US2666815A - Cathode-follower impedance maching device - Google Patents

Description

Jan. 19, 1954 w HAP|N 2,666,815

CATHODE-FOLLOWER IMPEDANCE MATCHING DEVICE Filed March 1, 1950 22 9 ll mm? W VOLTAGE l VOLTAGE OUTPUT *2 (9 a- I'M-'1 Lil NORMAL g RESPONSE l i- 9 i n DESIRED '5 RESPONSE Q INPUT VOLT GE Inventor:

William T. Chap His Attorney- Patented Jan. 19, 1954 PATENT OFFICE EfiTHflDE-FDIJLOW'ER "IMPEDANCE MATCHING DEVICE William Ehanin, Baldwinsville, .12, :assignor to General Electric Company, :axcorporation of :New York ApplicationMareh :1, 1950,S'e'ria'1No. 147,052

(olive-1'71) 2 fllaims.

. I This invention relates to an impedancematching-ar-rangement particularly applicalile"for-communicating electrical signals between two on- 'cuits.

"In the electrical signal transmission field, the problem often arises forcommunicatingelectrical signals between unbalanced impedance circuits, 'forexample, supplying-a*low impedance load from a high impedance source. The cathode follower type'circuithas been a'c'ommon dev'ice'iorachi'ev- 'ing this result. This circuit is-essentially an impedance lowering device having less than unity voltage gain bu't'capableof producing power gain. The output voltage of a cathode follower eircuit has good regulation because-of its low impedance while the input voltage may have a high impedance and relatively poor "regulation. However, the normalcathode follower circuit suifers from the fa'ct that'agiven amountof nc-signal bias is developed acrossits output. "That isjfor-zero signal applied to the input circuit of the cathode follower, there is acert'ain amount of spacecurrent "flow, resulting in a signal being developed across' its output. iils'o, the wariation of input signal'ever a given intensity range-produces a identically over 'a range including zerozmput volt- Y age.

'Anobjectof my invention is to provide an improvedlimpedance matching arrangement.

2% further object of -my invention is to provide an improvediarrangement fortefiecting 'communication of sig'n'als,'variablerovereaigiven rangeoi intensities, between circuits :of :different impedance Lat Lunity Igain.

still further obj-ectof my invention is ;to provide ian improved" cathode follower .circuit.

Thenovel features-which I believe to .bezcharacteristic of 'my' invention are set forthwith particularity in the appended claims. My invention itself, however, bothas to its organization and method of operation, utogetherawith further objects and advantages thereof :may .best .be umderstoodby reference.to 'thefollowing description taken in connection 'with the accompanying drawings wherein Fig. l shows a circuit diagram ofaantembodimentof nay-invention, and Fig. .12 shows response curves employed indescribing the invention.

,Briefly, an embodiment of my. invention comprises an improved :cathodefollower -.circuit rarrangement for supplying a voltage, variable= over arangeofintensitiesincludin'g zero voltsf-froma high impedance source to-a relatively low impedance load. The signals are applied through a coupling amplifier to a cathode follower circuit whose output circuit is coupled to the load circult and also-comprises a'portion-oi the cathode load circuitof the coupling device. The coupling device is provided with a feedback path to cornpensate for the eifectively reduced input signals due to plate'current variations in the output'circuit.

Fig.2 illustrates certain characteristics showing the operation and certain of the'advarrtages of my invention. In Fig 2 the curve Abearingithe legend Desired response "shows the relationship between the voltages at the output terminals 12 of the apparatus illustrated "in Fig. lplotted as ordinates and the voltages'at theinput terminals l of that apparatus plotted as abscissae. It will be observed thattheorigin of this curve A is zero and'th'at'the'output voltage at terminals? varies linearly from zero with variation in voltage at terminals 1.

The relationship of this curveA to curve B o": FigJZ'illuStIEi-WS the advantage of my invention over "that of the usual cathode follower circuit. Curve'B represents the corresponding characteristics of the usual cathode follower circuit not employing my invention. It is observed that .at zero input voltage a considerable output voltage appearsand that this voltage increases substan-- tially linearly only over a range .considerablyless than the range of'linear response .of my invention.

Referring to the Fig. 1,,positive and negative going voltages with respect to ground and available across terminals 4 fromahigh impedance source, not shown, are to be supplied to the output terminals 2 associated with a relativelylowiinpedance load, also-not shown. Toachieve this .result, a cathode follower circuitcomprising. a triode 3 having its anode-d directly coupledzto-a source of 13+ voltage and itscathodeelectrodet coupled by the cathode load impedance .\6 .to-a source of 3- Voltage. Underithe normal cathode follower operation, the outputdeveloped .acrossli would be applied to the outputterminals-E. ..In the present arrangement, the signalsavailable at l are applied between an'eifectiveintermediate point on lfiandthecontrolelectrode l of pentode 8, operating. as-a couplingamplifier, which inturn supplies thesignals in amplifiedform over coupling resistor 9 to the control electrode it of triode ll operating as a phase inverter. .Ther-invented, amplified signals .are =coup1ed from :the

output of II by means of coupling circuit l2 to the control electrode l3 of tube 3. The output of tube 3, available across 6, is thereupon supplied to terminals 2 by lead [4 connected to point l5. Thus any variation in the input voltage available at l, results in an output voltage variation available across terminals 2.

In order to compensate for the no-signal bias the output of tube 3 is included in the voltage divider circuit comprising pentode 8, thereby permitting cathode 5 to be operated at ground level. Thus for zero input voltage available at I, the tap on potentiometer I6 is adjusted to provide zero or ground level voltage across output terminals 2. Thus a positive or negative going voltage with respect to ground at terminal I results in a corresponding positive or negative going voltage respectively, properly referenced with respect to ground, being availabl at terminals 2. As the input voltage is increased, the output also increases by virtue of increased current flowing through 6. Most of this increased current is supplied by tube 3, resulting in a decreased grid to cathode bias of tube 3. This change in bias is produced by a change in the grid of cathode bias of tube 8 amplified by the gain of the circuit. Also, the increased current flowing through the cathode load of tube 8 comprising resistances I6 and I1 causes an increase in the voltage drop from cathode l8 to the output terminal 2. The

change in bias of tube 8 and the increased drop across [6 and I! both cause the output voltage available across terminals 2 to be less than the input voltage.

To compensate for this non-linearity in transfer of signals from terminals I to terminals 2, the voltage of screen 19 of tube 8 is arranged to be increased with increased input, hence increasing the plate current through tube 8 and the output resistance 6, thereby raising point 5 to its required level. This screen voltage variation is effected by a feedback arrangement coupling a portion of the output of the phase inverter available across the anode load potentiometer 28 through device 2| to the screen grid l9. Device 2| is arranged with its anode electrode coupled directly to B+ and its cathode electrode coupled directly to the screen electrode 59 with the output signals available across 28 being applied to its control grid. The rate at which the screen to cathode voltage of device 8 is increased is controllable by adjusting the tap on potentiometer 20. With the proper adjustment of the tap of potentiometer 28, the current through tube 8 is caused to increase even with a decrease in grid to cathode bias of tube 8. Ihe increase in current through tube 8, as previously mentioned, compensates for the decrease in cathode to output voltage of tube 8.

Recapitulating the series of events, the output of tube 8 is applied through the phase inverter tube II to the input of cathode follower tube 3 resulting in a corresponding current change through load impedance 6. For a zero input signal to tube 8, potentiometer I8 is adjusted to provide a zero signal output across terminals 2. If the input voltage to tube 8 is varied in a positive going direction, a corresponding positive going variation is experienced across terminals 2. However, it is found that the unity gain requirement is no longer obtained being due primarily to the fact that the variation in current through load resistor 6 controls the effectiveness of the input signal. To compensate for this, a feedback arrangement comprising tube 2| is provided to vary the screen grid voltage, thereby adjusting the plate current flow through tube 8, and hence compensating for the variation in effective bias.

To prevent instability in the arrangement shown by the single figure, tube 8 is arranged as a wide-band amplifier. Tube II has a capacitor 22 coupled between its plate and grid electrodes to reduce its bandwith relative to tube 8 such that the loop gain-phase shift characteristic never approaches the oscillatory state.

Applicant's arrangement provides a stable, unity gain coupling arrangement for feeding signals between different impedance stages. However, applicants arrangement lends itself readily to other gain requirements by merely adjusting the rate at which the space discharge current of tube 8 is to b increased in response to a given output current variation thereof. The manner in which the feedback signal is derived and used to control the space discharge current may be varied in accordance with the particular circuit requirements, although the use of a phase inverter II and coupling device I! has produced satisfactory results. The point of coupling of the load circuit to the cathode load resistance of tube 8 may also be varied to obtain a required input to output signal relationship.

While I have shown only a certain preferred embodiment of my invention by way of illustration, many modifications will occur to those skilled in the art and I therefore wish to have it understood that I intend, in the appended claims, to cover all such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An arrangement for applying a signal variable over a range of intensities from a high impedance source of a low impedance load comprising an electron discharge device including a cathode, a control grid, a screen grid, and an anode. a source of unidirectional operating potential including a positive and a negative terminal, a cathode resistance connected between said cathode and said negative terminal, means for applying said signal in circuit with said control electrode and said cathode resistance, an anode resistance connected between said anode and said positive terminal, a second electron discharge device including an input circuit, the electron discharge path of said second device being coupled across said potential source and including a portion of said cathode impedance, means for coupling voltage variations across said anode imedance to the input circuit of said second device, amplifier means including a portion of said signal coupling means and coupled between the anode and the screen electrode of said first device for feeding back voltage to said screen electrode in a manner to increase the potential difference between the cathode and said screen electrode of said first device as said signal voltage increases, whereby the current flow through said cathode resistance varies in correspondence with signal variations on said first control electrode, and means for coupling said low impedance load between an intermediate point of said cathode resistance and an intermediate point of said source.

2. An arrangement for supplying a signal, variable over a range of intensities, from a high impedance source to a low impedance load comprising a first electron discharge device, a plurality of input circuits for said device, a cathode load impedance and an anode load impedance, means applying said signal to one of said input circuits, a source of unidirectional voltage, said load ima .m n. l

pedances coupling the electron discharge path of said device across said voltage source, a second electron discharge device comprising an input circuit, means coupling the electron discharge path of said second device through a portion of said cathode impedance across said voltage source, signal coupling means coupled between anode load impedance and the input circuit of said second device, amplifier means including a portion of said signal coupling means and coupled between said anode load impedance and another input circuit of said, first device for feeding back a signal to said other input circuit of said first device to compensate for reduction in effectiveness of said input signal due to electron dis- 15 charge current flow through said cathode impedance to an extent to cause said current flow to vary in correspondence with said signal, and means for coupling said low impedance load between an intermediate point of said cathode irnpedance and an intermediate potential point of said source of unidirectional voltage.

WILLIAM T, CI-IAPIN.

References Qited in the file of this patent Froman Aug. 8, 1950

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