Broadcom G004 User guide

Brand: Broadcom

Model: G004

Type: User guide

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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

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

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

jumpers and the device was biased externally.

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.

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

shows both gain and input return loss from 300 kHz to 1 GHz with several

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shows both gain and input return loss from 300 kHz to 1 GHz with several

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

balanced trade-off between improved input match and reduction in gain. The

aberration in performance at the lowest frequency end of the plots is due to

the 500 pF capacitor (two 1000 pF capacitors in series) in the feedback loop.

A higher value for Cf would further extend the low end response.

It would be theoretically possible to limit the effect of the feedback, and thus

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

for both gain and input return loss for several values of shunt resistor.

A second approach to extending the low frequency performance of the

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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.

Japan:

(813) 3335-8152

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Japan:

(813) 3335-8152

Europe: Call your local Agilent sales office.

Data subject to change.

Broadcom G004 User guide

Broadcom G004 User guide

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