Updated July 1, 2012
to go to our MMIC page
to go to our page explaining K-factor and GMAX
here to learn how to use our S-parameter Utility spreadsheet
Quick links to the six examples
on this page:
Avago AMMP 6220
Eudyna FMM5704 (new for June 2012!)
One way to look at the suppliers
mentioned on this page is to think of them as "highly unlikely
sponsors of Microwaves101". Good thing for the rest of the
world that we aren't all about money here!
New for November 2011!
It turns out that if you read the fine print, there are amplifiers
out there that have been designed for conditional stability on purpose.
As "The Unknown Engineer" pointed out, there are special
cases where K might be less than unity but there is a good reason
for it and the manufacturer warns you:
I refer to the following
sentence "To verify that every design that they sell is unconditionally
stable" and the MAR-8SM discussion...
Microwave amplifying devices
that are unconditionally stable (k>=1) from DC to light usually
have internal stabilization. But stabilizing, especially over
wideband, with a series or a parallel resistor as described in
the oft-quoted Besser paper "Avoiding RF oscillations",
(AM&W, Spring 95) will either degrade the noise figure or
P1dB depending on whether connected to the input or the output.
So, an "unstabilized" transistor permits the designer
to choose which parameter to tradeoff. It also has the flexibility
to tailor the degree of stabilization to the application conditions
- e.g. a potentially unstable device may become stable after assembly
on a paper phenolic FR2 PCB because the substrate loss rolls off
the high frequency gain.
stability should not be made a prerequisite for marketing a device
because only a subset of amplifying devices will be interfaced
with a bandpass filter or an aerial with poor out-of-band match.
Specifically, the discussed "Mini-Circuits MAR-08SM"
was originally envisaged as a wideband gain block; i.e. to be
inserted between other stages, e.g. CATV-SATV IF amplifier. According
to the 1993 Hewlett Packard product brief AN-S002 "MODAMP™
MMIC Nomenclature" (page 2, under "Special Purpose"),
the MAR-08/MSA-08 "trades unconditional stability for gain,
high frequency, and low noise". Unconditional stability is
also not required for another of MAR-08/MSA-08's demonstrated
application as a C-band TVRO self-oscillating mixer (see 1997
Hewlett Packard appnote AN-S005 "Using Hewlett-Packard
MSA Series MMIC Amplifiers as Frequency Converters").
It is always good practice to
look at an amplifier's available gain and stability (K-factor)
before you use it. This is a check for conditional stability;
just because K is less than one doesn't necessarily mean you will
be chasing spurious oscillations in your design, but why tempt fate
when there are ton of unconditionally stable designs out
there. Conditional stability usually doesn't present a problem when
the manufacturer takes RF probe data for the data sheet, RF probes
present almost ideal load impedances to the DUT over a very wide
band. Trouble may arise when you put the device into an actual circuit
where you might just want to follow it with a bandpass filter for
example. Filters have high reflection coefficients outside their
passband, and soon the "fun" starts.
Here's a bone worth picking.
In spite of all the certifications that companies go though to verify
their procedures, like ISO-this, CMMI-that and "Six
Sigma", you'd think that an amplifier vender would have
a procedure in place to verify that every design that they sell
is unconditionally stable, at least in the samples that they measured
to provide data to customers. The examples shown on this page are
evidence that such a procedure is lacking, even at some well-known
and respected suppliers. If we were on their board of directors,
we'd make it happen and eliminate this constant source of completely
preventable grief. Please don't call us to complain about us displaying
your products' faults, call us to thank us and then take them off
the market or redesign them, this is a club you don't want to be
in. Good grief, Charlie Brown!
of the plots on this page were created using the Microwaves101 S-parameter
Utility Excel spreadsheet that you can download
Always do the
right thing... never use a potentially unstable amplifier!
Example 1: RF
Micro Devices FMA219
The FMA219 has been kicking around
for more than a decade. It's a pHEMT two-stage low noise amplifier
that provides remarkably flat gain response at X-band. Originally
a branded a Litton Solid State LMA219B back in the previous century,
it was passed across the Atlantic to Filtronic and became the FMA219.
Not long ago RF Micro bought the Filtronic fab and product line
and they still market this device but maybe they have never fully
examined the "performance" of it. Over the years the S-parameters
that have been posted by the manufacturers on this device may have
changed a few times, so we recently went to the RF Micro web site
and downloaded the data sheet. They don't make it easy to evaluate
the part, there are no downloadable S-parameter files for it on
RFMD.com, only a pdf document where the data has been pasted in
as an image and can't be processed into numbers. We typed the data
into a spreadsheet and checked it all (our optical character recognition
software seems to be missing in action); what a great way to spend
a couple of hours. If anyone wants the data in Excel format shoot
us an email and we'll save you the aggravation.
If you look closely, the image
that is on the most recent datasheet says "LMA219B" which
was Litton's original designation for the part.
In any case, below are the S-parameters
plotted in dB using our S-parameter
Utility Spreadsheet (very cool free
download). The parameter that should set off a mental red flag
is S11, which exceeds unity below 6 GHz (>zero value when plotted
in dB). Here comes trouble!
Below we plotted K-factor and
maximum available/maximum stable gain for the FMA219B, using our
all-powerful spreadsheet. K-factor dips well below 1 twice between
4 and 6 GHz. Look at all the available gain that is possible at
these frequencies... it's a good bet that this puppy is going to
oscillate if it doesn't see exactly fifty ohms between 4 and 6 GHz.
Apparently the FMA219 remains
stable (doesn't oscillate) when it sees fifty ohms at all frequencies
(like when the manufacturer RF probes
it.) For applications where the device is configured in front of,
or after a bandpass filter (like it might in a receiver
application), don't even think about using it. We speak from experience.
Example 2: Mini-Circuits
Please see relevant comments
at the top of this page, Mini-Circuits warns uses that this device
is conditionally stable.
This circuit is a Darlington
pair that uses "silicon technology" which might mean SiGe
but you can't tell from the data
sheet. Pieter tipped us off to the potential stability issue
of this device but we have no firsthand experience with it. Below
are the S-parameters, thankfully Mini-Circuits provides a text file
that you can easily convert to Excel format and enter into our Utilities
spreadsheet. The measured data shows a mess of bumps, could it be
that the part is oscillating when MC collected the data for the
Here's the best illustration
of the problem: the K-factor dips below 1 across a wide frequency
range. Good luck using it!
Example 3: Hittite
We received this tip from an
engineer in a country in Europe that is famous for beverages created
from malted barley. Danke!
This circuit is a "high
dynamic range GaAs InGaP HBT 1.6 Watt MMIC power amplifier operating
from 0.4 to 2.2 GHz and packaged in industry standard SOT89 packages"
according to Hittite. It is also conditionally stable, and we have
heard that it often blows up just as it is turned on. You probably
won't get that information from the data sheet. Note that stability
is also a function of bias point. Your design might be stable at
the manufacturer's recommended bias point, but the act of turning
it on sweeps it through a range of current and voltage, and at some
bias point the stability may be a lot worse than at the operating
Here's the S-parameters. We want
to thank Hittite for posting an "S2P" data file of the
S-parameters for this circuit and many of their other MMICs, which
is easily converted to Excel. It's a stretch to call this an "amplifier",
it's really an unmatched transistor. You need to follow the directions
on Hittite's data sheet to build matching networks and bias circuitry
from SMT components.
Here's the available gain and
K-factor. It's conditionally stable around 100 MHz, and just barely
stable over it's entire band. It also has a ton of available gain
right where it is conditionally stable, not a great combination!
Our advice is, if you have to use it, put some resistance in the
matching or bias circuitry. Better still, pick another part!
Here's a comment from the engineer
that was wrestling with this amplifier:
Finally I solved the stability
problem. The destructive oscillations in the VHF range are fixed
now. It was necessary to add some loss to the DC bias coil. I
used a 100 Ohm parallel resistor. It´s a compromise between
damping and system gain reduction. The system simulated stability
factor was now K>1 for all frequencies. The measurements showed
Example 4: Avago
If you go to Avago's web site,
it describes the AMMP 6220 thusly:
The broad and unconditionally
stable performance makes this LNA ideal for primary, sub-sequential
or driver low noise gain stages. Intended applications include
microwave radios, 802.16, automotive radar, VSAT, and satellite
Good one, guys. We took a look
at manufacturer's supplied S-parameters for the AMMP 6220 (below).
There seems to be a lot of noise on the measurement. You'd think
that Avago's cousins at Agilent would have taught them better measurement
techniques... and where is the data below 6 GHz? Is there a reason
they chose to hide it?
From the S-parameters,
it is easy to show that the AMMP 6220 is conditionally stable (K<1),
up around 24-26 GHz:
We found out about this part
from an anonymous user, who recently sent us this complaint:
I have built about 50 receivers
using this part and had six to date ring like a bell at 26 GHz,
with a "fairly" well matched output (definitely better
than 2.5:1 at any rate). Obviously the amp runs super hot in this
mode -- two burned up completely. It is very difficult to kill
the ringing. I'd prefer to kill the guy who specified these -
but hey, that's illegal.
Cure is to eat the slightly
higher power consumption of the AMMP-6222, which is unconditionally
stable but has slightly lower operating frequency range. I wonder
if the '6220 is merely an under-biased 6222.
Good luck with that... and thanks
Example 5: Maxim
This tip came to us from Another
Unnamed Designer, who tells us:
We're using this part right
around 130-180 MHz and have spent a lot of time getting it to
not oscillate with ill-defined input terminations! It's clear
now what the problem is... (he sent us a plot of the K-factor)
Here's what Maxim says about
the MAX2371 on their web site:
The MAX2371/MAX2373 wideband
low-noise amplifier (LNA) ICs are designed for direct conversion
receiver (DCR) or very low intermediate frequency (VLIF) receiver
applications. They contain single-channel, single-ended LNAs with
switchable attenuator and automatic gain control (AGC) intended
as a low-noise gain stage. These devices provide high gain-control
range (typically 60dB) at radio frequency (RF) with excellent
noise and reverse isolation characteristics.
What they don't
tell you is that the part is conditionally stable.
It turns out that this is really
not an amplifier in the true sense of the word, it requires an extensive
pile of off-chip parts to match it at a a particular frequency which
you will discover if you look for an app note. One of the parts
is a resistor, which we guess is need to stabilize the "amp".
Here's the S-parameters of the
device by itself. Neither the input nor the output is matched to
fifty ohms. Not that the data sheet or web site even mentions what
the system impedance is supposed to be (could it be intended for
75 ohms? Trust us, it would stink at any system impedance.
Below 100 MHz and above 1750
MHz the K-factor dives below unity, and the circuit has a mess of
low available frequency gain that likely spells trouble.
What a mess.
Example 6: Hittite
This is a wideband amplifier.
Unfortunately it is very sensitive to what it sees on the output,
below the operating band. Contact us if you want to learn how to
solve this problem, otherwise pick a stable amplifier!
Below the manufacturer's S-parameters
(magnitude) are plotted. From this view there doesn't seem to be
Let's zoom in on the reflection
coefficient down at the low end where both S11 and S22 look like
unity... indeed, they both go slightly above one, a telltale sign
of an unstable design. Could that just be measurement error? Not
likely, S-parameters measurements at low frequency are typically
So, what does K-factor look like?
Not good... actually, horrible.
gain spikes up below the band. If you use this amplifier in a real
system, you will be lucky if it doesn't oscillate. But in a pinch, it can be stabilized with a shunt resistor. Guess which RF port needs to loaded? It's supposed to be a power amp, so loading the output port will cut down the power. Too bad. Murphy's Law applies in this case. Try about 600 ohms.
Example 7: Eudyna FMM5704
Seven examples of trouble, without even trying.... hopefully these lessons are not being wasted and anyone using MMIC amplifiers will analyze stability before placing a purchase order...
In this example, we have added a shunt resistor to the input of the device to stablize it. The FMM5704 is a Ka-band LNA, obviously adding resistance to the input will increase noise figure, but you have to do what you have to do. Adding a resistor to the output did nothing to improve stability.
Before we get any comments about "real world resistors", we added some inductance to the resistor model, and it still made the indicated improvement.
Eudyna is now part of Sumitomo Electric Industries (SEI), and they still sell this part. Below is the schematic of the "stable amplifier" which is the two-port measured S-parameters provided by the manufacturer, stabilize with an 800 ohm resistor.
Below is the maximum available gain and stability factor (K) of the amplifier, with and without the resistor. Note that the resistor brings K>1 everywhere (or at least across the S-parameters that were available.) That nasty-looking peak in available gain disappears too.
In the close-up below you can see how the gain (S21) of the amplifier is reduced by about a quarter of a dB. You can expect an equivalent increase in noise figure, because we did the dirty work on the input.
The reflection coefficients below show another indication of instability and its cure. The FMM5704 spins outside the Smith Chart. Adding the resistor corrects that. S22 was fine without the resistor, and is unaffected by it.
Do you know of a commercially
available part that is conditionally stable? Send
us the info and win a cool pocketknife!