US2220770A - Apparatus for controlling the apparent resistance of an amplifier anode - Google Patents

Description

Nov. 5, 1940; H. F. MAYER 2,220,770

APPARATUS FOR CONTROLLING THE APPARENT RESISTANCE OF AN AMPLIFIER ANODE Filed Jan. 30, 1937 Inventor's Harry E Mayer,

Patented Nov. 5, 1940 UNITED STATES APPARATUS FOR CONTROLLING THE- AP- PARENT RESISTANCE OF AN AMPLIFIER AN ODE Harry F. Mayer, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application January 30,1937, Serial No. 123,213

3 Claims.

My invention relates to amplifier circuits which include an'electron discharge amplifier. In certain uses of such amplifier circuits it is desirable that the anode of the amplifier have a resistance which is materially different from its natural resistance; for example, in a radio receiver having an audio amplifier feeding a loudspeaker it is desirable that the resistance of the amplifier anode be low in order to provide the proper damping for the loudspeaker. In other cases it is desirable that the anode resistance shall be high, for example, in tuned high gain amplifiers where a low anode resistance is undesirable because it decreases the gain and the selectivity. Amplifiers with anodes having the desired low and high resistances are difiicult and expensive to construct if, indeed, they can be produced. 1 g

It is the object of my invention, therefore, to provide improved apparatus which is connected with the amplifier circuit for causing the amplifier anode, while the circuit is in use, to have an apparent resistance which is different from its actual resistance.

In accordance with my invention I cause the amplifier anode to have an apparent low or high resistance by providing circuit apparatus which functions to supply feedback voltages from the output circuit of the amplifier to the input circuit thereof, the value of one voltage being controlled by the output current and the value of another voltage being controlled by the output voltage. Where a low apparent resistance of the amplifier anode is desired, I make the current controlled feed-back voltage regenerative and,

the scope of the invention being pointed out in' the appended claims.

Referring to the drawing, Figs. 1, 2, and 3 are circuit diagrams illustrating various embodiments of my invention wherein the amplifier anode is caused to have an apparent low resistance; Fig. 4 is a circuit diagram illustrating another form of my invention wherein the amplifier is caused to have'a high apparent resistance; and Fig. 5 shows a modified detail of Fig. 4.

In'Fig. 1 where I have chosen to show my invention forming a part of a radio receiving sysv tem of well known form, I is the receiving an- 5 tenna, 2 is the radio frequency amplifier stage, 3 is the detector stage, 4 is the first audio amplifier stage, 5 is the last or power amplifier stage, and 6 is the loudspeaker. The power amplifier stage 5 includes the electron discharge power amplifier i which preferably is a pentode having the cathode 8,- the control grid 9, the screen grid 10, the suppressor grid H, and the anode l2. The input circuit of this amplifier connects with the audio amplifier stage A through the transformer M and the output circuit of this amplifier connects with the loudspeaker through the load transformer !5.

For the purpose of giving the anode 12 an apparent low resistance in order better to damp the loudspeaker, I have provided the voltage divider or potentiometer l'l across the anodecathode circuit of the amplifier I. This potentiometer preferably comprises the resistor l8 which may have a resistance, for example, of 10,000 ohms and the resistor 19 which for example may have a resistance of 2,000 ohms which latter resistor is engaged by the movable arm across this rheostat, which it will be understood is substantially in phase with the current in the anode-cathode circuit'and is proportional thereto, is applied to the primary of the step-up transformer 23, the secondary of which is connected in a series circuit between the potentiometer arm 20 and the control grid 9 of the amplifier 1 through the secondary of the transformer M. The secondary of the transformer 23 is so connected in the series circuit that at the instant at which the cathode end of the rheostat the transformer 22 and which is controlled by5 and is proportional to the anode-cathode current is a regenerative voltage; It Will be seen that the feed-back'voltage which is supplied from the potentiometer 1'! through the secondary winding of the transformer 23'rto the input cir-.-

cuit of the amplifier 7 is a degenerative voltage. With the apparatus disclosed in Fig. 1, therefore, the apparent resistance of the anode i2 is made less than the actual resistance by having 5 a regenerative feed-back voltage which is proportional to the current in the output circuit of the amplifier applied to the input circuit thereof and by having a degenerative voltage proportional to the output voltage of the amplifier 10 simultaneously applied to the input circuit thereof By the means which .I have described above the anode of the amplifier may be made to have as low a resistance as desired without loss of power sensitivity or increased harmonic distor- 15 tion. The apparent anode resistance may be reduced to zero or may even be made negative.

In the modified form of my invention illustrated by Fig. 2, I obtain correctly phased current-regeneration voltage by the use of a single 20 electron discharge amplifier which I use as a resistance coupled amplifier in lieu of the transformer employed in Fig. 1. This modified construction is of particular value with frequencies of the order of. 100 down to 40 cycles where con- 25 siderable phase shift occurs even with the best available transformers. In Fig. 2 I obtain the degenerative feed-back voltage by the same means as shown and described in connection with Fig. 1, namely the potentiometer H con- 30 nected across the output circuit of the amplifier. The regenerative feed-back voltage which is proportionalto the current in the output circuit of the amplifier is obtained by connecting the input circuit of the electron discharge amplifier 35 25 across the resistor 21 and connecting the output circuit of the amplifier 26, which includes the arm 25) of the potentiometer, with the input circuit of the amplifier l, a suitable blocking capacitor 28 being employed [to keep the direct current off the grid of that amplifier. In this figure the amplifier 26 is so shown that the phase of the current controlled feed-back voltage is properly phased to be regenerative when applied to the grid of the amplifier In the modification illustrated by Fig. 3 I have shown how the preceding audio amplifier stage may be employed in lieu of the separate amplifier 26 to reverse the phase of the current responsive feed-back voltage. In this case the po- ,50 tential drop across the resistor 39 in the anodecathode circuit is applied to the input circuit of the amplifier 3| comprising the preceding audio amplifier stage, the output circuit of which amplifier connects in the usual manner through 55 the capacitor 32 with the input circuit of the amplifier l. A voltage responsive degenerative feed-back voltage in this case is obtained by the voltage divider or potentiometer 33 which connects across the primary of the load transformer 0 I5. As will be seen from Fig. 3, the potentiometer arm 34 connects through the resistor 35 constituting the principal load on the amplifier 3| with the input circuit of the amplifier 1. In certain cases it may be found desirable to shunt the .65 resistor 30 with a capacitor or with a capacitor in series with a resistor in order to compensate for the internal impedance of the power supply, as

is obvious.

In the modification illustrated by Fig. 4 I have shown how the amplifier anode may be made to have an apparent resistance which is greater than the actual resistance thereof. This is done by supplying to the input circuit of the amplifier simultaneously a degenerative feed-back voltage .75 which is proportional to the current in the output circuit of the amplifier and a regenerative feed-back voltage which is proportional to the voltage of the output circuit of the amplifier. The amplifier in this case is illustrated as comprising a stage of intermediate frequency amplification and is represented at 7. The radio receiving system illustrated by Fig. 4 includes the converter stage 38, the intermediate frequency transformer 39 connecting stage 38 with the amplifier 1, the intermediate frequency transformer 40 in the output circuit of amplifier l, the second intermediate frequency amplifier stage 4|, the

second detector stage 42, the first audio amplifier stage 43, the second audio amplifier stage 44, and the loud-speaker 6. The transformer 40 is provided with the auxiliary or tertiary winding 45 which is wound in the same direction as, ooaxially with, and close to the primary winding of this transformer. Winding 45 connects in series with the cathode resistor 46 between ground and the cathode of the amplifier I. The suppressor grid connects with the ground through the by-pass capacitor 477 and with the cathode through the high resistor 48. In this form of my invention a degenerative voltage, which is the voltage drop across the resistor 45, is supplied to the input circuit of the amplifier I, which voltage is proportional to the current in the output circuit of the amplifier. Also, a regenerative voltage is applied to the input circuit of the amplifier I, which voltage is induced in the winding 45 and which is proportional to the alternating voltage applied to the primary of the transformer 4!). In that form of my invention shown by Fig. 4, therefore, the anode of the amplifier is caused to have an apparent high resistance as it is seen from the load by supplying to the input circuit of the amplifier a degenerative feed-back voltage which is proportional to the current in the output circuit of the amplifier, and a regenerative 40 voltage which is proportional to the voltage of the output circuit thereof. The modification illustrated by Fig. 4 is disclosed and claimed in my divisional application Serial No. 316,953, February 2, 1940, entitled Apparatus for controlling the apparent resistance of an amplifier anode.

In certain cases it may be desirable to reduce the amount of direct current cathode bias produced by the resistor 46 of Fig. 4. This may be done in the manner shown by Fig. 5 where the resistor 46 is replaced by two separate resistors 48 and 49 and the latter is connected across a resonant circuit comprising the capacitor 5i) and the reactor 5|. This circuit being resonant at signal frequency offers a high impedance to the signal frequency in the cathode circuit, but the reactor element 5| thereof readily passes direct current.

I have chosen the particular embodiment described above as illustrative of my invention and it will be apparent that various other modifications may be made without departing from the spirit and scope of my invention which modifications I aim to cover by the appended claims.

What I claim as new and desire to secure by 6 Letters Patent of the United States, is:

1. In combination with an amplifier circuit including a first and a second electron discharge amplifier connected together in cascade for the transmission of signals and said second amplifier having an anode, feed-back apparatus for causing said anode to have a lower apparent resistance comprising means for supplying a degenerative voltage to the input circuit of said second amplifier controlled by the voltage of the '15 output circuit thereof and means for supplying regenerative voltage to the input circuit of said first amplifier controlled by the current in the output circuit of said second amplifier.

2. In combination with an amplifier circuit including a first and a second electron discharge amplifier connected together in cascade for the transmission of signals and said second amplifier having an anode, feed-back apparatus for causing said anode to have a lower apparent resistance comprising a load device in the output circuit of said second amplifier, a resistor across said device, means for supplying the potential drop across a portion of said resistor degeneratively to the input circuit of said second amplifier and means for supplying a regenerative voltage to the input circuit of said first amplifier in response to the current in the output circuit of said second amplifier.

3. In combination with an amplifiercircuit including a first and a second electron discharge amplifier connected together in cascade for the transmission of signals and said second amplifier having an anode, feed-back apparatus for causing said anode to have a lower apparent resistance comprising a load device in the anode circuit of said second amplifier, a potentiometer in shunt with said device and having its arm connected to supply a degenerative voltage to the input circuit of said second amplifier, a resistorin the cathode circuit of said second amplifier and means for supplying the voltage drop in said resistor as a regenerative voltage to the input circuit of said first amplifier.

HARRY F. MAYER.

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