January 1962 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
play a vitally important role in electronic circuit design. Many people do not know the proper way for
deciding which capacitor or capacitors is/are needed for effective noise and/or signal bypassing without
either overdoing or underdoing it. Needs change over the years as frequencies and signal characteristics
occupy new realms of the spectrum. A Fourier analysis of some of today's complicated waveshapes for
switching power supplies shows how sometimes tailored responses to bypassing is required. This article
from the January 1962 Popular Electronics does not delve into the intricacies of complex filters, but
it does provide a nice introduction to the need for bypassing and how to stand a good chance of being
successful at it. In rare instances, no amount of bypassing will solve problems with spurious signals
and a more engineered approach is required where the functional circuit needs to be modified (I know
this from experience!).
Here is an article written by RF Cafe contributor
on the subject of bypassing.
Getting to know the... Bypass Capacitor
By John M. Doyle
National Radio Institute
Capacitors are used in modern electronic circuitry for such purposes as blocking, filtering, timing,
and bypassing. The last-mentioned application - bypassing - is by far the most common. It's also much
more critical than many people suspect, since the selection of a wrong-value bypass capacitor can result
in poor frequency response, phase distortion, circuit instability, or even outright oscillation.
Now, you may feel that this problem is no concern of yours, but is rather one for the design engineer.
"Shucks," you say. "If a capacitor-bypass or otherwise - goes bad, I'll replace it."
This approach works fine in most cases. But how many times have you wished you could help fix a piece
of equipment after a well-intentioned, but poorly instructed, do-it-yourself fan has been hard at work
with his trusty soldering gun? Or how many times have you felt like throwing that cheap "screech-box"
receiver out of the window, when a 15¢ bypass capacitor would do much to quiet the demon of temptation?
Finally, remember that despite our ultra-modern manufacturing methods, "goofs" are still made by people
who do the physical wiring and inspection but who know nothing about the workings of the circuit.
If you're now convinced of the importance of knowing a little more about bypassing (and shame on
you if you aren't), let's get on with the job.
Reactance. When a capacitor is used as a bypass, it must provide a low-impedance
path for electrical currents of certain frequencies and a high-impedance path for those of other frequencies.
The property which permits it to operate in this manner is called reactance. The value of reactance
for a given frequency is determined by the basic formula:
Xc = 1 / (2π f C)
where Xc is the capacitive reactance in ohms,
2π is a constant (approximately
6.28), f is the operating frequency in cycles per second, and C is the capacitance in farads.
This relationship tells us that the reactance of a given capacitor decreases as frequency increases.
For example, the value of Xc for a 0.01-μf. capacitor at a frequency of 500 cycles is
about 31,800 ohms. But at a frequency of 5000 cycles, the reactance has decreased to about 3180 ohms.
Not only must we be able to calculate Xc, but we must also know how to determine what
value capacitor is needed to obtain a certain reactance at some specified frequency. All that we have
to do is rearrange the above equation as:
C = 1 / (2π f Xc)
where all symbols have the same meaning as before. For example, if we want to know what value capacitor
will provide a reactance of 18 ohms or less at a frequency of 500 cycles, we just substitute known values
in the above formula. The calculated answer is 17.7 μf., approximately, but the next highest standard
capacitance value available will be okay for most applications.
Audio-Frequency Amplifiers. In a typical audio-frequency amplifier, such as that
shown in Fig. 1, a capacitor, C1, is used to bypass audio frequencies around the cathode resistor, R1.
If capacitor C1 is omitted or if it does not operate properly, the a.c. plate current component develops
a voltage drop across R1 which opposes the input signal applied to the grid. This effectively reduces
stage gain and results in inverse feedback or "degeneration."
Fig. 1. Cathode bypass for audio amplifier stage.
Now, let's see what requirements are placed on the capacitor if it is to prevent degeneration. Suppose
the amplifier is to pass all frequencies between 100 and 5000 cycles, and the value of cathode resistor
recommended by the manufacturer for class A operation is 1500 ohms. Because the reactance of the capacitor
decreases as frequency increases, a capacitor that satisfactorily bypasses the resistor at the lowest
frequency will work quite nicely over the entire range.
A rule-of-thumb used by circuit designers is that the reactance of the capacitor at the lowest frequency
to be passed should not exceed one-tenth the value of the resistor it bypasses. Using this rule, we
substitute known values in the equation developed for finding C:
C=1/(6.28 x 100 x 150) = 11 μf
An electrolytic capacitor is suitable for this purpose because its leakage resistance is not important
and high capacitance is obtained in a compact size.
In some applications, such as high-quality audio amplifiers, the ratio of resistance to reactance
at the lowest frequency passed is made 20 to 1 or even higher, but the ratio used in our example is
adequate for most cases. Needless to say, the working voltage of the capacitor selected for any bypassing
applications must be larger than the maximum voltage present.
Bypassing in the case of a transistorized audio-frequency amplifier is very similar. A typical pnp
transistor amplifier, using the common-emitter arrangement, is shown in Fig. 2. Base bias is obtained
from the voltage-divider network, consisting of R1 and R2, and the emitter is forward-biased (negative
in the case of a pnp transistor, and positive for the npn type). To prevent signal degeneration, the
emitter-bias resistor (R3) is bypassed with a high-value electrolytic capacitor (C1).
Fig. 2. Emitter bypass for transistorized circuit.
In either type amplifier discussed above, a certain amount of degeneration is sometimes intentionally
used. Therefore, before jumping to any wrong conclusion, always make sure that degeneration is in fact
undesirable before attempting to correct a case of "faulty" design. If bypassing is improved where degeneration
is needed, the circuit will not operate properly.
When a pentode-type tube is used, additional bypassing is needed in the screen grid which must operate
at ground potential, as far as all signal voltages are concerned, if degeneration is to be avoided.
A typical case is the television i.f. amplifier shown in Fig.3.
Fig. 3. Screen bypass for typical i.f. amplifier in a television set.
In this circuit, screen potential is obtained from the plate-supply source through the screen-dropping
resistor, R2. If bypass capacitor C1 fails to operate properly at any frequency, the gain of the amplifier
falls off at that frequency. The value of C1 is again determined by the rule-of-thumb that its capacitive
reactance at the lowest frequency passed should not exceed one tenth the value of the resistor it bypasses.
Generally, mica or ceramic capacitors, ranging in value from about 50 μμf. to 0.01 μf.,
are used for r.f. bypassing arrangements of this type. If the pentode is employed as an audio-frequency
amplifier, high-quality paper or electrolytic capacitors are used. Their proper value can be determined
in the same way.
Sometimes it is necessary to bypass radio but not audio frequencies.
A typical case is in the detector circuit of an AM receiver, as shown in Fig. 4. Assuming that the r.f.
carrier frequency is 455 kc., if the reactance of C1 is to be one-tenth the value of R1 at this frequency,
its value - using the formula previously given - is approximately 75 μμf. We would use a standard 100-μμf.
mica or ceramic capacitor. If the highest audio frequency to be passed is 5000 cycles, the reactance
of the capacitor at this frequency is better than 300,000 ohms.
Fig. 4. R.f. bypass for diode detector stage.
Another circuit in which bypassing is important is illustrated in Fig. 5, where three amplifier stages
are fed from a common plate-voltage supply. Since most power supplies possess a finite impedance, the
output of V3 will be returned to the plate circuit of V1 through load resistors R2 and R1. This effective
signal voltage is then fed to the grid circuit of V2 and then into V3. Naturally, if the gain of these
stages is high enough, oscillation occurs.
Fig. 5. Decoupling is required because of feedback through R1 and R2.
To prevent instability of this type, decoupling networks are used, a typical example of which is
shown in Fig. 6. The reactance of C1 and C2 at the lowest operating frequency is made very small compared
to the resistance of R3 and R4. Because R3/C1 and R4/C2 form voltage dividers, almost the entire voltage
developed across the common impedance is dropped by R3 and R4. Essentially, no feedback voltage is then
coupled into the plate circuit of V1 or V2.
Fig. 6. Networks R4/C2 and R3/C1 prevent undesired feedback between stages.
The values of R3 and R4 should be kept as low as possible to accomplish the job without dropping
a prohibitive amount of the d.c. plate voltage for V1 and V2. In cases where a very small drop in this
voltage is all that can be tolerated, R3 and R4 can be replaced by an inductance of low d.c. resistance.
The value of inductance needed for a given reactance at a specified frequency is determined by the formula:
L = XL / 2π f
where XL is the inductive reactance in ohms, and f is the operating frequency in cycles per
Chassis Grounds. Here's a final word about connecting bypass
capacitors. At frequencies of 30 mc. and below, the dimensions of the chassis are usually only a fraction
of a wavelength, and it can be considered a fixed reference. Above 30 mc., however, the chassis is essentially
a conducting sheet on which points of maximum current and voltage appear.
In the circuit of Fig. 7, grid and plate "ground" currents pass through the chassis to the cathode
of the stage. A good practice, generally, is to separate these ground currents from the chassis by returning
all leads to the cathode or a bus bar. Just be sure, however, that the leads are kept as short as possible
to prevent cross-coupling and undesirable feedback.
Fig. 7. Decoupling capacitors should be grounded at one point for best results at
Posted September 17, 2012