US2995667A - Transmission line driver - Google Patents

Description

G. L. CLAPPER ETAl TRANSMISSION LINE DRIVER Filed Dec. 23, 1957 Aug. 8, 1961 OUT INVENTORS GENUNG L. CLAPPER ROBERT L PRATT United States Patent 2,995,667 TRANSMISSION LINE DRIVER Genung L. Clapper, Vestal, and Robert L. Pratt, Johnson City, N.Y., assignors to International Business Machines Corporation, New York, N.Y., a corporation of New York Filed Dec. 23, 1957, Ser. No. 704,553

1 Claim. (Cl. 307-885) This invention relates to the transmission of information over lines and more particularly to the use of transistors to drive a transmission line.

In accordance with the embodiment of the invention, two transistors, a source of signal voltage, and other component elements are arranged to form a transmission network. The transistors are of the type having a base, a collector and an emitter. At the input to the transmission line, there is a transistor switch which is normally cut off but which, upon the application of an input signal of sutficient amplitude, closes and permits the current to flow along .the transmission line. At the output end of the transmission line, there is provided a transistor amplifier for amplifying the transmitted signal.

Such a circuit has the advantage of requiring negligible 1 power to drive the line. In addition, there is freedom from extraneous noise, no ringing due to poor line terminations, low power dissipation in the driver, and minimum phase shifting.

A principal object of the present invention is to provide an improved transmission circuit having a current switch at the input and a current operated voltage amplifier at the output of a transmission line.

A further object of the present invention is to provide a novel circuit having the ability to drive long lines at low signal levels with minimum capacitative loading effects.

Other objects of the invention will be pointed out in the following description and claim and illustrated in the accompanying drawings, which disclose, by way of example, the principle-of the invention and the best mode, which has been contemplated, of applying that principle.

In the drawings:

FIG. 1 is a schematic circuit diagram of a transmission circuit employing a current switch at the input and a current operated voltage amplifier at the output in accordance with the present invention.

FIG. 2 is a diagram of the waveforms illustrating the operating characteristics of the circuit of FIG. 1.

In semiconductive materials such as germanium or silicon, the electrical currents, according to presently accepted theory, are carried by electrons designated as excess electrons or by holes which are defect or missing electrons. According to the theory, a hole" may be viewed as a carrier of a positive electric charge and an electron as a carrier of a negative electric charge. Electron or hole" carriers are designated generically by the term mobile charges.

A semiconductive material is called excess or N type when the mobile charges normally present in excess in the material under equilibrium conditions are electrons. N type semiconductive material passes current easily when the semiconductive material is negative with respect to a conductive connected thereto.

A semiconductive material is called defect or P type when the mobile charges normally present in excess in the mateiral under equilibrium conditions are holes. P type semiconductive material passes current easily when the semiconductive material is positive with respect to a conductive connection thereto.

Two principal classes of semiconductor devices have been developed which have been referred to in the art as the point contact transistor and the junction transistor. The present invention employs, purely as a'matter of; choice, the junction transistor which, as the name implies, includes a semiconductive body having alternate zones of N and P type material forming between them junctions or barriers. Electrodes are placed in low-resistance contact with the discrete zones of the material and have been given the names of collector electrode, base electrode and emitter electrode. The

collector electrode and the emitter electrode are in contact with the end zones of the semiconductive body, and the base electrode is in contact with the intervening zone of the semiconductive body.

The alternate zones of the body of the junction transistor may be in the series N-P-N or in the series P-N-P. In a base-input, grounded emitter circuit, a positive input pulse applied to the base electrode to a P-N-P junction transistor will cause the current flowing from the collector to decrease; whereas, a positive input pulse applied to the base electrode of an N-P-N junction transistor will cause the current flowing into the collector to increase. Thus P-N-P and N-P-N transistors have complementary operating characteristics.

In accordance with the preferred embodiment of the present invention, a current switch is provided at the input to the transmission line and a current operated voltage amplifier is connected at the output of the line. Referring to FIG. I, the current switch comprises a junction transistor 10 which is of the N-P-N type. Transistor 10 has an emitter electrode 11 in contact with one N type zone, a base electrode 12 in contact with the P type zone and a collector electrode 13 in contact with the other N type zone. I he base electrode 12 functions as the control electrode and is connected, through an input resistor 14 and capacitor 15, to an input voltage signal terminal 16. A resistor 17 connected to a positive 10 volt terminal 18 serves to set the voltage at the base electrode 12. A source of operating voltage, such as positive 1.5 volt terminal 19, is connected to the collector electrode 13 through a resistor 20 and the emitter electrode 11, which functions as the output electrode, is connectedto the input terminal 21 of a transmission line 22. H

The current operated voltage amplifier comprises a junction transistor 23 which is of the P-N-P type. Transistor 23 has an emitter electrode 24 in contact with one P type zone, a base electrode 25 in contact with the N zone and a collector electrode 26 in contact with the other P type zone. The base or control electrode 25 is shown connected to ground and the emitter electrode 24 is connected to the output of the transmission line. A line terminating resistor 27 is provided between point D and ground. A diode 28 and resistor 29, connected between a negative 5 volt terminal 30 and a negative 15 volt terminal 31, set the voltage at the output or collector electrode 26.

Referring to the waveforms shown in FIG. 2, the circuit operates in the following manner:

When the input signal is at negative 5 volts, transistor 10 is cut oil? and no current flows from points C to D. When the input rises to ground potential, transistor 10 conducts and the resistor 20 in the collector circuit limits the current flow to around 8 milliamperes. As shown in FIG. 2, resistor 20 drops the voltage at point B to a positive 0.7 volt biasing the collector 13 more negative with respect to base 12 and driving transistor 10 to saturation. Coupled with the drop across the transistor, the voltage swing at point C is only 0.5 volt which is sufficient to drive the line 22 and receiver transistor 23 with minimum loading effects on the line.

Transistor 23 is a current operated voltage amplifier and when no current is flowing in the emitter circuit the output is at negative 5 volts. The 8 milliampere current pulse from the line driver transistor 10 is divided about equally betweenthe resistor 27 connection from point D to ground and the emitter electrode 24 of transistor 23. The voltage change at point D will then be about 0.4 volt and the voltage swing at the output will be about 5 volts due to the limiting diode 28. The use of a diode clamp at the output or collector electrode 26 eliminates any spurious signals which may occur in the line and produces a faster response. Hence, the output signal goes from approximately a negative 5 volts to volt and remains at 0 during conduction of transistor 10 and the transmission of current over the line 22. When the input signal drops sufliciently below 0 volt, transistor 10 will cut oif and current will cease to flow in the transmission line 22. As a result, the transistor 23 output will drop to approximately negative volts.

A major advantage of the present circuit is the negligible power required to drive the line. The energy stored in the line capacitance to ground (C is /2C;,E. Since the transient power required to charge the line through a given source impedance is proportional to this; reduction of the signal from 5 volts to 0.5 volt results in a transient power'reduction of or 5 Conversely, 400 times the power would be required if a 10 volt signal were used instead of 0.5 volt. The same reduction is also obtained in the standby power losses in the terminating resistance 27 of the line, since E p 'fi Also, as was previously mentioned, the circuit presents freedom from extranoeus noise, no ringing because of poor line terminations, low power dissipation in the driver even under shorted conditions, minimum phase shift, no adverse effects due to transistor variations, and simplicity of circuit design.

While there have been shown and described and pointed out the fundamental novel features of the invention as applied to a preferred embodiment, it will be understood that various omissions and substitutions and changes in the form and details of the device illustrated and its operation may be made by those skilled in the art,

without departing from the spirit of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the following claim.

What is claimed is:

A system, for transmitting information comprising, a source of relatively large signal voltage, a transmission line having an input terminal and an output terminal, a first junction type transistor having a base, emitter and collector electrode, said base electrode being connected to said signal source and said emitter electrode being connected solely to said input terminal, a source of bias voltage for said collector electrode, an impedance element connecting said bias voltage to said collector electrode, and a second junction type transistor having a base, emitter and collector electrode, said second emitter electrode being connected to said output terminal and said second base electrode being connected to ground, said first transistor elfecting a flow of current in response to said-signal voltage with'said impedance element allowing said first transistor to go to saturation whereby a measured current flow of known value will'be presented to the transmission line, said impedance element dissipating part of the available power output of said first transistor whereby the voltage swing at said input terminal is reduced to a minimum value to drive said transmission line and said second transistor, and said second transistor being responsive to said flow of current to produce an output voltage of the order of magnitude of said input signal voltage.

nan-a... Cited in the tile of this patent UNITED STATES PATENTS 2,246,331 White June 17, 1941 2,252,613 Bingley Aug. 12, 1941 2,256,084 Goodale Sept. 16, 1941 2,801,296 Bleeher July 30, 1957 2,803,758 Whitenack Aug. 20, 1957 2,839,619 Johnstone June 17, 1958 2,842,624 Marsh July 8, 1958 2,860,196 Schultz Nov. 11, 1958 OTHER REFERENCES Shea: Principles of Transistor Circuits, published by John Wiley and Sons, copyright 1953, page 189.

Lo et al.: Transistor Electronics, pages 82, 83, September 1955.

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