subject: bypass capacitors choice and applications
from: stuart rohre (rohre@arlut.utexas.edu)
date: wed oct 09 2002 - 19
22 edt
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good question richard,
here is what my elmers have said over the years. also, like you, i have
learned by observation of what was successfully used in various circuits.
now the general rule of thumb in picking coupling capacitor values is to
pick it large enough to pass the frequency of interest with a capacitor
reactance acting on the signal voltage at a value of 1/10 the circuit
impedance, thus passing the signal with minimal attenuation. you can get by
with larger reactance in some cases.
larger capacitance passes lower frequencies better, and so on. the formula
for capacitive reactance will show you this by inserting some trial numbers
for frequency. the reactance charts in older arrl handbooks or other
references such as the itt radio engineers handbook easily show the effect
of varying the capacitor value.
the same consideration is used for bypass capacitors. you either want to
pass all or most of the frequency to ground or common, or at least have that
path be only 1/10 the impedance of the circuit you are taming by the use of
the bypass. in other words, you want enough capacitance to short out the
frequencies of interest. for example, you want an rf path around a cathode
resistor, or an emitter resistor, so as to not attenuate the rf signal.
however, for bypass as well as coupling, the type of capacitor comes into
play for frequency considerations, as you touch upon. you do not want the
capacitor to resonate at the frequencies present.
thus, a rolled foil cap, such as the plastic tubulars might not be the
choice for rf circuits, where the rolled foil looks inductive. ceramic
capacitors are parallel plates, with dielectric layers, and thus are
preferred in rf circuit bypassing. the wide range of bypass values
developed from the use of smaller amounts of capacitance, such as 0.001 or
0.005 mf in tube circuit days when impedances were higher, or in high
impedance fet circuits.
audio and ac /dc power supplies are bypassed with electrolytic capacitors.
the "filter" capacitors in typical rectifier output filters are bypasses for
the ac ripple at 120 hz and 180 hz or whatever is left after the ac line is
rectified.
the values like 0.01 and 0.1 microfarad or even 10 mf are found in lower
impedance circuits typical of transistors. combined 0.01 and 0.1 are an
attempt (in parallel) to provide a low impedance bypass over a wider
frequency range than that of one capacitor alone. as stated above, old tube
circuits will bypass rf with 0.001 and 0.005 microfarad capacitors. in more
recent times use of the term nano farad has become popular, but with correct
movement of the decimal point, you can see the same capacitor noted as so
many (fraction of) microfarads in older literature.
that pretty much is it. you pick capacitors from manufacturers catalogs
that have low esr (equivalent series resistance) and low ohms of reactance
when you want bypasses. esr comes into play for audio and power supply
applications. capacitive reactance is a component of rf circuit impedance
and must be considered to find the capacitance suited to the rf bypass job
at hand.
reactance charts, (when you know or can guess closely the circuit
impedances) are a quick way of picking a capacitor value to pass a
frequency. knowledge of the preferred frequency ranges of certain types of
capacitor construction completes the knowledge base you need to pick
effective bypass capacitors.
finally, don't forget to consider the circuit dc voltage and the impressed
ac (rf) voltage when picking the voltage rating of the selected capacitance
value. in most transistor circuits today, anything over the dc power supply
voltage should be adequate until you get to high power rf stages, where you
might consider if the rf voltage is being raised by doubling, or by
modulation, in the case of am amplifiers.
72, stuart k5kvh
