US2085408A - Diode rectifier circuit - Google Patents

Description

June29, 1937. 1.. E. BARTON 2,085,408

DIODE RECTIFIER CIRCUIT Filed Jan. 51, 1955 RECTIFIER HIGH HMPLITUDE V LO W CHPHCI 7'1 7'0 GROUND 4' 50% now/v 200% UP 602' UP MODULATION MODULfl'r/ON MODULATION -INVEN7'OR Loy E.Barton Patented June 29, 1937 PATENT OFFICE DIODE V RECTIFIER CIRCUIT Loy E. Barton, Collingswood, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application January 31, 1935, Serial No. 4,243

1 Claim.

The present invention relates todiode rectifier circuits,'particularly adapted for detecting high frequency modulated signals, and has for its object to provide an improved diode rectifier circuit 5 for supplying rectified signals directly to abalanced audio frequency output stage for radio receiving apparatus and the like.

It is a further object of the present invention to provide an improvedradio receiving system wherein signals of relatively high amplitude are supplied to a diode rectifier, whereby the rectifier output signals may be applied directly to a push-pull output or power amplifier stage without the intermediary of a driver stage.

It is a still further object of the presentinvention to provide an improved diode rectifier circuit for modulated signals and the like providing a push-pull or balanced output circuit from a single diode rectifier of the hot cathode 20 typer r t It is also a further object of the present invention to provide an improved modulation indicator embodying a single diode rectifier device and a balanced output circuit therefor.

The invention will, however, be better understood fromthe following description when considered in connection with the accompanying drawing, and its scope will be pointed out in the appended claim. 7

In the drawing, Figure 1 is a schematic circuit diagram of a portion of a radio receiving system embodying the'invention, and

Fig.2 is a similar schematic circuit diagram of a modulation indicator embodying the invention.

Referring to Fig. 1, 5 is the output circuit from any suitable apparatus (not shown) for supplying modulated signals of high amplitude sufficient to operate an output amplifier stage 6 directly from a diode rectifier device 1 without the intermediary of a driver amplifier stage.

The rectifier device I is of the hot cathode type, comprising a filament 8 and an anode 9, the latter being connected to one side l0 of a tuned circuit suitably coupled to the output circuit 5 to receive signal energy therefrom.

The cathode or filament 8 is supplied with *heating currentfrom the secondary winding |2 of a heating transformer l3, the primary M of which is adapted to be connected to a suitable source of alternating current supply. The secondary of the transformer is designed to -have a low. capacity to ground, that is, to the core or frame indicated at It, and to the primary winding, M. The capacity is sufficiently low to substantially isolate the cathode or filament 8 from ground by capacity coupling at the signal modulation frequency which is usually within the audio frequency range.

The cathode or filament 8 is provided with a center tap connection through a center tap ter-' minal I6 provided on the secondary winding l2, and, through this connection and a lead H, the rectifier circuit is completed to the low potential terminal l8 of the input circuit through a rectifier output resistor l9. With this arrangement, signals received in the rectifier input circuit H from the output circuit 5 flow from the high potential terminal l0 through the rectifier and the cathode circuit forming the connection with the secondary l2, and, from the cathode circuit, the rectified signals return by way of the center tap I6 through the lead I! and the resistor |9 to-the low potential terminal l8 of said input circuit.-

The cathode circuit may be by-passed at radio frequencies by suitable by-pass condensers as indicated at 20a. The rectified signals appear across the terminals 2| and 22 of the output resistor I9.

Since the cathode 8 and its connection IT with the terminal 22 of the output resistor I9 is isolated above ground at the modulation or audio frequency, the cathode end or terminal 22 of the output resistor I9, therefore, may vary at the output frequency and 180 out of phase with the potential at the terminal 2| which is connected with the input circuit. A push-pull output circuit may, therefore, be connected with the terminals 2| and 22, and, for this purpose, the center tap '23 is provided on the output resistor I9 and connected to ground as indicated at 24. High frequency currents in the rectifier circuit are lay-passed around the resistor sections thus provided, by suitable by-pass condensers 25-25 connected in circuit between the grounded center tap 23 and the terminals 2| and 22. It has been found that radio signals of suflicient amplitude may be provided to drive the output stage 6 directly from the output terminals 2| and '22. For example, a signal potential of 100 volts or more may be provided between the terminals 2| and 22 with suitable input signal amplitude in the circuit The output stage comprises a pair of balanced output electric discharge amplifier devices or tubes 26-26 having a balanced output circuit 21 connected through a suitable coupling trans- I high-impedance type.

former 28 to a loudspeaker or other signal output device 29.

By reason of the high signal voltage available across the terminals of the output resistor I6, the tubes 25 may be coupled thereto by simple impedance coupling means comprising coupling condensers 30 in output leads 3| and 32, extending from the terminals 2| and 22 to the control grids 33 of the tubes 26, and suitable bias supply resistors 34 connected between the grids 33 and a bias potential supplylead 35.

A balanced push-pull input circuit is, there-' fore, provided for the tubes 26 of the output amplifier stage in connection with the balanced output circuit of the single rectifier device 1. This arrangement results in the audio frequencyamplification of the rectified signals without appreciable distortion and with comparatively simple amplifying means involving a minimum number of circuit elements.

With this arrangement, the cathode end 22 of the output circuit floats at audio frequency in connection with one output grid while the anode or input end 2| floats at audio frequency in connection with the other grid of the balanced or push-pull output amplifier stage. This is permitted by causing the cathode or filament of a hot cathode diode rectifier to float at low capacity to ground and with a push-pull or balanced output resistor connected between the floating cathode connection and the high frequency input circuit.

Referring now to Fig. 2, a circuit similar to that of Fig. l is shown wherein the electric discharge device 40 is the rectifier or detector which receives modulated signals through an input circuit 4| and supplies. rectified signals to the output terminals 42 and 43 of an output resistor 44, the center tap 45 of which is connected to ground as indicated at 46. High frequency by-pass condensers for each half of the resistor 44 toground are indicated at 41.

In the present example, the rectifier device40 is a hot cathode device of the equi-potential type comprising a cathode 48 and a heater element 49 therefor. The filament or heater 49 may be connected to any suitable source of heating current, for example, as shown in Fig. l and is sufficiently spaced from the cathode 48 to have a low capacity thereto, whereby the cathode 48 may float or be isolated from ground at audio or modulation frequencies. With this arrangement, the oathode is indirectly heated and the heating element may be connected to any suitable source (not shown), grounded or ungrounded, although the connection shown in Fig. 1 for the filament is at present preferred when an indirectly heated cathode is used. In case that the capacity between the cathode 48 and the heater 49 is sulficiently low, the heater cathode voltage is not important. The cathode is isolated from the heater by suflicient spacing to provide the desired low capacity.

The alternating current or signal output from the resistor 44 is applied to a pair of balanced amplifier devices 56 and 5| of the high-gain, In the present example, pentodes are illustrated and each comprises a cathode 52, a cathode heater 53, a control grid 54, a screen grid 55, suppressor grid 56, and an output anode 51.

The suppressor grids 56 are connected to the cathodes and the latter are connected together and to ground, as indicated at 58. The screen grids receive operating potential from a positive supply lead 59 through a potential reducing resistor 60. Between the screen grids and the cathode there may be provided a filter condenser 6| for smoothing the supply voltage.

The output anodes 51 receive operating potential through neon lamps 62 and 63, one lamp being in the anode circuit of each tube.

The control grids 54 are connected each with one terminal of the resistor 44 through grid leads 64 and 65, the latter being connected directly to the rectifier output terminal 42, while the former is connected to the rectifier output terminal 46 through a suitable coupling condenser 66. which acts primarily as a stopping condenser to prevent the application of direct current potentials from the terminal 43 to the grids of the tubes.

In the present example, biasing potentials for the tubes 50 and 5| are taken from the resistor 44 as the result of signal flow and the tubes may, therefore, be termed diode-biased. The potential applied to the grids is the drop in potential between the grounded mid-tap 45 and the terminals 42. This potential is applied directly to the grid 54 of the tube 5| through its connection with the lead 65 and the same potential is applied to the corresponding grid in the tube 50 through a direct current path provided by a leak resistor 61 connected between the lead 65 and the lead 64.

In each of the leads 64 and 65 there is provided a current limiting resistor 68 adjacent in circuit to the control grids 54.

In the circuits shown, it has been found that the resistance of each half of the resistors 44 may be approximately 25,000 ohms, the resistor 61 may be substantially 2 megohms and the limiting resistors 68 may each have a resistance of 200,000 ohms or more. The stopping condenser 66 may be of any suitable capacity having a relatively low impedance at audio frequency, such as .25 mfd., for example.

Signals from any suitable source, such as an antenna circuit 69, are applied to the input. circuit 4| by suitable coupling therewith as indicated by thetransformer 1!), and the modulation is indicated by the lamps 62 and 63, the lamp 62 at full brilliancy indicating percent up-modulation and the lamp 63 at full brilliancy indicated at 100 percent down-modulation.

The tubes 50 and 5| require low bias potentials for plate current cut-off, so that a normal signal potential of approximately 100 volts across proaches zero, at which point plate current may flow through the neon lamp 63, which indicates that the modulated signal has reached substantially zero instantaneous value.

It will be noted that at the instant the audio frequency signal at the terminal 43 reaches an upward value equal to thedirect current or recti- -fied signal potential at the terminal 42, the inv stantaneous voltage on the grid of tube 50 will be zero by virtue of the audio frequency coupling condenser 66 which efiectively maintains the terminal 43 and the lead 64 at the same audio frequency potential. At the instant the bias potential of the tube 50 is essentially zero the neon lamp 62 will glow, indicating 100% upward modulation, if the resistor 44 is grounded at its center During modulation, at 100% point. An upward modulation indication may be derived by connecting the lead 64 through the condenser '56 for any percentage modulation, to a corresponding position along the resistor 44 in the section connected with the terminal 43, for

percentages above 100%, as will be seen hereinafter.

For the indication of upward and downward modulation lower than 100% the audio frequency or modulation signal may be applied to the output circuit for connection with the balanced amplifier or to other output grid leads similar to leads 64 and 65, indicated at 10 and H, each of which is connected with one of the output terminals of the output resistor 44 through suitable coupling condensers 12. The percentage modulation below 100% at which the tubes are to provide full glow of the neon lamps, is adjusted by adjusting the grid bias connection along the re- 0 sistor Mi to the proper point, and is the percentage of the resistance section between the terminals 42 and 45, corresponding to the desired percentage modulation.

In the present example, the grid bias connection from each of the leads l0 and H is made through grid leak resistors 13 and 14, respectively, to a common connection 15 tapped as indicated at 6 with the mid-point of the section of the resistor 04 between the terminals 42 and 45 39 for 50% up and down modulation indication, the positive half wave of the carrier being modulated down toward zero amplitude and the negative half wave being modulated up toward zero amplitude. This is for the reason that the full signal output 35 is available on the leads 10 and H with only half the signal bias. For other percentage modulation indication, the tap '16 may be adjusted along the resistor to other points.

For modulation indications over 100% upward, 0 a reduced signal component may be derived from the resistor 44 as indicated by the tap connection Ti connected with an output lead 18 through a suitable coupling condenser 19. Full biasing potential is derived from the terminal 42 through a grid leak resistor 80. With this connection and the tap 71 at substantially the mid-point of the resistor section between the terminals 43 and 45, 200% upward modulation is required to cause full glow of an indicating device similar to that shown in connection with the leads 64 and 65. It will, therefore, be seen that by suitably tapping the balanced output resistor 44, the ratio of signal voltage, that is modulation or audio frequency voltage, to the direct current component thereof, may be adjusted to provide indication of any percentage modulation desired for full glow of the indicating means.

Fromthe foregoing description it will be seen that the modulation indicator comprises a single detector or rectifier device for supplying biasing potentials and rectified or demodulated signals to a pair of balanced amplifier devices, directly in connection with a balanced or push-pull output circuit in the detector circuit, and indicating means in the amplifier output circuit.

Furthermore, while providing a balanced or push-pull signal output, the rectifier circuit at the same time, provides diode biasing potentials for both of the amplifier devices, the biasing potentials varying in accordance with the signal strength.

It is obvious that the rectified signal component or direct current component of the signal flowing in the resistor M may be utilized to supply potentials for other purposes, as desired, without interfering with the operation as a balanced output means, through a suitable tap connection therewith as indicated at 8!, for automatic volume control supply, for example.

I claim as my invention:

In a radio receiving system, the combination with a single diode rectifier, of a balanced output amplifier comprising a pair of electric discharge amplifier devices having a balanced output circuit, and means providing a balanced input circuit therefor in connection with said rectifier, said last named means including an output resistor in circuit with the rectifier and having connection therewith whereby the terminal ends thereof receive rectified signal voltages which are substantially 180 out of phase with each other, a mid-tap connection for said resistor, connections between said amplifier devices and said resistor on opposite sides of said mid-tap, and means in said connections for supplying a biasing potential to said amplifier devices from one side of said mid-tap.

LOY E. BARTON.

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