US2931990A - Class b amplifiers - Google Patents

Description

April 5, 1960 J. E. BEST CLASS'B AMPLIFIERS Filed March 22, 1956 United States i atent CLASS B AMPLIFIERS John Edward Best, Streatham, London, England, assignor to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Application March 22, 1956, Serial No. 573,224

Claims priority, application Great Britain March 30, 1955 3 Claims. (Cl. 330-117) This invention relates to class B amplifiers and especially though not exclusively to audio output amplifiers.

The use of output transformers in high quality audio amplifiers is associated with several disadvantages. For instance such transformers are large and expensive and it is difiicult to reduce harmonic distortion in the low frequency region where amplitudes tend to be large and where harmonics tend in any case to be exaggerated due to the characteristics of the ear. Moreover it is diflicult to overcome overall distortion by the use of negative feedback since a transformer causes phase swings which tend to make the amplifier unstable. In consequence the degree of negative feedback which can be used has to be comparatively small. To overcome the disadvantages associated with output transformers it has been proposed to provide a transformerless output circuit comprising a pair of output valves connected in series across a source of high tension voltage and arranged for. operation in push-pull. In such a circuit, itis possible to use a high order of negative feedback for the purpose of obtaining a high degree of linearity. The two valves may be arranged to feed a high impedance loud-speaker via an output condenser, and in this case feedback can} be taken from the anode-cathode junction of the output 'valves to an initial amplifier stage driving a phase splitter which in turn drives the two output valves. One difficulty however with a circuit of this kind is that there is inevitably a large standingcurrent drain on the high tension supply if the series connected valves are operated in a class A manner, resulting in the need for a rectifier of high power rating and also causing difliculty in smoothing.

The main object of the present invention is to reduce the difliculties indicated in the preceding paragraph.

According to the present invention there is provided an amplifier comprising two valves connected in series across a source of voltage and set up for push-pull class B operation, and wherein said valves are included in a feedback loop which has a finite D.C. gain and is arranged to cause the voltage of said supply to be divided in'a substantially constant ratio between said valves.

- One example of the invention is illustrated in the single figure of the accompanying drawing.

Referring to the drawing, reference 1 represents an initial amplifying valve to which audio frequency signals are applied via a coupling capacitor 2 and a series resistor 3. The junction of 2 and 3 is connected via resistors 4 and 5 to the junction of potential dividing resistors 6 and 7 which are connected in series as shown between a positive voltage source 8 and ground. The voltage of the source denoted by arrow 8 may be 285 volts. Condensers 9 and 10 are smoothing condensers. The valve 1 has an anode load resistor 11 and the voltage variations set up across the resistor 11 are applied to the control electrode of a phase splitting valve 12 by means of a D.C. coupling circuit comprising a parallel combination of a resistor 13 and capacitor 14 connected in series with a resistor 15. The junction of 13, 14 and 15 is connected to a source of negativg voltage, denoted by the arrow 16, via resistor 17 and resistor 17 is dimensioned in relation to the resistors 11 and 13 to cause the standing bias maintained at the control electrode of the valve 12 to be of the order of 123 volts negative with valves in the present example. The valves 20 and 21 are connected in series between the voltage source 8 and ground, and in addition to being of the same kind the valves are also of the same type, each valve consisting of a double triode. These valves are driven in push-pull by coupling the control electrodes of the valve 20 to the anode of the valve 12 by a D.C. coupling circuit comprising a parallel combination of resistor 22 and capaci tor 23 connected in series with resistors 24 and 25, and by coupling the control electrodes of the valve 21 to the cathode of the valve 12 by way of a similar D.C. coupling circuit comprising resistor 26, a capacitor 27 and resistors 28 and 29. The upper end of the anode load resistor 18 for the valve 12 is connected via resistor 30.

to a positive voltage source denoted by the arrow 31 and. of the order of 400 volts. The junction of 30 and 18 is decoupled to ground by condenser 32, and is also connected to the junction of resistors 26, 28 and 29 by a resistor 33 which is dimensioned to maintain a standing bias of the order of 45 volts negative at the control electrodes of the valve 21. A bias of the same magnitude is maintained at the control electrode of the valve 20 by connecting the junction of resistors 22, 24 and 25 to the negative potential source 16 by resistor 34. The output of the valves 20 and 21 is applied via a storage capacitor 35 to the speech coils 36 and 37 of two loudspeakers, of high impedance type, the speechcoils being connected in parallel and shunted by a filter comprising capacitor 38 in series with resistor 39.

The circuit described is such that the valves 20 and 21 are operated in class B manner so that the normal current drain on the high tension voltage source 8 is small and reaches a maximum only on the transient peaks of the amplified waveform. It is desirable that the high tension voltage should always be divided equally between the valves 20 and 21, or in other words that the capacitor 35 should, in operation of the circuit, be chargedto a mean voltage which is approximately half the voltage of the source 8. However there is some tendency forthe voltage available at the terminal 8 to vary on account of the variable current drawn by the valves and this introduces a difiiculty in ensuring equal sharing of the high tension voltage. In accordance with the invention this difiiculty is reduced by coupling the anode-cathode junction of the valves 20 and 21 directly to the cathode of the valve 1 so that, having regard to the use of D.C. coupling circuits in the forward path from the valve 1 to the output valves 20 and 21, the valves 20 and 21 are included in a negative feedback loop which is conductive for direct current so that it has a finite D.C. gain. Moreover the bias applied to the valve 1 is adjusted so that initially the high tension voltage is shared equally between the valves 20 and 21, and this being the case the provision of a feedback loop which has a finite D.C. gain tends to maintain this condition under all operating conditions.

The elfect of the D.C. negative feedback loop in maintaining equal sharing of the high tension voltage between the valves 20 and 21 can be illustrated by considering the circuit when the audio frequency signal is zero. In this condition the standing current in the valve 1 is determined only by the voltage difference between its control electrode and cathode. The voltage at the control electrode is fixed by the voltage source 8 and by the values Patented Apr. 5, 1960 of the potential dividing resistors 6 and 7, whereas the voltage at the cathode is dependent upon the voltage across the storage capacitor 35. Assume that the latter voltage is tending to change due to unequal sharing of current in the valves 20 and 21, the change having the effect of increasing the voltage across the capacitor 35 The voltage difference between the control electrode and cathode of the voltage 1 then tends to diminish and the current in the valve 1 tends also to diminish. The resultant rise in the anode voltage of the valve 1 is transmitted to the control electrode of the valve 12 since the components 13, 14 and 15 constitute a DC coupling circuit; The anode to cathode current in the valve 12 increases so that its anode voltage decreases and its cathode voltage increases. The voltage decrease at the anode is transmitted by the DO coupling 22, 23, 24 to the control electrode of the valve and the voltage increase at the cathode which is transmitted by the DC. coupling 26,- 27 and 28 to the control electrode of the valve 21. The opposite voltage changes at the control electrodes of the valves 20 and 21 reduce the current in the valve 20 and increase that in the valve 21. In the absence of an audio frequency signal, the current in the valves 20 and 21 is restricted to a very small value because these valves are adjusted for class B operation, nevertheless the current changes are adequate to vary the charge in the capacitor 35. In the situation envisaged, the charge in the capacitor will in fact tend to diminish thereby counteracting the increasing voltage across the capacitor giving rise to the initial change in the voltage between the control electrode and cathode of the'valve 1. From-the foregoing explanation it is evident that, even when an audio frequency signal is present, the voltage across the capacitor 35 will tend to maintain itself at a fixed value determined by the voltage at the junction of the resistors 6 and 7, and this voltage is in fact selected to maintain the voltage across the capacitor 35 sensibly equal to half the voltage of the source 8.

What I claim is:

1 An amplifier comprising two amplifying valves of the same kind each having at least a current inflow elec-' trode, a current outflow electrode and a control electrode for controlling the current flow from said inflow electrode to said outflow electrode; a voltage source having a positive terminal and a negative terminal; a first connection from said positive terminal to the inflow electrode of one of said valves; a second connection from the outflow electrode of said one valve to the inflow electrode of the other valve; a third connection from the outflow electrode of said negative valve to said other terminal; a series combination of a storage capacitor and a load imped'ance connected from the inflow electrode to the outfiowelectrode of one of said valves; means for applying signals to be amplified in push-pull to the control electrodes of said valves; means for biassing the control electrodes of said valves for class B operation; and means for maintaining predetermined sharing of the voltage of said source between said two valves comprising a negative feedback path from said second connection to the control electrode of at least one of said valves, said negative feedback path being conductive for direct current to maintain the voltage across said capacitor equal to a substantially constant fraction of the voltage of said source.

2. An amplifier according to claim 1, said feedback path comprising an amplifier having an input electrode connected to said second connection and an output electrode connected to the control electrode of one at least of said first mentioned valves; means being provided for maintaining a bias at said input electrode responsive to the voltage of said source, thereby to determine said substantially constant fraction.

3. An audio amplifier comprising an initial amplifier having an input electrode and an output amplifying electrode; two output valves of the same kind each having at least a current inflow electrode, a current outflow electrode and a control electrode for controlling the current tlow from said inflow electrode to said outflow electrode; a voltage source having a positive terminal and a negative terminal; a first connection from said positive terminal to the inflow electrode of one output valve, a second connection from the outflow electrode of said one inflow valve to the output electrode of the other output valve; a third connection from the outflow electrode of said other output valve to said negative terminal; the series combination of a storage capacitor and a loudspeaker coil connected from the inflow electrode to the outflow electrode of said other output valve, means for applying audio signals to be amplified to the input electrod of said initial amplifier; phase-splitting means connecting the output electrode of said initial amplifier to the control electrodes of said output valves to apply initially amplified signals in push-pull to said output valves; said initial amplifier and phase splitting means being responsive to direct and audio frequency currents; means for maintaining biason the input electrode of said initial amplifier responsive to the voltage of said source; and means for maintaining predetermined sharing of the voltage of said source between said two valves comprising a negative feedback connection conductive for direct current from said second connection to the input electrode of said initial amplifier to maintain the voltage across said storage capacitor equal to a substantially constant fraction of the voltage of said source, said fraction being determined by the bias maintained at the input electrode of said initial amplifier.

References Cited in the file of this patent .UNITED STATES PATENTS 1,985,923 Gutmann Jan.'1, 1935 2,488,567 Stodola Nov. 22, 1949 2,659,775 Coulter Nov. 17, 1953 2,763,733 Coulter Sept. 18, 1956 2,773,136 Futterman Dec. 4, 1956 OTHER REFERENCES Article: General Radio Experimenter, October 1951, volume XXVI, No. 5, A New Push-Pull Amplifier Circuit, pages 1-8.

Mira- UNl'lED STATES PATENT OFFICE CERTIFICATE 0F CORRECTlON Patent No. 2,931,990 April 5, 1960 John Edward Best It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, line 50, for "negative valve to said other" read other valve to said negative column 4, line 23, for "inflow valve to the output" read output valve to the inflow Signed and sealed this 29th day of April 1969.

(SEAL) Attest:

EDWARD J. BRENNER Commissioner of Patents Edward M. Fletcher, Jr.

Attesting Officer

Images