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LNAs (preamps) and MMICs

Since writing the original article on the L & S band LNA, many new devices have become available for our use...the proliferation of cell phones likely contributed to this, making quite a few of these devices affordable and plentiful for amateur use.

Whether it is an MMIC, E-PHEMET or GaasFET, they are all similar in form, usually in a small surface-mount package. Their uses vary, from a simple gain block used to amplify low-level signals to more useful levels (pre-drivers), to Low Noise Amplifiers (LNA) used in receiving systems for satellite TV, EME, or weak-signal terrestrial work.

The small PC board shown to the left (next to a quarter) was designed to be adaptable to many of the common device form factors available to us. The next few photos show some examples....
 

Here's my favorite package (SOT-89), shown with a Minicircuits PGA103+ device, a $2 part. I Like this package the best because it has a large grounding tab at the top, which helps to draw heat from the device (these operate at 3 to 5v @ up to 100ma).

The PGA103 is quite easy to use...50 ohms in and 50 ohms out, just give it the required supply voltage and a few parts, and it works; not all of the part locations on the board are in use, as this type of device only needs the 8 parts shown (including the PGA103). The following measurement results are at 3v and 60ma when used with a 5v supply, and performed courtesy of W6QIW (Steve), who owns equipment able to make accurate NF measurements, and who was kind enough to oblige:

  • 144 MHz - 0.45 db and 24.7 db gain
  • 222 MHz - 0.45 db and 23.7 db gain
  • 432 MHz - 0.6 db and 21.0 db gain
  • 902 MHz - 0.6 db and 16.0 db gain
  • 1300 MHz - 0.81 db and 13 db gain

Steve also reports the NF did not change at higher currents, though gain was a bit higher. All in all, this one seems to be a very economical solution for the VHF and UHF ranges. The intercept point is high, so one can get away without an input filter in most cases, though a simple bandpass filter would be a wise addition between the LNA and the receiver.

The device used on this next one is just a broadband gain block, set up for use in the 100-3500 MHz range. Though the device used here has more than 20 db gain through 1500 MHz, the noise figure is also reasonable, just 2 db or so. The most common application for a device like this one is to amplify the output from a transmit mixer up to a useable level for a driver stage, in the range of 50 to 100 mw.
 

This one is an LNA designed for use on the 23cm band, and uses a E-PHEMET device requiring the few extra parts for bias and stabilization. NF on this one is in the range of 0.5 db at 1300 MHz.


Here's an MMIC (SGA-6386) in a SOT-86 package shown positioned where it should be mounted. When I finished building this one, it had 21.3 db gain and a NF of 1.7db at 1296MHz, a few tenths better NF than the manufacturer claimed. The schematic and values for the parts for whatever MMIC you will be using should be taken from the manufacturers data sheet.
 

Here's how to position one of the E-PHEMET or other devices in the SOT-343 package; note the wide lead is at the top, and the device is rotated about 45 degrees to make the proper connections.

The schematic for the SAV-541+ is below this photo.



And the final example, one of those small 6-lead packages that are so much of a challenge to install. This one is an MGA-82563 in the SOT-363 package. It fits, but it's close.
 

Now it's time to be controversial, an opportunity for me to express my personal feelings about LNAs (or pre-amps as they are more commonly called) for terrestrial receiving...this isn't about EME or satellite antennas pointed at quiet sky, that's a different situation. So here it is:

Most of us do not need a pre-amp ahead of our receivers. Here are some of the reasons:

  1. Our radio or transverter is modern, and overall gain is enough
  2. Feedline losses are low
  3. External noise is limiting our ability to hear the weak signals

If #3 is the dominant factor, placing an LNA in front of your receiver will not improve the Signal-to-Noise Ratio (SNR); all it will do is make both the signal and the noise stronger and make your receiving system more vulnerable to overload (crush) by strong out-of-band signals. You can test for this situation by doing the following:

  • disconnect your antenna and replace it with a dummy load
  • Note the noise level
  • reconnect your antenna

If the noise level increases by 2 or more db when the antenna is reconnected, you are limited by external noise and an LNA will do you no good. Even some set-ups with larger feedline losses are limited in performance by external noise, which can be a real problem in urban locations. Another issue may be low overall receiver gain...most of the radios we buy have the I.F. gain turned down too low for weak-signal work; I'm only speculating, but the manufacturers probably do this to keep the receiver from being crushed by strong local signals, some of which are out of band. Typically, these radios have decent low-noise front ends, but lack sufficient overall gain; this can always be corrected with an adjustment inside the radio...just turning up the I.F. gain will bring the receiver back to life.

If you do determine a legitimate need for an LNA for terrestrial work...perhaps you prefer not to fuss with the adjustments in an in-warranty radio, your receiver is older and has a poor front end, or your feedline losses are abnormally high, etc; you'll be happy to hear these things are quite easy to make, though you should be aware they require protection from your transmitter (see the LNA sequencing and protection) article.