Technical Feature: Capacitors in Broadband Applications

Proper selection of capacitors for RF broadband applications requires careful evaluation of frequency dependent parameters and circuit design requirements

By Richard Fiore
American Technical Ceramics

In today's rapidly expanding RF and microwave markets, numerous designs must operate over multiple octaves of frequency spectrum. Some of the more common of these include broadband bias networks such as transistor emitter and FET source bypassing, transistor collector and FET drain feed structures, as well as interstage RF coupling, DC blocking and wideband impedance matching.

This article will explore various ways to accommodate broadband application designs with the proper selection of capacitive elements. The first part of this discussion will address the implementation of a single capacitor solution followed by a multiple capacitor approach. Pertinent electrical design parameters, such as the magnitude of the impedance, insertion loss and the capacitor's parasitic elements, will be examined in detail for each method.

Single capacitor approach
There are many broadband applications with specific design requirements in which a single capacitor will provide an excellent functional solution. Given that the impedance of a capacitor, its equivalent series resistance (ESR), net reactance and quality factor (Q) are all frequency dependent, the designer must carefully consider these parameters before designing capacitive elements into a broadband application. Another critical parameter to take into account is the capacitor's insertion loss characteristic, i.e. the magnitude of S₂₁. By evaluating the insertion loss over the frequency band of interest, the designer can readily determine whether or not the subject capacitor is suitable for broadband DC blocking and coupling applications. This will also serve as a good starting point for selecting a broadband bypass capacitor. In contrast to DC blocking and coupling, a bypass capacitor also requires careful evaluation of its complex impedance over the entire frequency range of interest with emphasis on the inductive reactance resulting from the capacitor's parasitic inductance. The magnitude of both the real and reactive parts of the capacitor's impedance over frequency can easily be seen on a Smith chart presentation. Assessment of the net parasitic inductance will be discussed in greater detail later in this article….

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