Active bias
Passive biasing was used in this application note for
circuit simplicity and low component count. However,
active biasing is imperative for the ATF-50189 amplier in
volume production. Active biasing confers the ability to
hold the drain to source current constant over variations
in both gm and temperature. A very inexpensive method
of accomplishing this is to use two PNP bipolar transistors
arranged in a pseudo-current mirror conguration.
Due to resistors R1 and R3, this circuit is not acting as a true
current mirror, but if the voltage drop across R1 and R3 is
kept identical then it still displays some of the more useful
characteristics of a current mirror. For example, transistor
Q1 is congured with its base and collector tied together.
This acts as a simple PN junction, which helps temperature
compensate the Emitter-Base junction of Q2.
To calculate the values of R1, R2, R3, and R4 the following
parameters must be know or chosen rst:
Ids is the device drain-to-source current;
IR is the Reference current for active bias;
Vdd is the power supply voltage available;
Vds is the device drain-to-source voltage;
Vg is the typical gate bias;
Vbe1 is the typical Base-Emitter turn on voltage for Q1 &
Q2;
Therefore, resistor R3, which sets the desired device drain
current, is calculated as follows:
where, IC2 is chosen for stability to be 10 times the typical
gate current and also equal to the reference current IR.
The next three equations are used to calculate the rest of
the biasing resistors.
Note that the voltage drop across R1 must be set equal to
the voltage drop across R3, but with a current of IR.
(5)
R2 sets the bias current through Q1.
(6)
R4 sets the gate voltage for the FET.
(7)
Thus, by forcing the emitter voltage (VE) of transistor Q1
equal to Vds, this circuit regulates the drain current similar
to a current mirror. As long as Q2 operates in the forward
active mode, this hold true. In other words, the Collector-
Base junction of Q2 must be kept reversed biased.
Vdd
L3
L2
C8
C3
C4
R5
C5
C6
Q1
R2
Q2
R3
R4
R1
R6
Vds
Vg
VE