DESIGN APPLICATION NOTE --- AN010
SGA-2486 Gain Flatness Compensation Circuit
Abstract
New SiGe amplifier gain blocks exhibit a combination of wide bandwidth, high gain and IP3 with a
low noise figure. This application note describes circuits that have been developed to improve gain flatness over a wide frequency range.
Introduction
The SGA-2486 (datasheet # EDS-100629) is a high
gain, wideband cascadable SiGe gain block. It exhibits a combination of low power, high IP3 and low
noise figure. It does however show an expected
gain roll-off versus frequency characteristic of about
3 dB between 50 and 2150 MHz. This is easily corrected using the application circuit topology shown
in Figure 1 below. Circuit components Lcomp1,
Rcomp1, Lcomp2, Rcomp2 determine gain flatness
and must also be selected to preserve input/output
return loss. The value of Rcomp2 is further constrained by biasing requirements. The circuit below
operates with a 5 volt supply and draws 20 mA. It is
characterized for the 50 - 2150 MHz frequency
range.
Design / Performance Summary
The circuit layout and performance data are shown
on pages 2 and 3. The circuit was constructed on
our standard 86 package evaluation board according to the schematic shown in Figure 1. The performance highlights to be noted are :
• 16 dB of gain and 1 dB total gain Preliminary
variation
over the 50-2150 MHz frequency range.
• Greater than 17dB input / 20 dB output return
loss over the 50-2150 MHz frequency range.
• Flat Group Delay characteristic-only about 150
picoseconds added group delay over the 502150 MHz frequency range.
• Unconditionally stable over operating range.
• Pages 3 & 4 show how to use the circuit below
with an output RC network to achieve less than
0.1 dB flatness from 20-1000 MHz or 0.3 dB
flatness from 20-2150 MHz.
SGA-2486 @ 50-2150 MHz Application Circuit
+5V
Zc
Length
(Degrees @
1000 MHz)
12.3
+
Z1
50
-
Cd6
0.1 uF
Ref.
Des.
Z2
50
3.3
Z3
50
12.9
Z4
50
13.5
Z5
50
3.3
Z6
50
12.0
Cd5
Cd4
.01uF
1000pF
Rcomp2
120W
Lcomp2
15nH
SGA-2486
CC1
CC2
.01uF
.01uF
RF input
Z3
Z2
Z1
Z4
Z5
Z6
RF Output
Lcomp1
15nH
Notes:
1.0 See Parts List and layout on page 2.
2.0 All 0603 capacitors have an implied series inductance of
approx. 0.75 nanohenries.
3.0 Phase shift functional block elements between components
are calculated based on wavelength of 1000 MHz signal on FR4
board material with dielectric constant of 4.1 and microstrip
width and height dimensions of W=.061 inch and h=.030 inch.
Rcomp1
120W
+
Cd1
1000 pF
-
Cd3
0.1 uF
Cd2
.01 uF
Figure 1
The information provided herein is believed to be reliable at press time. 5)0' assumes no responsibility for inaccuracies or omissions. 5)0' assumes no responsibility for
the use of this information, and all such information shall be entirely at the user’s own risk. Prices and specifications are subject to change without notice. No patent rights or licenses to any of the circuits described
herein are implied or granted to any third party. 5)0' does not authorize or warrant any 5)0' product for use in life-support devices and/or systems.
Copyright 2002 5)0'. All worldwide rights reserved.
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EAN-101374 Rev B
DESIGN APPLICATION NOTE --- AN010
SGA-2486 Gain Flatness Compensation Circuit
Application Circuit Board Layout,
for 50-2150 Mhz
(Reference Figure 1 Schematic.)
Cd3
Parts List
REF.
DESIG.
DESCRIPTION
CC1,2
CD1,4
VALUE
PART NUMBER / STYLE
CAP.
.01 uF
0603 package (ROHM MCH 18)
CAP.
1000 pF
0603 package (ROHM MCH 18)
CD2,5
CAP.
.01 uF
0603 package (ROHM MCH 18)
CD3,6
CAP. (POLAR)
0.1 uF
"A" size package, 35V Tantulum
RCOMP1,2
RES.
120 Ohms
0603 package
LCOMP1,2
IND.
15 nH
TOKO LL1608-FH15NJ (0603)
+Vs
Cd6
Cd2
Cd5
Cd1
Cd4
Rcomp2
Rcomp1
SGA2486
Lcomp1
CC1
Lcomp2
CC2
Plot of P1dB, IP3 , Noise Figure and IP2 vs. Frequency
50
ECB-100330
86 Eval Board
NF
(dB)
Level (dBm)
40
Notes:
Board Material: FR4, e=4.1
Dielectric Thickness = 30 mils
Metal Thickness: 1Oz. (1.4 mil)
Backside fully plated
IP2
(high tone)
IP3
2
10
P1dB
1
0
500
S21 vs. Freq - Compensated vs. Uncompensated
Return Loss (dB)
Gain (dB)
19
18
Compensated
16
15
2050
0
-5
-10
-15
-20
-25
-30
-35
-40
-45
-50
2000
0
2500
Uncompensated
Compensated
50
2550
550
1050
1550
2050
2550
Frequency (MHz)
S22 vs. Freq - Compensated vs. Uncompensated
Group Delay vs. Frequency of Compensated Circuit
0
700
-5
600
Return Loss (dB)
Group Delay (psec)
1500
S11 vs. Freq - Compensated vs. Uncompensated
Uncompensated
1050
1550
Frequency (MHz)
1000
Frequency (MHz)
21
550
3
20
Figure 2
50
4
30
0
17
5
Noise Figure (dB)
OUT
IN
20
Preliminary
500
Group Delay of Compensated SGA2486
400
300
200
Delay of Microstrip length equivalent to Apps. Board length
100
-10
Uncompensated
-15
-20
-25
-30
Compensated
-35
0
-40
50
550
1050
1550
Frequency (MHz)
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2050
2550
50
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550
1050
1550
Frequency (MHz)
2050
2550
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EAN-101374 Rev B
DESIGN APPLICATION NOTE --- AN010
SGA-2486 Gain Flatness Compensation Circuit
Gain Dev. (Uncompensated)
1.5
Gain Deviation from
Average (dB)
Fine Tuning of Gain Flatness
Figure 3 shows how to add a pi-pad to the output of
the previously shown SGA2486 application circuit
to obtain finer gain flatness. The resistor values of
the pad are selected for a 50 Ohm match in either
direction with a shunt capacitor across the bridging
resistor. The d.c. attenuation value should be initially selected to be slightly larger than the gain difference between the frequency range endpoints. It
is then iterated as necessary. The value of the
shunt capacitor is then adjusted in a simulator to
obtain maximum flatness over the frequency range.
In the case of the 20-1000 MHz frequency range, a
flatness of less than 0.1 dB was obtained. Over the
20-2150 MHz band, the flatness was reduced only
to slightly less than 0.3 dB, mainly because the slope
of the gain/ frequency response lessened between
about 1000 and 1700 MHz, causing a response
peak.
Gain Deviation (from Average Value) from 20 - 2450 Mhz
2
1
0.5
0
Preliminary
Gain Dev. (Compensated)
-0.5
-1
-1.5
-2
0
500
1000
1500
Frequency (MHz)
2000
2500
Gain Deviation (from Average Value) from 20-1000 MHz
2
Gain Deviation from Average
(dB)
Gain Flatness Performance (Circuit in Figure 1.)
The three graphs in the right hand column show
gain deviation with frequency derived by taking the
difference between the gain at a particular frequency
and the average gain calculated over the indicated
frequency ranges.
1.5
1
Gain Dev. (Uncompensated)
0.5
0
Gain Dev. (Compensated)
-0.5
-1
-1.5
-2
0
Plot of Stability Parameters ( K & Delta) vs. Frequency
for Compensated SGA2486
2
1
200
400
600
Frequency (MHz)
800
1000
Gain Deviation (from Average Value) from 950-2150 Mhz
0.8
1.6
0.6
Delta
1.4
0.4
K
1.2
Delta Value
K value
1.8
0.2
1
0
0.0
0.8
1.6
2.4
3.2
Frequency (GHz)
4.0
Gain Deviation from Average
(dB)
2
1.5
1
Gain Dev. (Uncompensated)
0.5
0
Gain Dev. (Compensated)
-0.5
-1
-1.5
-2
4.8
950
1150
1350
1550
1750
1950
2150
Frequency (MHz)
(Note: For unconditional stability K>1, Delta<1)
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EAN-101374 Rev B
DESIGN APPLICATION NOTE --- AN010
SGA-2486 Gain Flatness Compensation Circuit
Schematic Diagram of SGA2486 Applications Circuit from Figure 1 combined with pi-pad R/C
network at output.Tables of component values for pi-pad at two frequency ranges are included
below, along with expected performance data for each configuration.
Ref.
Des.
Zc
Length
(Degrees @
1000 MHz)
Z1
50
12.3
Z2
50
3.3
Z3
50
12.9
Z4
50
13.5
Z5
50
3.3
Z6
50
12.0
Output pi-pad network enlarged
for clarity. See corresponding
tables of component values
below.
Preliminary
+5V
+
-
Cd6
0.1 uF
CA1
Rcomp2
Cd5
Cd4
.01uF
1000pF
120W
Lcomp2
15nH
SGA-2486
CC1
CC2
.01uF
RF input
.01uF
Z1
Z3
Z2
Z6
Z5
Z4
RF output
RA2
Lcomp1
RA1
15nH
RA3
Rcomp1
120W
+
Cd1
1000 pF
-
Cd3
0.1 uF
Cd2
.01 uF
Figure 3
Table of Component Values for f = 20-2150 MHz
(Use 0603 style resistors and capacitors)
(Use 0603 style resistors and capacitors)
6.8
Freq Range
(MHz)
Gain (dB)
39
-1.2
20-1000
15.7
Plot of Gain & Input / Output Return Loss vs.
Frequency (using pi-pad components above)
16
15.8
Gain (dB)
750
Low Freq.
Attenuation
(dB)
S 21(dB)
14.5
-10
14.1
S22(dB)
15.4
-30
S11(dB)
15.2
-40
15
-50
20
220
420
620
820
Frequency (MHz)
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1020
330
0
-20
15.6
RA1 (Ohms) RA2 (Ohms) RA3 (Ohms)
1220
Gain (dB)
750
CA1 (pF)
Return Loss (dB)
RA1 (Ohms) RA2 (Ohms) RA3 (Ohms)
15
330
CA1 (pF)
Low Freq.
Attenuation
(dB)
Freq Range
(MHz)
Gain (dB)
8.2
-2.6
20-2150
14.2
Plot of Gain & Input / Output Return Loss vs.
Frequency (using pi-pad components above)
S21(dB)
0
-10
13.7
S22(dB)
S11(dB)
13.3
12.9
-20
-30
-40
12.5
Return Loss (dB)
Table of Component Values for f =20-1000 MHz.
-50
20
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260
500
740 980 1220 1460 1700 1940 2180
Frequency (MHz)
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EAN-101374 Rev B
DESIGN APPLICATION NOTE --- AN010
SGA-2486 Gain Flatness Compensation Circuit
Conclusion
This application note has shown a simple solution using passive components to improve the gain
flatness of the SGA-2486 SiGe cascadable gain block. This device, with low cost, wideband performance, low power dissipation and high gain is an excellent choice for use in many applications.
For applications requiring flat gain over a specified frequency range the circuit shown in Figure 1
improves gain flatness significantly. The addition of the simple inductor/resistor matchingPreliminary
networks to
the normal device biasing topolology allows the gain flatness to be held to 1dB over a broad frequency
range (20 MHz to 2150 MHz). Adding an optional pi-pad to the output ( shown in Figure 3) improves
gain flatness to +/- 0.2 dB. Systems that require very flat gain from DC to 1000 MHz (such as CATV),
can achieve +/- 0.1 dB gain flatness using this circuit.
We hope that this application note and the products offered by 5)0' will assist you in
achieving your design goals.
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EAN-101374 Rev B