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Balanced Amplifiers
The single amplifier meets the specification for noise figure and again but fails to meet the
return loss specification due to the large mis-matches on the input & outputs. To overcome
this problem one solution is to use a balanced amplifier topography, which is shown in Figure
1.
NOTE Dashed lines are reflected signals
Figure 1 Schematic diagram of a balanced amplifier using two quadrature hybrids (eg
Lange Couplers).
The balanced amplifier employs two quadrature hybrids in this case two lange couplers
(although branchline couplers can be used). Any reflections of an incident signal on the input
due to the poor match of the amplifiers will be channelled back through the input lange to the
50 ohm load where they will be absorbed,and similarily on the the output.Therefore if we look
into the amplifier we will effectively ‘see’ the 50 ohm loads and will therefore present a good
match match.In addition this configuration will give us an extra 3dB’s of output power (less the
insertion loss of the Lange coupler ~typically < 0.25dB) and also the 1dB compression will be
approximately 3dB’s higher (In other words the circuit will be able to handle double the power
without distortion).The main drawback of this circuit is the power required for two amplifiers
instead of one.
Amplifier linearity
All amplifiers are designed to work over a given dynamic range where the amplifier should
behave linearily, and generally with LNA’s this is the case the input signals being received are
very small.However there may be large signals out of band that may de-sensitize the LNA (ie
reduce it’s gain and therefore effectively increase the Noise figure of the overall system) or
may generate intermodulation products that become mixed with other products further down
the receiver in the mixer forming spurious responses.
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The graph in Figure xx shows a typical 1dB compression characteristic for an amplifier. The
Dynamic range is characterised by linear gain ie where the output power rises linearly with an
input power. But as the 1-dB compression point is neared the output power begins to level out
at the saturated output power level. At this point further increases in input power fail to raise
the output power and in effect the gain has fallen to 0dB.
Of a greater consequence to LNA design is the level of intermodulation products that are
produced when two equal carriers are applied to an amplifier. The graph shows that the 3rd
order intermodulation products rise at a rate of 3 to 1 with the input power.
1dB
1dB
compression
point
Saturated output
power
Input power
Output
power
Intercept Point
Typ ~10dB
IM3
Figure 2 Typical amplifier linearity plot showing how the output power eventually rolls
off (saturated output power).
Therefore, if we use a balanced amplifier the input power is equally spilt ie is 3dB lower
therefore any IM3 products will be 9dB’s lower and the 1dB compression point for the whole
amplifier will be effectively 3dB’s higher.
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Lange Coupler Design
Lange couplers consist of very narrow coupled lines of a quarter wavelength coupled in
parallel to allow fringing on both sides of the line to contribute to the coupling. To increase the
coupling it is necessary to use very narrow gaps and to still further increase the coupling bond
wire interconnections are used. The resultant coupler will have a large bandwidth of at least
an octave and so for our application we will have plenty of bandwidth with a design centred on
5GHz (Lange bandwidth ~ 4 to 8GHz).
The initial analysis involves calculating the odd & even line impedances and then using a
graph to read off the finger spacing and line widths:-
( )( )
( )( )
[ ] ( )[ ]
( )
[ ] ( )[ ]
( )
( ) ( ) Ω==∴Ω==
Ω=+
+−+−=
+
+−+−=∴Ω=
=
⎟⎠
⎞⎜⎝
⎛ −−++−+=∴
==
=
===
⎟⎠
⎞⎜⎝
⎛ −−++−+==
52.5 .. 176.4
.
96.272
0.297861
10.29786140.2978614.05
.
1
111Z
. 50Z
0.29786 R
1410.7079/111
1)-1)(4(0.7079
0.7079 R
0.7079 10 C
10.2)r( Aluminaon lines coupled 4 using 5GHz @coupler 3dB afor Design
10 t coefficien Coupling C & lines coupled ofNumber N Where
11/111
1)-1)(N(c
C/
2
2
on
on
22
3/20-
dB/20
22
RZZZ
R
ZZ
Z
ZZ
R
RNRN
ZZ
NCRZZ
oeoooo
oeoo
oe
oeoo
oeoo
oeoo
ε
Using the calculated values of Zoo & Zoe we can use the graph of Figure xx to read off
Values of S/d and W/d given that d will be 0.635mm.
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Figure 3 Plot of Zoo againt Zoe
The resulting values of S/d & W/d were found to be:-
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5.7mm
9.64
0.06 lines coupled Lange of Length
9.6 where
4
0.06 )microstrip (in /4 06.0
95
83
f
c air
-: be willlines coupled the of length The
0.044mm. 0.07x0.635 Wand
0.0635mm 0.635x0.1 S therefore 0.07 W/d& 0.1 S/d
eff
eff
==∴
≅====
==
====
εελλ mE
E
50ohm lines W=0.635
S = 0.064mm
(0.054) mm
L = 5.7mm
( 5.9)mm
W=0.044mm
Optimised values in
brackets
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The data was entered into the Lange model on the CAD and analysed. It was found
necessary to slightly increase the length to lower the frequency response and to narrow the
spacing to increase the coupling such that there was slight over-coupling between the two
output ports. The frequency response of the two output ports is shown in Figure 4 and the
input return loss of the Lange is shown in Figure 5
Figure 4 Frequency response of Lange
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Figure 5 Input return loss of Lange
The amplifiers and Lange couplers were finally analysed to produce a full set of plots
characterising the performance.
The following page shows the final layout of the balanced amplifier. The amplifiers have been
arranged to ensure that the bias can be applied from the walls of the enclosure. The FET’s
are grounded using VIA holes but could be mounted directly to a metal carrier using two
separate substrates for the input & output circuits. To avoid thermal failure the carrier would
have to be made of Kovar the match the construction of the FET. The shorted stubs are in
fact RF shorted using very small (small electrical length) capacitors grounded using a via hole
pad. In reality a longer open circuit stub could be used but would be another 5.7mm longer (ie
one quarter wavelength), either of these two possibilities need to be done in order not to short
the applied DC bias to ground.
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Figure 6 Final Layout of the Balanced Amplifier
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3 PredictedResults
(I) Gain & Noise Figure response (passband)
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(ii) Input & Output return loss response (passband)
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(iii) Wideband Gain reponse
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(iv) Wideband return loss response
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4 Summary of results
The table below shows a summary table comparing the required specification with the
predicted analysed results from the CAD.In addition the power consumption and 1dB
compression point are given.
Parameter Specification Predicted Result Notes
Gain 10 ± 1dB 10.1 ± 0.3dB
Input return loss VSWR < 3 ie
> xx dB
> 18dB
Output return loss VSWR < 3 ie
> xx dB
> 21dB
Noise Figure < 1.8dB <1.5dB
+ve Power
consumption
- 2 * (10V * 11mA)=
220mW
-ve Power
Consumption
- 2 * ( 10V *1mA)
= 20mW
∴Total power
consumption =
240mW
1dB Compression
Point
- ~ 17.5dB
5 Circuit performance
The circuit is designed to be made on Alumina with a dielectric of 10 and will be etched to
produce the amplifier matching circuits and Lange couplers. There will be a tolerance in the
production due to errors in scaling the circuit using photo reduction and undercutting of
transmission lines during etching.
At microwave frequencies placement of components can be critical and a component that is
placed say 0.5 mm out of position will allow addition of another length of transmission line
slightly de-tuning the circuit. Opening out the gaps on the Lange will reduce the coupling
resulting in a under-coupling situation and hence cause an in balance in the amplifier
degrading it’s performance.
What is considered the most difficult parameters to predict in an amplifier is the spread of the
S-parameters from device to device and from batch to batch. A rule of ‘thumb’ is that the S-
parameters can vary by 5% from batch to batch. It is possible to use the CAD to calculate the
optimisation yield of the amplifier by varying all the S-parameters and re-analysing the gain &
noise responses. For flight applications active devices are supply as a batch with what are Lot
acceptance Test samples and individual test results.
These LAT samples are tested to ensure that the batch supplied meets with a procurement
specification, which basically agrees with the manufacturers data sheet. For critical
applications it is normal to characterise the LAT sample to produce a batch unique set of S-
Parameters that can be used to analyse the circuit. This way the circuit can be tweaked on
the CAD to ensure the amplifier will meet it’s specification with the given batch of devices.
With any manufactured item there will be variations in the dimensions of various items in our
case the lengths of the Langes or stubs, which will effect the frequency response of the
amplifier as will any variations in the dielectric constant of the Alumina.
With all these variations the circuit may well have out of specification results when the circuit
is built. No amount of yield analysis will ensure a compliant design and as a result some way
of altering the circuit is needed once built. This is done by adding ‘tuning pads’ to the layout,
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using allow pieces of gold tape or foil to be bonded in a position, so that the amplifier can be
tuned to be compliant at ambient.
After a few iterations of tuning it is normal to find a tuning pattern that can be used on all
amplifiers to give satisfactory performance assuming that all the active devices are from the
same batch.
In addition, to the production variations there are variations in the component tolerances in
the bias circuit that will in turn cause variations in the drain current and drain voltage.
Balanced Amplifiers
Lange Coupler Design
The data was entered into the Lange model on the CAD and ana