US2273997A - Negative feedback amplifier - Google Patents

Description

n Q m 5 INVENTOR. LEON RUB/N ATTORNEY.

Feb. 24, 1942. L. RUBIN NEGATIVE FEEDBACK AMPLIFIER Filed Aug. 16, 1939 Patented Feb. 24, 1942 NEGATIVE FEEDBACK AMPLIFIER Lon Rubin, Courbevoie, France, assignor to So,

cit Industrielle des Procds Loth., Neuilly- .sur-Seine, France, a corporation of France Application August 10, 1939, Serial No. 289,408 In the Netherlands September 23, 1938 4 Claims.

This invention relates to negative feed-back amplifiers, and has for its purpose to permit the application of negative feed-back coupling in' high-power amplifiers, for example in amplifiers of the B-class type, in which at least one of the stages, for example the final stage, is controlled by such a high alternating grid voltage that grid currents are produced.

As is well known, in such an amplifying stage the unpleasant phenomenon occurs that the grid-cathode impedance depends on the amplitude of the alternating grid voltage to be amplified, and consequently constitutes a variable charging impedance for the preceding amplifying stage which will be called control stage hereinafter. It is common practice, therefore, that the control stage is coupled to the following stage, which is controlled in grid currents and will be termed final stage" hereinafter, by means of a strong step-down transformerwhich steps up the impedance constituted by the gridcathode impedance of the final stage to a value which is great compared with the internal resistance of the control stage, the voltage across the primary, and consequently across the secondary winding, having a value which is independent of the grid-cathode impedance of the final stage.

With such amplifiers it is practically impossible, however, to apply negative feed-back coupling for reducing non-linear distortions, since the said transformer in conjunction with the output transformer between the control stage and final stage produces a phase displacement dependent on frequency, so that for certain frequencies the negative feed-back coupling will change to a positive feed-back coupling. Thus, the possible degree of negative feed-back coupling is limited to such an extent that the nonlinear distortions cannot be sufliciently eliminated.

According to the invention, instead of the usual coupling transformer between control stage and final stage, use is made of a coupling comprising a resistance which is interposed in the cathode lead of the first of the two amplifying stages to be coupled. w

This coupling has the property that the output impedance of the control stage measured between the terminals of the resistance in the cathode lead is low with regard to the variable grid-cathode impedance of the final stage, i. e. approximately equal to l/S, in which S is the slope of the amplifying tube of the control stage. Consequently, the variable control grid-cathode impedance of the final stage, which is in parallel to the resistance in the cathode lead, has a negligible influence on the amplification of the control stage so that the latter supplies to the grid of the final stage a voltage which is independent of the variable grid-cathode impedance of the final stage. Since, moreover, the phase displacement produced by this way of coupling is substantially independent of frequency, it is only with the use of such a coupling between the control stage and final stage that a strong negative feed-back coupling can be applied for suppressing non-linear distortions.

Due to the coupling resistance, being interposed in the cathode lead of the control stage, a negative feed-back coupling is already produced in this stage so that the non-linear distortions are reduced in this stage. In an amplifier according to the invention it is possible to use a very strong negative feed-back coupling without there being the risk of negative feed-back coupling being produced for certain frequencies which would lead to self-oscillation. The invention will be more clearly understood by reference to the accompanying drawing showing in Figs. 1 and 2, by way of example, two embodiments of an amplifier according thereto.

The amplifier in Fig. 1 has five amplifying stages i to 5, each of which comprises a pair of push-pull amplifying tubes. The final stage 5 is coupled to the control stage 4 by means of resistances 6 which are interposed in the cathode lead of the amplifying tubes 1 and 8 of the control stage 4, and whose extremities are connected to the grid and cathode of the amplifying tubes 9 and ll) of the final stage 5. This method of coupling, as was mentioned above has the property that the alternating voltage set up across the resistances 6 is substantially independent of the variable grid-cathode impedance of the tubes 9 and I0 which is a result of grid currents being produced in these tubes. Moreover, this method of coupling does not produce a phase displacement dependent on frequency which permits the use of a negative feed-back coupling. The negative feed-back circuit'extends from the primary winding of the output transformer H to the secondary winding of the inputtransformer I! of the amplifier, and comprises two potentiometers constituted-respectively by the resistances l3 and I5, and I4 and IS.

The amplifying stages I and 2, 2 and 3, 3 and 4 are coupled to one another in known manner by means of resistance-condenser coupling. In

- be amplified is maintained within narrow limits also in the preliminary stages so that neither these preliminary stages do produce for any frequency of the frequency range to be amplified such a phase displacement that the negative feed-back coupling changes to a positive feed-back coupling which may lead to self-oscillation.

In order to suppress the frequencies outside the frequency range to be amplified, a reducer constituted by a resistance 2| and a condenser 22 is provided between the third and the fourth amplifying stage. Owing to this reducer the amplification of very high frequencies is attenuated. Moreover, the reducer will produce a phase displacement which may be maximum 90. If this maximum phase displacement occurs at a frequency at which also the other stages of the amplifier produce a 90 phase displacement, it will yet be impossible for self-oscillation to occur, since for that frequency the attenuation is suiiiciently great.

Although the invention in Fig. 1 has been explained by reference 'to an embodiment in which the control stage is coupled to the final stage of an amplifier by means of a resistance in the cathode lead of the control stage, the application of the invention is not limited thereto. The said coupling can also be applied between two other stages of an amplifier, which are bridged by a negative feed-back circuit.

The improvement in Fig. 2 consists in that the resistance in the cathode conductor of the first of the two stages is replaced by a combination of resistances, inductances or capacities, preferably by a combination of resistances and inductances. In the amplifier circuit according to Fig. 1 a resistance is included in the cathode conductor of the first of the said two amplifier stages, the anode direct current of this stage brings about a voltage drop across this resistance, it being necessary for this voltage drop to be compensated for by the source of grid bias of the following amplifier stage. This voltage drop is avoided if in the cathode conductor the resistance or at least a part of the resistance is replaced by a combination of resistances, inductances and capacities and preferably by an inductance.

In the circuit according to Fig. 1 the non-linear grid current of the final stage will pass both via the source of anode voltage of the control stage and via the source of grid voltage of the final stage. The current which fiows via the source of anode voltage is inversely proportioned to the output impedance of the control stage, measured between the terminals of the resistance in the cathode conductor, said output impedance being about equal to 1/8, in which S designates the mutual conductance of the amplifier valve of the control stage. The current which flows via the source of bias of the final stage is inversely proportioned to the said resistance in the cathode conductor with the addition of the internal resistance of the source of bias.

If the resistance in the cathode conductor is large relatively to the output impedance of the control stage, measured between the terminals of the resistance, that part of the grid current which flows via the source of bias is smaller than that part of the grid current which flows via the source of anode voltage of the control stage. The source of bias of the final stage is thus not loaded appreciably.

According to Fig. 2 the cathode impedance is formed by the series combination of a resistance 30 and an inductance coil 3| it can be insured both that the source of bias of the final stage is not loaded and that the direct current voltage drop across this series combination is but small.

In the use of an impedance in the cathode conductor formed by a combination of resistances, inductances or capacities, amplification independent of frequency is obtained over a wide frequency range if the said impedance is high relatively to output impedances of the control stage, measured between the terminals of this impedance.

What I claim is:

1. In a signal amplifying system comprising signal input and output circuits, a pair of electron discharge devices, each device having at least a control grid, a cathode and an output electrode, said input circuit being connected to said grids to apply signals thereto in phase opposition, a load impedance in the space current paths of both of said devices, said load impedance being connected between the cathodes of said first pair of devices, a second pair of electron discharge devices each including at least a control grid, a cathode and an output electrode, said signal output circuit connecting the output electrodes of said second pair of devices in push-pull relation, the control grids of said second pair of devices being connected to respectively different points of said load impedance which are of opposite phase, and a degenerative signal voltage feedback path from said output circuit to said input circuit.

2. In a signal amplifying system comprising signal input and output circuits, a pair of electron discharge devices, each device having at least a control grid, a cathode and an output electrode, said input circuit being connected to said grids to apply signals thereto in phase opposition, a load impedance in the space current paths of both of said devices, a second pair of electron discharge devices each including at least a control grid, a cathode and an output electrode, said signal output circuit connecting the output electrodes of said second pair of devices in push-pull relation, the control grids of said second pair of devices being connected to respectively different points of said load impedance which are of opposite phase, and a degenerative signal voltage feedback path from said output circuit to said input circuit, said load impedance consisting of a resistor connected between the cathodes of said first pair of devices, said different points being the cathode ends of said resistor.

3. In a signal amplifying system comprising signal input and output circuits, a pair of electron discharge devices, each device having at least a control grid, a cathode and an output electrode, said input circuit being connected to said grids to apply signals thereto in phase opposition, a load impedance in the space current paths of both of said devices, a second pair of electron discharge devices each including at least a control grid, a cathode and an output electrode,

said signal output circuit connecting the output electrodes of said second pair of devices in pushpuli relation, the control grids of said second pair of devices being connected to respectively different points of said load impedance which are of opposite phase, and a degenerative signal voltage feedback path from said output circuit to said input circuit, said load impedance being a resistor connected between the cathodes of said first pair of devices, and means for applying negative bias to the control grids of said second pair of devices through said resistor.

4. In combination, a pair of push-pull amplifier stages in cascade, a signal input circuit connected to input electrodes of the first stage, an

output circuit connected to the output electrodes of the second stage, a resistive load connected in the common space current paths of the amplifiers of the first stage and between the cathodes thereof, means for connecting the input electrodes of the amplifiers of the-second stage to the respective cathode ends of said load, and a degenerative feedback path between the output and input circuits.

' LEON RUBIN.

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