Copyright: 1996 - 2024
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. Dial-up modems blazed along at 14.4 kbps
while tying up your telephone line, and a nice lady's voice announced "You've Got
Mail" when a new message arrived...
All trademarks, copyrights, patents, and other rights of ownership to images
and text used on the RF Cafe website are hereby acknowledged.
My Hobby Website:
small-signal, large-signal - RF Cafe Forums
RF Cafe Forums closed its virtual doors in 2012 mainly due to other social media
platforms dominating public commenting venues. RF Cafe Forums began sometime around
August of 2003 and was quite well-attended for many years. By 2010, Facebook and
Twitter were overwhelmingly dominating online personal interaction, and RF Cafe
Forums activity dropped off precipitously. If the folks at
phpBB would release a version with integrated
sign-in from the major social media platforms, I would resurrect the RF Cafe Forums,
but until then it is probably not worth the effort. Regardless, there are still
lots of great posts in the archive that ware worth looking at.
Below are the old forum threads, including responses to the original posts.
|-- Amateur Radio
Gripes & Humor
-- CAE, CAD, &
Test & Measurement
Post subject: small-signal, large-signal Posted: Wed Oct 05,
2005 2:16 pm
i'm a sophomore undergrad. and i've
a question to ask. what's meant by small-signal and large-signal. small-signal
means ac signal with small amplitude? and large-signal means ac signal
with large amplitudes? this is what i thought but in a book it says
'large-signal or dc'...now i'm confused. also, is small/large signal
referring to the Amplitude or Frequency? please respond....thanx
Post subject: Small-signal
vs. Large-signalPosted: Wed Oct 05, 2005 3:13 pm
"small enough that the performance of the circuit doesn't depend on
the amplitude of an AC signal". In other words, the circuit is essentially
linear - no AC voltages are large enough to change the operating conditions.
Large-signal means "big enough that the performance of the circuit
depends on the the signal amplitude". There may be harmonics or intermodulation
products, a change in bias conditions, or increased power consumption.
In RF, we generally are not interested in DC amplifiers. But if
we are, the definitions above still hold - if the circuit is linear,
it's small signal.
Post subject: Posted: Fri Oct 07, 2005 1:54 am
explanation helped but still begs the questions: why large signal is
same as dc. as i said earlier, a book says "....large signal or dc'...why?
Post subject: Small-signal/Large-SignalPosted:
Fri Oct 07, 2005 7:22 pm
My previous reply was from an RF point
of view. For any number of reasons, RF circuitry is almost always AC
coupled - thus eliminating any question of a "DC signal". (In a previous
job, one boss's definition of "DC" was "any frequency under 30 MHz".
That may be a bit extreme...)
There are cases where response
down to DC is needed. In this case, it's possible for either the AC
signal or the DC baseline (it's still hard for me to talk about a "DC
signal") to change the operating point of the circuit.
an opamp output circuit with a class AB output stage, driving a small-ish
load resistance, from a balanced positive and negative supply. When
the output signal is centered (near ground), both output devices are
carrying current. That current is the quiescent or resting current.
A small deviation, AC or DC, won't change the fact that both output
devices are active, and current now flows in three devices: the pull-up
transistor, the pull-down transistor, and the load resistor.
Now if the input raises the output voltage enough, whether temporarily
or permanently ("AC" or "DC", respectively), the pull-down transistor
turns off. (That's what class AB is, after all - less than 360 degree
conduction but more than 180 degrees). Now the operating point has changed,
and the term "large signal" applies.