Wednesday, April 3, 2013

AM RF Amplifier for External Antenna Using FET

It is would suggest that this RF amp would be most helpful when used with the AM loop antenna. In this case, this circuit is simply deleted T1, which is mounted off the board anyway. C1 will serve as the tuning capacitor for the loop. This is the figure circuit;


If a long wire antenna is used with (or without) a coupler, you will need T1. T1 is an AM RF transformer, and is a type that is becoming increasingly difficult to find. An AM loop stick antenna could probably be substituted, with the smaller number of turns being used for the secondary. I would suggest using a small ferrite slug, or breaking off the excess length of ferrite.

I have no idea whether the RCA 40468A FET is still available or not. It came in a 4 pin metal can with the 4th pin connected to the substrate. I have successfully substituted other N-channel devices, such as the HEP-802, which comes in a plastic TO-92 package. Simply ignore the substrate connection. Other good quality N-channel FETs can probably be used as well. Both articles stressed handling precautions for FETs. Even though FETs today are much better protected than in the past, static can still damage them. Use a wrist strap and grounded soldering station during assembly. I cann’t give enough praise to this design. It provides about 40dB of gain, and the main problem you will encounter is overloading.
Source; Bruce Carter


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