Termination Insensitive Amplifiers
Wes, W7ZOI and Bob, K3NHI, authored a seminal article on termination insenstive amplifiers which in simple terms means that what ever you hang on the output of an amplifier does not reflect back on the input impedance. This is especially critical where 50 Ohms is selected as the input /output impedance of amplifier blocks. [Hayward, Kposki " A Termination Insensitive Amplifier for Bidirectional Transceivers", 26 June 2009]
Their paper presented a bilateral configuration so that by biasing "on" the appropriate amplifier block it would be possible to change the signal path direction in an application such as a transceiver. This greatly simplifies the topolgy wherein a packaged double balanced mixer device (50 Ohms) such as an SBL-1, TUF-1, TUF-3 or ADE1-L could serve as a Receiver Mixer ahead of a pair of bilateral amps/crystal filter and on the back end another such devuce as mentioned could be the Product Detector. Reversing the signal path would now have the Product Detector act as a Balanced Modulator and the former Receiver Mixer is now the Transmit Mixer. The VFO (LO) and BFO (CIO) would always be connected to the same device.
The hidden beauty of their design is that the gain can be adjusted with a simple change of two resistors and the termination would always remain 50 Ohms. Below is a photo of my impelmentation of their circuit. It works really well!
In case there is wonderment about the pot on the second SBL-1 that is for carrier balance adjustment as this provides a means to unbalance the carrier for "Tune" purposes.
The following sequences of photos show the progression of the incorporation of the Termination Insensitive Amplifiers Mainbaord from a bread board (actually a 2X4 eighteen inches long) into a homebrew 20 Meter SSB Transceiver housed in a home constructed box. Extensive use of my CNC and Manual 3 Axis Mills made this all possible.
I am renaming this project the ZIA (thanks to N2CQR). Z being the symbol for impedance and thus Impedance Insensitive Amplifiers --the in/out is always 50 Ohms. Thanks Wes and Bob!
5 Watt Linear Amp using a MRF260. Link to Photo