US2302866A - Amplifier circuits - Google Patents

Description

Nov. 24, 1942. s. HUNT 2,302,866

AMPLIFIER CIRCUITS Filed Dec. 1, 1939 I I0 701. a) T E L 210 sou/20s 2 0577' p r0 PRIOR SIGNAL GRIDS) $7,552

E LIIII A VC RECTIFIER :Z 60 T0 6/2/05 OF PRIOR TUBES INVENTOR.

SEYMOUR HUNT Patented Nov. 24, 1942 2,302,866 AMPLIFIER CIRCUITS Seymour Hunt, Jackson Heights, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application December 1, 1939, Serial No. 307,001

'7 Claims.

My present invention relates to signal amplifier circuits, and more particularly to amplifier tube circuits adapted to have signal voltage induced in electrode circuits by the action of the electrons within a tube.

In the past it has been disclosed that a socalled virtual cathode eifect can be produced within a tube provided with at least a cathode, negative input grid, a positive screen grid, a negative suppressor grid and a positive output electrode. The virtual cathode efi'ect appears to exist between the screen grid and the suppressor rid. The efiect may be attributed to the slowing up of the electrons, emitted by the cathode and attracted by the screen grid, by the suppressor grid. The density of the virtual cathode varies with the amplitude of the signal voltage applied to the signal grid. The variable virtual cathode, in effect, acts like a source of electrons whose density varies at the signal frequency. The positive output electrode draws these electrons, and develops a signal output voltage in its output impedance.

Now I have discovered. and made use of, an induction phenomenon which occurs within the tube, and specifically at the suppressor grid. If an impedance is inserted in the negative suppressor grid lead, it is found that signal voltage is developed across the impedance. This voltage is produced by virtue of currents induced in the suppressor grid by the virtual cathode of varying intensity.

Accordingly, it can be stated to be one of the main objects of my present invention to provide a method of, and means for, deriving signal voltage from an amplifier, the method including establishing a virtual cathode effect within the amplifier, and inducing signal currents in an electrode located between the virtual cathode and output electrode, the desired signal voltage being developed by a load impedance in the path of the induced currents.

Another important object of this invention is to insert in the negative suppressor grid lead of a pentode amplifier a resonant circuit tuned to a desired signal frequency, and signal voltage being derived from the resonant circuit by virtue of signal currents induced in the suppressor grid by the electrons flowing to the amplifier output electrode.

Another object of this invention is to provide a signal transmission tube having in the order named a cathode, a signal grid, a positive screen grid, a negative suppressor grid and a positive plate, a signal output circuit being coupled to the plate, said suppressor grid being free of any external coupling to the input or output circuits, and a signal-tuned circuit being included in the suppressor grid circuit to develop signal voltage from signal currents induced in the suppressor grid by the fiow of electrons to the plate.

Still another object of the invention is to derive signal voltage from a signal amplifier tube having input and output electrodes by inducing signal currents in an auxiliary electrode located in the electron stream to the output electrode, and the auxiliary electrode being free of any coupling to the other electrodes externally of the tube.

Still other objects of my invention are to improve generally the efficiency of signal amplifier circuits, and more especially to provide a signal amplifier capable of simultaneous use as a source of amplified automatic gain control voltage.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawing Fig. 1 shows an amplifier with means for obtaining a control voltage from the suppressor grid circuit; Fig. 2 shows switching means in the suppressor grid circuit for varying the selectivity of the amplifier; Fig. 3 shows a modification of Fig. 2; and Fig. 4 shows how automatic selectivity control may be obtained with the present invention.

Referring now to the accompanying drawing, the invention is illustrated by embodiment in the intermediate frequency, (I. F.) amplifier oi. a superheterodyne receiver. The amplifier may consist of a tube I, as for example a pentode tube of the 6K? type, having a cathode 2, signal grid 3, screen grid 4, suppressor grid 5 and output plate, or anode, electrode 6. The cathode is connected to ground by the usual self-bias resistor l, the latter being bypassed for signal currents.

The signal grid 3 is connected to the high potential side of the intermediate frequency input circuit 8, while the low potential side of the input circuit is returned to ground through the condenser 9'. The numeral l0 denotes a resonant network, also tuned to the intermediate Irequency, which is to be understood as being coupled to any source of intermediate frequency voltage. Those skilled in the art know that superheterodyne receivers usually embody a signal collector which feeds signals to one or more tunable radio frequency amplifiers. then applies the amplified signals to a first detector tube,-and upon the detector tube are also impressed locally reproduced oscillations derived from a tunable local oscillator. The circuit It! may be located between output electrodes of the first detector, or it may be the output circuit of a prior intermediate frequency amplifier. Assuming that the receiver is of the broadcast type it can be varied over a range of 550 to 1500 kilocycles; the intermediate frequency may be chosen from a range of 75 to 450 kilocycles, and specifically it is preferred to use the 450 kilocycle value.

The numeral H designates the resonant output circuit of the amplifier, and this circuit is similarly tuned to the operating intermediate frequency. The plate 6 is connected through the coil of the output circuit II to a source of positive potential. The intermediate frequencytuned circuit I2 derives the amplified intermediate frequency voltage from circuit II, and impresses the amplified voltage upon a second detector. The demodulated voltage is then applied to one or more audio amplifiers, and finally the amplified voltage is reproduced in any desired type of loudspeaker. There is applied to the screen grid 4 a positive potential of the order of 100 volts, the screen grid lead being bypassed to ground for signal currents by the condenser 13. The suppressor grid 5 is connected to ground through a path which includes the lead l4 and a resonant circuit, the latter circuit comprising coil l5 shunted by condenser IS. The circuit l5--l6 is tuned to the operating intermediate frequency of the amplifier network.

As explained previously, the effect of the negative suppressor grid 5 is to create a virtual cathode effect between the screen grid 4 and the grid 5. Since the grid 5 is returned to ground it is at a negative potential with respect to screen 4. The intensity of the virtual cathode is varied because of the variable signal voltage applied to the signal grid 3. In other words, the density of the virtual cathode between screen grid 4 and suppressorgrid 5 will vary at the frequency of the signal voltage frequency. This means that electrons will fiow to the plate 6 through suppressor 5, and from the variable density virtual cathode. The passage of these electrons induces in the suppressor grid circuit currents of signal frequency, and since the signal frequency is the intermediate frequency there is caused to flow through the suppressor grid circuit intermediate frequency currents. These currents produce across the resonant circuit l5--l6 an intermediate frequency voltage which can be utilized in any desired manner. This voltage is of substantial amplitude, and is amplified with respect to the amplitude of the intermediate frequency voltage applied to signal grid 3. For a given input'signal voltage, say 1 volt, at grid 3, and with coils of like "Q" in the plate and suppressor circuits, the voltage developed across the suppressor coil will be about 80% that developed across the plate coil. If a 1 volt signal were applied to grid 3 and volts were developed across the plate coil, then about8 volts would be developed across the suppressor coil l5. The Q of eachcoil; the Rp of the tube; bias on signal grid, each affect the relative magnitude of voltages across the coils.

The latter The electrical circuit and functions disclosed above may be put to many uses. One of the uses to which the signal voltage across circuit l5--I6 can be put to is for automatically controlling the volume of the receiver. Thus, the rectifier H, which may be of the diode type, has its anode l8 coupled to the high potential side of circuit l5-l6 by condenser l9, while cathode 20 thereof is grounded. The diode load resistor 2| is connected between anode l8 and ground. There is developed across resistor 2| a uni-directional voltage which varies directly in amplitude with the intermediate carrier amplitude. The voltage is used for controlling the bias of the signal grid 3 of amplifier tube I. For this purpose, the direct current voltage connection is provided between the low potential side of circuit 8 and the anode end of resistor 2|.

The lead 30 is designated by the letters "AVC to show that this is the automatic volume control lead. The filter netw0rk'3l prevents the transmission to the controlled tubes of pulsating voltage components. The action of the AVG connection is well known; and it is believed sufficient to point out that as the intermediate carrier intensity increases above the desired level, the voltage across l5l6 increases due to a greater flow of induced current in the suppressor grid circuit. The rectifier output voltage across resistor 2| then increases, and the gain of tube l is reduced to an extent sufficient to bring down the receiver volume to normal. The lead 30 can be connected to the signal grids of prior signal transmission tubes, if desired.

It will be observed that the grid 5 is free of reactive coupling to any of the electrodes exterthe rectified audio output of diode I'I could be amplified and then utilized.

Further uses of the circuit will be discussed with reference to Fig. 2. The circuit shown is that associated with tube I, except that the input circuit l0, output circuit l2 and diode load circuit have been omitted. It is to be noted that the signal voltage applied to grid 3 causes the plate current to vary. Current is high in the suppressor grid circuit when plate voltage is low, and vice versa. If l8--l5 and l' are tuned to the frequency of input circuit 8 oscillation takes place between the suppressor and plate provided the bias on grid 8 is of .the order of zero to 4 or 5 volts. No external coupling is provided between circuit l and circuit |5-l6. Oscillation voltage could then be taken oil from either, or both, of circuits l8-I5 and I.

At grid biases greater than that causing oscillations, say 5 to -8 volts, the oscillations cease and regeneration takes place. Hence, in that negative bias range the circuit acts as a regenerative amplifier. At biases above -8 volts regenerarion ceases, a d the circuit behaves as a normal amplifier. The arrow through resistor 1 denotes an adjustable bias source.

In the regenerative, non-oscillation state brought about by tuning the suppressor load |5-l5 just above resonance it is found the selectivity and gain of the plate circuit I is considerably enhanced. The selectivity is several 'times sharper, and the gain about doubled. Circuits I6-I5 and I' may have identical Q5. The resulting circuit is very stable, and may be employed to advantage in amateur and com munication type receivers as an intermediate frequency amplifier.

If the grid bias is such that the circuit is not regenerating it is found that I6-I5 puts a dip in the resonance curve of circuit I, In other words, with circuit I6-I5 omitted in Fig. 2, and izcuit I tuned to the desired frequency, the circuit I will have a. single-peaked curve. The resonance curve will broaden out to a doublepeaked curve when circuit I6--I5, tuned to the frequency of /circuits 8 and I, is inserted in the suppressor lead. Hence, assuming the circuit in Fig. 2 to be an I. F. amplifier, a simple band width control can be provided by connecting a switch element 40 in shunt across the circuit Iii-l5. circuit'is sharp; the amplifier is broad when the switch is opened.

Fig. 3 shows a modified form of band width control mechanism wherein the secondary circuit I" of output transformer 4| maybe used as the suppressor tuned circuit. In this case when the transformer 4| is properly phased, and with switch contact element 42 on contact tap 43, the amplifier resonance curve is broad since circuit I is then functioning as the suppressor resonant circuit. Conversely, when element 42 contacts tap M the amplifier is sharp.

Automatic selectivity control is readily provided by the present invention. Fig. 4 shows such a selectivity control pressor circuit 50 is tuned to a frequency just above resonance. say 470 kilocycles (kc) when the I. F. circuits 8. I, I" are each tuned to the operating I. F. of 460 kc. The AVC rectifier is the usual signal rectifier, and delivers a direct -1 current voltage over AVC lead 6| to the signal grid 3 of amplifier tube I. The grid bias applied over lead 6| becomes increasingly more negative as the carrier amplitude at the signal collector (not shown) increases. The bias over lead 6| can be applied to the signal grids of earlier tubes concurrently. Those skilled in the radio art are fully aware of the manner of constructing such AVC circuits. The suppressor circuit 50 acts to aid the AVG action by virtue of the following functions. When the bias applied over lead 6| to grid 3 is small (weak signal reception) the gain of tube I is high, and the circuit of tube I is regenerative thereby enhancing the selectivity and gain of circuit I.

As the signal carrier amplitude increases the AVG bias increases and causes regeneration to cease. This reduces the gain and selectivity at circuit I. Finally, for very high carrier amplitudes, such as local reception, very high negative biases are applied to the signal grids. Not only would the gain of tube I be greatly reduced, but circuit 50 would absorb energy and would pro duce a dip in the widened amplifier resonance curve. This is the desired condition of sensitivity and selectivity for strong signal reception. The dip in the resonance curve could be smoothed out by a resistor across the coil of circuit 50, or by using a coil of low Q in circuit 50. This provides-a simple and most economical automatic selectivity circuit.

The suppressor grid current may be increased by the use of a slight positive potential on the suppressor. For example, a voltage of the order of zero to volts may be applied to the sup- When switch is closed the amplifier circuit. The 'suppressor grid. Further, in the circuit of Fig. the plate circuit II may be safeguarded from loading by the diode across the suppressor tuned circuit I6-I5. This is readily accomplished by biasing grid 3 so that tube I is regenerative,- when the diode II loads the tuned circuit it may bring the regeneration to non-regeneration.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the part cular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In a signal amplifier circuit of the type including a tube provided with at least a cathode, an output electrode, and a signal grid and positive screen grid arranged in succession in the electron stream from cathode to output electrode, a signal input circuit connected to the signal grid, a signal output circuit connected to said output electrode, means for applying a positive potential to the output electrode, an auxiilary electrode in said tube adjacent said screen grid and output electrode; means establishing the auxiliary electrode at a negative potential relative to cathode thereby to produce a virtual cathode at'a point between the screen grid and output electrode, an impedance in circuit with the auxiliary electrode for developing voltage of signal frequency from signal currents induced in said auxiliary electrode by electrons flowing from the virtual cathode to the output electrode and gain control rectifier means, responsive to said induced signal voltage, coupled solely to said impedance.

2. In a signal amplifier circuit of the typecincluding a tube provided with at least a cathode, an output electrode, and a signal grid and positive screen grid arranged in succession in the electron stream from cathode to output electrode, a signal input circuit connected to the signal grid, means for deriving amplified signal voltage from said output electrode, means for applying a positive potential to the output electrode, an auxiliary electrode to said tube intermediate said screen grid and output electrode, means establishing the auxiliary electrode at a negative potential relative to cathode thereby to produce a virtual cathode at a point between the screen grid and output electrode, an impedance in circuit with the auxiliary electrode for developing voltage of signal frequency from signal currents induced in said auxiliary electrode by electrons flowing from the virtual cathode to the output electrode means responsive solely to signal voltage across aid impedance for regulating the amplifier gain.

3. In a signal amplifier circuit of the type including a tube provided with at least a cathode, an output electrode, and a signal grid and positive screen grid arranged in succession in the electron stream from cathode to output electrode, a signal input circuit connected to the signal grid, means for applying a positive potential to the output electrode, an auxiliary electrode in said tube adjacent said screen grid and output electrode, means establishing the auxiliary electrode at a negative potential relative to cathode thereby to produce a virtual cathode at a point between the screen grid and output electrode, an impedance in circuit with the auxiliary electrode for developing voltage of signal frequency from signal currents induced in said auxiliary electrode by electrons flowing from the virtual cathode to the output electrode, means connected solely across said impedance for rectifying the developed signal voltage and controlling the amplifier gain.

4. In a signal amplifier circuit of the type including a tube provided with at least a cathode, an output electrode, and a signal grid and positive screen grid arranged in succession in the electron stream from cathode to output electrode, a signal input circuit connected to the signal grid, means for applying a positive potential to the output electrode, an auxiliary electrode in said tube adjacent said screen grid and output electrode, means establishing the auxiliary electrode at a negative potential relative to cathode thereby to produce a virtual cathode at a point between the screen grid and output electrode, an"

impedance in circuit with the auxiliary electrode for developing voltage of signal frequency from signal currents ,induced in said auxiliary electrode by electrons flowing from the virtual cathode to the output electrode, said impedance consisting of a signal-tuned resonant circuit and means connected across solely said last resonant circuit for controlling the amplifier gain.

5. In a signal amplifier circuit of the type including a tube provided with at least a cathode, an output electrode, and a signal grid and positive screen grid arranged in succession in the electron stream from cathode to output electrode, a signal input circuit connected to the signal grid, means for applying a positive potential to the output electrode, an auxiliary electrode in said tube adjacent said screen grid and output electrode, an impedance in circuit with the auxiliary electrode for developing voltage of signal frequency from signal currents induced in said auxiliary electrode by electrons flowing from, the cathode to the output electrode, said impedance being a resonant circuit tuned to the frequency of said input circuit, and rectifier means coupled solely to the latter to derive a control voltage for varying the amplifier gain.

6. In a high frequency circuit, an electron discharge tube provided with a cathode, a positive anode and at least two control gridsarranged successively in the electron stream to the anode, means for establishing each of said grids at negative direct current voltages relative to the cathode, a high frequency input circuit connected between the cathode and the grid adjacent thereto, an output circuit, tuned to the input circuit frequency, connected to the anode, an auxiliary resonant circuit, tuned to the input circuit frequency, connected to the second grid adjacent the anode, and each of said output and auxiliary circuits having currents of said input frequency flowing therein, a rectifier coupled across said auxiliary resonant circuit to rectify high frequency currents flowing therethrough, and means responsive to the rectified currents for regulating the gain of said tube.

7. In a high frequency circuit, an electron discharge tube provided with a cathode, a positive anode and at least two control grids arranged successively in the electron stream to the anode, means for establishing each of said grids at negative direct current voltages relative to the oathode, a high frequency input circuit connected between the cathode and the grid adjacent thereto, an output circuit, tuned to the input circuit frequency, connected to the anode, an auxiliary resonant circuit, tuned to the input circuit frequency, connected to the second grid adjacent the anode, and each of said output and auxiliary circuits having currents of said input frequency flowing therein, and gain control rectifier means, responsive to high frequency currents flowing through solely said auxiliary circuit.

SEYMOUR HUNT.

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