US3651266A

US3651266A - An amplifying system with reduced feedback

Info

Publication number: US3651266A
Application number: US849016A
Authority: US
Inventors: Daniel L Queen
Original assignee: Chamberlain Manufacturing Corp
Current assignee: Chamberlain Manufacturing Corp
Priority date: 1969-08-11
Filing date: 1969-08-11
Publication date: 1972-03-21
Anticipated expiration: 1989-03-21

Abstract

An amplifying system, particularly for application in which the input and output transducers are closely coupled acoustically, for example, hand-held power megaphones and the like, as a result of which feedback howl may occur, in which an amplifier stage thereof is constructed for class B operation and is provided with a capacitor in the input circuit thereof, arranged to be charged by an input signal and apply a reverse bias to said input circuit operative, in the presence of a sustained feedback howl, to effect a reduction in the gain of such stage and the amplifier sufficient to cut off such howl.

Description

United States Patent Queen [4 1 Mar. 21, 1972 [541 AMPLIFYING SYSTEM WITH 5 a 3,536,836 10/1970 Pfeiffer ..179/1 vc REDUCED FEEDBACK 3,187,268 6/1965 Bauer et al... ..l79/l 3,324,406 6/1967 Fristoe ..330/l5 [72] Inventor: Daniel L. Queen, Chicago, Ill. 7 [73] Assignee: Chamberlain Manufacturing Corporation, 'f' Exami'fer xathleen Claffy Chicago, In. Assistant Examiner-Douglas W. Olms AttorneyHill, Sherman, Meroni, Gross & Simpson [22] Filed: Aug. 11, 1969 [21 Appl. No.: 849,016 [57] ABSTRACT An amplifying system, particularly for application in which the input and output transducers are closely coupled acoustically, (gl. ..l7 9/l FS, 179/ l k: for example, hand-held power megaphones and the like, as a l 58] Field Search 330/15 76 129 139 179 A result of which feedback howl may occur, in which an amplifi- 179 Fs 325/462 A 2 1. er stage thereof is constructed for class B operation and is provided with a capacitor in the input circuit thereof, arranged to be charged by an input signal and apply a reverse bias to said [56] References cued input circuit operative, in the presence of a sustained feed- UNITED ATES PATENTS back howl, to effect a reduction in the gain of such stage and the amplifier sufficient to cut off such howl. 3,531,728 9/1970 Visher ..330/l5 3,534,268 10/1970 Mazziota ..325/475 10 Claims, 4 Drawing Figures PATENTEUhMZI I972 3.651.266

flV/PdTT lNPl/ 7' INVEN'I UR jazgziglji pa e 7M w a WTTORNEYS AN AMPLIFYING SYSTEM WITH REDUCED FEEDBACK BACKGROUND OF THE INVENTION Speech amplification systems of the small, self-contained type, such as the hand-held, so called power megaphone have numerous advantages which have contributed to their increasing popularity. Among these are the small size of the complete self-contained unit, capable of being readily held in the hand and in effect forming a megaphone, the self-contained power supply eliminating connecting lines, etc., as well as the transport and operation of relatively large bulky amplifiers and the like. However, such amplifying systems, because of their compact size, have one undesirable characteristic. As the microphone associated with the amplifier input and the loudspeaker associated with the amplifier output are contained in a unitary housing, in relatively close proximity, a relatively short in-phase feedback oftentimes occur. This is particularly true with respect to a howlback which oftentimes takes place when no audio signal is being fed into the microphone. Obviously, the use of transistors in such an amplifier is desirable from the point of size, power consumption, etc., and as such types of speech reinforcing systems are normally used in locations of relatively large expanse as for example outdoors or in a relatively large enclosure, such amplifiers are normally of relatively high gain. As a result it is comparatively easy to have the amplifier gain set at a value which will result in an in-phase feedback factor greater than unity and the production of an undesired howl.

BRIEF SUMMARY OF THE INVENTION The present invention is directed to the production of an amplifier system, of the type referred to, which is so designed that upon application of a signal the gain of the amplifier will be automatically reduced by a desired predetermined amount, sufficient in the presence of an undesirable in-phase feedback have to eliminate the same.

This is accomplished in the example of the invention illustrated by the utilization of a class B transistor amplifier as one of the stages of the amplifying system, such stage being illustrated herein as a push-pull amplifier employing two transistors, which is biased for class B operation whereby the stage is biased in the exponential gain region near the origin of the transistor gain characteristic. Disposed in the input circuit is a suitable storage element such as a capacitor which is adapted to be charged when a signal is applied to such input circuit and which is operative to provide a reverse bias to such input circuit whereby the gain of the amplifier stage is thereby reduced.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, wherein like reference characters indicate like or corresponding parts, and wherein a preferred form of the invention is illustrated, which, as will be apparent therefrom and from the following description, may be capable of various immaterial variations and modifications without departing from the spirit and scope of the novel concepts of the disclosure:

FIG. 1 is a block diagram of an amplifying system of the type here involved;

FIG. 2 is a circuit diagram of a push-pull amplifier stage constructed in accordance with the present invention;

FIG. 3 is a graph illustrating the gain characteristic of the amplifying stage illustrated in FIG. 2; and

FIG. 4 is a graph illustrating the gain-time relation of the amplifier illustrated in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION As illustrated merely diagrammatically in FIG. 1, amplification systems of the general type here involved comprise an amplifier l to the input of which is connected a microphone 2 and to the output of which is connected a loudspeaker 3, the three components being suitably mounted in a housing 4 or the like, indicated in broken lines in FIG. 1, which forms a self-contained unitary structure which is sufficiently small in size that itmay be hand-held, as for example by means of a suitable handle 5. In the embodiment illustrated, the housing 4 is depicted as being generally conical in shape having the microphone 2 disposed at the small end thereof and the large end being open and communicating with the loudspeaker 3. Thus the user may hold the device by means of the handle 5 in the same manner as a megaphone, with the microphone 2 positioned to enable the user to speak directly into the same. The physical usage thereof thus is similar to that of the usual megaphone but enables a power amplification of the sound received at the input end thereof.

The amplifier 1 may consist of any desired number of amplifier stages, consistent with the requirements of the particular unit and FIG. 2 illustrates one such stage, that depicted being illustrated as being the output stageof the amplifier with its output connected to the speaker 3.

The amplifier stage of FIG. 2 is illustrated as being of the push-pull type having two transistors T1 and T2 disposed in a grounded-emitter type circuit with the respective bases of such transistors being connected to the ends of the center tapped secondary winding 6 of an input transformer 7 having a primary winding 8.

The emitters of the transistors T1 and T2 are connected together and adapted to be connected to the positive side of a battery 9 or other power source.

The collectors of the transistors T1 and T2, in the embodiment illustrated, are connected to the respective ends of the primary winding 10 of a transformer 11, illustrated as comprising an output transformer, the secondary winding 12 of which is operatively connected to the loudspeaker 3. Obviously, the amplifier stage illustrated in FIG. 2 may comprise the output'stage of the amplifier 1, as illustrated, or may comprise another stage of such amplifier. Likewise, if desirable, the input transformer could be omitted and the amplifying stage capacitively coupled with the preceding stage, in conjunction with a phase inverter, as is well known in the art. Also, if the amplifier stage is to be employed as an intermediate stage and coupled to the input of a following pushpull stage, the collectors of the transformers T1 and T2 may be capacitively coupled to the respective inputs of such following stage.

The transistors TI and T2 are provided with a bias voltage determined by resistances l3 and 14, the values of which are so selected that the respective transistors are biased for class 8 operation. Referring to FIG. 3 which illustrates the gain characteristics t, it will be noted that the gain initially increases rather slowly and then makes a relatively sharp increase forming a bend or knee in the characteristic curve and the bias of the amplifier stage thus is set in the exponential gain region near the origin of the transistor gain characteristic whereby the stage will have a characteristic curve corresponding substantially to that of FIG. 3.

Connected between the center tap of the winding 6 and the emitters of the transistors T1 and T2 is a capacitor 15. The latter thus is so disposed that in the presence of an input signal the capacitor will charge, whereby a reverse bias voltage results in the base circuits of the respective transistors T1 and T2, operative to effect a reduction in the gain of the amplifier. This is graphically illustrated in FIG. 4 which illustrates the du/dt slopeat the bend or knee of the curve illustrated in FIG. 3.

It will be apparent that in the operation of the circuit illustrated in FIG. 2 the amplifier stage will function as a class B amplifier whereby the collector current of the respective transistors will in the absence of an applied signal at the input, be approximately or close to zero; upon the appearance of a signal at the input, the amplifier gain initially will increase, in accordance will class B operation, and then as the capacitor 15 charges, the gain will decrease from its initial level to a somewhat lower level, as indicated in FIG. 4. While this will have no noticeable effect on the operation of the system with respect to the desired signals to be amplified, the gain reduction is sufficient, in the absence of an input signal, to cut off any sustained howlback as the gain will be reduced to a value sufficient to reduce the in-phase feedback factor necessary to sustain the howlback.

Theinvention thus enables the production of an amplifying system of the type here involved which will pennit an increase in the amplifier gain, before the creation of a howlback, beyond that heretofore possible.

It will be appreciated from the above description that the invention is not directed to an automatic level control which is intended to automatically maintain substantially uniform output level with varying input levels, but rather to provide an increase in the usable gain before a feedback howl will occur, and more particularly to effect an automatic cutoff of a sustained howlback.

l claim as my invention: 7

l. A threshold controlling circuit, particularly for use in a speech reinforcement system to reduce feedback howl in the absence of an audio signal, comprising an amplifier stage having an electronic amplifier device having a pair of electrodes including an output electrode, between which electrodes a current may flow, an input electrode cooperable with said pair of electrodes by means of which such current flow between said pair of electrodes may be controlled, an input circuit connected to said control electrode and an output circuit connected to said output electrode, means connected to said input circuit for operatively connecting thereto a signal at said stage input, means for applying a biasing voltage to said input circuit of a magnitude to effect class B operation of said stage, and means connected to said input circuit, responsive to an input signal thereat,-for producing a reverse bias voltage at such input circuit increasing the gain threshold of said amplifier stage, operative to effect a reduction in the low-level gain of said amplifier, said reverse bias-producing means comprising a capacitor so disposed in said input circuit that it will charge upon application of such an input signal and apply a reverse bias voltage to such input circuit.

2. An amplifier stage as defined in claim 1, wherein said electronic device comprises a transistor having base, emitter and collector electrodes, the base electrode comprising said control electrode and said collector electrode comprising said output electrode.

3. An amplifier stage as defined in claim 2, wherein said reverse bias-producing means comprises a capacitor so disposed in said base circuit that it will charge upon application of such an input signal and apply a reverse bias voltage to such base circuit.

4. An amplifier stage as defined in claim 2, wherein said stage comprises in further combination a second transistor, said transistors being circuited for push-pull operation, and

said reverse bias-producing means being connected in a common portion of the base circuits thereof.

5. An amplifier stage as defined in claim 4, wherein said reverse bias-producing means comprises a capacitor so disposed in said common portion of the base circuits of said transistors that it will charge upon application of such an input signal and apply a reverse bias to the respective base circuits of said transistors.

6. An amplifier stage as defined in claim'5, wherein an input coupling transformer is provided having a center-tapped secondary winding, the ends of which are connected to the respective bases of said transistors, said bias voltage for efi'ecting class B operation being applied to said center tap, one side of said capacitor also being connected to said center tap.

7. An amplifier stage as defined in claim 6, wherein said bias an out ut electrical-audio transducer respective] connected to san amplifier input and output, the distance etween and relation of said transducers being such that an in-phase feed-- back may occur during operation of said system, said amplifier including a class B transistor stage circuited for push-pull operation, having base, emitter and collector circuits, means connected to the base circuit of said stage for operatively conducting an input signal thereto, means for applying a voltage to said base circuits of an magnitude to effect a class B operation of said stage, and means connected to said base circuit, responsive to an input signal thereat, for producing a reverse bias voltage at such base circuit increasing the gain threshold of said stage, operative in the presence of sustained feedback howl, to efiect a reduction in the low-level gain of said amplifier sufficient to cut off such howl, said reverse biasproducing means comprising a capacitor so disposed in said common portion of the bias circuits of said transistors that it will charge upon application of such an input signal and apply a reverse bias to the respective base circuits of said transistors.

9. An amplifying system as defined in claim 8, wherein an input coupling transformer is provided having a center-tapped secondary winding, the ends of which are connected to the respective bases of said transistors, said bias voltage for effecting class B operation being applied to said center tap, one side of said capacitor also being connected to said center tap.

10. An amplifying system as defined in claim 9, wherein said bias voltage for effecting class B operation is derived from a voltage divider having an intermediate point thereof connected to said center tap, said capacitor being connected across a portion of said voltage divider.

Claims

1. A threshold controlling circuit, particularly for use in a speech reinforcement system to reduce feedback howl in the absence of an audio signal, comprising an amplifier stage having an electronic amplifier device having a pair of electrodes including an output electrode, between which electrodes a current may flow, an input electrode cooperable with said pair of electrodes by means of which such current flow between said pair of electrodes may be controlled, an input circuit connected to said control electrode and an output circuit connected to said output electrode, means connected to said input circuit for operatively connecting thereto a signal at said stage input, means for applying a biasing voltage to said input circuit of a magnitude to effect class B operation of said stage, and means connected to said input circuit, responsive to an input signal thereat, for producing a reverse bias voltage at such input circuit increasing the gain threshold of said amplifier stage, operative to effect a reduction in the low-level gain of said amplifier, said reverse bias-producing means comprising a capacitor so disposed in said input circuit that it will charge upon application of such an input signal and apply a reverse bias voltage to such input circuit.

4. An amplifier stage as defined in claim 2, wherein said stage comprises in further combination a second transistor, said transistors being circuited for push-pull operation, and said reverse bias-producing means being connected in a common portion of the base circuits thereof.

6. An amplifier stage as defined in claim 5, wherein an input coupling transformer is provided having a center-tapped secondary winding, the ends of which are connected to the respective bases of said transistors, said bias voltage for effecting class B operation being applied to said center tap, one side of said capacitor also being connected to said center tap.

7. An amplifier stage as defined in claim 6, wherein said bias voltage for effecting class B operation is derived from a voltage divider having an intermediate point thereof connected to said center tap, said capacitor being connected across a portion of said voltage divider.

8. An amplifying system comprising an amplifier having an input and an output, an input audio-electrical transducer and an output electrical-audio transducer respectively connected to said amplifier input and output, the distance between and relation of said transducers being such that an in-phase feedback may occur during operation of said system, said amplifier including a class B transistor stage circuited for push-pull operation, having base, emitter and collector circuits, means connected to the base circuit of said stage for operatively conducting an input signal thereto, means for applying a voltage to said base circuits of an magnitude to effect a class B operation of said stage, and means connected to said base circuit, responsive to an input signal thereat, for producing a reverse bias voltage at such base circuit increasing the gain threshold of said stage, operative in the presence of sustained feedback howl, to effect a reduction in the low-level gain of said amplifier sufficient to cut off such howl, said reverse bias-producing means comprising a capacitor so disposed in said common portion of the bias circuits of said transistors that it will charge upon application of such an input signal and apply a reverse bias to the respective base circuits of said transistors.