US2273680A

US2273680A - Amplifier control circuit

Info

Publication number: US2273680A
Application number: US322460A
Authority: US
Inventors: Hendrikus T J Wiegerinck
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1939-04-26
Filing date: 1940-03-06
Publication date: 1942-02-17
Anticipated expiration: 1959-02-17

Description

H. T. J. WIEGERINCK- AMPLIFIER CONTROL CIRCUIT Feb, 17, 1942. 2,273,680

Filed March 6, 1940 'AMPL/F/ER NEGATIVE FEEDBACK DETECTOR .L

f I /1 f2- a;\

AVC

IVEGA 77 W FEEDBACK WEAK SIGNALS STRONG SIGNALS AUDIO FREQUENQZ INVENTOR HENDR/KUS Z W/EGER/NCK ATTORNEY Patented Feb. 17, 1942 AMPLIFIER CONTRGLG'IRCUIT Hendrikus "T. J Wiegerinck, Eindhoven, Netherlands, assignor to Radio Corporation of America, a corporation of Delaware Application March 6, 1940,.SerialNo.322,460 In Germany April26,.1939

6 Claims.

As is well known, in radio receiving apparatus comprising automatic volume control the phenomenon occurs that during reception of weak signals interfering noise is reproduced by the receiver.

.It is'known to decrease this noise'by constructing the receiving apparatus in such a manner thatthe highffrequencies of the frequencyrange to be reproduced are suppressed during reception of'weak signals. This measure is based on recognition of the factthat the frequency spectrum of the interfering noise substantially comprises high audio frequencies, 1. e., higher than 4000 to 5000 cycles. Due to the reproduction of these high audio frequencies being rendered impossible, undisturbed reception is obtained. Another known interfering phenomenon occurring in short wave reception is that hum having. frequencies in the vicinity of 40 to 60 cycles occur due .to the rapid fadingphenomena in the short wave region and the action of the automatic volume control. To suppress this hum it is necessary during receptiton of weak signals that the low tones of the frequency range to be reproduced should besuppressed.

The present invention relates to a radio receiving apparatus in which measures are taken to suppress automatically the high and the low audio tones during reception of weak signals, whereas during reception of strong signals the whole frequency range to be reproduced is uniformly reproduced.

This is achieved according to the invention by controlling the amplification of the low frequency amplifier, which includes a negative feedback couplingpath, in dependency on the amplitude of the received signals. Thiscontrol acts insuch manner, and'the negative feedback coupling'path impedances dependent on frequency being interposed in the negative feedback circuit in such a manner that the negative feedback coupling is greater for the low and the high frequencies than the negative "feedback coupling for the intermediate frequencies of the frequency range to be am- The invention will be more clearly understood by reference to the. accompanyingv drawing wherein Fig. 1 shows a feedback amplifier.

Fig. .2 showsan embodiment of the invention,

Fig. 3 graphically shows the operation of the invention.

Fig. 1 shows an amplifier I with input terminals 2 and 3 and output terminals .4 and 5. A potentiometer constituted by impedances Z1 and Z2 is connected between the output terminals. If the voltage between the output terminals is c2 and the ratio is taken a voltage peg is produced across impedance Z2 and this voltage is supplied to the .input circuit of the amplifier in phase opposition with the voltage e1 to be amplified which is produced between theinput terminals. The total voltage produced in the input circuit of the amplifier consequently is 6o=61B62.

If the amplification of the amplifier is V, the output voltage e2 is consequently determined by e2=Veo=V(e1-cc2). From this .follows:

2 V 71* 1+Vc in which V is the amplification of the amplifier withnegative 'feedbackcoupling.

If the, amplification V and the value [3 of the negative fedback coupling are so chosen that which means that the amplification is almost exclusively determined by the negative feedback coupling. If, on the contrary, V and c are so chosen that V,8 1, V becomes equal to V, or in other words the amplification of the amplifier with negative feedback coupling is substantially equal to the amplification without negative feedback coupling.

According to the invention, in a radio receiving apparatus the amplification of the low frequency'amplifier, comprising anegative feedback coupling path,.is so controlled in dependency on the amplitude of the received signals that during reception of a weak signal the amplification V is so ,great or during reception of a strong signal so small that the condition VB 1 and B,8 1 respectively is fulfilled. Furthermore, the im pedances Z1 and Z2 are made so dependent on frequency that the negative feedback coupling for the low and the high frequencies is greater than the negative feedback coupling forthe intermediate frequencies. Since in this case during reception of weak signals the amplification of the low frequency amplifier is determined by the negative feedback coupling and the latter is great for the low and the high frequencies, these frequencies are suppressed so that interferences due to rapid fading and noise are not reproduced, or at least reproduced with but small amplification. During reception of weak signals the amplification of the low frequency amplifier is substantially equal to the amplification without negative feedback coupling. The latter amplification may have any desired variation of frequency, for example in such manner that all frequencies are reproduced uniformly.

Fig. 2 of the drawing shows a radio receiving apparatus embodying the invention. The signals received in an aerial 6 are amplified in the high, and intermediate-frequency, portion of the receiving apparatus indicated by I and supplied to a diode 8. The network can be the usual presecond detector circuits of a superheterodyn receiver. The low, or audio, frequency oscillations obtained by rectification, which are produced across a potentiometer resistor 9 shunted by a condenser H], are supplied through a condenser II and a resistance IE to the grid of a low frequency amplifying tube l3, in the output circuit of which is connected an output transformer l4 supplying the loudspeaker [5. For the purpose of obtaining a control voltage for automatic volume control of amplifiers I a diode I6 is provided, and the latter has intermediate frequency oscillations supplied to it. The control voltage is produced across a resistance H, which .is bridged by a condenser i8, and the control voltage is smoothed by means of a resistance i9 and a condenser 28 and controls in known manner the amplification of the high, and intermediate frequency, portion I of the receiver. The letters AVC denote the automatic volume control lead.

The control voltageis at the same time supplied, through a resistance 2|, to the control grid of the low frequency amplifying tube I3, and controls the amplification of this tubein such a manner that during reception of weak signals the amplification is great and during reception of strong signals the amplification is small. In the output circuit of the amplifying tube [3 are connected impedances dependent on frequency which are constituted by resistances 22 and 23 and

condensers

24 and 25. These frequency-dependent elements of the feedback path are purely passive; that is, the path includes no electron discharge device. To block the anode direct current a condenser 26 is provided. The resistance 23 is connected through a conductor 21. to the control grid of the tube with the result that a negative feedback coupling path is provided.

The condenser is so chosen that it exhibits a high impedance for the low frequencies and a small impedance for the high frequencies and in the intermediate region of the frequency range to be produced. The condenser 24 only exhibits a small capacity, and consequently constitutes a high impedance for the low frequencies. The negative feedback coupling for the low frequencies is almost exclusively determined by the ratio between the condenser impedance and the resistance 22 and this ratio is made such that a strong negative feedback coupling is produced for the low frequencies. region of the frequency range to be reproduced the impedance of the condenser 25 is small and the impedance of the condenser 24 high so that In the intermediate the negative feedback coupling in this region is small. For high frequencies the impedance of the condenser 25 is very small so that for these frequencies the negative feedback coupling is determined by the ratio of the resistance 23 to the impedance of the condenser 2A. This ratio is such that the negative feedback coupling is great for the high frequencies.

The resistances 22 and 23 and the

condensers

24 and 25 may thus be adjusted in such manner that the factor 6, which determines the negative feedback coupling for the low and the high frequencies exhibits the value and for the frequencies in the intermediate region the value Upon designing the impedance dependent on frequency in the negative feedback circuit, the impedances 9, Ill, I I and I2 etc., in the grid circuit of the amplifying tube I3 should be considered, which must also be given such values that the desired dependency on frequency of the negative feedback coupling is obtained. During reception of weak signals the amplification V of the amplifying tube is great and is, for example, 100. For the low and the high frequencies VB=10 in this case and for the intermediate region V5=l.5; consequently, the condition Vc 1 is satisfied so that during reception of weak signals the amplification is determined by the negative feedback coupling. Since the negative feedback coupling of the low and high frequencies is great, these frequencies are amplified to a smaller extent than the frequencies in the intermediate region. In this case the amplification V varies as a function of the frequency f as shown by curve A in Fig. 3.

With an increase in amplitude of the received signals the amplification decreases due to the control voltage supplied to the control grid of the tube I3; in this case the amplification V is no longer determined exclusively by the negative feedback coupling. At a very great amplitude of the received signals the amplification V is still but small, and is, for example, 3 so that for the low and the high frequencies V;3=0.3 and for the frequencies of the intermediate region Consequently, the condition VB 1 is fulfilled and the amplification V is substantially independent of the negative feedback coupling and equal to the amplification V of the amplifier without negative feedback coupling. If this amplification V is independent of frequency in the frequency range to be reproduced, the amplification V in the case of large amplitudes of the received signals may be represented as a function of the frequency by the line B in Fig. 3.

In the arrangement of Fig. 2, the low frequency amplifier only comprises a single amplifier tube I3. The invention is also applicable, however, to low frequency amplifiers comprising a plurality of stages, and in which the negative feedback coupling takes place through two or more of these stages. 1

What is claimed is:

1. In a radio receiver of the type including a modulated carrier wave transmission network.

7 2,273,680 a demodulator, a modulation voltage transmission network and means for controlling the transmission efliciency through said carrier network in response to received carrier amplitude variation; the improvement which comprises purely passive means for degenerating transmission of modulation voltage through said modulation network at the low and high portions of the modulation frequency range, and means responsive to said control means for minimizing said degeneration at relatively high received carrier amplitude.

2. In an audio-modulated carrier wave receiver provided with an audio amplifier having input and output terminals, means for deriving from received modulated carrier waves a control voltage whose magnitude varies directly with carrier amplitude, a degenerative audio voltage feedback network between said output and input terminals, said network consisting of purely passive elements with constants chosen to provide a substantially dominating degenerative feedback at the low and high audio frequencies, and means for controlling said audio amplifier gain with said control voltage whereby said degenerative feedback is greatly reduced at high received carrier amplitude.

3. In an audio-modulated carrier wave receiver provided with an audio amplifier having input and output terminals, means for deriving from received modulated carrier waves a control voltage whose magnitude varies directly with carrier amplitude, a degenerative audio voltage feedback network between said output and input terminals, said network having its constants chosen to provide a substantially dominating degenerative feedback at the low and high audio frequencies, said network comprising a condenser arranged in series with a resistor between said output terminals, a second condenser in series with a second resistor arranged in shunt with the first resistor, and said feedback path being connected to said second resistor, and means for controlling said audio amplifier gain with said control voltage whereby said degenerative feedback is greatly reduced at high received carrier amplitude.

4. In an audio-modulated carrier wave receiver provided with an audio amplifier having input and output terminals, means for deriving from received modulated carrier waves a control voltage whose magnitude varies directly amplifier gain is reduced with carrier amplitude, a degenerative audio voltage feedback network between said output and input terminals, said network consisting of purely passive reactive elements with constants chosen to provide a substantially dominating degenerative feedback at the lowand high audio frequencies, and means for controlling said audio amplifier gain with said control voltage whereby said degenerative feedback is greatly reduced at high received carrier amplitude and said audio for all audio frequencies.

5. In an amplifying system having at least one tube, said system having input and output terminals, means for applying signals of a wide frequency range to said input terminals, means for adjusting the gain of said tube over a wide range of values thereby to vary the transmission of said signals through said system, a negative signal feedback network connecting the said output and input terminals, said feedbacknetwork comprising frequency-dependent impedance elements whose constants are so chosen that feedback discriminates in favor of the low and high portion of said signal frequency range at high values of said tube gain, and said adjusting means being adapted substantially to eliminate feedback through said network at low values of said tube gain, said impedance elements comprising a first series condenser-resistor path across said output terminals, and a second series condenser-resistor path connected in shunt with the resistor of the first path.

6. In an amplifying system having at least one tube, said system having input and output terminals, means for applying signals of a wide frequency range to said input terminals, means for adjusting the gain of said tube over a wide range of values thereby to vary the transmission of said signals through said system, a negative signal feedback network connecting the said output and input terminals, said feedback network comprisingi solely passive frequency-dependent impedance elements whose constants are sochosen that feedback discriminates in favor of the high portion of said signal frequency range at high values of said tube gain, and said adjusting means being adapted substantially to eliminate feedback through said network at low values of said tube gain.

HENDRIKUS T. J. WIEGERINCK.