US2151829A - Valve amplifying circuit - Google Patents

Description

March 28, 1939. EfBART'QN l l2,151,829

VALVE AMPLIFYING CIRCUIT Filed July 24; 1937 Patented Mar. 1939 UNITED sjrATEfsf VALVE AIHPLIFYING CIRCUIT 'Loy E, Barton, Collingswood, N. J., assignor to 1 Philco Radio and Television Corporation, Philadelphia, Pa., a corporation of Delaware Application July 24, 193?, Serial No. 155,553 u In Great Britain August 4, 1936 11 m1111111. 4(c1. 25o-2o)A 'Ihis invention relates to a combined detector and audio amplifier system employing a feedback connection-from the output circuit of the audio system to theinput circuit thereof and also to the detector circuit, whereby distortion of the system may be minimized and certain other ad'- vantageous features may be obtained, ls set forth hereinafter.

Generally speaking, the invention k adapted for use in the detectonand audio alplier circuits of any system in which intelligence is transmitted by the use of a modulated carrier signal, such as a radio receiver. The principal object of the invention is to provide means for minimizing distortion in theloutputsystemand means for improving the performanceof the detector circuit by which the carrier signal is demodulated.

The invention further provides other advantageous features, including a low impedance zo source of feed-back signal and means for obtaining a simple frequency or tonel control by which the frequency characteristic or response of the system may be modied.

The'mode of operation of the circuit may beV understood by reference to the accompanying drawing illustrating the application of the invention to a systemincluding a diode detector, an audio ampliiier, and a loudspeakenin which a degenerative feed back is utilized in the audio an amplier part of the system.

In the drawing:

Fig. l illustrates an embodiment in which the detector is regenerative; l Fig. 2 illustrates a modification iin which the detector is degenerative; and

Fig. 3 illustrates a further modiiication. Referring generally to the several figures ofthe drawing, it will be seen that there is shown a source of-signal Eo which might be modulated 1 o carrier signal source, a tuned transformer T for supplying this signal to a diode D, an audio load impedance P--C for supplying the rectied or modulation signal to an amplifier tube V1, which in turn supplies the amplified signal to a power amplifier tube V2, which energizes a suitable utilization means, such as thel loudspeaker LS, through the output transformer T1, whose primary P.is connected to the output circuit of tube V2 and whose secondary S is connected to the loudspeaker. A signal obtained from the second.

load circuits will be of the order of magnitude of thousands of ohms, whereasthe impedance of the loudspeaker itself may be a few ohms. Thus, the resistances R1 and Rn may be'of the order of several hundred ohms without placing an ap- 5 preciable load upon the output of the circuit and yet they may constitute asignal source whose impedance is`negligible as compared with other impedances in the circuit of the diode and the l iirst audio amplifier. It willA be noted that the 10 sum of the signals E1 and E2 equal the signal across the input circuit ofv the loudspeaker and will include any distortion signals which may =have occurred anywhere Within the circuit. It

will be apparent that the derivation of the feed- 15 back signal from the output 'circuit of the audio system is an important feature of the invention. 'Ihe amplier tube V1 may be supplied with an audio signal obtained from a portion of the diode leak P and its associated condenser C. It will zo be` understood, ofcourse, that the audio load P-C isillustrated in simple form and.may,fif desired, include the more complicated resistance capacitance filters and volume controls usually employed in a. detector circuit. The signal supplied to V1 will then-comprise the portion of the signal across the potentiometer P ,plus the signal E1 fed. back from the output circuit. 'Ihe polarity ci the signals should besuch that the signal E1 tends4 to decrease-the total signal between the so grid and cathode of tube V1. When this condition obtains, that portion of the amplier including tubes V1, V2 and the transformer T, is degeneratively connected, with a resulting decrease in the amount of distortion transmitted to the output circuit. Considering a specific example, if the circuit is degenerated to the extent that twice as much signal must be supplied across the diode leak with degeneration, as that which must besupplied across the same ieak without o degeneration in order to obtain the same output, then the distortion incurred in the audio amplier will be reduced to one half that' in the latter case.

If desired, the amount of signal fed back to V1' may be caused to vary with frequency, thus caus- 5 ing a variation in the frequency response of the circuit as a whole. For example, if the resistance R1 is shunted by a condenser C1, whose impedance at higheraudio frequencies is 'comparable with or less than that of R1, then the signal 50 E1 will decrease with increasing frequency, since the impedance across which E1 is obtained decreases with frequency, while the impedance R2 does not change. Thus, the use of a condenser in this position will cause the amount of signal .u

fed back to decrease with increasing frequency, which in turn will reduce the amount of degeneration with increasing frequency, and thus cause the amplifier to have a rising frequency characteristic. On the other hand, if the condenser C1 were in shunt with the resistance Ra and the other connections remained the same, then the reverse would be true and the amplier would have a frequency response curve which would fall off at higher frequencies.

It will be noted that in Fig. 1 the cathode of the diode is connected to the cathode of the rst audio amplifier tube V1 and hence the feed-back voltage E1 is included lin the diode circuit. It appears that the amplitude of E1 is substantially determined by the amount of signal supplied.to V1 and lf En remains constant, as would be the case for a receiver having automatic volume control, then for low output volume levels when only a small portion of the total signal across the diode leak is supplied to an audio amplifier, the amount of signal reinserted in the diode circuit would be quite small as compared with that built up across the leak. On the other hand, if the volume control were adjusted so tlat the full signal across the leak is supplied to the input circuit of V1, then the voltage E1 would be of a magnitude comparable with that across the leak,

Jso

although it could never be greater than.I the latter voltage. In some cases, it may be desirable that the voltage fed back to the diode circuit be constant with respect to the signal across the diode leak, and in this instance, the potentiometer may be replaced by a fixed resistance. Consideration of the actual voltages in the circuit shows that when the diode anode is positive with respect to its cathode and the diode is drawing current and energy from the system, the reinserted signal is such as to increase the voltage between the diode anode and cathode on modulation peaks. whereas it will decrease the voltage amplitude between the diode anode and cathode on modulation valleys. In other words, this signal effectively tends to increase the effective degree of modulation ofthe signal and for that reason tends to reduce the detector distortion on signals where the modulation level is low. Generally speaking, 'where the average modulation level is low, it is desirable and usual to supply more of the detected slgnalto the amplifier in order to obtain the desired output signal. Consequently, a relatively large signal will be inserted in the diode circuit and the desired minimization of detector distortion will obtain. On the other hand.- where the percentage modulation is high, it is generally desirable to reduce the amount of vsignal supplied to the audio amplifier, and hence the amount of signal inserted in the diode circuit will be small and will have al negligible effect, as far as overloading the diode is concerned. Thus. where it is desirable to increase the effective degree of modulation of the incoming signal, the detector may be made lregenerative by the circuit shown in Fig. 1. Y

'I'he circuit of Fig. 2 is generally similar to that of Fig. 1 except that the cathode of the diode D in Fig. 2 is connected to resistance Rz instead of R1. In this instance,l therefore, the signal inserted in the detector circuit will be of, opposite polarity as compared with-that inserted in the` input circuit of the amplifier V1. Under these conditions, both the detector and the amplifier will be degenerated. The reduction of distortion within the audio amplifier itself will occur as before and the same variation in frequency response may be had. In this case. however, in the detector circuit the'connections are such that' considering only the time intervals during which the diode anode is positive as compared with its cathode, on modulation peaks the reinserted signal will tend to make the anode less positive with respect to its cathode, whereas on modulation valleys the circuit will tend to make the diode anode more positive with respect to its cathode. Thus, this circuit is particularly suited for use in special circuits, for example, those of the type in which a signal has its volume range compressed and later expanded to enable the transmission of a greater volume range. Since the circuit of Fig. 2 tends to decrease the effective degree of modulation, a larger degree of modulation may be used at low levels to keep the actual modulation signal above the noise level, while the degenerative detector shown in Fig. 2 may be used to reduce the degree of modulation and thus increase the effective volume range. Preferably such a circuit should be used with an audio amplifier containing a volume modfying circuit. In the latter case it is likewise desirable that a substantially linear detector should be employed.

In Fig. 3, the system of Fig. 2 has been modiiled to include an additional resistance R3 in series with R1 and R2 insofar as the output of the transformer T1 is concerned. 1f any of these three resistors are made non-linear, for example, by using a pilot light as a resistor or by using any other resistor whose resistance varies with current, a. convenient volume expander or volume contractor circuit may bev obtained, If, for example, R3 were the tungsten filament of a pilot light. its resistance would increase as the output voltage increased. Consequently, the voltages E1 and En would constitute only part of the total voltage across the output circuit,` and the ratio between the feed-back voltage and the output lvoltage would decrease as the' output voltage increased. Thus, this circuit would act as a volume expanding circuit in that the gain of the amplifier would be greater for larger output voltages. If, on the other hand, R3 were made linear and R1 or Rn made non-linear in the same way, then the portion of voltage fed back would increase with larger output voltages, and the circuit would act as a volume contracting circut in that the gain of the amplifier would decrease with larger output voltages. Such a circuit is, of course, well adapted for use with the degenerative detector of Fig. 2 in that using the expander circuit the detector will be highly degenerative for low signal levels, whereas only slightly degenerative for high signal levels. Thus, for low signal levels the effective percentage of modulation will be markedly decreased, whereas for high modulation levels-there will be a smaller change and thus vthe diode will further aid in the volume expension function of the apparatus as a whole. Minimization of distortion and reduction of the apparent noise legel will obtain as before.

A tungsten filament is particularly suited for use such as-above-mentioned in audio Icircuits, since its thermal time lag is suiliciently low to permit rapid changes in volume and yet is not fast enough to cause distortion of low notes.

Thus, the invention contemplates an audio amplifier having an input circuit, that is, the control grid-to-cathode circuit of valve V1, and

an output circuit such as the secondary of transnal across the secondary terminals, between the 7G 30 l amplifier Vchannel including an input circuit, an

cathode and-ground of V1. In addition, a signal from the same source is supplied in the rectifier circuit, in which the incoming modulated signal is demodulated, and the audio signal Abetween ground and some point on the audio load obtained by the demodulation process is supplied to the control grid of V1. Thus the audio ampliner is degenerated while the rectiiier circuit is degenerated or regenerated depending upon the circuit connections.

The invention further contemplates the use of a non-linear element in a degenerative feedback circuit by which the amount of degeneration may be made dependent'upon the signal level and a volume expansion action obtained.

It win be apparent that the feed-back signal' applied to the diode in the above-described sysltems varies the effect of the diode as an im.

y lincluded-in a .single envelope to provide a single tube or valve. Y

Although several embodiments have been illustrated and described, the inventioniscapable of further modifications, as will be apparent to 'those skilled in the art.

I claim: 1. In a carrier wave receiving system, an audio output'circuit and anamplifier valve, a source of modulated signal energy, rectier means for dethe ampliiier and to vary the effective degree of l modulation of the audio signal.

2. In a carrier wave receiving system, an audio ampliiier channelincluding an input circuit, an output circuit and an ampliiler valve, a. source of modulated signal energy, rectiiier means for demodulating a carrier wave from said' source, a potentiometer in the rectier circuit andL connected to the input circuit of theaudio amplifier for supplying a selectable portion of the audio signal voltage to said input circuit, and means for deriving a signal from said output circuit,

and forv supplying it to the vinput circuit of the audio amplifier and to the rectiier circuit to thereby control the amplification of the ampliiier and to vary the eective degree of modulation of the audio-signal.

3. In a carrier wave receiving system, an audio ampliier channel including an input circuit, an output circuit and 'an amplier valve, a-,source of modulated signal energy, rectier means for Qdemodulating a carrier wave from said source, connections for supplying the audio demodulation products to the input circuit of the audio amplier, an impedance at least a portion of which is connected in both the rectier circuit and said input circuit, andmeans for deriving a signal from said output circuit, and for supplying it to said impedance to thereby control the amplification of the amplier and to vary the effective degree of modulation of the audio signal.

4. In a carrier. wave receiving system, an audio amplifier channel including .an input circuit, an

output circuit and an amplifier valve, a source oi v the rectifier circuit and another portion con- 5 nected in said-input circuit, and means for deriving a signal from said output circuit, and for supplying it to said impedance to thereby control the amplication of the amplifierand to vary the eiective degree of modulation of the 1o audio signal, the connections of said impedance portions being such as to apply signals of opposite polarity to said rectifier circuit and said input circuit.

5. In a carrier wave receiving system, an audiol ampliiier 4channel including an input circuit, an output circuit and an ampliiier valve, a source of modulated signal energy, rectifier means for demodulating a carrier wave from said source,

connections for supplying the audio demodula- I tion products to the input circuit` of the audio amplifier, an impedancev at least a portion of which is connected in both the rectiiier circuit and said ,input circuit, meansV for deriving a signal from said output circuit, and for supplying it to said impedance to thereby control the, amplification of the amplifier and to vary the eifective degree of modulation of the audio signal,

and means associated with said impedance for causing the amplitude of the derived signal to vary with'frequency.

6. In a carrier wave receiving system, an audio ampliiier channel including an input circuit, an

output circuit and an ampliiier valve, a source of modulated signal energy, rectifier means for. de-

modulating a carrier Wave from said source, connections for supplying the audio demodulation products to the input circuit of the audio ampliiier, an impedance having a portion connected in the rectifier circuit and another portion con- 4o nected in said input circuit, means for deriving a signal from said output circuit; and for supplying it to said impedance to thereby control the ampliiication of the amplifier and to vary the effective degree of modulation oi' the audio signal, the 45 connections of said impedance portions being such as to apply signals of opposite polarity to said rectier circuit and-said input circuit, and means associated with said impedance for causing the yamplitude of the derivedsignal to vary with 5 frequency. j l l '1.' In a. carrier wave receiving system, an audio ampliiier channel including an input circuit, an output circuit and an amplifier valve, a source of modulated signal energy, rectifier means for de modulating a carrier wave from said source, connections for supplying the audio demodulation products to vthe input circuit of the audio am'- ,pliiien a feed-back path comprising means for deriving a signal iromisaid output circuit, transfer l8. In a carrier wave receiving system, an audio ampliier channel including an input circuit, an output circuit and an amplier valve, a source of modulated signal energy, rectier means for demodulating a carrierwave from said source, connections for supplying the audio demodulation products to the input circuit oi' the audio ampliiier, means for deriving a signal from said output circuit,means for passing the derived signal through an impedance having a non-linear characteristic, to thereby set up across said impedance a control signal whose amplitude varies with the volume level oi' the system, and means for supplying said control signal to the input circuit of the audio amplifier and Ito the rectiiier circuit to thereby control the amplification of the amplier and to vary the effective degree of modulation of the audio signal.

9. In a. Acarrier wave receiving system. the provision of an audio amplifier channel including an input circuit, an output circuit and an amplifier valve, a source of modulated signal energy, rectier means for demodulating a carrier wave from said source, connections for supplying the audio demodulation products to the inputv circuit oi the audio amplifier, an impedance at least a portion of which is connected inboth the rectifier circuit and said input circuit, means for deriving a signal from said output circuit, and for supplying it to said impedance to thereby control the ampliiication of the amplifier and to vary the effective degree of modulation of the audio signal, and another impedance in circuit with said first-named impedance, at least one of said impedances having a non-linear characteristic,

whereby the amplitude of the derived signal is caused to vary with the volume level of the system. Y

10. In a carrier wave receiving system, an audio amplifier channel including an input circuit, an output circuit and an amplliier valve, a source o1 modulated signal energy, rectiiier means for demodulating carrier wave from said source, connections for supplying the audio demodulatlon products to the input circuit of the audio amplitier, animpedance having a portion connected in the rectifier circuit and another portion conf nected in said input circuit, means for deriving a signal from said output-circuit, and for supplying it to said impedance to thereby control the ampliiication of the amplifier and to vary the effective degree of modulation of the audio signal, the connections of said impedance portions being such as to apply signals of opposite polarity to said rectifier circuit and said input circuit, and another impedance in circuit with said iirstmentioned impedance, at least one of said impedances having a non-linear characteristic, whereby the amplitude of the derived signal is caused to vary with the volume level of the system.-

1l. In a carrier wave receiving system, a modulation signal ampliiier channellncluding an input circuit, an output circuit and an amplifier valve, a source of modulated signal energy, rectiiier means for demodulating a carrier wave from said source, connections for supplying the mo'dulation signal to the input circuit of the modulation signal ampliiier, and means for deriving a signal from said output circuit, and for supplying it to the input circuit of the modulation signal amplifier and to the rectliier circuit to thereby control the ampliication of the amplifier and to vary the effective degree of modulation of the modulation signal.

LoY E. BARTON. 5

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