relatively "and the most favorable loop gain and phase shift characteristic for the amplifier. Fig. designates two alternative points of connection of the grid, to either a or b. Point a is the more advantageous point to which to make connection.

point of maximum signal voltis concemed but favorablefrom the feedback standpoint in the case of the a connection. Regarding the feedback impedance I9 as a source of voltage that is to be applied to the grid, the parasitic capacity 2l isdirectly in shunt across 19 and this cannot be avoided with this type of circuit. Following on from there, with the a connection the capacity I8- is in parallel with the branch comprising resistance l'| and capacity 24 in series (at very' high frequencies where this efiect is important the inductance' 28 can be disre garded). These two parallel branches (18 and H, 24) join at a. leading to the grid, with parasitic capacity 23 shunted across to the ground return side of the circuit just traced. The fed back voltage reaching the grid is, therefore, principally determined in the asymptotic frequency region by the potentiometer eiect of the capacities 18 and 23. With connection to b, it will be seen that the fed back voltage at extreme high frequencies is determined by the potentiometer effect of capacities I8 and 24 potential dividing impedance and capacity 23 as the other and that the voltage reaching the grid is smaller inthis case. This same'reasonin is applicable to the output transformer circuit also.

The limit of loop gain of the amplifier in the useful band is determined by the asymptotic gain and phase margin, so that by-increasing the feedback voltage reaching the grid at extreme high frequencies, the loop gain in the band is similarly increased.

The feedback circuit shown in Fig. 5 is of the series type. Fig. 6 shows for illustrative purposes another known type of feedback amplifier using cathode mpedance feedback in which the cathodes of stages 1 and 3 are connected toimpedance 29 to ground, the cathode of stage 2 being grounded. In this type of circuit the high side of the input and output transformers are grounded so that the capacity 21 of Fig. 5 is absent. In this circuit, connectiou to terminal b gives a poor termination for the input coupling network since it causes capacity 25 to appear directly in shunt across the terminating resistance ll. Connection of the grid to terminal a places capacity 25 across capacity I8 or in effect incorporates it into capacity I 8 so that it can be taken entirein series as one transformer,

2,35o,44e the fed back voltage 'to the grld for the reasons given in connection with Fig. 5.

The fact that capacity 25 appears next to the tube, while capacity I8 is associated with the makes it possible to keep these two capacities separate by inserting a series inductance between them, thus making possible a higher transformer step-up with the same parasitic elements. inductance 30 is connected between capacities I8 and 25. H inductance 30 has parasitic capacity the network can be M-derived (using a different value of M than used above) to include such .a capacty in the design. In this flgure the position of resistance I1 andcoil and condenser 28, 24 have been interchanged in order to place the letter elements nearer to ground, since the parasitic capacity of the resistor I! can be held small and that of 28 can be. made part of the capacity 24.

In an actual desisnthe shunt 3| must be transferred to the low-side since it cannot physically be connected between the ideal transformer and its lea.kage inductance IS. In so transfering it, its impedance is reduced in the ratio of the imped ce transformation ratio of the transformer. The actual configuration of. an input or output coupling circuit used by applcant in a -kilocycle to 2-megacycle amplifier is-giyen inFig. 8 with the circuit constants marked on it.

This is of the type of network shown in Figs. 4 and 5. A sinlar diagram for the type of input coupling shown in Fig. 7 is given in Fig. 9. The capacity inductance shunt across the low impedance side of thetransformer is in practica trimmed or made variable to correct for manufacturing irregularities. In this latter flgure the 80 pp.f. condenser was inserted in series with the coil28 (see Fig. 7) to improve the attenuation and phase charaeteristic of the amplifier at its low frequency cut-oif region, this in combination with the terminating resistance equalizing the transformer impedance so as to extend the range to lower frequencies. This was put in for use especially in systems in which television is to be transmitted, using frequencies extendnz downward in the frequency scale close to zero. The inclusion of this capacity for this p 'P se is being shown for completeness of disclosure but in and of itself it forma no part of the present invention.

What is claimed is:

1. In combination, a stabilized feedback broad band amplifier having cathode. grid and anode elemerits, a feedback impedance having one end connected to the cathode and its opposite end adapted for connection to an amplifier grid terminal and to an amplifier anode terminal, to

provide a feedback path for said amplifler, a

capacityofwhi ch together form inherently a tei section connected in said circuit,

tion at a also results in better transmission of low-pass filter section, a flrst lVl-derived filter section includina as an element a portion of said leakage inductance, and a second lll-derived filfacins said shunt capacity, for flatteninx the response characteristic of'the circuit up to naar the cut-ofl frequency, a terminnting resistance for said second M-darived filter section, and connections from the terminals of said second M-derived 'section facing said shunt capacity to, respectively, the said opposite end of said feedback This is illustrated m Fig. where impedance and one of the tw said ampliiler terminals.

2. A circuit for coupling a stabilized feedback amplifier to a highlv resistive low impedance circult including a two-winding non-unity ratio transformer having lts 1ow impedance side connected to said circuit and its high impedance side connected to said feedback amplifier, sald coupling circuit comprising together with said transfopmer a mid-seties equivalent M-derived half section f filter, a half-section constant-K filter, a mid-series equivalent M-derived half section of fllterand a. terminating impedance in the order named, the connection from said coupling circuit to said amplifler being made at the iunction between said two last-mentioned half sections of filter.

3. The combinaticn of a broad band filter havconnected to saidfllter, sald transformer being l0 built out to provide on its high impedance side half-section M-derived filter. in

an equivalent tandem .with other equivalent half-filter sections .and a terminating resistance, and connectlons from the junction points of two cfsaid 15 half sections to, respectively, one of said two ilrst-mentioned tegminals of sald amplifler. and said feedback terminal. E. CAMPBELL.

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