Ecleree PAM2600 Service Manual page 6

SYMMETRY ADJUST
The threshold voltage varies much, even
between MOSFETs of the same kind. When connecting
them in parallel we must be careful that they all have the
same conduction current if we want equal currents
circulating in all of them. If the conduction voltage of P
an N channels MOSFETs is not the same they will
conduct different currents, even when we apply identical
gate-source voltages. As the bias current of the N
MOSFETs must be identical to the one of the P
MOSFETs the feedback will correct the continuous
voltage at the operational output to polarize the
MOSFETs with different voltages until both conduct
equal currents.
If the operational output is not 0 V its capacitity
to give voltage and current is not the same in both
senses. To avoid this we must put a symmetry adjust. It
is just an adjust which allows to vary the collector
resistance of one of the current sources (R111).
The symmetry adjust does not correct the
asymmetrical clipping saturation of the power amplifier
with real load. This happens because the conduction
resistors (Ron) of the MOSFETs N and P are not equal.
Channel P has a higher Ron than channel N. This
characteristic depends on the MOSFET's physical
construction.
POWER MOSFETs
The MOSFETs used are IRFP9240 (P) and IRFP240 (N). They are assembled in a common source
configuration so they can be completely saturated.
This kind of configuration has two drawbacks compared to a common drain one: less stability (because
of the configuration gain itself) and high output impedance in open loop.
The source resistances (0.22 Ω ) are needed for the MOSFETs to work in parallel. E.g.: Two MOSFETs
excited by the same Vgs voltage (gate-source voltage) of 5V. If they have different transconductance curves (Id
function Vgs) they will conduct different drain currents; let's say 1A and 3A. The second one will dissipate more
power and will get hotter.
The use of source resistances tends to match the current that each of the MOSFETs connected in
parallel is conducting.
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