Preamps are class A amplifiers, but if they are driven hard enough,
they are effectively being "biased" by the RF signals they're handling
toward class A-B & B. The DC biasing network can "store" this energy
with time constants ( due mostly to large bypass capacitors ) for
significant periods in the aftermath of a pulse event. Effectively,
the non-linearities of the device has rectified the RF and charged
(or discharged) the capacitors in the DC bias circuit.
For an example, A FET has near zero current in its GATE circuit,
but when overdriven can develope mA's of GATE current which changes
the voltage on the bypass capacitors. This different voltage still
is present following the removal of the overdrive condition, and
may persist for considerable time if the DC bias circuit isn't low
enough impedance. The FET itself may draw uA's or less of GATE
current and relies on the bias circuit to recover quickly.
Active bias circuits can exhibit even more significant problems
unique to their transient response from elements of stored energies
and gain.
The point here is that the RF device is inappropriately biased in
the period following an event of being overdriven. Therefore,
it is not the same amplifer during that period and is actually
changing performance as the bias circuits are recovering.
BCNU DE N2LO~>
|