US2331708A

US2331708A - Resistance network for amplifiers

Info

Publication number: US2331708A
Application number: US442298A
Authority: US
Inventors: John E Maynard
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1942-05-09
Filing date: 1942-05-09
Publication date: 1943-10-12
Anticipated expiration: 1960-10-12

Description

Oct. 12, 1943.

J. E. MAYNARD 2,331,708

RESISTANCE NETWORK FOR AMPLIFIERS Filed Ma 9, 1942 ITfiveTTt-or: John E. Maynard,

3 WW6 Kim/Z 1 His Abtorne g.

speaker 3.

Patented Get. 12, 1943 2,331,708 RESISTANCE NETWORK FOR AMPLIFIERS John E. Maynard, Schenectady, N. Y, assignor to General Electric Company, a corporation of New York Application Mayfi, 1942, Serial No. 442,298

5 Claims.

My invention relates to audio amplifiers and particularly to such amplifiers in which a portion of the output voltage is supplied to the input circuit of the amplifier to improve the operation thereof.

One object of my invention is to provide manual volume control means connected in the input of such an amplifier in such a way that its action does not affect the action of such feed-back voltage supply means. v

In amplifiers used in monitoring equipment for broadcast studios, for example, it frequently happens that the output impedance of the amplifier is adjusted to desired values by properly proportioning the feed-back voltages. For example, if two components of feed-back voltage be supplied to the input of the amplifier. one in degenerative phase and varying as the output voltage, and the other in regenerative phase and varying as the output current, the amplifier output impedance may be made to appear as zero if these components are properly related in magnitude. Similarly, by properly proportioning these components and adjusting their polarities, the amplifier output impedance may be made to appear large if desired. Once the proper proportion is established, however, it is desirable that it be maintained constant. Hence it is another object of my invention to provide means whereby adjustment of the manual volume control means will not disturb the proportion of the components of the feed-back voltage.

The features of my invention which I believe to be novel are set forth with particularity in the single figure represents an embodiment of my invention.

Referring to the drawing, I have illustrated therein a portion of the audio frequency circuits of a radio receiver to which my invention is applied. This circuit comprises an audio frequency amplifier l which amplifies the reproduced audio signal and supplies it to a subsequent audio power amplifier 2, whose output. in turn, is coupled to output means, for example, the loud A signal voltage supplied across the terminals 3 is impressed .upon the control electrode 5 of the amplifier I. The amplifier i is shown as a triode'having in addition to the control electrode 5, an anode 6 coupled to the cathode l by meansof resistor 8 and bypass capacitor 9; however, any other suitable type of discharge device may be used. The output of the anode 6 of amplifier i is coupled to a first control electrode H] of power amplifier 2 by means of capacitor H and a volume control potentiometer 12. This potentiometer comprises two equal branch resistors i3 and I i and a contact bridging member I5. The audio power amplifier 2 may be of the pentode type having an anode l6, a cathode I1, and control electrodes II], I 8, and I9. Control electrode 18 may be coupled to the cathode, as by means of bypass capacitor 2!, and connected to a source of potential by means of resistor 20 to act as the usual screen grid. Control electrode l9 may be directly connected to cathode H in a well-known manner to act as the usual suppressor grid. The output of the anode of power amplifier 2 is supplied through an audio transformer 22 to I Potential for the anode of the the loud speaker. discharge device 2 is furnished by battery 23 through the primary winding of transformer 22.

In order to improve the frequency response characteristics of the audio circuit described, a combination of regenerative and degenerative feed-backs is employed. As shown in the drawing, a feed-back voltage in degenerative phase and proportional to the audio output voltage supplied to the loud speaker 3 is developed across resistor 26 connected acrossthe secondary winding of transformer 22. Another feed-back voltage, in regenerative phase and proportional to output current, is developed across the resistor 25 in series with the secondary winding of transformer 22. The voltages across those portions of these resistors between the tap 26 and ground are supplied to the control electrode in of the audio power amplifier through a feed-back network including the capacitor 2.! and a high resistance potentiometer having an upper section 30 and a lower section 3i. If these components are properly related in magnitude and polarity, the amplifier output impedance may be made to appear as possessing a desired value, either large or small. Thus, by properly adjusting the values of

resistors

24 and 25 and the polarity of the voltages applied to these resistors in a well-known manner, this output impedance may be made to appear as zero.

if desired, or may be made to appear as having any larger value. This characteristic is of particular advantage where, instead of being supplied to a loud speaker as illustrated in the drawing, the output of the amplifier is supplied to some other type of output means, such as a filter or line and cathode ll, which is presented to the feed back voltages developed in the output circuit, be maintained substantially constant. Such a constant impedance is obtained in my system by means of the volume control potentiometer l2. With respect to alternating currents, the discharge device 2 has its cathode connected to one terminal of resistance 13 and its control electrode l connected through the variable resista ance H to a variable point on resistance 13. The connection between the variable resistance I4 and the variable point on resistance I 3 is effected by the bridging member l5. Since the two branch resistances l3 and I4 have substantially equal ohmic values per unit of length, as the contact bridging member I5 is varied, a desired fraction of the audio voltage from the audio amplifier I is impressed upon control electrode I0 without varying the impedance between electrode I0 and ground. Thus, as the bridging member l5 removes a part of resistance [3 from the circuit between control electrode l0 and cathode II, it inserts into that circuit a part of resistance 14, equal in ohmic value thereto. Consequently the performance of the feed-back network is not altered by adjustment of potentiometer l2.

While I have shown a particular emodiment of my invention, it will of course be understood that I do not wish to be limited thereto since various modifications may be made, and I contemplate by the appended claims to cover any 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. In combination, a source of signals, an electron discharge device having an anode, a cathode, and a control electrode, an input circuit connected between said control electrode and cathode,- an output circuit connected between said anode and cathode, means for impressing signals from said source between said control electrode and cathode in adjustable intensity, means connected in parallel with said first means for impressing a feed-back voltage derived from said output circuit between said control electrode and cathode, and means for maintaining the impedance between said control electrode and cathode presented to said feed-back voltage substantially constant as said first means is adjusted to vary the intensity of said signals.

2. The combination in a radio receiver, of an audio amplifier having an anode, a cathode, and a control electrode, an input circuit connected between said control electrode and cathode including a source of signal voltage to be amplified, an output circuit connected between said anode and cathode, a transformer in said output circuit having primary and secondary windings, means connected in parallel with said input circuit for impressing a feed-back voltage derived from the secondary winding of saic transiormer between said control electrocir and .l.7.-

ode, and means for adjusting the intensity of signal voltage amplifier by said amplifier, said means comprising a resistance connected across said source, the cathode of said discharge device being connected to a terminal of said resistance and said control electrode being connected through a variable resistance to a variable point on said resistance, and means to increase said variable resistance as said variable point is varied toward said terminal thereby to maintain constant impedance between said control electrode and cathode.

3. The combination with a source of signals, of an electron discharge device having an anode, a cathode, and acontrol electrode, an input circuit connected between said control electrode and cathode, an output circuit connected between said anode and cathode, means for impressing signals from said source between said control electrode and cathode, feed-back means connected in parallel with said first means for impressing a voltage derived from said output circuit between said control electrode and cathode, and means for adjusting the intensity of signals impressed on said control electrode, said means comprising a resistance connected across said source, the cathode of said discharge device being connected to a terminal of said resistance and said control electrode being connected through a variable resistance to a variable point on said resistance, and means to increase said variable resistance as said variable point is varied toward said terminal thereby to maintain constant impedance between said control electrode and cathode.

4. In combination, a source of signals, an electron discharge device having an anode, a cathode, and a control electrode, an input circuit connected between said control electrode and cathode, an output circuit connected between said anode and cathode, means for impressing signals from said source between said control electrode and cathode in adjustable intensity, a resistance connected between said control electrode and said output circuit and a resistance connected between said cathode and said output circuit thereby to supply voltage derived from said out putput between said control electrode and cathode and in parallel with said signals, and means for maintaining the impedance between said control electrode and cathode presented to said voltage substantially constant as said first means is adjusted to vary the intensity of said signals.

5. In combination, a first electron discharge device having an input circuit and an output circuit for supplying signals, a second electron discharge device having an anode, a cathode, and a control electrode, an input circuit connected between said control electrode and cathode, an output circuit connected between said anode and cathode, means for impressing a feed-back voltage derived from said output circuit of said second discharge device between said control electrode and cathode in parallel with the input circuit of said device, a resistance connected in said output circuit of said first discharge device, said cathode being connected to a terminal of said resistance and said control electrode being connected through a variable resistance to a variable point on said resistance, and means to i rea e aid variable resistance as said variable point is rant-:1 n said terminal thereby to maintain constant impesance between said control electrode and cathode.

JOHN E. MAYNARD.