US2398596A - Limiter circuit - Google Patents

Description

April 16, 1946. T.'H.PR|CE- v 2,393,596

LIMITER CIRCUIT Filed March 24, 1943 k viii.

' INVENTOR THOMAS H. PRICE Patented Apr. 16, 1946 Thomas Henry Price, Chelmsiord, England, asslgnor to Radio Corporation of America, a corporation of Delaware Application March'24, 1943, Serial No. 480,343 In Great Britain January 16, 1942 i 2 Claims.

The present invention relates to thermionic circuit arrangements, and particularly to limiter arrangements therefor.

It is irequently required that there shall be a linear relationship between the input signal and output signal of a thermionic valve circuit for all values of input signal up to a given amplitude thereof, and that for values of input signal above the given amplitude there shall be no further increase in the value of the output signal. That is, that the output shall be limited. There are, of course, a number of circuit arrangements designed to satisfy this requirement, but, so far as my experience goes, they suffer from the defect that appreciable and objectionable distortion occurs before the point is reached at which limi tation of the output occurs. What is really a diflerent aspect of the same phenomenon, limitation 01' output takes place at a point which is undesirably higher than the point at .which that the grid current has negligible effect upon the input circuit. The first and second impedances may be given such values that the hallcycle of input signal voltage which sweeps the anode current negatively will, by reason of the coupling of the grid and anode circuits in the second impedance, greatly reduce distortion effects until such an input signal voltage is applied that the falling anodecur'rent is too low ior the prevailing degree of feedback to eifect appre-' ciable correction, so that'sharp limitation ocarrangement of Figure 1, there is connected bethird and fourth impedances connected between minal, connections whereby an input signal may be applied across the fourth impedance between the Junction point of said third and fourth impedances and the negative terminal, and connections whereby an output signal may be taken from across the series-connected anode to cathode space of the valve and the second impedance. The values of the first and second impedances are such that for an input signal of given voltage, grid current just does not flow, but for an input signal or voltage higher than said given voltage, grid current fiows in said second, third, and fourth lmpedances in such direction as to increase the effective negative bias oi the control and due to the fiow oi anode current in the second impedance. Moreover, the "values ofthe third and fourth impedances are such that the grid currentcauses a very sharp discontinuity in the change in anode current.

The third impedance may be given such value tween the positive terminal of a source of direct current and the anode'of a valve AI, having an anode, control grid and cathode, an adjustable resistance RI. Between the cathode of the valve and the negative terminal of the current source there is connected asecond adjustable resistance R2. A reservoir or decoupling condenser C is connected across the said current source; that is, from the positive end of the first resistance to the negative end or the second resistance. A third resistance R3 is connected at one end'of it to the control grid of the valve, and at the other end of it to one end of a fourth resistance R4. The other end of the fourth resistance R4 is con nected to the said negative terminal. An input circuit IP is connected across resistance R4, and an output circuit OP is connected across the anode to cathode space of the valve in series with resistance R2.

Resistance R2 efl'ects a coupling between the anode to cathode circuit or the valve and the grid to cathode circuit, and also, provides for negative feedback. By suitably choosing the values of resistances RI and R2 it is possible to obtainthe condition that a given sinusoidal input signal voltage having an amplitude u will be Just below that value at which grid current flows, but sweeps the anode current fiow into the bent tude greater than '0 will cause grid current to flow through resistances R3, R2 and R4 in such a direction as to increase the negative grid bias voltage obtained by the flow of anode current through the resistance R2. If resistances R3 and R4 be given suitable'values this flow of grid current will cause a very.sharp discontinuity in the change of anode current. Eurthenif resist-. ance R3 be given a suitable value this grid current can be made to havenegligible effect "on the circuit which supplies the input voltage 12.

If, also, theresistances RI and R2 be suitably chosen the half-cycle of input signal voltage which sweeps the anode current negatively will, by virtue of'the coupling of the grid and anode circuits in resistance R2, experience the purifying effect of the feedback principle which is brought into play in such a circuit arrangement.

During this half-cycle, therefore, distortion of the form of the input wave is greatly reduced until such an input signal voltage is applied that the falling anode current is too low for the prevailing degree of feedback to effect appreciable correction, Sharp limitation, then, is also produced for this half-cycle. By asuitable choice of the comp nent values it is possible to arrange that the two limiting actions occur, as indicated in Figure 3, for closely equal amplitudes of positive and negative half-cycles, and that the distortion of -the form ofthe output waveyof voltage V is neg- 'ligible until limiting occurs. This good limiting componentsmust' be so chosen that an anode current I flowing with a steady grid bias voltage V should be exactly doubled if the grid bias voltage be made zero. This permits of such limitation of grid current and of zero anode current, as to produce the symmetrical limitations for the positive and negative half waves of signal to operate for input signals in excess of 0. From this it follows that for the circuits above and below described, the product of In. and R2 (where R2 is the value of resistance R2) should be substantially. equal to v, the input signal level above whichit is desired that limitation of the amplitude of the output signal shall occur.

In a preferred circuit arrangement accordin to the invention;illustrated in Figure 2, the valve Al is preceded by an amplifier stage comprising a further valve A2. The latter has at least an anode, control grid, and cathode, and is connected between the positive and negative terminals of the source of anode current. A fifth "resistance R5. is connected between the positive terminal of the current source and the anode of the valve A2,.an'd a sixth resistance R6 is connected between the cathode of valve A2 and the negative terminal of the current source. A feedback circuit, including a blocking condenser CBI and a resistance R1, extends from the anode of valve Al to the cathode of valve A2. The anode of. valve A2 is coupled, through a blockin condenser CB2, to th junction point of the resistances R3 and R4. The input signal is applied to valve A2 across a resistance R in the grid to cathode cir-' 30 some positions.

level of inputvoltage v to the valve A2. The additional feedback is controlled by suitably proportioning the values of resistances R6 and R1. It, also, permits of such limiting action that less than 1% distortion of the output signal can be' 10 obtained for all signal levels less than 10% below that at which severe limitation has set in. This represents a considerable improvement over known' existing limiter circuits.

Suitable vvalues for the various components in a set-upusin a Marconi valve designated in Great Britainas type H63 for valve A2, and a Marconi valve designated as type KT63 for valve Al, are as below: 7

Resistive value in ohms RI 5,000 R2 Approximately 1,110 R3 v 100,000 R4 500,000 R5 250,000

R6 3,000 R1 Notless than 100,000

Various minor modifications are possible. Impedances other than resistances may be used in A variable resistance R8 may be inserted in the feed connection between the source of anode current and the reservoir or decoupling condenser C. This is illustrated, for example, in Fig. 1. In this manner a useful auxiliary control is obtained over the input voltage which prow duces limitation.

What I claim is: 1.. A thermionic valve circuit arrangement including in combination, a thermionic valve hav- 4 ing at least an anode, control grid and cathode,

a source of anode current, a first impedance connected between the positive terminal of said current source and anode, a second unbypassed resistive impedance connected between the cathode 5 and current source negative terminal whereby anode current flow therethrough provides 2. voltage which is degeneratively applied to said grid, series-connected third. and fourth impedances connected between the control grid of the valve and said negative terminal, connections for applying an input signal across the fourth impedance between the junction point of said third and fourth impedances and said negative terminal, connections whereby an output signal may be taken from across the series-connected anode to cathode space of the valve and the second impedance, the values of the first and second impedances being so chosen that for an input signal.

of given voltage, grid current just does not flow b and the valve operates at the lower bend of its anode current-grid voltage characteristic, but for an input signal of voltage higher than said given voltage, grid current flows in said second, third, and fourth impedances in such direction as to increase the eifectivenegative bias of the control change in anode current, an amplifier stage preceding said valve, said amplifier stage comprising a second valve having at least an anode, control grid, and cathode connected between the positive and negative terminals of said source of anode current, a fifth impedance between the positive terminal of the source and the anode of the second valve, a sixth impedance connected between the cathode of the second valve and the negative terminal of the current source, a signal source coupled to said control grid of the second valve, said input signal connections of the first valve being made to said fifth impedance, a degenerative feedback circuit, including a blocking condenser and a seventh impedance, extending from the anode of the first valve to the cathode of the second valve.

2. In an amplitude limiter, a tube having at least a cathode, output electrode and control grid,

a signal input circuit, an amplifier network coupling said input circuit to said grid and cathode, a signal output circuit coupled to said output electrode and cathode, an unbypassed resistor in the space current path of the tube common to the grid circuit and output electrode circuit, said resistor developing thereacross signal voltage apat least onc resistor in circuit with said grid and plied as negative ieedback voltage to said grid,

unbypassed resistor to provide a voltage from grid current thereby to increase the eflective negative bias of said grid over and above that due to said :ieedback, and means in circuit with said output electrode to provide a relatively low positive potential, said amplifier network including a second tube having at least a cathode, control grid and output electrode, said signal input circuit being coupled to the grid and cathode of the second tube, an unbypassed resistor in the second tube space current path, means coupling the second tube output electrode to the grid oi! the first tube, said last resistor being common to the grid circuit and output circuit 01 the second tube whereby voltage developed across the resistor is applied in degenerative phase to the grid of the second tube, and an auxiliary negative feedback path from the output electrode of the second tube to the cathode of the first tube.

" THOMAS mNRY PRICE.

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