US2269694A - Uniform response wide band amplifier - Google Patents

Description

Jan. 13, 1942. o. H. SCHADE Fi led Oct. 26, 1939 TUUUTPUT 15 NETWORK l TOSUURCE 0/ W050 7 :5 3 vozmas *5; 32, 3 V I6 :5 T 8 INYENTOR. 077 0 HSCHADE ATTORNEY.

Patented Jan. 13, 1942 UNIFORM RESPONSE WIDE BAND ANIPLIFIER Otto H. Schade, West Caldwell, N. J assignor to Radio Corporation of America, a corporation of Delaware Application October 26, 1939, Serial No. 301,220

1 Claim.

My present invention generally relates to wide band amplifiers, and more particularly to a novel method of, and means for, providing constant gain over the lower part of the frequency range of a wide band amplifier.

Self-bias networks are highly desirable for use, in amplifiers having high gm. Degenerative effects at zero frequency (D. C.) stabilize plate current and operating bias. This is particularly true in circuits of the type disclosed by me in application Serial No. 211,155, filed June 1, 1938, now Patent No. 2,243,442, May 2'7, 1941, wherein the self-biasing resistor is given a large value, and the grid resistor is returned to a direct current potential point which is positive with respect to ground, but is negative relative to cathode potential. If such an arrangement, however, is used for a wide band amplifier extending to low frequencies such as 60 cycles, the bypass condenser shunting the bias resistor causes phase shift effects at the low end of the frequency range; decreased again at the said low end also occurs. Large values of capacity, of the order of 250 mf., have therefore been used in television video amplifiers to extend the low frequency range; in such amplifiers the aforesaid problem was encountered.

One of the main objects of my present invention is to provide a compensation network in the output circuit of such a high gain amplifier, which network completely eliminates phase distortion or gain variation down to zero frequency.

Another important object of my invention is to provide a high gain amplifier operating in the video frequency range and employing a self-bias network utilizing a bypass condenser; the amplifier output circuit utilizing a network to prevent gain reduction at the low end of the frequency range and permitting reduction of the value of the bypass condenser capacity to a very low value.

Still other objects of this invention are to improve generally the efliciency and response characteristics of wide band, high gain amplifiers, and more especially to provide such amplifiers in a form such that they are reliable in operation and economical to manufacture and assemble.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

"In the drawing- 1 Fig. 1 shows an amplifier stage embodying the invention,

Fig; 2 illustrates two stages of a video amplifier embodying the invention.

Referring now to the accompanying drawing, wherein like reference characters in the two figures designate similar circuit elements, there is shown in Fig. 1 an amplifier tube l which may be a high gain pentode tube of the 1851 type. The tube is provided with a cathode 2, a signal grid 3, screen grid 4, a suppressor grid 5 and a plate Gall in the order named. The cathode 2 is connected to ground through a bias resistor l 'oflarge value, the resistor being shunted by the bypass condenser 8. A current source (D? C.) 9 has its negative terminal at ground potential, while its positive terminal is connected to the plate 8 through a path including the load resistor! and compensation resistor l l arranged in series. The condenser l2 shunts resistor H. The screen grid 4 is connected to an intermediate point on source 9 to establish the screen at aproper positive potential. The signal grid 3 is connected, through the grid leak resistor l3 and lead I4, to a point on source 9 which is positive relative to ground. The positive potential applied to grid 3 reduces the excessively large negative bias developed across resistor l to the desired negative bias value. As previously ex plained in my aforesaid patent application, the use of the type of biasing arrangement disclosed herein stabilizes plate current and operating bias, and isof especial advantage in circuits using high gm tubes.

For example, where the amplifier is used to amplify video frequencies, say in a range up to 9 megacycles (mc.), the video voltage will be appliedbetween the input terminals marked Input, and the amplified voltage will be taken from across the series resistors l0-I I. =Smaller values. of condenser 8 cause phase shift and decreased gain at low video frequencies; hence, large values of capacity (of the order of 250 mf.), are employed at 8 to extend the low end of the frequency range. The inclusion of compensation network Il-I2 in series with load re,- sistor H] eliminates the aforesaid phase distortion and gain reduction, and moreover permits reduction of the value of condenser 8 to a moderate'value, such as 1 mf. or less.

Without compensation the gain is substantially reduced at the low frequency end of the range,

because the input voltage must overcome the phase-opposed cathode voltage developed across resistor 1, and the cathode voltage is proportional to the cathode transconductance 91110;) and the magnitude R1 of resistor I. The network ||--l2 inserted in the plate circuit does not affect the gain at the high frequency end of the range, but zero frequency gain Go increases to the value:

Im n( 1+ 2) m(k) l Where gmm) is the plate transconductance of amplifier tube l; R1 is the value of resistor l;

R2 is the value of resistor I I.

The value of R2 necessary for a zero frequency gain equal to the high frequency gain is obtained from the following expression:

where I}: is the cathode current; Ib is the plate current.

In order to obtain further constant gain at other frequencies the condenser I2 must produce the same phase shift of voltage across resistor H against voltage across resistor H], as the phase shift caused by condenser 8 of voltage across bias resistor 1 with respect to the grid to cathode voltage of tube I. This is the case when the time constants are equal:

where C1 is the capacity magnitude of condenser 8, and C2 is the capacity value of condenser [2. The correction network ll-l'2 is fully determined by Equations 2 and 3. The minimum value of capacities C1 and C2 depends on the shunt capacity Cs across the entire plate load (tube and circuit capacities). C2 should be a short-circuit when the shunt-capacity C5 begins to aifect circuit performance. For a 1% error there is obtained C2=100 Cs.

By Way of specific illustration, and in no way restrictive or limiting, the following constants are given for an amplifier embodying'the invention:

gm p =10,000 micro-mhos gm(k)=13,000 micro-mhos Ib= milli-amperes (ma.)

Ik=13 milli-amperes (ma) E (min.)=100 volts (v.) Eb=300 volts Resistor 10=R1=3000 ohms Cs=20 micro-micro-farads (mmf.) Ec1(signal grid bias =-1 volts Resistor 11=R2=17,000 ohms Resistor 7=R1=436 ohms Condenser 12=C2=0.002 micro-farads (mf.) Condenser 8=C1=0.078 micro-farads (mf.)

A practical choice of values for the biasing and compensation networks comprises assigning values of 0.1 mf. for C1 and 400 ohms for R1, while C2 is made equal to 0.00256 mf., and R2 is given a magnitude of 15,600 ohms.

In Fig. 2 there is shown the amplifier circuit of Fig. 1 embodied in a multi-stage video amplifier. The amplifier, in actual operation, had a flat frequency response from 20 cycles to 9 me. Here, as in Fig. l, amplifier tube I is of the 1851 type, and includes biasing network 18 in its grounded cathode lead. The signal grid 3 is connected to a coupling condenser I5 which acts to transmit video voltage to the grid; the grid leak resistor l3 returns to ground through resistor l6, and condenser I1 bypasses the junction of resistors l 3 and Hi to ground. Screen grid 4 is connected to the positive terminal of the 300 volts current source 9 through resistor 18, and bypass condenser l9 grounds the grid end of the resistor.

The junction of resistors I3 and ['6 is connected to the positive terminal of current source 9 through resistor 20. The suppressor grid 5 is connected to the positive terminal of current source 9 through a resistor 2|, while the grid end of resistor 2| is connected to ground through a resistor 22. The plate 6 of the amplifier tube is connected to the positive terminal of current source 9 through a path which includes coil 30, coil 3|, resistor l0, resistor II and resistor 32 all arranged in series. The junction of resistors II and 32 is connected to ground through the condenser 33, while the condenser I2 is connected to ground from the junction of resistors 10 and II. The succeeding amplifier 50 is coupled to the junction of coils 30 and 3| through the condenser 40; the desired negative bias is provided for the signal grid of amplifier tube through a path which includes resistor 5|. The plate shunt capacity Cp is shown in dotted lines asconnecting the plate end of coil 30 to ground, whereas the reference character Cg2 denotes the dotted line capacity connected between the junction of coils 30 and M to ground.

The network Il-|2 of Fig. 2 acts as the compensation network which corrects for the phase shift and reduced gain at the low frequency end of the video range caused by biasing network 'l-8. The compensation network I l| 2 will not afiect the performance of additional networks to extend the high frequency end of the range, nor will it prevent operation of compensation circuits for correcting voltage losses and phase shifting due to the coupling condenser 40 and the leak resistor 5|. In other words, there is shown in Fig. 2 in the plate circuit of amplifier tube l various other compensation networks such as 32-33 and 303l which act to compensate for other Voltage losses at low frequencies and at the high frequency end of the video range. The following table of constants is given by way of illustration for the various elements utilized in Fig. 2:

C15=0.05 mf. (317:0.004 mf. C8 =0.1 mf. C19=1 mf. C o cgzg l4 mmf. 012:0.004 mf. 033:8 mf. 040:0.05 mf. R13=0.5 megohms R16=l0,000 ohms (variable between 5000 and 15,000 ohms. R2o=7,000,000 ohms R7 :400 ohms R1a=75,000 ohms R21=200,000 ohms R22=10,000 ohms R32=3,0O0 ohms R11=10,000 ohms R10=2,000 ohms R51=0.5 megohms L3o=60 microhenries L3l=30 microhenries The resistor I S in Fig. 2 is adjustable in order to vary the signal grid bias, which in turn varies the values gim and gm(k) of pentode 1. This variation effects Equations 1 and 2 permitting therefore, adjustment for perfect compensation in case the values R1,, R1 or R2 are somewhat in error as may be the case in practical circuits.

Resistors 20 and I6 provide a potential divider to obtain a small positive bias from source 9 to counteract the excess negative voltage produced across element 1. Resistors 2| and 22 provide, also, a potential divider to obtain a small positive suppressor grid bias from source 9. The latter small bias causes the knee of the plate currentplate voltage curves to occur at a lower plate voltage thereby permitting higher resistance values with consequently lower plate voltage in the plate circuit wthout loss of gm. The voltage drop across resistor I in Fig. 2 is in bucking relation to the voltage drop across bleeder "5. Assuming a drop of +5.2 v. across resistor I and a drop of +4.2 v. across bleeder I 5, then the effective bias of grid 3 will be -1 v. Actually these values are not critical because of the degenerative action which causes the cathode current to readjust itself. The network 32-33 compensates for phase shift and voltage drop at low frequencies caused by coupling condenser 40 and resistor Coils 3| and 30 together with tube capacitances Cp and Cg2 provide a low pass filter section, and this network is operative at high frequencies only and extends the frequency range, but does not interfere with the low frequency compensation. In the absence of network I|-l2 the frequency response would drop at about 100,000 cycles to approximately 16% response at 1000 cycles.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claim.

What I claim is:

In a video amplifier network capable of amplifying frequencies in a range up to approximately 9 megacycles, an electron discharge tube provided with at least a cathode, a signal grid and an output electrode, a source of video voltage coupled between the signal grid and cathode a self-biasing network comprising a resistor of relatively high magnitude of the order of 400 ohms located between said cathode and at a point of relatively fixed potential, means for applying the direct current voltage developed across said resistor to said signal grid, means for applying a direct current potential of positive polarity to said signal grid in opposition to said bias resistor voltage thereby to reduce the voltage applied to the signal grid to a normally low negative bias value, a capacity of the order of 1.0 microfarad shunted across said bias resistor, a pair of inductances and a load resistor serially connected between the output electrode and cathode of said tube, a correction network consisting of a resistor arranged in series with said load resistor, a condenser shunting said correction resistor, said correction network having its constants chosen to compensate for a decrease in response at the low frequency end of the video range, said correction network and biasing network having substantially equal time constant values, and a second amplifier tube having its input electrode coupled to the common terminal between said pair of inductances included in the output circuit of the first tube.

O'I'IO H. SCHADE.

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