US2312748A - Volume control circuit - Google Patents

Description

March 2, 1943.

TOSOURCE 0F MOM/L4 TED SIGNALS H. A. BROOS VOLUME CQNTROL CIRCUIT Filed June 22;.1959

INVENTOR.

' HENR/CUS A. BROOS ATTORNEY.

Patented Mar. 2, 1943 VOLUME CONTROL CIRCUIT Henricus Adrianus Broos, Eindhoven, Netherlands, assignor, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application June 22, 1939, Serial No. 280,507 In the Netherlands October 26, 1938 2 Claims. (01. 250-27) My present invention relates to amplifiers with so-called negative feed-back, and it consists in that the control of the amplification is automatically accompanied by a control in the inverse sense of the negative feedback.

Thus, one obtains the desired state in which with small signals the sensitiveness of the amplifier is not reduced by negative feedback, while in the case of strong signals the negative feedback enters into operation and improves the timbre of the sound.

According to the invention, use may be made for this purpose of a voltage divider from which the voltage to be amplified is taken and which, in addition, is traversed by a current which is withdrawn from the output circuit, the arrangement being such that the voltage maximum of the signal to be amplified coincides with the voltage minimum which is fed back.

According to the invention, in order to insure that in the case of the maximum amplification the negative feedback is completely switched out, the voltage divider is preferably connected in one of the branches of a Wheatstone bridge, the negative feed-back voltage being applied between the ends of one diagonal while in the case of the minimum amplification the control grid and the cathode of the back-coupled tube are connected to the ends respectively of the other diagonal. This arrangement offers the further advantage that by connecting a tone filter in the first-mentioned diagonal, tone correction is obtained which depends on the position of the volume control device.

The invention will be more clearly understood by referring to the accompanying drawing which represents, by way of example one embodiment of the invention applied to the final stage of a radio receiver.

The modulated high, or intermediate, frequency oscillations are transmitted by a coil I to an input circuit 2, 3 of a diode-pentode tube 4, which input circuit is tuned to the frequency of these oscillations. This tube comprises a cathode 5, three grids 6, I and 8, an auxiliary anode 9 and a principal anode 25. The upper end of the circuit 2, 3 is directly connected to the auxiliary anode 9, the lower end being connected through a small condenser It to the cathode 5, and through a resistance l I to one of the diagonal points of a bridge-connection in the four branches of which are located a resistance I2, windings l4 and I5 and a resistance IS. The windings l4 and I5 are located on the same core of transformer T as a primary winding l'I which constitutes the output impedance of the tube 4, and as a secondary winding l8 which supplies the coil IQ of a dynamic loudspeaker 20.

The resistance I2 is formed as a voltage divider having a slide-contact l3 which is connected through a blocking condenser 2| to the control grid 6 of the tube 4. This grid is connected to earth through a leakage resistance 22, while the cathode is earthed through a resistance 23 and a large condenser 24 connected in parallel therewith. The voltage drop across the resistance 23 is used in well known manner as the negative bias voltage of the grid 6.

The modulated oscillations coming in at l are detected by the diode 9-5, the low frequency component taking its way through the resistance II and the bridge-connection, l2, l4, l6, l5 to the cathodefi, a variable proportion of the loss of voltage thus produced in l2 being applied, by means of the slide-contact [3, through the condenser 2| to the control grid 6. The tube 4, whose grids land 8 act in well known manner as a screen grid and a suppressor grid respectively, amplifies these low frequency oscillations and supplies them for the greater part through I1 and I8 to the loudspeaker IS. The remainder is transmitted to the windings l4 and I5 and thus made operative in the closed circuit l2, l4, l5, l6 and this in a sense such that negative feedback occurs. The values of the four previously mentioned impedances are, therefore, so chosen that:

'its highest position, which corresponds to the maximum amplification, negative feedback will not occur. The latter, however, always increases accordingly as the contact 13 is slid downwards.

An analogous effect, that is to say a negative feedback which varies inversely with the ampli-' fication, is obtained when the above-described arrangement is simplified by omitting the winding l5 and the resistance It. In this case, however, the negative feedback is never zero since the diode 95 represents a resistance which is not negligible. By connecting the free ends of the windings l4 and I5 through the intermediary of a switch 28 to a tone filter 26-41, there is librium of the bridge l3, l4, I5, IS.

The invention may be applied in the same purpose and with the same effect to many Ether circuit arrangements without departing from the spirit of the invention.

What is claimed is: 1. In a signal voltage transmission circuit having a pair of input terminals and a'pair' ofv out;

put terminals, a source of signal voltage, a bridge network having one pair of inductive arms, one of i the input terminals being connected to the June tion of the inductive arms, the second pair or bridge arms being resistive, a frequency. discriminatory path connected in shunt with both said resistive arms and acting as a diagonal of: said bridge network, an adjustable element connect-" ing the second of the input terminals to one of said resistive arms, and a signal voltage transmission path coupling said output terminals to said inductive arms.

2. In combination with an audio modulated carrier detector provided with a load impedance, an audio voltage transmission tube having at least a control grid, cathode and output electrode, a bridge network comprising a pair of impedance arms, one of said arms being said load. a se eondpair of impedance arms having its junction c onnected to said cathode, an adjustable element variably coupling said control grid to slaid'load impedance, means degeneratively applying signal voltage in the output electrode circuit'to said second pair of arms, a tone filter network connected across said second pair of arms as a diagonal of said bridge, said arms having, theconstants thereof so chosen that the biidgeis balanced and no degenerative voltage is applied to said. control grid; when. the adjustable. element is, at. theljunction of said first, pair- Of: arms HENRICUS BROOS;

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