Ecler PAM1400, Service Manual

Page: 6 / 183

3
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 capacity 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.