Avago Technologies MGA-81563 - Manual

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DesignTip G004
Extending the Low Frequency Response of
the MGA-81563 and MGA-82563 RFIC
Amplifiers
Bob Myers
Application Engineer, Wireless Semiconductor Division
Agilent Technologies
Abstract: The MGA-81563 and 82563 are GaAs MMIC amplifiers with an
intrinsic bandwidth of 0.1 to 6 GHz. Some applications require operation at
lower frequencies. This note discusses two methods for extending the low
frequency response of these RFICs.
Introduction
The low frequency response of the MGA-81563 and 82563 is limited by an
on-chip, internal feedback circuit. This RF feedback circuit consists of a
resistor in a shunt feedback configuration in series with a DC blocking
capacitor. The design value of the blocking capacitor is fairly small (several
pF) to minimize overall die size commensurate with low-cost wireless
components. As a result, the resistive feedback becomes ineffective at lower
frequencies. The decreased feedback has the effect of increasing gain and
degrading input impedance match at frequencies below approximately 100
MHz.
Two approaches may be used to extend low end performance by
compensating for the low frequency limitation of the internal feedback
circuit. One approach is to add an external feedback circuit that parallels the
internal feedback, but contains a higher value blocking capacitor. A second,
and simpler, approach is to merely add shunt resistance at the input of the
MGA-81563 / 82563. Test results for both of these methods are presented.
External Feedback Method
By adding a series R-C from the RF output to the RF input of the MGA