Separation of Component Parts

Author: J.B. Hoag

Complex currents are often encountered in electronic circuits. It is often desirable to separate the components one from the other.

Fig. 5 B. To illustrate the separation or "filtering" of d.c., a.f. and r.f. currents from a complex current

In Fig. 5 B we assume that a combination wave of d.c, 60 cycles, and 60 kHz is applied to the terminals at the left of the circuit. The coil L₁ called a radio-frequency choke (r.f.c.), has sufficient reactance to stop most of the 60-kHz current, but permits most of the 60-cycle current and the d.c. to pass on to the right. However, the condenser C_1; called an r.f. bypass, has low reactance to the r.f. of 60 kHz so that this current passes through it and is indicated on the r.f. meter. The reactance of C₁ is sufficiently high for the 60-cycle current and is infinite for the d.c, so that these current components do not pass through C₁. Thus we have separated the 60 kHz from the other two components. The larger condenser C₂ serves similarly to shunt the 60-cycle a.f. currents through it, while the large audio frequency choke coil (a.f.c.) prevents them from passing on to the third arm of the circuit. The d.c. can pass through L₂, with only a small loss in the coil's resistance.

Later we shall study, under the title of "Filters", more elaborate circuits which are of value in separating currents whose frequencies are not so greatly different as in the elementary case just discussed.