US2331360A - Peak limiting amplifier - Google Patents

Description

Oct- 12, 1943- L. P. TUCKERMAN 2,331,360

PEAK LIMITING AMPLIFIER 'l FiledAug. 18, 1942 PatentedOt. 12, 1943 UNITED STATES PATENT OFFICE PEAK LIMITING AMPLIFIER Lucien P. Tucker-man, washington, D, c., assignor to Federal Telephone and Radio Corporation, a corporation of Delaware Application August 18, 1942, Serial No. 455,19l

Claims.

This invention relates to' ampliers of the automatic volume control type, and more particularly to such amplifiers including a pushpull stage of amplification.

While automatic volume control circuits in combination with ampliers of the kind referred matic volume control circuit. -Since the effective amplification or output of the two tubes, as purchased in the open market, or as obtained within Y the `limits vof commercial tolerances for such to. are in general very old, the available forms of such circuits appear to be subject to quite rigid limitations which cannot be overcome by ordinary means without undesirably affecting the stability of operation of the circuits. Diihculties have been found in cases where the gain of the y amplifier` is veryv high, or where a very high quality of wave transmission is required over va wide frequency range, or where the speed of tubes, is generally unequal when the tubes are operation, an undesirable amount` of experimentation, extra skill, effort, ortime may be or interruption of transmission. For example, in

the case of a relatively high gain amplifier employing a push-pull amplifying stage and having the usual volume control circuit for transmitting a rectified impulse from the output of the amplier to an'input coupling common to the two input circuits of the push-pull stage, it has been found that a sudden large increase of amplitude of the signarwave, may entirely paralyze the operation of the amplier for a brief period at least, or may cause self-oscillation of the amplifier, or may otherwise interfere with the normal intended operation of the amplifier.

Some ofthe objects of the present invention are to counteract one or more of the above dif-l culties, to provide an improved wide frequency,

. replacements of tubes inpush-pull amplifiers of the automatic volume control type .referred to above, where any two tubes generally dlifersomewhat in their characteristics and thus interfere with the production of a perfect balance in the push-pull stage, I have found that it is important to position the tubes in the push-pull stage in I such relation to each other that the resulting unbalance of the circuit has an eiect whose phase is in degenerative relationwith the autorequired in trying to make the amplifier operate vwith. suitable stability under theconditions re- Y ferred to. f

In accordance with the present invention, these difficulties may be counteracted eectively and the range of stable operation of the amplii'ler extended by providing an auxiliary unbalance of suitable amount and suitable phase in the transmission circuit which includes the push-pull amplifier and extends from' the volume control input terminals ofthe amplifier to the rectifier in the automatic volume control circuit. This unbalancing means may, for example, consist of in the push-pull stage, to reduce the effective amplification of one side of the push-pull amplifier, so that the resulting unbalanced characteristic is greater than the unbalanced transmission produced by the inherent predetermined' difference in the tubes themselves, and the phase relation of the unbalanced transmission characteristic is such as to produce degeneration with respect to the feedback path formed by the automaticvolume control circuit. 'I'he resulting amplifier is adapted to quickly and efficiently limit peak amplitudes in the, amplied signal wave.

These and other objects and features of the invention will be understood more clearly from the following detailed description in connection withl the accompanying drawing and the ap- 'pended claims.

The drawing shows a schematic circuitdiagram of an amplier illustrative of one form of the present invention.

In the drawing, the amplier I, includes a push-pull amplifying stage 2, and the extra stages 3, the push-pull stage including amplifying units '4 and 5 which are shown, by way of example,vas being of the electron discharge type, although it will be understood. that the invention is applicable to amplifying units of other types.

' symmetry and circuit balance. The input output terminal 38 for amplifier 35.

' duced,

`The secondary Each electrondischarge device includes an anode, an electron emitting cathode, and an electron control element.

The source of waves I impresses signal 4waves to be amplified on the primary of transformer I I, the secondary of which has a circuit in shunt therewith, including resistance elements I2, I3, I4 and I5 in series', the resistance I3 being made equal to resistance `I4, and resistance I2 being made equal to resistance I for the purpose of signal voltage across resistance I3, is transmitted to the inputterminals I8, I1 of amplifying unit 4, and input signal voltage across resistance I4, is transmitted to input terminals I8 and I9 of amplifying unit 5, terminals I1 and I9 being connected together at ground 2U. Terminal I5 connects directly with the control grid of unit 4, and terminal I8 connects directly with the control grid of unit 5, the terminals I6 and IB constituting push-pull input terminals for the push-pull amplifier stage 2. The grid-cathode circuit for each unit 4 and 5 has avportion in common including y the resistance coupling 2| having the automatic volume control terminals BI and 62, the condenser 63 being in shunt therewith for opposing too sudden changes in the automatic volume control grid bias.

The resistance 22 is connected between the cathode of unit 4 and ground 20, and the corresponding resistance 23 is connected between'the cathode of unit 5 and ground 20, resistances 22 and 23 serving in well known manner to provide a normal negative biasing potential for the respective control grids of units 4 and 5. Space current is suppliedto the anodes of units 4 and 5, through the coupling resistances 24 and 25. Output current from the anodes of units 4 and 5 is transmitted through the respective direct current blocking condensers 28, 21 to the -output terminals 28, 29 across which is connectedin series, the resistor 30, auto-transformer coil 3l,

and the resistance 32, the mid-point 33 of4 the coil 3l being grounded. Amplified output current passing through circuit 83, 3|, 32 transmits iinpulses to the control grid amplifier of the electron discharge type having a capacity-resistance coupling beween its anode andthe control of amplifier 35 of the electron discharge grid which connects with output type, the anode of terminal 38, to which is connected one terminal of theprimary of transformer 31, the other terminal of said primary being .connected with the It will be seen that terminals 38 and 38 serve as output terminals for the amplifier I. The number of extra amplifier stages 3 may obviously be varied to suit requirements as indicated by the dotted line 39, at which point additional of ampliiication not shown maybe introand similarly itv willl be apparent that additional push-pull stages of amplification similar to push-pull stage 2, may be introduced immediately following stage 2 atthe points indicated the dotted lines 48. The output'transformer 31, may obviously have its terminals 38, 38, connected directly with terminals 28, 2 8, when the number of extra stages 3 are reduced to zero. winding 4I, of 4transformer 31, is-connected with the load 42, which may be any desired form of utilization circuit for the amplined signal waves. The secondary winding 43 of transformer 31, is coupled with rectifier 44, the direct current output conductors 45 of the rectiner being connected directly with the terminals stages 6I, 62 of the coupling resistance 2| constituting the automatic volume control input terminals for the amplifier.

Rectifier 44, preferably includes a full-wave rectifier tube 46 having two cathodes 41 connected respectively with opposite terminals of coil 43. 'I'he anodes 48 being connected together with one of the conductors 45, the other conductor 45 being connectedrthrough potentiometer resistances 49, by way of the adjustable tap 53 with the mid-point of coil 43. Potentiometer 49 is connected across the direct current source of potential 5I, poled in a 'direction to oppose the output potential of the rectier. Adjustment of potentiometer 49, permits of adjustment of the threshold amplitude of amplied output signals below which no rectification takes place in automatic volume control circuit extending from coil 43 to coupling 2|, and below which' no volume control impulses are transmitted to the input of the amplier, this operation being ln accordance with the well-known practice for limiting the 'operation of the automatic volume' control to apply to only those Iamplified output signal amplitudes which exceed a predetermined value. The capacity 63 is made small enough to permit of a relatively quick application of the automatic volumel control biasing potential to the grids of amplifying units 4 and 5, when the amplitude of the output signal waves tend to exceed the desired value. The variable resistance 52 may be included in one of the conductors 45 for damping the volume control impulses.

When the requirements of the amplifier are such that a very high gain of amplification is to be obtained, or where a very wide range of signal frequencies are to be amplied, or where a relatively quick response is to be made. by the automatic volume control circuit and especially where two or more of these conditions are to be met in a given amplifier, I have found that small unbalances in the push-pull stage 2 tend to cause serious ,dinicultiea 'I'hese difllculties are apparently due in part to inherent differences between the characteristics of available amplifier tubes for substitution in units 4 and 8. These tube differences produce an unbalance which tends to cause instability .of operation of the amplifier and may momentarily paralyze operationv of the amplifier or cause it to produce self-oscillations.

or cause the production of peak amplitudes momentarily far in excess of the limit set by the automatic volume control circuit, or may otherwise interfere with the stable operation ofthe ampliiler when there is a sudden increase inthe amplitude of signal waves from the source I8. andothe amplitude reaches a value sumciext i sense with vrespect to the automatic volume control or feedback circuit 45. The resulting stabilization of operation serves to limit peak ampli- .tudes satisfactorily to the maximum set by the automatic volume control circuit 45.

It will be apparent that when the number of stages of amplification is changed from that shown in the drawing, or when the phase of any to cause operation of the automatic control ciruitf the other side of the vcircuit so that the unbalance of the circuit is reversed 180 in phase, in order to produce degeneration around the closed path formed by the amplifier in combination with the automatic volume control circuit 45.

The value of the resistance 60, or other unbalancing means introducedin one side of the transmission circuit, referred to above is preferably not less than that necessary to compensate for any undesired unbalance due to inherent differences between amplifying units of the same type utilized in the push-pull stage 2, and .is preferably of sulcierlt value to produce a safe margin of unbalance in excess-of that necessary to compensate for said tube or amplifying unit dierences, so thati the desired stability is achieved irrespective of the relative positions of the unequal amplifying units or tubes in the push-pull stage 2.

The invention is not limited to the use of ampliers of the space current type in the pushpull stage 2, but is applicable to various other types of ampliers, and the form of the unbalancing 'means and its position in the transmission circuit may have any ofa wide range of variations without departing from the invention as set forth in the appended claims. l

What is claimed `is:-

1. In an amplifying system having a push-pull amplifying stage, wherein two amplifying umts have individual input circuits providing pushportion of each said input circuit being in common and including a coupling having automatic volume controlv input terminals, anda feedbackcircuit including rectifying means responsive to an amplified wave derived from the outputpf said push-pull stage and connected with s/aid volumev Icontrol input terminals for transmitting volume control impulses thereto, the combination wherein there is provided a transmission circuit including said push-pull stage and extending from said volume control input terminals to said rectifying meanssaid transmission'circuit includingpredetermined circuit unbalancing means 4. In an amplifying system according to claim 1, the combination wherein said rectifying means includes a rectifier of the full-wave type for transmitting rectied impulses to said volum control input terminals.

5. In an amplifying system according to claim 1, the combination wherein said rectifying means includes a rectifier of the full-wave type for transmitting rectified impulses to said volume control input terminals, a source of potential being provided in said feedback circuit between said rectier and said volume control input terminals in opposition to the output of said rectifier.

stage including two amplifying units having individual input circuits vproviding push-pull input terminals for signals to be amplified, a portion f each said input circuit being in common and but having an inherent predetermined difference in characteristics whereby there is produced an unbalanced transmission characteristic for impull input terminals for signals to be amplified, a

for producing a circuit unbalance not less than 1 opposite sides of `said transmission circuit, the. y

resistancesof said elements being substantially different from each other, the dinerence between saidresistances constituting said circuit unbalancingmeans. 'A

3L In an amplifying system according to claim 1, the combination wherein each of said amplifying umts includes an electron-discharge device having an anode, a cathode, an electron controlelement, andan anode-cathode circuit external] ing a diierence ofresis r to said device, said anode-cathode circuits havpulses applied tosaid volume control terminals,

`and auxiliary transmission Aunbalancing means responsive to waves transmitted to said amplier,

corresponding conductor of greater resistance in the other side of said circuit.

8. An amplifier according to claim 6, wherein said amplifier includes a cathode, an anode, and

electron control means, and" said auxiliary un.

balancing means includes means for making the normal resistance of the anode-cathode circuit of one of said amplifying units greater in value than that of the anode-cathode circuit of the other of said amplifyingunits. I

9. An amplifier according to claim 6,'whereln the said auxiliary unbalancing means includes a partially balanced circuit having a conductor of given resistance in one side of said circuit. and a corresponding conductor of greater resistance in the `other side thereof, said rectifier being of the full-Wave type for Atransmitting rectified current to said volume control terminals.

10. An amplifier according to claim 6, wherein said auxiliary unbalancing means includes a. partially balanced circuit having a conductor of given resistance in one side of said circuit anda i corresponding conductorof ldifferent resistance in the other side thereof, said rectifier being of the full-wavetype for transmitting rectified current to said vvolume control terminals, said direct current connection including a sourceof direct current potential opposed to the output of said rectier.

, LUCIEN P. TUCKERMAN.

6. Anamplifler having a push-pull amplifying

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