Small-Signal, Large-Signal - RF Cafe Forums
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
subject: Small-signal vs. Large-signalPosted: Wed Oct 05, 2005 3:13
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.
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
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
subject: Small-signal/Large-SignalPosted: Fri Oct 07, 2005 7:22
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.
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.