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A Power MOSFET TRF Radio Circuit

Updated on January 2, 2013

Introduction


It can be an interesting and useful experience to build a simple radio receiver for the AM broadcast band. While it is possible to buy a dedicated IC such as the MK484 to simplify construction in practice it is hardly worthwhile going to such trouble and expense. In particular you can construct an equally sensitive circuit using common junk-box components.


Figure 1

Make a useful AM receiver using simple components.
Make a useful AM receiver using simple components.

AM Radio Circuit


The key creating a very sensitive Tuned Radio Frequency (TRF) receiver is to make use of the very high transconductance of common Power MOSFET devices. Transconductance is the change in current flowing through the device for a given change in input voltage.

For modern Power MOSFET devices a transconductance of 30 amps per volt is typical and for some exceptional devices it can exceed 90 amps per volt. A considerable difficulty occurs because of the very high internal capacitance values of Power MOSFET devices. If you can meet the challenges thought you will have a very simple but effective AM radio.

Figure 1 shows a circuit that works very well in practice. Transistor Q1 presents a very low impedance at its emitter. This prevents the voltage at the drain of M1 from changing by any significant amount at radio frequencies. This prevents loss of gain from M1 by the Miller effect through the large drain-gate capacitance of the device and also prevents negative feedback through resistor R3. There is still some loss of gain because of the ratio of the tuning capacitor C1 to the large gate capacitance of M1. Still that is not so much to worry about. C1 and the inductor bar antenna L1 form a resonant circuit that tunes the circuit.

Q1 in fact forms a common base amplifier circuit biased by R1 and C3. C3 should be a multilayer ceramic capacitor though you could improvise by using a low leakage 10uf electrolytic capacitor in parallel with a 100nf ceramic capacitor. If the value of C3 is too low the circuit will become excessively sensitive to supply line noise.

The output of Q1 goes to the base of Q2 which is arranged as a sensitive transistor square law AM detector. R8 and C6 provide some additional filtering to remove any remaining radio frequency remnants.


Conclusion


This circuit is very sensitive and will provide a large output even in a poor reception area for AM broadcast band signals. Of course it is not particularly selective as must be the case with a simple TRF radio. Nevertheless it is very usable.

Normally you would connect the output to an audio amplifier IC and loudspeaker for room filling volume or you could just connect the output to a crystal ear-piece for personal listening.


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