US2365575A - Electron discharge amplifier - Google Patents

Description

Dec. 19, 1944. D., E MAXWELL 2,365,575

ELECTRON DISCHARGE AMPLIFIER Filed Deo. 3l, 1941 Inventor: Y Donald-E-MaxWell,

His Attorney.

Patented Dec. 19, 1944 ELECTRON DISCHARGE AMPLIFIER.

Donald E. Maxwell, Scotia, N. Y., assignor to I' General Electric Company, a corporation o! New York Application December 31, A1941, Serial No. 425,111

Claims.

My invention relates to electron discharge ampliiiers and particularly to such amplifiers employing degeneration. vThe advantages resulting from the use of degeneration in amplifiers -to rel duce distortion are well known.`

An object of my invention is to provide, in such amplifiers, which are required to have a denite anode impedance, means whereby degenerative voltage may be supplied to the input of such ampliiiers without-affecting such anode impedance. Such means adapts the amplier to use in circuits where definite 4predetermined relationships between the amplifier anode impedance and other circuit impedancesare important. An object of my invention is to provide means whereby degeneration may be employed in such an amplifier without affecting such impedance relationships and at the same time increasing the undistorted power which the ampliiier supplies to its load circuit. 4

Degenerative voltage proportional either to the output voltage or to the output current of the amplifier may be supplied to the input thereof. If it be proportional to the output voltage, such voltage has the eilect of decreasing the output impedance of the amplifier. If it be proportional to output current of the amplifier, it has the effect of increasing the output impedance of the amplifier. An object of my invention is to supply to the input of the amplifier two components of degenerative voltage, one proportional to output voltage and the other proportional to output current, said components being so proportioned that the output impedance of the amplifier remains at stood by reference to the following descriptionv taken in connection with the accompanying drawing in which Fig, 1 represents a circuit in which myinvention finds utility. Fig. 2 representsone embodiment of my. invention and Fig. 3 is a.

schematic representationof the circuit of Fig. 2

to facilitate analysis.

Referring to Fig. 1, I have shown therein at I,

2, 3 and l a plurality of sources of signal electromotive force to be amplified. These sources may comprise microphones, for example, such as are Vused in the studios of a radio broadcasting station. Each of these sources supplies signal electromotive force to a corresponding amplifier 5, B, 1, 8, each of which supplies its output through a respective attenuator 9, III, II, I2 and impedance matching network I3, I4, I5, I6, each of which may comprise a simple resistance, to a. common line I1. v'Ihis line may also include an impedance matching network Y I8 and an attenuator I9 through which the signal electromotive forces are supplied with desired intensity to any suitable apparatus 20. This apparatus may comprise the modulation system of a broadcast transmitter.

In such a system the attenuators 9, I0,- I I, I2 and I9, which are arranged to control manually the intensity of signal voltage supplied therethrough. are provided with a control member, which may be manipulated by the operator, and which cooperates with a calibrated scale. Such a control member is represented by the arrow 2l which is shown as cooperating with a scale 22, for example.

It is necessary that the calibration of these controlv members be accurate and the accuracy thereof is dependent on the impedance matching which exists in the circuit on either side of the attenuator. That is, the calibration of these attenuators is dependent upon the use of the attenuator between an input circuit having an im#- pedance, which matches the input impedance of the attenuator, and an output circuit which has an impedance, which matches the output impedance of the attenuator. If these impedance -matches do not exist, then the calibration oi the attenuator is inaccurate. This, ofcourse, means that the impedance of the ampliiiers 5, 6, 'I and 8, which feed the attenuators 9, III, I I and I2 respectively, must have an impedance which matches the input impedance of the respective attenuators. 1f it does not match the input impedance of the attenuator, not only is the calibration of respective attenuator inaccurate, but the impedance looking from the junction point 23 toward the respective attcnuator is such that it does not match the impedance of the other attenuators I0, II, I2 and .I9 in parallel. with the result that the calibration of these other attenuators is also disturbed, and the impedances thereof do not match that of the respective amplifiers.

Thus the adjustment of any attenuator affects the gain, not only of the channel in which it is connected, but also of every other channel thereby requiring readjustment of all of the other attenuators. 'Ihis effect is most pronounced when the attenuator is adjusted near the position for minimum attenuation. This effect is, of course, objectionable. It is desirable that the adjustment of all of the attenuators be entirely independent' of each other.

Therefore, it is necessary that the output impedance of each amplifier-match the 'input impedance of the respective attenuator into which it operates. If the amplifier employ a discharge device having a transformer connected between its output and the input of the attenuator then the output impedance of the discharge device must match the impedance of the attenuator as seen through the transformer. This, of course, assumes a perfect transformer, a condition which obtains within practical limits of accuracy. It is desirable that the amplifiers represented by the rectangles 5, 6, 1, and 8 employ degenerative feedback to reduce distortion and to secure certain other advantages. As commonly employed, such degenerative feed-back either reduces or increases the internal impedance of the amplifier with respect to the impedance that exists when no feedback is present.

In accordance with my invention, a feed-back circuit is provided which secures the advantages common to degenerative feed-back systems in general, and which at the same time does not affect the internal output impedance of the amplifler.

One circuit which may be employed in accordance with my invention ls shown in Fig. 2. The circuit comprises a pair of electron discharge amplifiers 25 and 26. The electron discharge amplifier 25 is supplied with voltage to be amplified through a transformer 24, the secondary of which is connected between the control electrode and cathode rthereof through a bias resistor 21, which is shunted for currents of the frequency to be amplified by a condenser 28. The amplified electromotive force appears upon the anode of this device and upon the resistance 29 and is thus supplied through condenser 30 to the control electrode of the discharge device 26. Discharge device 26 operates tog-amplify these signal electromotive forces and to supply them through transformer 3| to the output circuit.

Anode operating potential may be supplied from a source which may be connected between ground and the terminal 32, this current being supplied to the anode of discharge device 26 through resistance 33 and the primary winding of transformer 3|, and to the anode of discharge device 25 through resistances 34 and 29. It may also be supplied to the screen grid 35 of discharge device 25 through a resistance 35. The resistance 34 cooperates with condenser 34' and resistance 33 with condenser 33' to form the usual anode isolation filters commonly employed in such circuits to prevent coupling between the amplifiers through the power supply source. The screen grid 35' is connected to the cathode through a large by-pass condenser 36 having negligible impedance at any frequency to be ampliiied. This device 25 is also provided with a suppressor grid 31 which is connected to the cathode thereof directly. Device 25 has a high anode impedance such that it operates as a constant current source of signal voltage for the discharge device 26. That is, signal current flowing in the space path of device 25 is not affected to any material degree by the external impedance connected 'between the anode and Cathode thereof.

The cathode of .discharge device 26 is connected to ground through a resistance 40, which is in series with a resistance 4|, the latter of which is by-passed for currents of the signal frequency by condenser 42. The resistances 40 and 4| are proportioned to provide a suitable operating bias upon the control electrode of the discharge device 26. The control electrode 41 is connected to ground through a resistance 43. Resistance 40, unlike resistance 4|, is not bypassed and thus the signal current flowing through it produces a certain amount of electromotive force, which is impressed between the control electrode and the cathode of the discharge device in degenerative phase. This electromotive force is proportional to the signal current flowing between the anode and cathode of the discharge device.

A second component of voltage from the anode of the discharge device 2-6 is supplied to the grid thereof through a blocking condenser 44, which is connected between the anodes of the two discharge devices through a properly proportioned resistor 45. Thus the voltage supplied from the anode of discharge device 26 through condenser 44 and resistance 45, appearing upon resistance 29, is supplied through condenser 30 to the control electrode of the device 26 in degenerative phase. This voltage is proportional to the signal voltage existing between the anode of discharge device 26 and ground.

The secondary winding of the transformer 3| is connected to the load impedance which,l in the equipment of Fig. 1, may be the input terminals of the attenuators 9| 2. It is necessary that the internal impedance of the discharge device 26 as reflected by the transformer 3| to the attenuator match the impedance of the attenuator. Accordingly, it is requiste to satisfactory operation of the system that the degenerative feed-back employed be so controlled that it does not affect the anode impedance.

In accordance with my invention the two components of degenerative voltage supplied to the input of the amplifier are so proportioned relative to each other, to the amplification of the amplifier, and to the anode impedance thereof that they have no effect upon that anode impedance.

In order better to explain the operation of my invention, I have represented in a more schematic way in Fig. 3 the circuit which is illustrated in Fig. 2. In this circuit I have shown at 46 and 41 the cathode and control electrode of the discharge device 26. This control electrode 41 is represented as connected to the cathode through resistances R1 and Ra. The resistance R1 may be considered to represent-the resistance of the paths comprising the resistances as and 29 and the anode-cathode path -tf discharge device 25 of Fig. 2, all in parallel. The resistance Ra :may represent the resistgnce of the resistor 40 of Fig. 2. The internal-f anode impedance between the anode and cathode of the discharge device 26 is represented in Fig. 3 at rr and the resistance of resistor 45 of Fig. 2 is represented in Fig. 3 'as R2. The impedance looking from the discharge device to the primary winding of the transformer 3| of Fig. 2 is represented at R in Fig. 3. This impedance, of course, is equal to the impedance of the load which is connected to the secondary winding of transformer 3| multiplied by the turns ratio of the transformer, if transformer 3| be considered an ideal transformer and it must equal the resistance rr.

In order to analyze the system as thus described let us introduce a search voltage E into the circuit at the point indicated by that character in Fig. 3 and write the resulting equations.

If we let V represent the resulting voltage between the control electrode and cathode of amplifler 2l, then aV represents the voltage produced in the anode circuit of the amplifier, where a represents the amplification of the amplifier, and the following relation applies:

where `ia represents the signal current in the circuit Ra, rp, Rz, R1, and i1 represents the current in the circuit Ra, rp, R.

Then, applying Kirchoifs laws to the circuits carrying the currents il and is, we obtain the following relationships:

The denominator -AB-l-C of this Expression 5 represents the total impedance across which the voltage E appears and includes the resistance R and may be called Zt. When expanded, Expression 5 becomes:

sented to the primary of transformer 3i, and

(7) Z=TP+R3+ALR" [TP-i-Rs'iuRal [TP-l-Rs-FMRi-iR/Q] `RV Rz+TP|Ra+l(R1 +123) Now, since the anode impedance fr oi the discharge device must equal the load impedance, Z must equal fr, or

Equation 8 for R1', R2, and R: individually. If

certain entirely practical assumptions be made, however, this equation reduces toa very simple expression which islsatisfactory for many practical purposes in the design of such circuits. 'I'his expression is:

The assumptions which `must be made to effect this simplification of the expression of the relatlonslfips between these quantities are:

(a) That p. is very much greater than 1 (b) -That Ra is very much smaller than R1 (c) That rp is very much smaller than R2 (d) That n is very much smaller than R1 These conditions hold approximately true in the usual practical circuit design.

Accordingly, if the system be designed to satisfy Equation 9, the anode impedance of the discharge device will be, within a reasonable error,

not greater than about ten per cent. the same as it would be if no degeneration were employed.

Moreover, as seen from Equations 6 and 7, Z is not affected by the quantity R. Accordingly, if the load impedance varies for example with frequency over the range of frequencies to be amplified,l it does not affect the impedance rr.

As a practical example, let us assume that the discharge devices 25 and 28 are of the 6.17 type. This discharge device connected as shown at 25 has an anode to cathode impedance of approximately one megohm. At 26 it is shown with its y screen and suppressor grids connected directly to its anode. It thus acts as a triode and when operated with a supply voltage between ground and point 32 of 225 volts in a circuit as shown, with circuit constants presently to be given, has an anode to cathode impedance rp of approximately 15,000 ohms and a of approximately 20. These circuit constants are as follows:

Resistance 00 ohms-- Resistance 0 do 2,220 Resistance 0l ..-do 570 Resistance 00 megohms 1 Resistance 20 ohms-- 100,000 Resistance 05 do 600,000 Condenser 02 microfarads 40 Condenser 00 e ..-do. .1 Condenser 00 do. i5

Using these values, the value R1 of Fig. 0 is R1 ohms-- 03,300 R2 d0 600,000 dn 2,200

Equation 7 may now be written or Znrr y This means that the impedance seen by the load circuit is not affected by the presence of the degenerative feedback voltage.

This also follows from Equation 9 which may be written:

approximately 4 out any substantial complication of the apparatus or the addition of parts which materially in'- crease the cost thereof.

Moreover, as seen from Equation 9 the con- .stants of the circuit may be proportioned to produce any desired amount' of feed-back voltage which may be necessary to correct such distortion as .may be encountered in any particular application. This contrasts. for example, to a system employing a high impedance output tube, such as a pentode, with degenerative feed-back which reduces its anode impedance to a. value which matches that of its load circuit. In such an arrangement only the amount of feed-back necessary to such matching may be employed irrespective of the distortion present.

While I have shown a particular circuit designed to operate in accordance with my invention, it will be understood that this is by way 6i' illustration of the principles involved and that many modifications may be made both in the circuit arrangement and in` the instrumentalities employed and I contemplate by the appended claims to cover any such modications as fall within the true spirit and scope of my' invention.

What I claim as new and desire to secure by n Letters Patent of the United States, is:

1. In combination, an electron discharge amplier having input electrodes between which voltage to be amplified is supplied and having output electrodes between which the amplified` voltage appears, a. circuit between said output electrodes, means to supply voltage from said circuit to said input electrodes in degenerative phase, and means to prevent the voltage supplied by said last means from ail'ecting the impedance between said output electrodes.

2. In combination, an electron discharge amplifier having input electrodes between whichr voltage to be amplified is supplied and having output electrodes between which the amplified voltage appears, a load impedance connected between said output electrodes having a value equal to the output impedance of said amplifier, means to supply degenerative voltage from said output electrodes to said input electrodes thereby to increase the capacity of said amplifier to supply undistorted power to said load impedance, and means to maintain the impedance between the output electrodes of said amplifier substantially constant irrespective of the presence of said degenerative voltage.

4. In combination, an electron discharge amplifier having input electrodes between which voltage to be amplified is supplied, and output electrodes, a load circuit connected between said output electrodes, means to supply from said output electrodes to said -input electrodes two voltages in degenerative phase, one of said voltages being proportional to current in said load sensu 5. In combination, an electron discharge ampliner having an anode, a cathode, and a control electrode, a circuit between said anode and cathode including a resistance, a circuit between said grid and cathode including said resistance and a v second resistance, and a third resistance connected between said anode and control electrode. said first resistance having a value substantially equal to the ratio of the product of said second resistance and the internal resistance between said anode and cathode to said third resistance.

6. In combinatioman electron discharge amplifier having an anode, a cathode, and a control electrode, a circuit between said anode and cathode including a resistance, a circuit between said grid and cathode including said resistance and a -second resistance, and a third resistance connected between said anode and control electrode, the values of said resistances satisfying the equation said resistance and additional impedance, said circuit and the other of said two voltagesl being resistance producing between said control electrode and cathode a degenerative voltage proportional to the intensity of current in said outrput circuit, and a resistance between said anode and control electrode to supply additional voltage proportional to the voltage between said anode and cathode in degenerative phase to said control electrode and cathode, said resistances and impedance being so proportioned that said degenerative voltages do not affect the internal anode impedance of said discharge device.

8. In combination, an electron discharge amplifier having input electrodes between which voltage to be amplified is supplied, and output electrodes between which the amplified voltage appears, the impedance of said amplifier between said output electrodes matching the impedance to which said amplified voltage is supplied in the absence of any degenerative feed-back in said amplifier, and means to produce degenerative feed-back voltage in said amplifier of proper magnitude to overcome such distortion as said amplified voltage may be subjected to and means to prevent said feed-back voltage from aecting said impedance between said output electrodes of said amplifier.

9. In combination, an electron discharge amplier having input electrodes between which voltages of the entire range of audio frequencies are supplied,` and output electrodes between which the amplified voltages appear, a -load impedance connected between said output elec.

trodes having a value equal to the output impedance of said amplifier, means to supply at least'two degenerative voltages from said output electrodes to said input electrodes thereby to increase the capacity of said amplifier to supply undistorted power to said load impedance over said entire range of frequencies, one of said degenerative voltages being proportional to the current flowing between said output electrodes and another of said degenerative voltages Ibeing proportional to the voltage between said output electrodes, and said degenerative voltages being so proportioned that the impedance between said output electrodes is maintained substantially constant irrespective of the presence of said degenerative voltages.v

10. In combination, an electron discharge amplier for voltages of a Wide band of frequencies and having an anode, a cathode, and a control electrode, an input circuit for said voltages connected between said control electrode and said said anode and cathode, means connected in series with said anode and control electrode for supplying to said input circuit a degenerative voltage proportional to the intensity of current in said output circuit, means connected in shunt with said cathode and control electrode for producing in said input circuit a degenerative voltage proportional to the voltage between said anode and cathode, said degenerative voltages being of such values that the internal impedance of said amplifier between said output electrodes is of substantially the value existing in the absence of-said voltages, and said voltages being effective to increase the capacity of said amplier to suplply undistorted power to said output circuit over v cathode, an output circuit connected between l5 said entire band of frequencies.

DONALD E. MAXWELL.

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