ATF-521P8 Avago Technologies, ATF-521P8 Datasheet - Page 15

• Manufacturer Part Number: ATF-521P8
• Description: Atf-521p8 Gaas Field Effect
• Manufacturer: Avago Technologies

15
Amp
Figure 4. High Pass Frequency Response.
The second solution is a low pass configuration with a
shunt capacitor and a series inductor shown in Figure 5
and 6.
RF
Figure 5. Low Pass Circuit Topology.
Amp
Figure 6. Low Pass Frequency Response.
The actual values of these components may be calcu‑
lated by hand on a Smith Chart or more accurately done
on simulation software such as ADS. There are some
advantages and disadvantages of choosing a high pass
versus a low pass. For instance, a high pass circuit cuts
off low frequency gain, which narrows the usable band‑
width of the amplifier, but consequently helps avoid
potential low frequency instability problems. A low pass
match offers a much broader frequency response, but it
has two major disadvantages. First it has the potential
for low frequency instability, and second it creates the
need for an extra DC blocking capacitor on the input
in order to isolate the device gate from the preceding
stages.
Amp
Figure 7. Input and Output Match for ATF-521P8 at 2 GHz.
in
Input Match
L1
C1
Frequency
RF in
RF
Frequency
Frequency
ou t
C1
+
Amp
ATF-521P8
C2
Frequency
Zo
+
Amp
52
Output Match
Frequency
Zo
=
Amp
Figure 7 displays the input and output matching se‑
lected for ATF‑521P8. In this example the input and out‑
put match both essentially function as high pass filters,
but the high frequency gain of the device rolls off pre‑
cipitously giving a narrow band frequency response, yet
still wide enough to accommodate a CDMA or WCDMA
transmit band. For more information on RF matching
techniques refer to MGA‑53543 application note.
Passive Bias
Once the RF matching has been established, the next
step is to DC bias the device. A passive biasing example
is shown in Figure 8. In this example the voltage drop
across resistor R3 sets the drain current (Id) and is calcu‑
lated by the following equation:
R3 = V
where,
V
V
I
I
A voltage divider network with R1 and R2 establishes
the typical gate bias voltage (Vg).
R1 = V
R2 = (V
Often the series resistor, R4, is added to enhance the
low frequency stability. The complete passive bias ex‑
ample may be found in reference [1].
C3
ds
bb
dd
ds
is the device drain to source current;
for DC stability is 10X the typical gate current;
is the device drain to source voltage;
is the power supply voltage;
Total Response
I
bb
I
g
dd
ds
Frequency
dd
p
+ I
– V
L1
– V
[1]
RF out
bb
V
ds
g
g
) x R1
p
(2)
(3)
(1)