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small-signal, large-signal - RF Cafe Forums
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shane
Post subject: small-signal, large-signal Posted: Wed Oct 05,
2005 2:16 pm
hi guys,
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
shane
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Guest
Post subject: Small-signal
vs. Large-signalPosted: Wed Oct 05, 2005 3:13 pm
Small-signal means
"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.
Good Luck!
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shane
Post subject: Posted: Fri Oct 07, 2005 1:54 am
thanx...your
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?
thanks
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Guest
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.
Consider
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.
Clear yet?
Good Luck!
Posted 11/12/2012
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