Ecleree PAM2600 Service Manual page 5

BIAS CURRENT ADJUST
The bias current adjust is performed through the
variable resistor connected between the emitters of the
current sources R110 (5K Ω ). It delivers a supplementary
current (it does not go through the operational) which
simultaneously increases the voltage which falls in the Rc load
resistors.
This is the easiest way of acting with just one adjust
over both branches at the same time. In order to adjust the
bias current the adjustable resistor must be varied until a
current of about 80mA circulates through each MOSFET. So,
for instance, for a PAM2600 in which there are six MOSFETs
it will be 80 x 6 = 480mA.The bias current depends on the
MOSFETs temperature and the stabilizing circuit transistors
temperature.
TEMPERATURE STABILIZING CIRCUIT
Temperature affects MOSFETs conduction in two different
ways: first, the conduction threshold voltage has a negative
temperature coefficient; second, the drain-source conduction
resistance increases with temperature. Depending on which of the
two things is predominating the temperature coefficient of the drain
can be positive or negative. In our case, in which the gate-source
voltage in the MOSFETs is very low when they conduct, the
temperature coefficient of drain current -which is positive- is
predominating.
To avoid thermal runaway in the polarizing current we must
decrease the gate-source voltage as the MOSFETs get hot.
Temperature stabilization is performed by modifying the reference
voltage of both sources. If the temperature increases the Vref must
decrease so that Ic and VRc decrease and, as a consequence, the
gate-source voltage also decreases.
The circuit used is shown in figure 3. The base-emitter Vbe
temperature/voltage feature is used to obtain the final result we need.
The main idea is adequately choosing R1 and R2 to obtain the right
temperature coefficient.
2