



Copyright: 1996  2024 Webmaster:
Kirt Blattenberger,
BSEE  KB3UON
RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling
2 MB. Its primary purpose was to provide me with ready access to commonly needed
formulas and reference material while performing my work as an RF system and circuit
design engineer. The World Wide Web (Internet) was largely an unknown entity at
the time and bandwidth was a scarce commodity. Dialup modems blazed along at 14.4 kbps
while typing up your telephone line, and a nice lady's voice announced "You've Got
Mail" when a new message arrived...
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Basic Circuit Theory
Answers to RF Cafe Quiz #19 
All RF Cafe Quizzes make great fodder for
employment interviews for technicians or engineers  particularly those who are
fresh out of school or are relatively new to the work world. Come to think of it,
they would make equally excellent study material for the same persons who are going
to be interviewed for a job. Bonne chance, Viel Glück, がんばろう,
buena suerte, удачи, in bocca al lupo, 행운을 빕니다,
ádh mór, בהצלחה, lykke til, 祝你好運.
Well, you know what I mean: Good luck!
Click here for the complete list of
RF Cafe Quizzes.
Note: Some material based on books have
quoted passages.
Return to RF Cafe Quiz #19
1.
Which circuit law states that the sum of all currents into and out of a node equals zero?
c) Kirchhoff's
Current Law.
This diagram illustrates the principle.
2. How much power is dissipated by
an ideal 100 pF capacitor fed by a 1 V_{pkpk} sinewave at 10 MHz? Hint: Z = 1/(2πfC)
d) 0 W.
An ideal capacitor or inductor dissipates no power, since power is a function of the real part
of a complex impedance (R ± jX). Ideal capacitors and inductors store and release energy without consuming it (by
releasing heat).
3. What causes a ground current loop?
c) More than one ground potential in
a circuit.
A ground current loop occurs when "ground," or the supposed 0 V point, exists at more than one
potential in a common circuit. The difference of potential causes a current to flow, often creating a noise
component.
4. What
type of filter does this circuit represent?
c) Bandpass.
An ideal series tank circuit passes energy
at its resonant frequency with zero impedance (short circuit), and a parallel resonant circuit presents infinite
impedance (open circuit). So, if both are tuned to the same frequency (although usually there is an offset), then
at resonance the circuit appears as a short between the upper terminals and an open circuit wrt the lower
terminals.
5. How many poles does the filter in Q4 have?
c) 2 poles.
Each resonant LC tank generates a
pole in the transfer function.
6.
What is the configuration of the opamp shown here?
a) Integrator. The transfer function is
7. What is the secondary voltage
of this transformer?
d) 2 V.
The secondary:primary turns ratio is 20:100, or 1:5. Voltage ratio is
proportional to the turns ratio, so the secondary voltage is 10/5 = 2V
8. What is the impedance of
the transformer secondary in Q7?
d) 4 Ω.
The impedance ratio is proportional to the square of the
turns ratio, so the secondary impedance 100/5^{2} = 4 Ω.
9. In an ideal system, what is the
minimum sampling rate to prevent aliasing of a signal with a maximum frequency of 100 kHz?
b) 200 kHz.
The Nyquist Sampling Theorem demonstrates that a bandlimited signal must be sampled at a rate at least twice the
highest frequency in the original signal in order to prevent aliased content in the reconstructed signal. Here is
a nice little applet that illustrates the
sampling principle. Set the original signal for 150 Hz, and the sample rate to 300 Hz (2x the original) to see
that the reconstructed signal is an exact duplicate of the original. Ditto for 350 Hz. Now, set the sample rate to
200 Hz or 100 Hz and note the added frequency components in the reconstructed signal.
10. Which
type of diode typically has the lowest forward bias voltage for a given current?
d) Schottky.
A
Schottky diode is very similar to a standard PN junction diode, except instead of using an implanted player, the
rectification occurs at the interface between a barrier metal and the silicon layer. Here are typical IV charts
for the four types of diodes (compare at 25°C) .
GaAs Diode
Germanium Diode
Silicon Diode
Schottky Diode



