AN10714_1
© NXP B.V. 2010. All rights reserved.
Application note
Rev. 01 — 26 January 2010
10 of 21
NXP Semiconductors
AN10714
Using the BLF574 in the 88 MHz to 108 MHz FM band
The
−
2.2 mV/
°
C at its base is generated by Q2. This is then multiplied by the R14 : R15
ratio for a temperature slope (i.e. approximately
−
15 mV/
°
C). The multiplication function
provided by the transistor is the reason it is used rather than a diode. A portion of the
−
15 mV/
°
C is applied to the potentiometer (R1).
The amount of temperature compensation is set by resistor R4. The ideal value of which
proved to be 2 k
Ω
. The values of R11 and R13 are not important for temperature
compensation. However, they are used for baseband stability and to improve IMD
asymmetry at lower power levels.
3.3 Amplifier alignment
There are several points in the circuit that allow performance parameters to be readily
traded off against one another. In general, the following areas of the circuit have the most
impact on the circuit operating frequency and P
L(1dB)
performance. The modification areas
are listed in order of sensitivity, with the most sensitive tuning elements listed first.
Effect of changing the output capacitors (C12 and C13):
•
This is a key tuning point in the circuit. This point has the strongest influence on the
trade-off between efficiency and linearity.
Effect of the length of the output balun (B2):
•
The frequency can be shifted by modifying this element. Typically, the longer the
balun, the more the response is shifted to lower frequencies. Conversely, a short
balun shifts the response to higher frequencies.
Effect of changing the output 4 : 1 transformers (T3 and T4):
•
The frequency can be shifted by modifying these elements. In general, longer
transformers shift the whole response to a lower frequency. Shortening the
transformers shifts the response to higher frequencies. Changes in efficiency and
P
L(1dB)
is seen when the characteristic impedance of these transformers is changed.
Effect of changing the output capacitor (C14):
•
Changing this output capacitor has an effect of tilting the response over the band. The
efficiency or P
L(1dB)
performance can be made more consistent over the band by
modifying C14.
Effect of adding capacitance off the drain (C10, C11, C15, and C16):
•
A small adjustment in the trade-off between efficiency and P
L(1dB)
performance can be
made by changing these capacitors.
