Wednesday, 9 March 2016

An IF Strip Using Homebrew ICs.

In a recent post I described a way to homebrew your own mixer ICs. It actally contain a differential single ended mixer which can be used in many ways as it has been stated in the end of that post. So, why not build a simple cascode amplifier with these sub circuits? It seemed to be a good idea and I tried it. A two stage IF stage amplifier with AGC was wired as shown in schematic diagram and was tested.

The parts in dotted squares are homebrewed subcircuits. Using this two modules, I tried to prototype 4MHz IF amplifier strip of a two-stage cascade IF amplifier. Output tuning circuit of each stage is consists of an RFC of 4.7uH alongwith a capacitor of 330pF. These values can be altered for the frequency of interest or use. I used standard axial chokes for inductors but obviosly toroids are a better option. AGC is derived through a voltage doubler rectifier a portion of the output in 1N60 or 1N914 diodes,and a common PNP transistor. 

The initial tests show a maximum IF gain of about 68dB, AGC  effectiveness started from an input of about 20dBuV EMF, that is quite an impressive figure for such a simple circuit with inexpensive devices. AGC versus gain characteristics are plotted in the following diagram:
Indeed good designs can be evolved empirically with inexpensive components and simple thoughts.

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