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.
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shows both gain and input return loss from 300 kHz to 1 GHz with several
balanced trade-off between improved input match and reduction in gain. The
the 500 pF capacitor (two 1000 pF capacitors in series) in the feedback loop.
It would be theoretically possible to limit the effect of the feedback, and thus
values of feedback resistance. Performance without any external feedback is
also shown for comparison.
Figure 3. Test Results with External Feedback.
A value for the feedback resistor between 240 and 510 ohms provides a
aberration in performance at the lowest frequency end of the plots is due to
A higher value for Cf would further extend the low end response.
the reduction in gain, to only the low frequency portion of the band by
adding a series inductor to the feedback loop. In practice the combination of
high value capacitors (e.g., in the mF range) and their associated parasitics
in combination with the inductor would lead to undesirable resonances.
The high output power feature of the MGA-82563 is not affected by the
external feedback approach. An output power of 17.7 dBm at 1-dB gain
compression was measured on this circuit using a feedback resistor of 240
ohms. (This measurement was made at 40 MHz due to the limitation of
external bias tees used in the particular power test system.)
Shunt Input Resistor Method
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A second approach to extending the low frequency performance of the
MGA-81563 / 82563 amplifiers is to add a shunt resistor at the input as
shown in Figure 4. Since the input of the MGA-81563 / 82563 amplifiers is
at a DC potential of zero volts, it is possible to add a shunt resistor directly
to the input of the amplifier without disturbing the amplifier's internal DC
bias. The value of the shunt resistor may be selected to provide as good a
match at the input as desired. The trade-off is, of course, lower gain and
higher noise figure. The shunt resistor approach is the simplest and has the
advantage of improving performance down to DC frequency.
Figure 4. MGA-82563 Circuit with Input Shunt Resistor.
Figure 5 shows a test circuit with a shunt resistor placed at the input of the
MGA-82563.
Figure 5. Test Circuit with Input Shunt Resistor.
The test results of the shunt R circuit are shown in Figure 6. Data is shown
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for both gain and input return loss for several values of shunt resistor.
Figure 6. Test Results With Shunt Input Resistor.
A shunt resistor value of between 82 and 150 ohms provides a good input
match with minimum loss of gain. The output power is not affected by the
addition of the input shunt resistor.
Summary
Two methods of extending the low frequency response of MGA-81563 and
MGA-82563 RFIC amplifiers have been presented. Of these two choices,
the second method using a shunt R at the input is the simplest to implement
and provides a solution effective to DC. The first method using external
shunt feedback is more complex but may not degrade noise figure as much
as the shunt R technique.
RLM110299
For technical assistance call:
Americas/Canada: 1-800-235-0312 or 408-654-8675
Far East/Australasia: Call your local Agilent sales office.
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Japan:
(813) 3335-8152
Europe: Call your local Agilent sales office.
Data subject to change.
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