US3343003A - Transistor inductor - Google Patents

Description

Sept. 19, 1967 R. E. ARSENEAU TRANSISTOR INDUCTOR Filed Jan. 24, 1964 m m m F \J \V W B| I T l mmm WW Ill-M pupa DIODE-9 F IG 2 ATTORNEY United States Patent O 3,343,003 TRANSISTOR INDUCTOR Roger Edward Arseneau, Elkgrove Village, 11]., assignor to International Telephone and Telegraph Corporation Filed Jan. 24, 1964, Ser. No. 340,011 4 Claims. (Cl. 307-885) This invention relates to circuits for performing functions which are equivalent to the functions of an inductor, and more particularly to transistorized circuits for providing such functions.

An inductor has both desirable and undesirable characteristics. Among other things, an inductor has desirable characteristics which may be used to provide a low D.C. impedance and a high A.C. impedance. An inductor also provides a current lag which facilitates the design of certain circuits such as choke coils, filters, and the like. In addition to the desirable characteristics, an inductor also has undesirable characteristics which sometimes cause problems. For example, the design and construction of an inductor may become extremely critical when conflicting demands are placed on it. To illustrate this point, consider the requirements of a repeat coil or transformer for coupling a source of voice signals to an electronic switching network. If the transformer has good voice transmission characteristics, current may occasionally reach levels which cause the switching system to fail. On the other hand, if the transformer passes a current having characteristics which insure proper switching, the quality of voice transmission may deteriorate. Of course, it should be understood that this transformer example is cited only to illustrate the problem. Many other examples could also be cited to illustrate other problems which are encountered when an inductor is used.

The functions of an inductor are similar to the functions of a flywheel since they both add mass to maintain a steady state condition in a kinetic energy system. For example, an inductor maintains a uniform delivery of current despite non-uniform variations (an A.C. signal) in voltage applied across the inductor. Thus, one of the most important considerations in the design of an inductor is the provision of a proper mass which is well matched to the needs of the system in which it is used. This matching is done by a selection of the physical dimensions of the inductor. It must be large enough to govern the electrical system in which it is used, and small enough not to overload the source which powers the system. Moreover, it also should be tuned for proper operations in response to characteristic variations of the kineticenergy of the electrical systems. Since the inductor characteristics depend upon its physical dimensions, it has been expensive to provide a great number of in ductances having a wide variety of characteristics. Severe economic burdens result if a great number of different sized piece-parts must be made and stocked in inventory.

In addition to mass considerations, there are other non-related design considerations which inherently occur in many electronic circuits. Therefore, a real economy results if an electronic circuit which is primarily designed toprovide the functions of an inductor also provides a solution for some of the other design problems. For example, almost all circuits require amplification of signals. Thus, a circuit combining inductance and amplification would be quite valuable.

Accordingly, an object of the invention is to provide new and improved circuits having the electrical characteristics of an inductor. A more particular object is to provide a transistorized circuit having both an inductors characteristics and an amplification capability.

Yet another object of the invention is to provide new and improved devices for coupling dissimilar circuits. Here an object is to eliminate expensive and involved 3,343,003 Patented Sept. 19, 1967 circuitry requiring critical components, especially transformers. In this connection, an object is to substitute an inexpensive capacitive coupling for an expensive transformer coupling.

Still another object of the invention is to reduce the cost of designing and building circuits. In particular, an object is to reduce the cost of electronic switching telephone systems. Here an object is to accomplish the above and other objects by a circuit which not only eliminates problems caused by inductor design considerations, but also provides amplification in an electronic switching network.

In accordance with one aspect of this invention, an electronic network is provided with a single terminal which functions as both an input and an output. Connected thereto are a pair of electronic devices, both having input, output, and control electrodes. These devices are coupled in a feed back loop relation so that one transistor controls the output of the other transistor as a function of A.C. voltage variations at the single terminal. This control is accomplished in a manner such that the other transistor applies a constant DC. current to the terminal despite A.C. variations appearing thereat.

The above mentioned and other features of this invention and the manner of obtaining them will become more apparent, and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic circuit diagram showing a transistorized circuit which has characteristics that are equivvalent to the characteristics of an inductor; and

FIG. 2 is a schematic circuit diagram showing how the circuit of FIG. 1 may be used in an electronic switching telephone system.

FIG. 1 discloses a preferred embodiment of the invention which is here shown as a transistorized circuit having the electrical characteristics of an inductor. The circuit is an electronic network with a single terminal 50 which functions as both an input and an output. Connected thereto are a pair of electronic devices, 51, 52, both of which have input, output, and control electrodes a, b, and 0 respectively. Of these, device 51 is here shown as a PNP transistor and device 52 as an NPN transistor, both operating in their active range, to function when positive current flows into terminal 50. If polarities are reversed to cause a negative current at terminal 50, the transistor types are also reversed. These devices are coupled in a feed back loop relation so that one transistor controls the output of the other transistor as a function of A.C. signal variations appearing at the single terminal 50. This control is accomplished in a manner such that the other transistor. supplies a constant DC. current to the terminal. Thus, there is, in effect a low impedance for DO. and a high impedance for A.C.

In construction, this circuit is arranged with both the emitter a and base c of the PNP transistor 51 individually coupled to the input-output terminal 50, the emitter a via a resistive element 53, and the base 0 via a capacitive element 54. The emitter a is also connected to ground by a resistor 55 which helps control the impedance at point 50, and which cooperates with the resistor 53 to form a voltage divider. The base 0 is further biased to a quiescent point of operation by a voltage divider 56, 58 connected between positive battery and ground. A resistor 59 is coupled between a 12 volt source and the collector b of transistor 51 and base 0 of transistor 52. This resistor 59 stabilizes the collector current when the collector junction is reverse biased (i.e. the 1 The circuit works this way. The voltage divider 56, 58 normally establishes a potential at the base of the transistor 51 which is a little less positive than the potential desired at the terminal 50. Thus, when current flows into terminal 50, transistor 51 is normally forwardly biased to drive a control current into the base of the transistor 52. The circuit values are such that the voltage drop across resistor 53 is enough to cause the transistor 52 to operate in its active region and conduct the current entering terminal 50 to ground G1 while keeping the potential at terminal 50 more positive than the potential at the base of transistor 51. The potential at the base of transistor 51 is high enough to keep both of the transistors 51, 52 from saturating.

Means are provided for causing the circuit to present a low D.C. impedance and a high A.C. impedance. To understand how this occurs, assume that an A.C. signal at the point 50 is such that the incoming voltage begins to increase. This increase in voltage is applied to one end of both resistor 53 and capacitor 54. If this voltage change occurs much faster than the RC time constant of the network of capacitor 54 and resistors 56, 58, the same change is applied to both the base and emitter of the transistor 51. Since both of these electrodes experience the same voltage variations, there is no change in the voltage across resistor 53, and, therefore, no change in the collector current of transistor 51. Thus, the current driving into the base of transistor 52 remains the same so that there is no change in its collector current. In a like manner, the voltage across resistor 53 and the collector current of transistor 52 remain constant when the potential at terminal 50 decreases. It should now be apparent that since a change in voltage at terminal 50 does not produce a corresponding change in the current into terminal 50, there is a very high impedance to the A.C. change.

The well known inductor causes a current lag. This effect is seen at terminal 50 where a current change also lags after a voltage change. In greater detail, for a very slow increase in potential at terminal 50, the charge on capacitor 54 increases, thus increasing the voltage across resistor 53. With these changes, the bias on transistor 51 is effectively shifted to increase the collector current of transistor 51. This means that the current driving into the base of transistor 52 increases to cause an increase in the collector current of transistor 52. This increase in co1- lector current lags after the increase in voltage at terminal 50 because it takes time after the voltage change occurs at terminal 50 before the charge on capacitor 54 can change.

For a change in the DC. level of the current at terminal 50, the voltage at terminal 50 increases just enough to increase the voltage across both resistor 53 and capacitor 54. This causes an increase in the base current to transistor 52 to cause it to increase its collector current to terminal 50.

Means are provided for using these functions to cause electrical effects which are equivalent to changing the physical dimensions of an inductor. More particularly, for a given A.C. signal, no elfect is seen in the current flow from point G1 through the emitter-collector of the transistor 52 to the point 50; or stated another way, there is a low D.C. impedance and a high A.C. impedance. If the undulations of the A.C. signal become more rapid, there are no significant voltage variations at the base of the transistor 51, with respect to the voltage on its emitter. As the undulations slow, the capacitor 54 comes closer to receiving a full charge, and the voltage variations at the base of the transistor 51 become more significant with respect to the voltage on its emitter. If the undulations are slow enough, a significant A.C. signal effect is felt in the output of the transistor 52. If the capacitor 54 charges sufficiently, the voltage in the base circuit of the transistor 51 changes by an amount which is adequate to produce a relatively great change in the current through the transistor 52. The answer to the problem of what is rapid or slow is determined by the circuit values which establish the RC. time constant. A large time constant causes a slower response. Thus, by varying the size of the capacitor, one gets results which are the same as those which are obtained by varying the size of the inductor. Thus, by selecting the value of capacitor 54, relative to the value of other components, the circuit may be given an output stimulating the output of any one of a great variety of inductors.

In one exemplary construction, the following circuit values were used with good results.

Transistor 51 Type, 2N1309. Transistor 52 Type 2N1308. Resistor 53 3909. Resistor 55 39K.

Resistor 56 K. Resistor 58 10K. Resistor 59 100K. Capacitor 54 l0 mf.

With the circuit constructed from components having the above circuit values, it was found that a good inductive characteristic was achieved. from zero through 5,000 cycles per second. The circuit was designed to operate over this range of frequencies because the transistor inductor circuit was used as a substitute for a voice frequency choke coil. Obviously, the invention contemplates a selection of any appropriate circuit values as may be required by the particular frequency band of interest for a given application.

Means are provided for giving a negative resistance which is comparable to amplifying the A.C. signal that appears at terminal 50. In greater. detail, an external circuit driving into terminal 50 sees an impedance which is fixed primarily by resistors 53, 55. By reducing the resistance at 55, the impedance seen at terminal 50 may be raised to infinity, or even higher into a negative impedance zone. Thus, the signal source (not shown in FIG. 1)

sees a smaller effective load which amounts to a signal.

amplification.

This negative impedance occurs because the resistors 53, 55 form a voltage divider connected between terminal 50 and ground. Here the ratio of the divider arms are selected so that the A.C. voltage variations experienced in the circuit of the emitter of transistor 51 are less than the A.C. signal caused voltage variations experienced in the circuit of base of transistor 51. When the potential raises at terminal 50, it also raises at the common tie point between the emitter of transistor 51, the resistor 53, and the resistor 55. This causes an increase in the current flowing through the resistor 55. The potential across resistor 53 remains constant because emitter a and base c experienced the same rise in potential; thus, the current through resistor 53 also remains constant, and the increase in current through the resistor 55 must occur at the expense of a decrease in current into the emitter of transistor 51. This decrease in emitter current causes a decrease of the current driving into the base of transistor 52. Therefore, there is a reduc-. tion in the collector current of transistor 52. Hence, an.

increase in the potential at terminal 50 causes a decrease in the current into terminal 50, thereby producing an effect which is as if there were a reduction in impedance or more conventionally, a negative impedance appears at terminal 50.

With the'circuit values given above, it was found that the input impedance at terminal 50 could be varied from 2K through infinity to 5009 by varying resistor 55 from 18K to 200K.

There are, of course, many .uses for the invention. One exemplary use is found in an electronic switching telephone system shown in FIG. 2. The principal divisions and a link 63. The subscriber line 60 terminates at one end in a telephone sub-set 64 .(which may be a calling subscriber station), and at the other end in a repeat coil signal caused 65. The switching network 62 terminates at one end in the transistorized inductor 61 and at the other end in a constant current source 66. The components 67 indicate another identical path to a called subscriber station.

A low D.C. impedance high A.C. impedance path may be traced from ground G2 through inductor 61, network 62, and constant current source 66 to battery B1. Thus, at capacitor 68, the speech path may be capacitively coupled from line 60 to network 62. This means that the repeat coil 65 may be an inexpensive item of standard design. Its design does not have any effect on DC. current flow through the network 62.

Heretofore, the DC. holding path for the network 62 extended through the secondary winding of repeat coil 65. The network was subject to malfunctions if the holding path was not controlled quite closely. The subscriber lines, such as 60, are notorious for their non-uniform characteristics. Thus, the design of the transformer 65 has been very critical.

While the principles of the invention have been described above in connection with specific apparatus and applications, it is to be understood that this description is made only by way of example and not as a limitation on the scope of the invention.

I claim:

1. An electronic network having a single input-output terminal, a pair of electronic devices connected to said terminal in a low-D.C.-resistance-high-A.C.-impedance configuration, one of said devices being connected to control the conductivity of the other of said devices, both of said devices having base, emitter, and collector electrodes, said one device having at least a capacitive element connected between the base and emitter electrodes, and means comprising said two devices and responsive to a change of current into said terminal for compensatingly changing the conductivity of said other device to counteract the change in current into said terminal, whereby said network supplies a constant direct current to said terminal despite the variations in an A.C. signal appearing at said terminal, wherein a first of said devices is a PNP transistor and a second of said devices is an NPN transistor.

2. The network of claim 1 wherein the emitter and base electrodes of said PNP transistor are individually coupled to said terminal via a resistive element and said capacitive element respectively, and the collector electrode of said PNP transistor is coupled to drive current into the base electrode of said NPN transistor, a potential point, and a circuit extending from said potential point through the emitter and collector electrodes of said NPN transistor to said terminal, said last named circuit being the path for supplying said direct current to said terminal.

3. The network of claim 2 and a resistor coupled to cooperate with said resistive element and form a voltage divider for establishing the input impedance at said terminal, the value of said resistor being selected to provide a decrease in current flowing into said terminal responsive to an increase of voltage appearing at said terminal, said emitter electrode of said PNP transistor being coupled to a point on said voltage divider.

4. The network of claim 2 wherein the Value of said capacitive element is selected to provide a predetermined time lag after which said A.C. signal may cause a change in said direct current.

References Cited UNITED STATES PATENTS 2,892,164 6/1959 W011 30788.5 2,892,165 6/1959 Lindsay 307-885 3,001,157 9/1961 Sipress et al. 333-241 X 3,152,309 10/1964 Bogusz et al. 307--88.5

ARTHUR GAUSS, Primary Examiner.

I. ZAZWORSKY, Assistant Examiner.

Claims (1)

1. AN ELECTRONIC NETWORK HAVING A SINGLE INPUT-OUTPUT TERMINAL, A PAIR OF ELECTRONIC DEVICES CONNECTED TO SAID TERMINAL IN A LOW-D.C.-RESISTANCE-HIGH-A.C.-IMPEDANCE CONFIGURATION, ONE OF SAID DEVICES BEING CONNECTED TO CONTROL THE CONDUCTIVITY OF THE OTHER OF SAID DEVICES, BOTH OF SAID DEVICES HAVING BASE, EMITTER, AND COLLECTOR ELECTRODES, SAID ONE DEVICE HAVING AT LEAST A CAPACITIVE ELEMENT CONNECTED BETWEEN THE BASE AND EMITTER ELECTRODES, AND MEANS COMPRISING SAID TWO DEVICES AND RESPONSIVE TO A CHANGE OF CURRENT INTO SAID TERMINAL FOR COMPENSATINGLY CHANGING THE CONDUCTIVITY OF SAID OTHER DEVICE TO COUNTERACT THE CHANGE IN CURRENT INTO SAID TERMINAL, WHEREBY SAID NETWORK SUPPLIES A CONSTANT DIRECT CURRENT TO SAID TERMINAL DESPITE THE VARIATIONS IN AN A.C. SIGNAL APPEARING AT SAID TERMINAL, WHEREIN A FIRST OF SAID DEVICES IS A PNP TRANSISTOR AND A SECOND OF SAID DEVICES IS AN NPN TRANSISTOR.

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