Dec. 18, 1956 . F. J. BURGER 2,774,866 AUTOMATIC GAIN AND BAND WIDTH CONTROL FORTRANSISTOR CIRCUITS Filed Jan. 30, 1956 s log 00 0 0-0 000 & S. l INVENTOR, firANCIS. J. B/RGER 9. BY alid. A77ORNEYs 2,774,866 United States Patent Office Patented Dec. 18, 1956 2 reduced gain conditions. In order to prevent such dis tortion some means is required to reduce the signal level 2,774,866 prior to its application to the first intermediate frequency stage. In addition to the distortion problem, as is known, AUTOMATIC GAN AND BAND WIDTH CONTROL 5 the band width of the intermediate frequency amplifier FORTRANSISTOR CIRCUITS varies as a function of the magnitude of the automatic Francis J. Burger, Leonia, N. J., assignior to Enuerson gain controi voltage. As the gain of the transistor is Radio & Phonograph Corporation, Jersey City, N. , reduced, the loading which it presents to the intermediate a corporation of New York frequency coils decreases, thus increasing the Q of the O circuit and narrowing the band width. There also results Application January 30, 1956, Serial No. 562,072 a change in reactive loading provided by the I. F. tran 6 Claims. (C. 250-20) sistor which causes the intermediate frequency coils to shift their frequency center to a higher value. The purpose of this invention is to provide a variable This invention relates to improvements in transistor damping system which will vary the primary damping circuits, particularly transistor radio receivers with re of the converter intermediate frequency coil as a func spect to means for providing automatic gain and band tion of the automatic gain control voltage. With a suit width control. able proportional variable damping factor System the The general object of this invention is to provide an signal handling capacity of the receiver is increased, the automatically controlled variable damping circuit for 20 intermediate frequency band width change is reduced, and transistor circuits such as radio receivers by means of the shifting of the frequency center of the intermediate which the gain and band width of the main circuit is auto frequency amplifier is minimized. matically controlled. These objects are accomplished through the use of the More specifically, an object of this invention is to pro connections described above involving the diode D, vide a variable damping system for controlling primary capacitor C and resistor R. damping of the converter intermediate frequency coil With this arrangement under small input signal condi as a function of the automatic gain control voltage where tions the voltages at the points A and B are but slightly in the signal handling capacity of the receiver is increased, different and are of such polarity that the diode D is for changes in the intermediate frequency band width are practical purposes open circuited and has very little damp reduced and the shifting of the frequency center for the 30 ing effect on the primary of the input transformer and the intermediate frequency stage is minimized. intermediate frequency amplifier. Other and more detailed objects of the invention will However, with large input signals the change in the be apparent from the following disclosure of the em automatic gain control voltage changes the base to emit bodiment thereof illustrated in the attached drawings. ter potential on the transistor i4 of the intermediate In the drawings the single figure is a diagrammatic and 35 frequency amplifier, thus reducing its collector current schematic illustration of one application of the damp and gain. As the collector current of the transistor 14 is ing system of this invention as applied to a transistor thereby reduced, the voltage drop across the resistor R2 radio receiver. decreases, causing an increase in the voltage difference The system selected for illustrating the invention in between the points A and B in such a direction as to cludes a converter stage 12 having an input transistor 10 40 decrease the internal impedance of the diode D. The supplied from any suitable signal source 18 which, of resulting decrease in impedance of the diode D reduces course, can be an antenna system. The details of the the gain in the output of the converter stage and prevents converter stage are for the purposes of this invention to the Q of the intermediate frequency coupling transformer be taken as conventional and illustrative since they are from rising due to the unloading action of the low col capable of wide variation within the knowledge in the art. 45 lector current operating point of the intermediate fre The output of the converter stage is applied to an in quency amplifier. It follows, of course, that the shifting termediate frequency amplifier circuit 14 through a cou of the frequency center of the intermediate frequency pling transformer in the usual manner. The output of amplifier is minimized by this change in Q and loading the intermediate frequency amplifier is fed to subsequent by the additional damping provided by the diode D. circuit elements, which can be widely varied, through 50 Although the foregoing will be completely understood a coupling transformer 16. The connection 20 is from by those skilled in the art, this material is added as further the detector stage, not shown, to provide an automatic explanation of the illustrated example of the invention gain control voltage to the point C, in accordance with herein disclosed. - established practice. The value of C1 is not critical providing it is within The invention itself consists in applying a suitable diode 55 the limits 0.05 and approximately 1 mtd. A typical D, which may be of the semi-conductor type by way of value is 0.1 mi?d. Resistor R1 also is not critical in value example, in Series through a capacitor CE between a and can be selected anywhere in the range of 500 to 5000 tapped point on the primary of the transformer which ohms. Obviously the larger the value of R1 the greater couples the converter 12 with the intermediate frequency 60 the decoupling effect and the more limited the current amplifier 14 and the point A, that is one end of that fioW through the diode D becomes. As an example, how primary. The junction point of the diode D and the ever, a given differential voltage between the points A and capacitor C is connected through an isolating and filter B, if R is Zero, maximum diode current will flow; con ing resistor R1 to the point B which is connected to the versely as R1 is made larger the magnitude of the diode primary of the intermediate frequency output coupling 65 current becomes smaller. From this it can be seen that transformer 16. Ri has a limiting effect upon the maximum amount of In the usual circuit of this type, without the variable diode current that can flow. C2 and C3 are merely by damping system of this invention, when the automatic pass capacitors having values between 0.01 and 0.1 mfcd. gain control voltage is applied to reduce the gain of the as found in conventional transistor radio receiver circuits. first intermediate frequency amplifier, distortion occurs 70 R3 and R2 are merely B-- isolating resistors which in upon the application of the input signals due to the small conjunction with their respective capacitors C2 and C3, signal handling capacity of this amplifier stage under the decouple the two stages to prevent regeneration and inter 2,774,866 3. 4. action effects. The values of R2 and R3 are usually be cuit comprising a diode and capacitor connected in series tween 200 and 2000 ohms. These values are preferably across the primary of said transformer and a connection kept as low as possible so as to avoid excessive battery from the common point of said diode and capacitor in voltage drop. In this particular application Ri is kept cluding an impedance to said first impedance. low in value but R2 is kept on the high side, so that for 4. In the combination of claim 1, said damping cir a given current change in the I. F. amplifier collector cir cuit comprising a diode and a capacitor connected in cuit there will be a sufficiently large change in voltage at series across the primary of said transformer and a con the point B. It is this change in voltage which provides nection from the common point of said diode and capaci the required differential voltage between the points. A tor including an isolating and filtering resistor to said and B for operation of the damping diode D. 10 inpedance. Those skilled in the art will therefore appreciate by the 5. The combination with a transistor radio receiver simple expedients herein disclosed, that the defects comprising a converter stage, an intermediate frequency pointed out above are eliminated or minimized, thereby amplifying stage, a transformer for coupling said stages, improving the overall operation of the circuit by reduc and an automatic gain control voltage connection to said ing distortion over a relatively wide practical input signal amplifier stage, of variable damping means connected be variation range, the band width of the interneciate fre tween the primary of said transformer and the output quency amplifier is maintained, as required under these circuit of said amplifier stage. conditions. 6. The combination with a transistor radio receive Those skilled in the art will appreciate that some vari comprising a converter stage, an intermediate frequency ations in the details of the invention herein disclosed can amplifying stage, a transformer for coupling said stages, be made without departure from the novel subject mat and an automatic gain control voltage connected to said ter thereof.