Transformer-coupled source follower in an active antenna

Pieter-Tjerk de Boer, PA3FWM pa3fwm@amsat.org

(This is an adapted version of part of an article I wrote for the Dutch amateur radio magazine Electron, January 2017.)

For receiving radio signals in the long and medium wave band for research of the Luxemburg effect (which I'll write about in the future), I made a battery-powered active antenna, using a somewhat unusual source-follower circuit.

Figure (a) shows a "normal" source follower. A voltage divider applies a such a voltage to the gate, that at rest about half the supply voltage is across the source resistor. Thus, the output voltage range is maximised: plus and minus (almost) half the supply voltage.

Figure (b) shows the source follower I used. The essential difference is that at the place of the source resistor, it has an inductor or the primary of a transformer. Ideally, at rest, i.e., for DC, 0 volts fall across this. Thus, the full supply voltage is across the FET itself.

The output voltage range for AC is now (almost) plus and minus the full supply voltage. The plus half of this clear: if the transistor is driven more into conduction by its gate voltage, the voltage across is can decrease to (almost) 0, so (almost) the entire supply voltage is across the inductor. For the minus half the output voltage must go negative. Due to the inductor, this is possible: if the gate voltage closes the FET, at first the current in the inductor just continues, pulling the output voltage negative. This of course cannot continue indefinitely, because the current through the inductor decreases if there is a negative voltage across it; so this only works for AC of sufficiently high frequency. The mean output voltage stays 0, because across an inductor there cannot be a DC voltage (component). So, with this circuit we get double the output voltage range for the same supply voltage.

Figure (c) shows the essence of my active antenna's schematic, inspired by a design I found somewhere on the Internet but unfortunately can't find back. It consists of two source followers, one after the other. The first transformer gives "free" and distortionless voltage amplification. The second transformer acts as a balun to drive the symmetric feedline to my receiver.

The circuit actually is very simple, with particularly few resistors. This is partially caused by the simplicity of the DC biasing of the FETs in this circuit: with the gate at ground potential the current equals the FET's I_DSS, which is often a good bias point for a source follower. For the BF862 FET I used, it is about 20 mA, albeit with some spread, so selecting the FET can be needed. Two rechargeable penlite batteries in series give enough voltage.

I did not measure the circuit's large-signal behaviour. Probably it will be less good than that of a more advanced design (higher supply voltage, emitterfollower for the final stage, etc.), but it was good enough for my purpose.