An AM RF Amp for External Antennas

by Bruce Carter

This page is based on two articles:

  1. "Pre-amp for the Broadcast Band", Victor Kell, Electronics Illustrated, March 1969, pages 56-59
  2. "Big Booster for the BCB", Herb Friedman, Electronics Illustrated, March 1971, pages 77-80

The two articles referenced above, although appearing two years apart and done by different authors, describe an almost identical circuit, shown below:

Figure 1: AM radio RF Amplifer

The first article had a 3PDT switch that not only acted as a power switch, but also allowed the user to bypass the amplifier. This would be very useful for areas with strong stations, to avoid overloading the receiver. The second article utilized a ferrite bar antenna, which severely compromised sensitivity. It did, however, include PCB artwork:

Figure 2: RF Amplifer PC board artwork

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

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 loopstick 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 FET's can probably be used as well. Both articles stressed handling precautions for FET's. Even though FET's 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 cannot give enough praise to this design. It provides about 40dB of gain, and the main problem you will encounter is overloading.