Avago Technologies MGA-81563 User Manual

DesignTip G004

Extending the Low Frequency Response of

frequency response of these RFICs.

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

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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

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By adding a series R-C from the RF output to the RF input of the MGA

jumpers and the device was biased externally.

Test results for the external feedback circuit are shown in Figure 3. This plot

amplifier, the low-end frequency response becomes limited only by the
value of the DC blocking capacitor used in the external RF feedback loop.

Figure 1. MGA-82563 Schematic with External Feedback.

A circuit based on the MGA-8-A evaluation PCB was assembled with an
MGA-82563 to test the external feedback method. The DC blocking
capacitors normally used at both the input and output were replaced with

The assembled test circuit is shown in Figure 2. The external feedback
resistor, Rf, is mounted in between two 1000 pF chip capacitors on either
side of the amplifier package. Capacitor Cf was implemented with two
capacitors instead of one to simplify physically bridging around the MGA-

82563.

Figure 2. Test Circuit with External Feedback Circuit.