Introduction: Using a Monolithic Microwave Integrated Circuit (MMIC) As a 50 MHz Amplifer
MMIC IC's have been around for about 25 years and are versatile and affordable. Their small size 2mm, with leads that are "surface mount" as opposed to "through hole" deters a lot of electronics and radio amateurs from using them. If you have patience and practice using a small soldering iron while using tweezers and a magnifying glass, you will get satisfactory results. The IC that I chose was a SNA-586 originally manufactured by Sirenza and now replaced by the Stanford Microdevices, NGA-586. The SNA-586 is capable of amplifying with useful gain up to 5 GHz so can be used for many microwave designs. With just five capacitors, a bias resistor and choke, you can have a very good amplifier with easily 20 dB of gain. All the amplifier needs is to be biased, so that it will have ideally 65 milliamperes of current at 5 volts. The application sheets tell you how to pick the bias resistor so that it will drop the right amount of voltage at 65 milliamperes. I chose a 47 ohm resistor to give me a 3 volt drop from 8 volts to 5 volts. The specifications sheet gives the values of capacitors and inductor to used at 500 MHz. I wanted to see if the SNA-586 would work at 50 MHz so I experimented with values that would be appropriate for 50 MHz and I was surprised to see that it worked quite well at 50 MHz!
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Step 1: Schematic Diagram With Values Appropriate for 50 MHz.
Note that the black grounds should all be tied together, ideally on a ground plane but I was just experimenting and wanted to see if the SNA-586 would work put on a perfboard with grounds soldered together with wire. Even at 50 MHz, all leads should be kept short to minimize stray inductances! I found that instead of 3 decoupling capacitors, 1 uF, 1000 pF and 100 pF, I was able to get away with dropping the 1000 pF capacitor.
Step 2: Using a Magnifying Glass, I Was Able to Solder the Components in Place.
This board was approximately 1 inch by 1 inch, around the size of a standard postage stamp. The IC is approximately 2 mm across with leads that make it around5 mm from tip of lead to tip of lead. I used ceramic capacitors for the input and output with a 1 uF electrolytic in parallel with the 100 pF ceramic for decoupling the power supply. The SNA-586 is designed to be used individually as a single 20 dB amplifier or they can be cascaded so that many can be added together to give much higher gain. The amplifier is designed to see 50 ohm input and outputs.
NOTE: THESE GALLIUM ARSENIDE CHIPS ARE VERY SENSITIVE TO OVERHEATING, SO ONLY USE THE AMOUNT OF HEAT NEEDED TO SOLDER THEM IN. A HEATSINK SHOULD BE USED TO TRANSFER THE HEAT AWAY FROM THE BODY OF THE COMPONENT. PROPER ANTI STATIC PROCEDURES SHOULD BE TAKEN ALSO.
Step 3: 1-200 MHz Frequency Response.
As you can see in this frequency response graph. The area from 40 to 60 MHz gives approximately 20 dB of gain with an area of instability below 25 MHz. The amplification drops off abruptly until the level at 200 MHz where it is approximately 35 dB down from the 60 MHz level. This circuit could be modified with feelback circuits to create a more effective filter that passes one range of frequencies and gives more effective performance. The results however proved to be better than expected and shows how easy these MMIC IC's are to design with and very effective amplifiers can be built using techniques well within the abilities of most electronics enthusiasts.
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