19
Bias Networks
One of the major advantages of the enhancement
mode technology is that it allows the designer to be
able to dc ground the source leads and then merely
apply a positive voltage on the gate to set the desired
amount of quiescent drain current Id.
Whereas a depletion mode PHEMT pulls maximum
drain current when V
gs
=0V, an enhancement mode
PHEMT pulls only a small amount of leakage current
when V
gs
=0V. Only when V
gs
is increased above V
th
, the
device threshold voltage, will drain current start to ow.
At a V
ds
of 2.7V and a nominal V
gs
of 0.47V, the drain
current I
d
will be approximately 10 mA. The data sheet
suggests a minimum and maximum V
gs
over which the
desired amount of drain current will be achieved. It is
also important to note that if the gate terminal is left
open circuited, the device will pull some amount of
drain current due to leakage current creating a voltage
di erential between the gate and source terminals.
Passive Biasing
Passive biasing of the ATF-551M4 is accomplished by
the use of a voltage divider consisting of R1 and R2. The
voltage for the divider is derived from the drain voltage
which provides a form of voltage feedback through
the use of R3 to help keep drain current constant. In
the case of a typical depletion mode FET, the voltage
divider which is normally connected to a negative
voltage source is connected to the gate through
resistor R4. Additional resistance in the form of R5 (ap-
proximately 10KΩ) is added to provide current limiting
for the gate of enhancement mode devices such as
the ATF-551M4. This is especially important when the
device is driven to P1dB or Psat.
Resistor R3 is calculated based on desired V
ds
, I
ds
and
available power supply voltage.
V
DD
– V
ds
I
ds
+ I
BB
R3 =
(1)
Figure 37. Typical ATF-551M4 LNA with Passive Biasing.
ATF-551M4 Applications Information
Introduction
The ATF-551M4 is a low noise enhancement mode
PHEMT designed for use in low cost commercial appli-
cations in the VHF through 10 GHz frequency range. As
opposed to a typical depletion mode PHEMT where the
gate must be made negative with respect to the source
for proper operation, an enhancement mode PHEMT
requires that the gate be made more positive than
the source for normal operation. Therefore a negative
power supply voltage is not required for an enhance-
ment mode device. Biasing an enhancement mode
PHEMT is much like biasing the typical bipolar junction
transistor. Instead of a 0.7V base to emitter voltage,
the ATF-551M4 enhancement mode PHEMT requires a
nominal 0.47V potential between the gate and source
for a nominal drain current of 10 mA.
Matching Networks
The techniques for impedance matching an en-
hancement mode device are very similar to those for
matching a depletion mode device. The only di erence
is in the method of supplying gate bias. S and Noise
Parameters for various bias conditions are listed in
this data sheet. The circuit shown in Figure 37 shows a
typical LNA circuit normally used for 900 and 1900 MHz
applications. Consult the Broadcom web site for appli-
cation notes covering speci c designs and applications.
High pass impedance matching networks consisting
of L1/C1 and L4/C4 provide the appropriate match for
noise gure, gain, S11 and S22. The high pass structure
also provides low frequency gain reduction which can
be bene cial from the standpoint of improving out-of-
band rejection.
Capacitors C2 and C5 provide a low impedance in-band
RF bypass for the matching networks. Resistors R3 and
R4 provide a very important low frequency termination
for the device. The resistive termination improves low
frequency stability. Capacitors C3 and C6 provide the
RF bypass for resistors R3 and R4. Their value should be
chosen carefully as C3 and C6 also provide a termina-
tion for low frequency mixing products. These mixing
products are as a result of two or more in-band signals
mixing and producing third order in-band distor-
tion products. The low frequency or di erence mixing
products are terminated by C3 and C6. For best sup-
pression of third order distortion products based on
the CDMA 1.25 MHz signal spacing, C3 and C6 should
be 0.1 uF in value. Smaller values of capacitance will
not suppress the generation of the 1.25 MHz di erence
signal and as a result will show up as poorer two tone
IP3 results.
INPUT
C1
C2
C3
L1
R4
R1 R2
Vdd
R3
L2 L3
L4
Q1
Zo
Zo
C4
C5
C6
OUTPUT
R5
