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Decibels; Impedance Matching

In amplifier work, the ratio of two voltages E1 and E2 at the same impedance level is often stated in decibels (db) according to the definition

db = 20 log10 (E1/E2) [59]

Amplifier voltage gain, transformer ratio, frequency response, and noise levels all may be expressed in decibels. Volume, voltage, or power in decibels must be compared to a reference level; otherwise the term is meaningless. A standard reference level is 1 milliwatt. This is expressed as zero dbm. Across 600 ohms, the voltage for zero dbm is ; for 20 dbm the voltage is 7.75 volts.

Transmission lines at audio and higher frequencies exhibit properties commonly ignored at 60 cycles. Line wavelength, characteristic impedance, and attenuation are important at audio frequencies; so is the matter of matching impedance. If a long transmission line has no attenuation, its characteristic impedance is given by

[60]

where L and C are the inductance and capacitance per unit length. If such a line terminates in a pure resistance load equal in ohmic value to Z0, all the power fed into the line appears in the load without attenuation or reflection. This is called matching the impedance of the line. It is very desirable to save audio power and avoid reflections; therefore impedance matching of lines is the usual practice wherever possible. The notion has been extended to include the loading of vacuum tubes, but this is stretching the meaning of the term matching. A vacuum tube has its optimum load impedance, but the value depends upon the conditions of tube operation and is not necessarily the same as the tube internal impedance.

Power transmission lines operating at 60 cycles are rarely long enough to act as appreciable source impedances. When a short-circuit or low-impedance fault occurs on the load side of a power transformer, the load current is limited mainly by the transformer short-circuit impedance. In a vacuum-tube amplifier, the load current delivered into a short-circuited load is limited mainly by the vacuum-tube internal resistance rather than by the transformer. At certain frequencies the transformer itself may contribute to low load impedance. But the greatest difference between power and amplifier transformers is the difference in source impedance. Even the use of the word impedance in the two fields of application reflects this difference. In power work, transformer impedance denotes the short-circuit impedance; in amplifier work, the same term refers to the load or source impedance.



Last Update: 2011-01-25