WTB (cheap) audio output transformer 5K plate to 3.2 or 8 ohms. Cheap is the operative word!
There are many forms of Bi-directional amplifiers and of course we have a segment of our ham homebrewer demographic who have a TIA Tattooed on their butt. But a bi-directional amplifier can take other forms. I will share some of my approaches with you.
TIA (Termination Insensitive Amplifier) enjoys great popularity in that the gain can be adjusted by the resistors you choose for the circuit. Then there is the all important 50 Ohms that remains 50 Ohms no matter what you hang on the front end of the amp. They are particularly useful on either end of a crystal filter as that has the overall effect of improving the signal quality of that stage, because of the constant impedance factor. Of course using a TIA makes you feel a kinship with one of its inventors, Wes Hayward, W7ZOI.
Transceiver Module with two SBL-1's, a set of TIA's and a 9MHz Filter
Quite honestly the TIA's worked great, BUT not something so spectacular that you would only use this circuit. The Plessey Module (also recognized by W7ZOI) does a great job without any noticeable difference and is 50 Ohms in and out and a lot less parts. Then again I don't have a tattoo of a TIA on my butt.
Long ago I came up with a project idea called LBS (Let's Build Something) which along with AI6YR, Ben we co-authored a series for QRP Quarterly. We started with a Direct Conversion Receiver and using most of that hardware turned it into a 40M SSB Transceiver.
I came up with an idea for a Bi-directional Amplifier that had gain pots in each leg so you could optimize that stage. Typical use might be as a Rx RF Amp in one direction and Tx Pre-Driver in the other. On Rx you want some gain BUT not max gain and on Tx you want the pedal to the metal.
Here was the neat feature of this design. The output off of the secondary of the matching transformer is always terminated in 50 Ohms with the 100 Ohm resistor in parallel with the 100 Ohm pot (a forced 50 Ohm termination). The center wiper is the tap point for the output and provides a variable output without disturbing the 50 Ohm termination. No one noticed this, but it works well! Relays are used to steer which amp is in-line depending on whether in Receive or Transmit.
I have used bi-directional amps that were diode steered and you saw some of that in yesterday's post on Crystal Filters. Look at the 4 Diodes around the 3N211 and certain Points are either biased on with +12VDC or Grounded. Note the A and B arrows to show how the signal gets routed around the DGM. On the right most diode if the upper RH point is grounded and 12VDC applied an the Lower RH point the signal form the upper RH core gets routed to gate #1 and since the upper LH point has +12VDC on it, the Drain is supplied juice for direction B and the output appears on the upper LH matching transformer. Cool Beans guys! This circuit was found in SPRAT authored by my friend Ron Taylor, G4GXO.
Currently my approach is to use Relay steering where two relays can reverse the In's and Out's and it can be a small foot print. Here a whole IF Module is steered in two directions.
.jpg)
We earlier mentioned the Plessey Circuit and many of my homebrew transceivers have that functionality (but only one has the TIA*). I like the Plessey Module since I also have a CNC layout using SMD Parts. So, it is pretty easy to cut a board -- just punch the Start Button. This makes for a compact rig. Good to 6 Meters with 17dB of Gain and 50 Ohms in/out. No toroids to wind.
I have also used MMIC devices in a bidirectional module that also has a small footprint. Good to 3.5 GHz with 20dB of gain and 50 Ohms in and out.
So as a designer and you as a builder there are many bi-directional forms aside from a TIA. * My take on the TIA is like the Nano VNA and Tiny SA, it certainly has excellent qualities but used primarily, I think because of the identification with Hayward and Kopski perhaps more so than termination insensitivity! Hey guys this rig uses TIA's from W7ZOI. Do cool kids have a Nano VNA Tattooed on the other side of their butt?
This is sort of like wearing Nike Tennis shoes, where you have a strong identity by using the brand. You know when you take your morning walk in the hood the widows and divorcee's spot you and approve because you have your Nike Tennis shoes on for the walk. But you can wear Skechers and they cost a lot less and will get you around the hood, but you might not be noticed!
My P3ST design specifically had as a goal No Bitx, No TIA, No EMRFD and No Facebook Groups. That radio works well and is now a kit from K7TFC. There another piece of a SSB Transceiver project has been provided to you so get off the couch you couch potato!
73's
Pete N6QW