small-signal, large-signal - RF Cafe Forums
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Post subject: small-signal, large-signal Posted: Wed Oct 05, 2005 2:16 pm
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
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.
"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
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
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.