US2270764A

US2270764A - Amplifier coupling circuit

Info

Publication number: US2270764A
Application number: US328045A
Authority: US
Inventors: Donald E Norgaard
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1932-04-28
Filing date: 1940-04-05
Publication date: 1942-01-20
Anticipated expiration: 1959-01-20
Also published as: US1941345A; DE629207C; GB545827A; US1930339A; GB397076A; FR51771E; FR754811A; GB419914A; FR44573E; US1990781A; NL39142C

Description

Jan. 20, 1942. D. E. NORGAARD 2,270,764

AMPLIFIER COUPLING CIRCUIT Filed April 5, 1940 7 Fig.2.

ourPur mm m/Pur FREQUENCY Inventor":

- Donald E.Nor aard, by Wan/1 7 His Attorney.

Patented Jan. 20, 1942 AMPLIFIER COUPLING CIRCUIT Donald E. Norgaard, Cohoes, N. Y., assignor to General Electric Company, a corporation of New York Application April 5, 1940, Serial No. 328,045

5 Claims.

This invention relates to an amplifier coupling circuit and more particularly to a coupling circuit for use in a wide frequency band amplifier.

It is an object of my invention to provide an improved wide band coupling device in which phase shift and variations in amplitude response at low frequencies are substantially eliminated without altering in any way the characteristics, such as high amplification and constant phase shift, at high frequencies.

-It is another object of my invention to provide such an improved coupling device in which the ratio of output voltage to theinput current and the time delay therebetween are substantially constant for all transmitted frequencies and in which a large amount of amplification is attained.

It is a further object of my invention to provide a wide band coupling device, as set forth above, which is substantially independent of power supply impedance and which reduces the effect of interfering power supply ripple.

It is as well an object of my invention to provide a wide band coupling circuit which is easily applied in the mechanical design of an amplifier, and which can be adjusted by simple methods to give substantially uniform response over the desired range of frequencies.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 illustrates a circuit embodying my invention; Fig. 2 illustrates a simplified circuit in which certain characteristics are similar to those of the circuit of Fig. 1; and Fig. 3 represents certain operating characteristics of the circuit of Fig. 1 both before and after the application of my invention.

Referring to Fig. 1, the amplifier illustrated therein comprises a source II] of a Wave whose frequency lies in a very wide band, and a pair of electron discharge amplifying devices II and I2, which are connected in cascade to supply the amplified wave to a load device I3. The source I is connected in series with a source I4 of grid bias potential between the control grid I5 and the cathode I6 of the device II. The anode I! of the device II is connected through an inductance I8, a resistance I9, and a resistance in series to the positive terminal of a source of potential 2 I. An intermediate point of the source 2| is connected to ground and to the cathode It.

A by-pass condenser 22 is connected from ground to a point between the resistors l9 and 20, so that high frequency currents from the anode I'I flow through the inductance I8, resistance I9 and condenser 22. This condenser is sufficiently large that little high frequency voltage is produced therein. A screen electrode 23 of the device II is maintained at a substantially constant positive potential through a resistor 24 connected between the electrode 23 and the positive terminal of the source 2|. A high frequency by-passing condenser 25 and a gas discharge regulating device 26 are connected in shunt from the electrode 23 to the cathode I 6 to aid in maintaining the potential of the electrode 23 more nearly constant at both high and low frequencies.

High frequency voltage variations appearing on the inductance I8 and the resistance I9 are transmitted to the discharge device I2 through a small coupling condenser 21 which is connected between the anode I1 and control grid 28 of the device I2. A grid resistor 29 is connected from the grid 28 to the negative terminal of the source 2| which maintains the grid at a suitable negative bias potential. The negative terminal of the source 2| is by-passed to ground and to the cathode 3B of the device I2 by a large by-passing condenser 3|. The load device I3 is connected between anode 32 of the device I2 and the positive terminal of the source 2|. Screen electrode 33 of the device I2 is connected through resistor 34 to the positive terminal of the source 2| and is by-passed to ground and to the cathode by a by-passing condenser 35 and a gas discharge voltage regulating device 36 in shunt, which maintain the potential of the screen electrode 33 substantially constant.

The wave from the source I0 is amplified and transmitted to the load device I3 as follows: Voltage variations produced by the source I0 on the control grid I5 excite corresponding current variations in the anode I! through the anode load circuit comprising the inductance I8, the resistance I9, and the shunt combination of resistance- 20 and the condenser 22. The discharge device II possesses an extremely high impedance, sothat voltage variations of the anode I1 caused by current flow in the anode load circuit have substantially no effect on the anode current. These voltage variations are transmitted through the coupling network comprising condenser 27, the condenser 38, and resistors 31 and 39 to the control grid 28 of the device I2 where they are again amplified and appear as large variations in current in the anode 32 and the load device l3.

The coupling condenser 2! is made small in order to minimize its stray capacity to ground. It is therefore incapable of transmitting faithfully low frequency components of the wave from the source H1. The means which are provided in accordance with my invention, in shunt with the coupling condenser 21 for transmitting these low frequency components, comprises, in order, a series connection of the resistor 31, the large coupling condenser 38, and the resistor 39. The

- components of the entire coupling circuit described are designed, as will be explained later, to transmit all frequencies within an exceedingly wide range with high gain and at the same time with substantially uniform response.

Fig. 2 illustrates the circuit of Fig. 1, simplifled to produce ideal operation with constant phase shift and a constant ratio of input current to output voltage at any frequency, if it be assumed that stray capacity to ground is negligible. In practice this simplified circuit responds ideally only at low frequencies, because stray capacity is inevitably present. For the lower portion of the frequency band to be' transmitted the circuit of Fig. 2 is therefore equivalent to that of Fig. 1, with certain reservations to be made clear as the explanation proceeds.

It will be noted that the condenser 38, which separates the resistors 39 and 3! of Fig. 1 is not shown in thisfigure. For purposes of this explanation it is assumed that this condenser is infinitely large, This introduces no error except at the very lowest frequencies, which are not intended to be transmitted. It has been found that the circuit of Fig. 2 allows a wave covering a frequency range occupying the lower part of the desired band to be transmitted therethrough with uniform voltage response and substantially zero phase angle, provided the resistance-capacity products of the left and .right portions of the circuit be made equal. In other words, the condenser 21 and resistors 3'! and 39 produce a phase shift which varies with frequency, but the condenser 22 and resistors 19, and 29 produce a complementary phase shift, so thatthe overall phase shift through the circuit of Fig. 2 is substantially proportional to frequency. That is, if the ratio -of resistance 29 to resistance I9 be equal to the ratio of capacity 22 to capacity 21, and also be equal to the ratio of resistance 31, 39 to resistance 20, any current made to flow between input conductors 64 and 45 must divide between the lefthand and the right-hand branches in a certain fixed proportion regardless of frequency, since their impedances have been made proportional.

In practice, the inclusion of the condenser 38, Fig. 1, between the resistance 31 and 39 introduces some error at very low frequencies, since the combination of these three elements is not purely resistive. However, the condenser 38 can be chosen large enough with respect to the resistors 3? and 39 so that the phase angle of the combination can be made so small at any specified frequency that the error introduced by use of condenser 38 can be neglected. It is desired to use the condenser 38 in order to prevent amplification of any direct current component of the source ID, of Fig, 1.

The amount of amplification actually available in the circuit of Fig. 2 is directly proportional to the magnitude of the resistor I9,.this

resistor being given a value in ohms determined by the function 2 1r f 6 Where f is the maximum frequency which it is desired to transmit with uniform response and c is the total stray capacity in farads to ground in the coupling stage. The stray capacity includes only the combined capacity effective to ground from the anode H, the condenser 21, and

grid 28, and from all intermediate wiring. The

inductance I8 is given a value in henrys which is times the value of the resistance l9, which inductance compensates substantially for the presence of the stray capacity 0. It is therefore apparent that it is very desirable to minimize the stray capacity in the coupling circuit since the value of the resistance l9 may thereby be made a maximum with a corresponding maximum gain of the stage for a given maximum frequency, f.

The choice of resistor 29 must be made with due regard to the maximum direct current impedance which should be utilized in the control grid circuit of the device I2. The condenser 22 is made sufficiently large to act effectively as a decoupling filter in conjunction with resistor 20, which is given as high a value as will permit sufficient continuous voltage to be applied to the anode ll of device II to insure normal operation. The values of the inductance l8, the resistances I9, 29, 20, and 37, 39, and the condensers 22 and 2'! are thus determined through use of the proportionalities pointed out above.

As stated above, it is highly desirable to minimize the stray capacity to ground from the coupling circuit and the components of the coupling circuit are fixed in size with this end in view. As" indicated by dotted lines in Fig. 1, which represent the case or shell of the condenser 38, and by a dotted capacitance 40, the condenser 38 may have a substantial amount of stray capacity to ground because of the possibility that it will have large physical dimensions. The dotted capacitance 40 is intended to represent the effective shunt capacity from the capacitor 38 to ground, whether the case be grounded or not. If the resistors 31 or 39- are not made sufiiciently large and resistor 20 is omitted, this stray capacity affects the coupling circuit so as to reduce the amplification therethrough at high frequencies. This tendency is illustrated by curve 4| in Fig. 3 in which values of the ratio of the output voltage applied to grid 28 of device l2 to the input voltage applied to grid I5 of device H for such a coupling circuit are plotted as ordinates against values of frequency as abscissae. This curve shows clearly that if the stray capacity 40 be allowed to affect such a circuit, otherwise properly adjusted, the intermediate frequencies are transmitted in too large amount and the highrfrequencies are transmitted in too small amount.

In my invention, the ratio which is established between resistors 29 and I9 must be preserved between resistors 37, 39 and resistor 20 in accordance with the previous explanation concerning Fig. 2 so that the low frequency response of the coupling circuit is made highly uniform. It is thus necessary to choose both the ratio between resistors 20 and' I9 and the value of resistor 20 high enough to prevent loss of highfrequen'cy response through the shunting effect of the network comprising resistors 31, 39 in series with condenser 40 to ground. This network would otherwise have sufiiciently low impedance with respect to resistor I9 to by-pass a relatively large amount of high frequency current.

If the resistors 31 and 39 be made too small as a result of too low a value for resistor 20 or too low a ratio between resistors 29'and I9, or if, for example, resistor 39 be omitted (resistor 31 being then increased to maintain uniform low frequency response as previously shown) and the remaining components in the circuit be properly adjusted in accordance with the above explanation, taking into account the stray capacity 40 which is thus added to the existing stray capacity, c mentioned above, the response of the coupling circuit is made substantially linear for all frequencies but does not permit a maximum amount of amplification. This fact follows from the necessity of reducing the value of resistor I9 to take account of increased effective shunt capacity when the amplifier is made to pass a given maximum frequency f with uniform response. The curve 42 of Fig. 3 illustrates the frequency response of the circuit of Fig. 1 when it is adjusted in such a fashion.

In accordance with my invention, it is best practice to make the resistors 31 and 39 of roughly equal value and sufficiently large to serve as decoupling resistors for high frequency currents. By making these resistors of such a size,

the effect of the stray capacity may be made negligible, so that, if the remaining components of the circuit be proportioned properly (taking into account all stray capacity except that represented by the capacity 40), a maximum amount of amplification is obtained. The characteristics of a circuit constructed in such a manner are represented by the curve 43 of Fig. 3.

Any inherent internal impedance of the power source 2I in Fig. 1 is effectively in series with the resistor 20, which is made as large as possible without dropping the direct current plate potential of device I I to such a value that its operation is impaired. Thus, the internal impedance of the source 2| is normally small compared with the resistor 20, and its effect on the operation of the circuit is negligible. Likewise, any low frequency ripple appearing in the source 2I must pass through the high resistance 20 to the bypass condenser 22, through resistor I9 and inductance I8 in order to appear on the anode I! of the device II. Moreover, the network consisting of condenser 21, resistor 31, condenser 38, and resistor 39 operating in conjunction with resistor 29 serves further to attenuate low frequencies which may appear in the source 2|. The amount of such attenuation approaches as a limit the quotient of resistor 20 plus resistor I9 divided by resistor I9. The combination of the resistor 20 and the condenser 22 serves as a re sistance-capacity filter which itself reduces the amount of ripple that appears on the anode I I of device II at the outset. This same filter action takes place at high frequencies where the attenuation of the right hand branch of the coupling network is small, but the attenuation of the combination of resistor 20 and condenser 22 is correspondingly greater. Since the anode circuit is made up of elements whose impedance is independent of current, voltage, or frequency, high frequency currents from the anode I! of device I I are prevented from reaching the power source 2| in any great amount.

Since it is possible to neglect the stray capacity 40 of Fig. l by the application of my invention, the mechanical design of the wide band amplifier is greatly simplified, both as to placement of the circuit elements and as to the necessity of shielding to prevent unwanted stray couplings in a multiple-stage amplifier. The only require ments of the condenser 38, for instance, are that its stray capacity to case or ground be small with respect to its nominal capacity and that its nominal capacity be large enough to make the series circuit consisting of resistors 3'! and 39 and condenser 38 essentially resistive over the low frequency portion of the frequency band it is desired to transmit. Because condenser 38 is decoupled from the signal circuit at high frequencies by resistors 31 and 39 there is no need to isolate it or shield it from other parts of the amplifier; moreover, its nominal capacity can be made large, so that the low frequency response can be extended downward to practically any point. Since the value of the coupling condenser 21 can be made small by proper choice of the other constants of the circuit, its stray capacity to ground and to other parts of the circuit can be made a minimum with the result that higher gain or wider bands of amplification can be had than with other coupling circuits which have equivalent low frequency response.

In a typical example in which the discharge devices II and I2 are of the RCA type 1852, the various components of the circuit may be given the following values:

Resistor l9 2000 ohms (depends on stray capacity) Inductance l8 0.05 millihenry (depends on stray capacity) Capacity 27 0.02 microfarad Resistor 29 1.0 megohm Condenser 22 10.0 microfaraols Resistor 37 2.5 megohms Resistor 39 2.5 megohms Resistor 20 10,000 ohms Condenser 38 1.0 microfarad Source 21 350 volts positive with respect to ground, and 2.0 volts negative with respect to

ground Devices

26 and 36 volts A coupling circuit having components with these values gives substantially constant response (see Fig. 3, curve 43) from 20 cycles to 3 megacycles with a 20 cycle phase shift substantially less than 0.1 degree. If it be desired to make the low frequency transmission and phase shift even better, the condenser 38 may be increased in magnitude to 2 microfarads. The 20 cycle phase shift is then substantially less than 0.05 degree, and the decoupling provided by the resistors 3'! and 30 for the stray capacity is still ample, so that the high frequency characteristic is not measurably altered.

While I have shown a particular embodiment of my invention, it will, of course, be understood that I do not wish to be limited thereto, since different modifications may be made both in the circuit arrangement and instrumentalities employed without departing from the principle disclosed above, and I aim by the appended claims to ,cover any such modifications as fall within the true spirit and scope of my invention.

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

1. Coupling means for transmitting a wide frequency band wave between two terminals, comprising a first resistor connected between one of said terminals and a fixed potential point, a second and a third resistor connected between the other of said terminals and a fixed potential point, a point between said second and third resistors being connected to a fixed potential point through a by-pass capacitor in shunt to said third resistor, a small capacitor and a large capacitor connected in a shunt circuit around said small capacitor to provide respectively high frequency and low frequency coupling between said terminals, said large capacitor having substantial stray capacity tending to reduce transmission of high frequency components between said terminals, and a decoupling resistor in said shunt circuit on each side of said large capacitor providing sufficient decoupling between said stray capacity and said small capacitor to maintain transmission of said high frequency components substantially unaffected, the ratios of said first resistor to said second resistor, of the sum of said decoupling resistors to said third resistor, and of said by-pass capacitor to said small capacitor being substantially equal.

2. Coupling means for transmitting a wide frequency band Wave between two terminals, comprising a first resistor connected between one of said terminals and a fixed-potential point, a series connected group including an inductance, a second resistor and a third resistor connecting the other of said terminals to a fixed potential point, a point between said second and third resistors being connected to a fixed potential point through a by-pass capacitor in shunt to said third resistor, a small capacitor and a large capacitor connected in a shunt circuit around said small capacitor to provide respectively high frequency and low frequency coupling between said terminals, said coupling means including said large capacitor having "substantial stray capacity tending to reduce transmission of high frequency components between said terminals and said inductance tending to compensate therefor, and a decoupling resistor in said shunt circuit on each side of said large capacitor providing sufficient decoupling between said stray capacity and said small capacitor to maintain transmission of said high frequency components substantially unaffected, the ratios of said first resistor to said second resistor, of the sum of said decoupling resistors to said third resistor, and of said by-pass capacitor to said small capacitor being substantially equal.

3. Coupling means for transmitting a wave having components within a predetermined band of frequencies, comprising a small condenser and a large condenser connected in a shunt circuit around said small condenser, said large condenser having substantial stray capacity tending to reduce transmission of high frequency components within said hand through said condensers, decoupling means in said shunt circuit on each side of said large condenser for decoupling said stray capacity from said small condenser at the frequencies of said high frequency components so that such components remain substantially unaffected, said coupling means being effective to transmit a wave having components within said band of frequencies through said condensers so that the phase shift of said waves passing through said coupling means is not proportional to frequency, and means for producing a phase shift in said wave complementary to the phase shift of said wave through said coupling means so that the overall phase shift through said coupling means and said last means is substantially proportional to frequency.

4. Coupling means for transferring a signal represented by the intensity of a current from an input terminal to an output terminal on which said signal is represented by a voltage, said signal having frequency components within a predetermined band of frequencies, and said terminals being at different continuous potentials, said coupling means comprising a small condenser connected between said terminals, a large condenser connected in a shunt circuit around said small condenser, said large condenser having substantial stray capacity tending to reduce transmission of high frequency components through said condensers, and decoupling means in said shunt circuit on each side of said large condenser for decoupling said stray capacity from said small condenser at high frequencies within said predetermined band to maintain transmission of said signal substantially unaffected by said stray capacity, said coupling means being effective to transmit said signal through said condensers so that the phase shift between variations of said input current and corresponding variations of said output voltage is not proportional to frequency, and means for producing an additional phase shift in said output voltage complementary to said first phase shift through said coupling means so that the overall phase shift between said input current and said output voltage is substantially proportional to frequency.

5. Coupling means for transmitting waves having components within a predetermined band of frequencies, comprising a small condenser, a large condenser connected in a shunt circuit around said small condenser, said large condenser having substantial stray capacity tending to reduce transmission of waves of high frequencies within said band through said coupling means, decoupling means in said shunt circuit on each side of said large condenser to reduce the effective stray capacity of said coupling means for said high frequency waves to a relatively smaller residual stray capacity, means for broadly tuning said residual stray capacity at such high frequencies thereby to achieve a large voltage response through said coupling means at such frequencies, said coupling means having such characteristics that the phase shift of low frequency waves within said band passing through said coupling means is not proportional to frequency, and a compensatory phase shifting network in circuit with said coupling means, said network including impedance elements which are proportioned to produce a phase shift in said low frequency waves complementary to the phase shift of such waves through said coupling means and which are of such magnitude with relation to said coupling means as to produce a net voltage response through said coupling means and said network for such low frequency waves substantially equal to the large response at said high frequencies.

DONALD E. NORGAARD.