The MMIC (Monolithic Microwave Integrated Circuit) is an excellent vehicle to represent the overall homebrew process from the Idea to the Design to the Finished result.
So, this posting is about a process and not so much about MMIC's specifically.
Several Blog postings ago I showcased a Bilateral MMIC Amplifier stage used in a SSB Transceiver and likely there are many blog readers who would like to "know" more before building one.
It all started over 15 years ago when I spotted a piece about MMIC Amplifiers. It might have occurred with something unrelated like hunting for packaged Double Balanced Mixers and a trip to the Mini Circuits website. It was an experience much like a kid in the candy store!
While searching for SBL-1's I spotted the MAR line of MMIC devices. That info was tucked away in the grey matter. My brain is warped and several 3 AM Brain Awakening Realizations (BAR) said Pete you need to look more at MMIC's.
Also, the craze at that time were bilateral circuits. (Although bilateral circuits go back to the mid 50's with the Cosmophone 35 and the early 60's with the SBE-33.)
About 15 minutes' worth of "noodling time" convinced me to pursue a Bilateral MMIC Gain Block. What convinced me? Well, for starters, minimal part count (engineering economics) coupled with high gain wide bandwidth, low cost (economics) and physically small size. Did I mention termination insensitivity at 50 Ohms in/out. It is like the pairing of a fine wine with some good old Italian Pasta!
More research turned up the company Tri-Quint which was in lock step with Watkins Johnson who made Commercial Military Receivers and was a former division of Tektronix. In researching their Catalog I found the AG303-86G MMIC device.
I was not smart enough to scratch design a total Bilateral MMIC amplifier but smart enough to contact their Engineering Department. It turns out their Director of Engineering was a ham and so much needed help was forthcoming. A Key Point is to not be afraid to ask questions.
A reply from the Director resulted in this circuit.
A better photo here
A couple of noteworthy points about this bilateral circuit with the 1st being Diode Steering and DC Power Switching to cause the Bilateral action. Each leg of the Amp is powered through a 5 Volt Regulator and is only turned on either in Transmit or Receive but not both simultaneously.
Anticipating the issue that will be raised by the nerdiest of the Blog readers and that is transit time from power on to full operation. In practice for SSB and QSK at Moderate CW Speeds (25WPM) that is not an issue. For more demanding applications then another method may be warranted.
The power application via the 5VDC three terminal regulator not only powers on the MMIC but provides steering voltage to the 1N3070 didoes to route the signal as either an Input to the Gain Block or as an Output.
Tri-Quint was specific about the 1N3070 not so much about the Voltage or Current capabilities as its RF properties. I bought a bag and at that time were pennies but now one unit is about $7. Luckily there is a cheap substitute a 1N4935T about 17 Cents. The same applies to the AG303-86G which is out of production thus the MAR-6SM+.
A first piece of "knowing": what is a MMIC Amplifier and what is inside of the amp? This is the MAR-6SM+
Basically, a MMIC is a block amplifier and for the MAR-6SM+ is two transistors, Darlington connected for higher gain, along with the usual bias resistors. The MMIC Transistors typically have materials Like Gallium Arsenide or Gallium Nitride or even a Combo Silicon Germanium or just plain Silicon.
The Gallium variants are especially liked by the military as they work in the Microwave region and tend to be RAD hard (Survive Electro Magnetic Pulses from Nuclear explosions). Better processes now make Silicon OK for the higher frequencies of operation but not RAD hard. The MAR-6SM+ is a Silicon device.
What are the Specifications for the MAR-6SM+? Essentially DC to 2gHz, 50 Ohms in/out, 20dB gain and 2.3dB Noise Figure. Not bad for less than $2 in single lots.
Smaller than a pencil eraser, they present certain challenges for mounting and installation. If you have not graduated to Big Boy Pants and only deal with glue down Manhattan pads, then stop here. These devices need a carrier board or custom PC Board. Fortunately, K7TFC has a mounting board including the device in the works. So, start reading again.
That leads us from a MMIC device to a circuit diagram to a working module. In our travels we did the upfront engineering evaluations and considered carefully our application. No, we are not doing microwaves, but this unit has such great specs at HF and has low part count, no tuned circuits, is 50 Ohms and Termination Insensitive -- what other evidence do you need. Soon there will be a board from K7TFC --so no impediment from a mounting perspective.
I know just because it appeals to me and has for some time does not make it a clear choice for all. Will your well-worn bag of 2N3904 devices work in an amp circuit? Why of course they will. But the MMIC devices have so much more capabilities. Sure, when you were a kid, you could talk to your neighbor using a string and two tin cans. But to have a QSO today, across the globe, takes some more sophisticated hardware.
The 1uH chokes at the input and output ports have an impedance close to 90 Ohms at 14MHz and provide a DC path to ground for the Diode Steering. (2*Pi*F*L)
Always slightly ahead...
73's
Pete N6QW