DEMO-ATF-5X1P8 Datasheet, PDF (15/23 Pages) Broadcom Corporation. – High Linearity Enhancement Mode[ Pseudomorphic HEMT in 2x2 mm2 LPCC[3] Package

Amp
Frequency
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_in L1
RF_out
C1
Figure 5. Low Pass Circuit Topology.
Amp
Frequency
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
bandwidth 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.
Figure 7 displays the input and output matching
selected for ATF-521P8. In this example the input and
output match both essentially function as high pass
filters, but the high frequency gain of the device rolls off
precipitously 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 ^[1]
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_dd – V_ds
(1)
I_ds + I_bb ^p
where,
V_dd is the power supply voltage;
V_ds is the device drain to source voltage;
I_ds is the device drain to source current;
I_bb for DC stability is 10X the typical gate current;
A voltage divider network with R1 and R2 establishes
the typical gate bias voltage (Vg).
R1 = V_g
(2)
I^p
bb
R2 = ^(V_dd ^{– V}_g^{) x R1} (3)
V_g
Often the series resistor, R4, is added to enhance the
low frequency stability. The complete passive bias
example may be found in reference [1].
RFin C1
Zo
52
C2
C3
Zo
RFout
L1
Input Match
Amp
Amp
+
ATF-521P8
Amp
+
Output Match
Frequency
Frequency
Frequency
Total Response
Amp
=
Frequency
Figure 7. Input and Output Match for ATF‑521P8 at 2 GHz.
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