Matching Impedance

Radio Record, Volume II, Issue 32, 22 February 1929, Page 30

Matching Impedance

-==77-_-_- =F A CORRESPONDENT in "Questions and Answers" has asked, "How can you calculate the impedance of a coil, given the guage of the wire and the diameter of the coil?’ Actually, the question cannot be answered, but the following discussion from Drake’s Radio Cyclopedia throws a= great deal of light on this problem. "Any electrical device ‘which produces or delivers power, such as a valve, will put forth the greatest effort and will deliver the greatest possible power when the impedance of whatever unit forms the external load is at least equal to the internal impedance of the source of power, the valve in this case. "In the earlier days of radio reception, little, if any, attention was ‘paid to this subject of matching impedances. The results were manifested in poor performance. The principal concern. in this matter of matching impedance is to obtain a balance between the plate impedance of a valve and the impedance of whatever unit is connected as a load in the external plate circuit. The plate of a valve may be.eonnected to any form of coupler such as an audio frequency transformer, and audio frequency impedance or an audio frequency resistance coupler. The plate of the yalve

may also be connected -to a loudspeaker. to a pair of headphones, or to a coupling transformer for a loudspeaker. In any of these cases, the impedance of the plate circuit in the valve measured in ohms, should be equalled or exceeded by the impedance of the coupling ' deyice or speaker. . "To attain this object is not quite so easy as it sounds, because the impedance of the winding in any’ transformer, in any loudspeaker. or in any other unit, containing inductance or capacity changes with every change of frequency. The higher the frequency the higher becomes the impedance of a transformer, a choke or a speaker in which inductance is the chief factor in its impedance. "The impedance in the valve is composed principally of the resistance between plate and filament. In tables of valve characteristics it is the plate resistance which is usually specified and for the work of matching. impedances sufficient accuracy will be obtained by matching the impedance of the load with the plate resistance of the value, both being measured’ in ohms. In the following paragraphs the plate. resistance will be spoken of as representing the plate impedance. "The impedance of the external load changes greatly with change of the frequency being handled, but there is comparatively little change of the | valve resistance with change of fre-

quency. Therefore, it is impossible to match the load impedance with the plate resistance at all frequencies. The parts of the audio frequency range which are used the: most run between 100 cycles and 3000 cycles. The impedance match may be made at a medium frequency, say around ‘1000 ‘to 1200 cycles, and the results at lower and higher frequencies left to care for themselves. This is the most economical method because it allows the use of small inductance in the external load, but it is not the most satisfactory. "Better all round results will be mad by matching the impedance and ‘resistance at much lower frequencies. This will call for a larger inductance in whatever umt follows the valve. At higher frequencies the impedance four times the resistance of the valves but this is an advantage rather thar mh in the load. will then be two, three, or t a disadvantage. . "The plate resistance of a valve is changed by changes in the plate yol‘tage or "B" battery voltage. For exumple, a certain amplifying valve has a plate resistance of -5500'° ohms with 185 volts on the plate, while with only 90 volis on the plate the registance rises to $800 ohms. , It is generally quite easy to learn from information published by valve manufacturers the resistance of their yarious valves with different plate voltages."