US 3612780 A
Description (OCR text may contain errors)
United States Patent Henry R. Beurrier Chester Township, Morris County;
Harold Seidel, Warren, both 01 NJ. 864,695
Oct. 8, 1969 Oct. 12, 197 1 Bell Telephone Laboratories, Incorporated Murray Hill, Berkeley Heights, NJ.
Inventors Appl. No. Filed Patented Assignee ACTIVE FOUR-PORT 10 Claims, 7 Drawing Figs.
U.S. Cl 179/170 T, 330/30 110313/62 179/ 170,
Field of Search  References Cited UNITED STATES PATENTS 2,733,304 1/1956 Koenig 179/170 T 3,271,528 9/1966 Vallese.... 179/170 T 3,401,351 9/1968 Ellestad 330/69 Primary Examiner-Kathleen H, Claffy Assistant ExaminerWilliam A. Helvestine Attorneys-R. J. Guenther and Arthur J. Torsiglieri ABSTRACT: This application describes an active four-port having directional transmission properties. The four-port comprises two active members whose respective emitting, control and collecting electrodes are connected by means of separate networks characterized in that the symmetric mode transfer gain and the antisymmetric mode transfer gain, as measured between the control and collecting electrodes, are equal.
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WVENTORS H. R. BEURR/ER H. SE/DEL ig/K 013mg ACTIVE FOUR-PORT This invention relates to active four-ports having directional transmission properties.
BACKGROUND OF THE INVENTION It is very often desirable to divide a signal into'two components, or to combine two signals in a manner which also senses the direction of signal propagation. A typical device for doing this is the directional coupler, of which there are two general classifications. One classification is the traveling wave type of coupler whose overall length is a fraction of the signal wavelength. The second classification is the lumped-element coupler whose dimensions are substantially independent of signal wavelength, but which requires a lumped inductor.
The object of the present invention is to perform the abovedescribed circuit functions by means'which do not use inductors and whose size is independent of signal wavelength.
SUMMARY OF TI-IE INVENTION An active four-port in accordance with the present invention comprises an amplifier having equal symmetric mode and antisymmetric mode transfer gains, where the term transfer gain denotes a matched output-load and a matched input source. In one of the illustrative embodiments of the invention, the emitter, base and collector electrodes of two transistors are coupled together, respectively, by means of three electrically identical, matched two-ports. The two base electrodes and the two collector electrodes define the ports of the active four-port.
In an alternate embodiment of the invention utilized to combine two signals, two-ports having different transfer characteristics are used.
While the three networks can, in the most general case, comprise resistive, inductive and capacitive circuit elements, it is an advantage of the invention that directional coupler characteristics can be realized using only resistive elements. This has the advantage that the complete coupler can be readily fabricated using standard printed circuit techniques.
These and other objects and advantages, the nature of the present invention, and its various features, will appear more fully upon consideration of the various illustrative embodimen'ts now to be described in detail in connection with the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows an active four-port in accordance with the invention;
FIGS. 2A and 28, included for purposes of explanation, show the four-port of FIG. 1 excited in the symmetric mode and antisymmetric mode, respectively;
FIG. 3 shows an alternate embodiment of the invention wherein the two-ports are matched at different impedance levels; I
FIG. 4 shows an'active' four-port utilized to combine two signals for propagation along a common direction; and
FIGS. 5A and 53, included for purposes of illustration, show two networks of the type that can be used in the present invention.
DETAILED DESCRIPTION Referring to the drawings, FIG. 1 shows a first embodiment of an active four-port 9 in accordance with the present invention, comprising two active elements 10 and 11, and three identical two-port networks 12, 13 and 14. For purposes of illustration, active elements 10 and 11 are depicted as transistors, each of which has an emitter electrode 1, a collector electrode 2, and a control, or base electrode 3. It will be recognized, however, that other types of active elements, such as vacuum tubes, can just as readily be used.
One of the networks,'l2, is connected between the collector electrodes 2, which form two of the ports 0 and d of the active four-port. A second of the networks 14 is connected between the base electrodes 3, which form the other two ports a and b of the four-port. The third network 13 is connected between the emitter electrodes 1. Direct current biasing sources and connections have been omitted so as not to complicate the circuit diagram.
In accordance with this first embodiment of the invention, networks l2, l3 and 14 have the same transfer characteristics and are matched with respect to an arbitrary impedance level Being matched, they can be characterized by a bisected symmetric impedance Z, and a bisected antisymmetric impedance Z where As indicated hereinabove, the active four-port of FIG. I can be used as a directional coupler to divide a signal into two components, or to combine-two signals. As a power divider, a signal is applied to either port a or b, and an output is obtained at the other of these two ports, and at one of the other ports c or d. For purposes of illustration, a signal source 15, having an output impedance 2,, is shown connected to port a. Ports b, c
' and d are each terminated by means of a load impedance Z,
which, typically, would be a transmission line.
The operation of the tour-port is conveniently analyzed by separately examining its responses to the symmetric mode of excitation and to the antisymmetric mode of excitation, and then superimposing the two results. This is now done referring to FIGS. 2A and 2B which show the four-port excited in the symmetric mode and in the antisymmetric m'ode, respectively.
Referring more specifically to FIG. 2A, signal sources 20 and 21 of amplitude E/2 and output impedance Z,, excite ports a and bin phase. Assuming for the purposes of this analysis that both transistors have an a=l (infinite base impedance), the symmetric mode signal voltage at the base of each transistor is b'=( where, by definition, Z, is the bisected symmetric impedance of each of the networks 12, 13 and 14.
The symmetric mode emitter current is then I,=(e,,/Z,)==(E/2)[l/Z,,+Z,]. (3) Since a=l, the emitter and collector currents are equal. That is !,-=I,'. (4) mode load current I is then given by The symmetric (4) Since the excitation signals derived from sources 20 and 21 are in phase, the load currents at ports 0 and d are also in phase.
From a similar analysis of FIG. 2B, wherein ports a and b are excited by means of two I80 out of phase signal sources 22 and 23, we obtain for the antisymmetric mode load current L a/( o+ u) ]v where 2,, is the bisected antisymmetric, network impedance. Substituting the value of Z, obtained from equation (I) in equation (7) yields L# I .I( O+ I)L (3) where the load current at port 0 is out of phase with the load current at port d.
A comparison of equations (6) and (8) shows that the symmetric load currents and the antisymmetric load currents at ports c and d are equal in magnitude. Applying the principle of superposition to both the inputs and outputs, the signals applied to port a sum to E, whereas the signals applied to port b sum to zero,'thus simulating the excitation conditions shown in FIG. 1 wherein a signal source 15 is coupled to port 0. Similarly, the load currents at port c sum to to produce an output voltage E, =(EZ,,Z,/( Z,,+Z,)). (10) The load currents at port d,
on the other hand, sum to zero. Thus, an active four-port in accordance with the present invention has directional properties in that a signal applied to port a couples to port c but not to port d. correspondingly, a signal applied to port b y will coupleto port d but not to port 0.
In addition to th e above there is also coupling between ports a and b throughnetwork 14. Applying the same technique of energizing network 14 in the symmetric and antisymmetric mode and then superimposing the resulting currents and voltages, it can' be shown that the signal E, trans mitted from port a to port bis The coupled signal component, given by equation l), and
the transmitted signal component given by equation (I I), can
' also be expressed in terms of the coefficient of transmission of the transmitted signal eomponentE becomes =(E/ and the coupled signal component B, becomes E, =(E/4) (l-t (13) Thus, an active four-port having any arbitrary power divithe networks l2, l3 and 14. Designating this coefficient as t,
sion ratio can be realized simply by designing the networks N to have the symmetric impedance Z, defined by equations or (I l or, alternatively, to have the transmission coefficient defined by equation (12) or (13).
It will be noted that the active four-port described above is basically an amplifier that responds equally to both the symmetric and antisymmetric modes of excitation. More specifically, it is an amplifier that is characterized by equal symmetric mode and antisymmetric mode transfer gains, where transfer gain is defined as the ratio of the output signal into a matched load to the input signal from a matched source.
FIG. 3 shows a secondembodiment of the invention. Basically, this embodiment has the same general circuit c'o'iln u'ra:
tion as the embodiment of FIG. 1, comprising two active members l0 and 11 interconnected by means of three two-port networks 12, 13 and 14. As in the embodiment of FIG. 1, networks 12, 13 and 14 are matched networks having the same transfer characteristics. However, in the embodiment of FIG. 3, the networks are matched at different impedance levels. As illustrated, network 14 is matched at impedance level 2,, and is characterized by the bisected symmetric and antisymmetric mode impedances Z,and Z,,. Network 13 is matched at impedance level 122,, where n is any number, and is characterized by mode impedances nZ, and nZ,,. Similarly, network 12 is matched at impedance level m Z where m is any number, and is characterized by mode impedances m2, and M2. v i
A modalanalysis of this network shows that signal source 15, having an open-circuit voltage E and output impedance Z5, applied to port a will produce a coupled signal E, at port 0 given by i v E =EZ ,,Z,/(Z,,+Z,)- (m/n). 14 In terms of the coefficient of transmission 1, which is the same for all three networks,
' It will be noted that the value of E, given by equations (14) and (I5) differs from that given by equations l0) and I 3) by -In operation, signal E, is eoupled'throughnetwork 12 to port d, producing a first output signal component E,t, where l is the coefficient of transmission for each of the networks. Signal E, produces a second signal component (Li /4) (l-t). (m/n) at port d, in the manner explained hereinabove. The two components combine to produce an output signal propagating along a common direction.
Signal E: also coupled through network 14 to produce an output signal 1-3,! at port a. If the latter signal is not needed, it
. can either be absorbed in termination Z, or, alternatively. the
two ports can be modified so that their transfer characteristics are no longer the same. In particular, if network 14 has a coefficient of transmission 0, there will be no signal coupled between ports b and a. In this situation, directional coupling between ports b and d is obtained when the coefficients of transmission t, and of the other networks 12 and 13, are related by is then given by V r a sm H. (mm).
, A comparison of equations (17) and (15) shows that when the transfer characteristics are different, but tailored in the manner described, the coupled signal given by equation (I7) is greater than that given by equation (15) by the factor In the various embodiments described hereinabove, the two-ports were characterized as having the same transfer characteristics. This was based, in part, upon the assumption that the active members are ideal transistors whose base and collector impedances are infinite and whose emitter im-' pedance is zero. Obviously, this is not the case, and to the extent that any deviation from these ideal conditions becomes significant, this will be reflected in the details of the two-ports. For example, two-port 13 will, in general, not be identical to two-port 12 to the extent thatvthe emitter resistance of the transistors is not zero. Thus, when characterized as having the same transfer characteristics, it will be understood to mean that the transfer characteristics of the networks are the same when the transistor parasitic impedances are also taken into account. Advantageously, each of the networks will include adjustable elements to compensate for the actual transistor impedances.
FIG. 5A, included for purposes of illustration, shows a twoport network of the type that can be used in the present invention. Designating the series elements Z, and the shunt element ZZ the bisected symmetric impedance Z and the bisected an- 2,, are given by Z,=Z,+2Z and Z,,=2Z,. To satisfy the match condition that i Z, and Z can include solely resistive elements; combinations of inductive and compactive'elements; or combinations of inductive, capacitive and resistive elements.
two matched transmission paths having different-charac-- terist'ic impedances.
FIG. 4 shown an active four-port utilized to combine components of two signals E, and E,, where signal E, is coupled to port c andsignal E,.is coupled to port b. Networks 12, 13 and 14, as in the embodiments of FIG. 1 and FIG. 3, have the same transfer characteristics, and are matched at impedance levels FIG. 5B shows a two-port network which is matched at impedance level Z, and has zero transmission. This two-port can be used at network 14 in the embodiment of the invention shown in'FIG. 4Q
It will be noted from the description of FIG. 4 that a signal coupled to port a (or b) will divide to produce signal components at ports b and c (or a and d). A signal coupled to either port 0 (or d) on the other hand, is only coupled to port d (or c). In this sense, the active directional coupler described herein is nonreciprocal and, as such, differs from a passive atmmimml cnunlel' which is reciprocal.
As indicated hereinabove, one advantage of the present invention is that directional coupling can be realized without inductors or transformers. Thus, the four-port can be readily fabricated using current printed circuit and integrated circuit techniques. lf, however, there are no limitations on the fabrication techniques employed, networks 12, 13 and 14 can include, as noted above, inductors as well as capacitors and resistors. The networks can also include lengths of transmission line. For example, if each of the networks is a length of transmision line, the coefficient t becomes imaginary whenever the line is an odd integral of a quarter of a wavelength long. Being imaginary, l is negative, and the coupled signal, given by equation 13) is maximized. Thus, the coupler, under these conditions, has a maximum response which varies as a function of frequency and can be used as a filter. Thus it is evident in all cases that the above-described arrangements are illustrative of only a small number of the many possible specific embodiments which can represent applications of the principles of the invention. Numerous and varied other arrangements can readily be devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention.
l. in combination:
two active members, each having an emitting electrode, a
control electrode and a collecting electrode;
first, second and third two-port networks connected,
respectively, between the emitting, control and collecting electrodes of said members;
said networks characterized in that the symmetric mode transfer gain and the antisymmetric mode transfer gain, measured between said control electrodes and said collecting electrodes, are equal.
2. The combination according to claim 1 wherein said networks have the same transfer characteristics.
3. The combination according to claim 2 wherein said networks are matched to the same impedance level.
4. The combination according to claim 2 wherein said networks are matched to different impedance levels.
5. The combination according to claim 1 wherein said networks have different transfer characteristics.
6. The combination according to claim 5 wherein said networks are matched to the same impedance level.
7. The combination according to claim 5 wherein said networks are matched to different impedance levels.
8. An active four-port network having directional transmission properties comprising:
two active members, each having an emitting electrode, a
control electrode and a collecting electrode;
a first two-port network connecting the emitting electrode of said members;
a second two-port network connecting the control electrodes of said members;
and a third two-port network connecting the collecting electrodes of said members;
said control electrodes and said collecting electrodes constituting the four ports of said network;
said two-port networks characterized in that equal symmetric and antisymmetric mode signals connected to said control electrodes produce equal symmetric and antisymmetric mode signals in said collecting electrodes.
9. The four-port according to claim 8 wherein an input signal is coupled to the control electrode of one of said members;
and wherein output signals are produced at the control electrode of the other of said members and at only one of said collecting electrodes.
10. The four-port according to claim 8 wherein a first signal is coupled to the collecting electrode of one of said members;
a second signal is coupled to the control electrode of the other of said members;
and wherein components of both of said signals are extracted from the collecting electrode of said other member.