US2397850A - Amplifier gain control - Google Patents

Description

. PatentedApr. 2, 1946 AMPLIFIER. GAIN coN'moI.

John a. Ford, Narberth, Pa.., assignor a Radio Corporation of America, a corporation of Delaware Application August 22, 1942, Serial No. 455,133

V 2 Claims.

My present invention relates to amplifier gain control circuits, and more particularly to tuned radio frequency amplifiers provided with automatic gain control.

One of the main objects of my present invention is to avoid any change in the shape of the frequency response curve of a radio frequency amplifier due to change in input, ortgrid, capacity as the gain of the amplifier is varied.

Another important object of this invention is to provide aradio frequency amplifier provided with a band pass input circuit and means for varying the gain of the amplifier; there being included in the band pass input circuit sumcient loading to prevent a change in the shape of the band pass response curve of the input circuit as the gain of the amplifier is varied.

. Another object of the invention is to provide an amplifier of the pentode type provided with a pair of coupled tuned input circuits, the coupling between the tuned circuits being chosen to provide a substantially flat top band pass curve, and the tuned circuits being individually loaded to a suflicient, extent to prevent a sub- .stantial change in the response curve shape as in an economically manufacturable manner.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims; the invention 20tHere, again, any other value of LF. may be em- 25 The control grid 3 thereof is connected to the high 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 a circuit organization whereby my invention may becarried into ef-' Fig. 3 illustrates one change in the band pass response curve in the absence of the invention,

Fig. 4- illustrates'a difierent type of undesirablechange in the band pass response curve in the absence of the invention.

Referring now to Fig. 1, and assuming for the purposes of illustration that the invention is embodied in a superheterddyne receiver of the broadcast band covering a range of 550 to 1.700

kilocycles (kc.), reference is made to numeral! which denotes the usual mixer, or converter, stage'of the receiver. Those skilled in the art are fully acquainted with the circuit details of a mixer stage, and are also fully acquainted with the manner of connecting the input terminals thereof to one or more preceding tunable radio frequency amplifiers. signal collecting device which feeds the first of the amplifiers. It is to be clearly wider-stood that the invention is not limited to a receiver of the superheterodyne type, nor to the particular specified operating range. For example, the invention may be used in the me'gacycle range if desired."- The output circuit of the mixer develops the intermediate frequency (I. F.) ener y, andlet it be assumed that the operating 1. F. value is 465 kc.

P ye

The numeral 2 designates a pentode amplifier tube, although any other type of tube may be used which functions well as an I. F. amplifier.

potential side of the resonant input circuit which comprises coil'Lz shunted. by condenser 4. The input circuit 1.2-4 m tuned to the operating 1. F. value. Coil 1a is magnetically coupled, as indi cated at M, to the primary coil L1. The latter is shunted by condenser 5. The circuit Ll5 is tuned to the'operating I. F. value. The primary resonant circuit of the I. F. transformer is arranged in the output, or plate,circuit of the mixer tube. Hence, the low potential end of coil L1 is connected to the plus B terminal of the direct current supply source. A resistor R1 is connected in shunt with the primary circuit.

The low potential end of coil L: is connected I through a filter resistor ito a point on the load impedance of an AVC (automatic volume control) of the manner of constructing an AVC circuit,

and, therefore, it is not necessary to describe in detail the AVG circuit. It is pointed out that there will be applied to the control grid 3 of amplifier tube 2 a variable negative bias which functions to control the gain of the amplifier tube.

Inthe specific illustration disclosed in this appli- I cation, the variable negative bias is that produced 'by the usual carrier rectifier of the diode type,

and a point of increasing negativepotential with on increased carrier amplitude is connected to the The latter usually have a I point becomes increasingly load resistors R1 and R2.

magnitude.

2 lower end of coil In through the carrier filter resistor 8. Gain may also be controlled by varying the voltage of another electrode, as for example thescreen grid.

The secondary circuit In-l is shunted by resistor R2. The low potential end of the secondary circuit may be bypassed to ground by condenser I for I. F. currents. In the plate circuit of ampli- According to'my present invention the effects represented in either of Fig. 3 or 4 are prevented. The band pass shape of the curve is maintained,

" as indicated by the dotted linecurve in Fig. 2,

fler 2 there is arranged an I. F. transformer Ml whose primary and secondary circuits are individually tuned to the operating IJF. value. The remaining parts of the circuit shown are purely conventional, and need not be described in further detail. It is sufllcient to point out, however, that the secondary circuit of transformer Ml. may be coupled to the input electrodes of a following I. F. amplifier 2. The following amplifier maybe suc= ceeded by ademodulator which is followed in turn by'one or more stages of audio frequency amplification "terminated by a reproducer.

I have indicated in Fig. 2 a full line curve P which shows the band pass response curve of the I. F. resonant networkbetween the. mixer stage and the input electrodes of I. F. amplifier tube 2.

maintained when the gain of ampifier tube 2 1s varied. As the bias voltage appliedto the con-" trol grid 3 is increased in a negative sense on the control grid of amplifier tube 2, its grid input capacity decreases in magnitude. I have represented the input grid capacity in dotted lines as 01. This inherent grid to cathode capacity shunts the secondary circuit Iii-d. It is pointed out that the inherent grid to cathode capacity Crwill have a minimum value for um AVC bias magnitude, while the capacity 01 will have maximum value ,with minimum AVG bias In other words, the magnitude of the inherent capacity 01 varies inversely with the magnitude of AVG bias. Merely by way of example, it is pointed out that the capacity 01 may vary as much as 3 micromicrofarads in the case of an I. F. amplifier tube. The capacity efiect C1 is known as the "Miller effect, and is due to feedback through the plate to grid capaclty of tube 2. This feedback is a function of tube gain.

The effect of the input capacity 01 on the shape of the band pass response curve, as the gain of amplifier tube 2 is varied, is illustrated in Figs. 8 and 4. Taking the case of R1 having infinite value, while Ra is adjusted to a value such as to give a desired flat top band pass curve, the curve will appear as shown in Fig. 3 ifthe AVG bia is increased thereby decreasing .the magnitude of grid capacity C1. It will be noted that the upper left portion of the curve is raised while the opposite right hand portion is lowered. This results in distortion of the modulation side band frequencies.- 0n the other hand if the resistor Rz is infinite, and R1 is chosen so as to provide the desired band pass curve, an increase in AVC bias will cause theband pass curve to appear as shown in a for all values of the capacity C1. It will be noted, however, .that frequency displacement of the mid-band frequency value occurs. However, this frequency-displacement is so small as to be inappreciable. When receiving modulated carrier energy in the broadcast band, the band pass circuit generally has a width of 10 kc. With such a frequency band value, a slight shift in mid band frequency is not observed in the reproduction of the modulation frequencies. What is more essential is that the shape of the band pass curve be maintained.

In order to maintain the band pass response curve for all values of 01, it is merely necessary critically to load the winding L2 as describedv above. It is now possible to find a resistance load for the opposite coil such that the frequency curve will remain flat-while shifting upward in frequency. For example, if loading of coil L1 so chosen that R1 is infinite, then the magnitude of R2 will be such as to prevent the change in curve shape which is shown in Fig. 3. What will occur will be a slight shift in mid-band frequency while the response curve itself has its shape maintained.

To enable one skilled in the art to practice the invention the following specific description is given. First design an I. F. ,transformer to be loaded on one coil, and having the proper coemcient of coupling to give the desired band width. Determine the value of R1 necessary to give a flat top response with no loading on the grid coil. Then determine R2 necessary to give a fiat top response with no loading on the plate coil. With R2 connected across the grid coil a resistor can be found greater in value than R1 which, when connected across the plate coil, will permit the gain to be changed without altering the shape of the response curve. The response curve will now be slightly more rounded than before. A. slight increase in the value of R: will restore the original curve.

Suppose the tube 2: is a high mutual conductance pentcde operating in an I. F. stage at 30 megacycles, and that the band width is 2 megacycles. Capacitors 4 and 5 are 68 and 10 mmf., (micromicrofarads) respectively. The following relations between R1 and R2 result;

R1 Ra Result when grid bias is changed Infinite 470ol1ms Gain falls at high end of pass band. 2200ohms Infinite-.. Gain falls at low end of pass band. 68000 560ohms Gain remains flatover pass band which shifts upward about kc.

tem for carrying my inventioninto efiect. it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shownand described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In combination with a high frequency amplifier tube provided with input electrodes and 7 output electrodes, a bandpass input network connected to said input electrodes, said network comprising a pair of resonant circuits each tuned to a frequency of the order of.3c megacycles.

While I have indicated and described a sys aacmso means for coupling said resonant circuits to provide a response curve for the amplifier which has a substantially fiat top over a band width of the order or 2 megacycles, means for varying the gain or said amplifier tube thereby introducing a variable magnitude of input electrode capacity across the resonant circuit directly connected to said input electrodes, a resistance of the order of 6800 ohms connected across one 01 said resonant circuits, a second resistance of the order of 560 ohms connected across the second of the resonant circuits, said two resistances functioning to permit the aforesaid. band pass response curve maintain its shape over a midband frequency shift up to 100 kilocycles regardless of the magnitude of said input electrode capacity.

2. A method of constructing a band pass amlifier of the type comprising an amplifier tube having an input networkconsisting of a pair or resonant circuits each'tuned to a common desired Irequency; the method including the steps of reactively coupling said circuits in cascade to provide a band pass response curve of a desired width for the amplifier, first loading the first v resonant circuit with resistance suflic'ient to provide said response curve with a fiat top while maintaining the second resonant circuit tree of loading, removing the loading of the first circuit during the step of loading the second circuit with sufilcient resistance to secure said flat top response curve, and loading again saidfirst resonant circuit while, maintaining saidsecond reslstive loading, to a resistance value sufliciently in excess of said first loading resistance value to maintain said fiat top response curve over a relatively substantial range of variation of the gain of said amplifier tube.

' v I JOHN R. FORD.

Images