balanced amplification technique using Wilkinson and 90 degrees

LNA
Article 1 (Wilkinson)
An ideal balanced configuration provides low input and output VSWR in addition to a 3 dB
improvement of the power and linearity performance.
The Traveling-Wave Divider/Combiner(1) is a planar structure used to increase output power
by combining parallel amplifier stages. The simplest implementation is a 2-way Wilkinson type
divider/combiner with a 90transmission line in one arm. The Wilkinson divider provides an
equal-power, equal-phase power split and the transmission line shifts the phase of one path
by 90° so at the output of the divider we have two equal power signals in quadrature (phase
shifted by 90°) as shown in Figure 1. Two identical power amplifiers with gain A are placed in
each arm. Any reflected power at the input to the amplifier is terminated in the 100-isolation
resistor, since the reflected power of the two arms at the resistor is 180° out of phase. A
combiner with similar properties is placed at the output to combine both output signals from
the amplifiers, but the extra 90° line is placed in the opposite arm so that the output signals
add in phase through the combiner. Assuming no loss and perfect combining, the 2-way
combiner will increase output power and linearity by 3 dB. Given a real life application, the
typical increase is 2.5 dB. Even though this power combiner is comprised of multiple quarterwave
lines, the Wilkinson type divider/combiner with a 90° transmission line has a broad
bandwidth, significantly wider than a branch-line type combiner, and has low loss. Since the
combiner is planar, it is much easier to fabricate using standard PCB technology. The
example we show in Figure 1 has a 50-impedance at the amplifier input.
However, lower impedances can be achieved by increasing the transformation ratio in the
quarter wave lines. Unlike branch line or Lange type combiners, the Wilkinson type
divider/combiner easily scales to 4-way combining for even higher power. The performance of
the balanced stage is 2.5 dB to 2.8 dB better than performance of a single-ended unit under
the same conditions. This novel design approach for combining two power amplifiers offers
simple low-cost implementation, wide bandwidth (approximately 40% versus 15% for a
branch-line approach).

Article 2 (90deg)
The absence of S-parameter information in low temperatures precluded the design of a
feedback amplifier, so a balanced configuration was adopted. This has the advantage of
providing a good input match even though the amplifiers in the two arms of the balanced
circuit are poorly matched. However, there are disadvantages. The loss of the input hybrid
degrades the noise temperature and coupling errors in the hybrids, and differences between
the amplifiers reduce the gain and result in a noise contribution from the input load.
Article 3 (Narda 90° Hybrids)
A 90° Hybrid (Hybrid Junction) is a network having the
electrical characteristics of a 3 dB directional coupler
whose branch line is not terminated. The four terminal network
can be considered to have two pairs of terminals
called conjugate pairs. In most packages, each conjugate
pair is located on either side of the device (Figure 1). The
two terminals that make up the conjugate pair are isolated
from each other. Therefore, power flowing into one terminal
of the pair does not appear at its conjugate, but is
equally divided between the terminals of the opposite conjugate
pair. When used as a power divider, any one of the
four terminals can be used as the input. With the conjugate
port of the input terminated in 50 ohms, the two outputs
at the opposite conjugate pair will be of equal
amplitude and in quadrature (90° apart in phase).
The primary advantage of the hybrid junction is its power
handling capability. Since the isolated port (conjugate of
the input port) is terminated externally, the only limitations
to power handling are heat generated by the internal dissipation
losses and the power handling capability of the external
termination. As stripline losses are typically low,
hybrid junctions can be designed to handle up to 500
Watts average power in special versions. Another advantage
of the hybrid junction is that it maintains its quadrature
relationship over the full operating frequency range of
the device. This characteristic is highly desirable in Polar
Frequency Discrimination and Circularly Polarized antenna
circuits.
Article 3
The 90o Balanced Hybrid
Coupler consists of two 50 strip lines
arranged to be mutually coupled. The strip
lines are made in various ways including PC
board traces, copper or brass strips placed
between high dielectric insulation materials.
A signal applied to the Port 1 & 2 line will be
coupled to Ports 3 & 4. Since the lines are
electrically ¼ wavelength long the currents and
voltages are changed in phase between Ports
1 & 2 and Ports 3 & 4. Forward loss is 3 dB
plus incidental conducted losses.
Note that this places Ports 1 & 3 and Ports 2
& 4 at 180o in phase relationship. If a signal is
applied to port 1 and Port 4 is terminated with
a 50 load termination the applied power will

e equally split between Ports 2 & 3.
Conversely, power applied to Ports 2 & 3 will
be split between Port 1 and the termination at
Port 4.
With this device two transmitters can be
coupled to a common antenna. All
impedances will be matched and a high order
of isolation is provided between the transmitter
P.A. circuits, at least 30 dB for the hybrid
coupler alone.
The strip lines are transposed at one end to
facilitate the best chassis layout for a Hybrid –
Ferrite transmitter combiner. Note that the T1
and T2 input connectors are on the same side
of the hybrid housing. We have also included
a network at the antenna port that when
properly adjusted, will produce more than 65
dB of isolation between the T1 and T2 input
ports.
The Wilkinson Splitter employs two 70
sections of line, each ¼ wave length. Each
line will produce a phase change of 90o
resulting in a 180o phase relationship between
Ports 1 and 2. The 100 terminating resistor
provides an effective match at the isolated
ports and the conjunctive impedance of the
two 70 lines in parallel is 50 at Port 1.
Wilkinson Splitters are mostly used for dividing
signal power to feed a number of devices from
one source. One example (Figure 3) is an 8-
way signal power divider for receiver
multicoupling service. Of course, it works in
reverse, to combine low power transmitters,
provided that the 100 termination resistors
and related heat sinks are capable of
dissipating ½ of all power applied to the inputs.
Insertion loss is 3 dB plus conducted circuit
losses and isolation is usually 25 dB or better
between ports.
Article 4 (wikipedia)
90 deg. The phase difference between the two output ports of a hybrid coupler should be 0,
90, or 180 degrees depending on the type used. However, like amplitude balance, the phase
difference is sensitive to the input frequency and typically will vary a few degrees.

The phase properties of a 90-degree hybrid coupler can be used to great advantage in
microwave circuits. For example in a balanced microwave amplifier the two input stages are
fed through a hybrid coupler. The FET device normally has a very poor match and reflects
much of the incident energy. However, since the devices are essentially identical the
reflection coefficients from each device are equal. The reflected voltage from the FETs are in
phase at the isolated port and are 180 degrees different at the input port. Therefore, all of the
reflected power from the FETs goes to the load at the isolated port and no power goes to the
input port. This results in a good input match (low VSWR).
Wilkinson. The Wilkinson power divider has low VSWR at all ports and high isolation
between output ports. The input and output impedances at each port are designed to be
equal to the characteristic impedance of the microwave system.
Article 5
A balanced amplifier design is defined by two amplifiers of equal gain, 1dB compression point
(P1dB) and Third-Order Intercept (IP3), arranged in the configuration shown to the right. The
couplers are 3 dB hybrids, where the input power is split equally between a 0°and a 90° port.
The unused ports are terminated in the system impedance – typically 50 Ω. Reflections from
the input and output ports of the amplifiers are shunted to the unused port of each coupler,
giving the entire arrangement a matched impedance.

Article 6
The Wilkinson power divider
At higher frequencies (above 500 Mhz) these devices are usually
realized as a microstrip or stripline Wilkinson design. All
Browadwave reactive power dividers are Wilkinson types. Figure
5 shows a simple 2-Way Wilkinson power divider. Being a
lossless reciprocal three port network, it inherits all its properties
which state that this type of network cannot have all the
ports simultaneously matched. To solve this an isolating resistor
is placed between the two output ports, since no current
flows through the resistor (there is no potential difference between
the output ports), this resistor does not contribute to any
resistive loss. This makes an ideal Wilkinson a 100% efficient
device. This resistor also provides excellent isolation even
when the device is used as a combiner. Another property of
the Wilkinson divider is that it is broken down into quater wavelength
(l/4) sections. This device is useful for limited bandwidth
applications, but to achieve a wider bandwidth a multi
section Wilkinson design is used as shown in Figure 6. As a
general rule, the greater the bandwidth the more sections
added to the design. But by doing so, makes devices become
larger and more importantly lossy. These devices can be designed
for octave bandwidths and are sometimes cascaded to
form higher order devices.

Article 7
Balanced amplifier

Article 8
For the distributed 90-deg. hybrid, there are basically four quarter-wavelength distributed
transmission lines connected in a "square" arrangement (Fig. 2). Two opposite transmission
lines have an impedance of 50 Ω(assuming a characteristic impedance of 50 Ω) and the other
two lines have an impedance of 35.35 Ω[(50 Ω) 0.5 ]. It is very important to get the orientation of
the coupler input and isolated port correct (see ref. 2).
As noted previously, there are two simple lumped-element equivalent circuits in a pi or tee
arrangement. Either arrangement will work, although the choice may depend on other factors:
for example, MMIC inductors tend to have more loss than MMIC capacitors. By choosing the
pi arrangement to reduce the number of inductors, the lumped-element circuit of Fig. 3
results. Note the combining of capacitors at the "corners" of the 35- and 50-Ω branches.
The branchline or 90-deg. hybrid can be used for many functions. Some of these include
image-reject mixers, attenuators, phase shifters, modulators, and power combiners/dividers.
This is true of both the lumped element and distributed implementations of the branchline
hybrid. However, the distributed equivalent will repeat at three times the fundamental
frequency. Also, the bandwidths of the two implementations are different near the
fundamental frequency, F 0 . The input match of the hybrid is good provided the terminations at
the direct and coupled ports are nearly identical. Mismatches at the direct and coupled ports
reflect to the isolated port. By adding a switch or variable resistor to the direct and coupled
ports of the branchline hybrid, one can create a switched or variable attenuator using the
input and isolated ports. Likewise, a switch or variable capacitor can be used to build an
analog or digital phase shifter or phase modulator. In an image-reject mixer, one branchline
hybrid at RF helps distinguish between the upper sideband and the lower sideband of the
signal. An additional 90-deg. hybrid at the intermediate frequency (IF) combines or cancels
the two mixed signals to select just the upper or lower sideband.
The hybrid can also be used as a combiner or power splitter with the properties that the input
match is good provided that the loads at the coupled and direct ports are matched. As a
combiner, reflections from the coupled and direct port are absorbed by a resistor at the
isolated port. The difficulty with the hybrid as a combiner/divider is controlling the impedances
and lengths of the 35- and 50-Ω transmission lines to obtain an equal 3-dB split at the two
ports.
The Wilkinson coupler is often used as a power combiner or splitter. It divides an input signal
equally between two outputs, or can be used to create unequal split or an n-port divider. In a
Wilkinson, two quarter-wavelength 70.7-Ω lines—assuming a 50-Ω characteristic
impedance—split the input to two output ports (Fig. 4). A 100-Ω resistor is tied between the
two output ports to provide isolation in the odd-mode case. Placing this resistor can be much
easier in a MMIC lumped-element layout than in a distributed layout (Fig. 5). For this
example, a pi arrangement was chosen to reduce the number of lossy inductors. The input
shunt capacitors combine into a single capacitor yielding two inductors, one shunt capacitor at
each port, and the 100-Ω isolation resistor plus interconnect for the MMIC Wilkinson.
As a splitter, the input is divided into two equal in-phase outputs, ideally at −3-dB levels from
the input signal level. When fed at the outputs by two signals in phase and of comparable
signal level, the Wilkinson acts as a power combiner. The major differences between using a
Wilkinson as a divider/combiner versus the branchline hybrid is that the input match now
depends on the match at the other two ports. However, it is much easier to get an equalphase,
equal-power split, as well as wider bandwidth, with the Wilkinson than with a hybrid
combiner.
A 90-deg. lumped-element MMIC hybrid coupler is a useful for a variety of designs, such as a
phase modulator MMIC developed by a student in the Johns Hopkins University MMIC
Design Course. 3 Students in that course learn to develop practical MMIC layouts that are then

fabricated at the TriQuint Semiconductor foundry. Those students developed several lumpedelement
hybrid layouts, planned around a central substrate via shared by four capacitors and
four spiral inductors to make up the four transmission lines of the lumped element equivalent
hybrid (Fig. 6). Using the pi arrangement for the lumped-element branches and combining the
capacitors at the ends, the layout has a single capacitance value and two inductance values
that can be tuned for performance. Arranging the layout allows performance trade-offs by
tuning the single capacitance and the size of the two inductors. Careful use of symmetry
makes it easier to tune the circuit without "breaking" the layout. A 2.1-GHz hybrid coupler
fabricated on a 34 × 54 mil die is an example of the several hybrid couplers fabricated with
the TriQuint process (Fig. 6). Hybrid couplers for other frequency ranges can use the same
topology by changing the capacitor and two inductor values (plus interconnect).
The performance of the hybrid coupler was simulated (Fig. 7) with the Advanced Design
System (ADS) software from Agilent Technologies and the TriQuint TQTRX device library, as
well as with EM simulation software from Sonnet Software (Liverpool, NY). Only the "core" of
the hybrids were simulated and assumptions were made that the effects of the ground-signalground
probe pads and off-chip wire bonds were minimal at these frequencies. Given
additional time, the matches can be tuned to offset the off-chip wire bond inductance and
provide a better 50-Ω termination.
A 7.5-GHz Wilkinson divider/combiner was also fabricated with the MMIC process. It consists
of two 71-Ω transmission lines and a 100-Ω resistor to provide isolation for the coupled ports.
Symmetry was used to ensure proper equal phase and equal amplitude split. The hybrid pi
lumped-element equivalent was chosen for the MMIC implementation since it has the least
number of lossy inductors. Capacitors on the input side can be combined resulting in one
value for the two inductors and two values for the three capacitors—the first capacitor is twice
the value of the other two. Optimizing the simulation is done by tuning the one inductor value
and "one" or "two" capacitor values. A single shared substrate via was used for the shunt
capacitor to ground connections. The Wilkinson was also computer simulated, although the
ADS simulations did not include the isolation resistor in the layout because its effect was
considered to be minimal. The layout for the 7.5-GHz X-band Wilkinson looks similar to half of
the hybrid coupler layout (Fig. 8). The 100-Ω isolation resistor was added to the layout along
with the ground-signal-ground probe pads in the final layout using the ICED layout software.
The 7.5-GHz Wilkinson measures 34 × 29 mils, and measured performance compared
closely with ADS and EM simulations (Fig. 9). Various branchline hybrids from 2.1 to 8.4 GHz
were all fabricated on a 34 × 54 mil MMIC tile with room to spare. The higher-frequency
hybrids had some additional room for test circuits. Of course, the great advantage of MMICs
over MICs is size, and a quarter wavelength on an alumina substrate (dielectric constant of
9.8), for example, is almost 600 mils. If one needs to incorporate additional circuits such as
switches, varactors, diodes, FETs, etc., the size, weight, and power savings of a MMIC over
an MIC circuit can be substantially higher, although for small volumes, MICs still offer cost
advantages compared to the high price of a MMIC wafer run.