TRF Radio

A Current Mirror Based TRF Radio

Circuit description:

Q3 and Q4 form a cascode RF amplifier. Since only small signal levels are involved a biasing hack is used for the upper cascode transistor to reduce the component count. L2 and C4 form the resonant frequency selective circuit and should be coupled to an antenna in some way, for example by light coupling via a gimmick capacitor (2 insulated wires twisted together.)

The resonant circuit is impedance matched into the base of Q3 by over-winding 1/10 the number of turns of the resonant circuit.

It is greatly preferred in the biasing arrangement for Q3 that one end of impedance matching coil goes to ground as this reduces hum pick-up to nothing.

Q1 and Q2 form a current mirror at DC since L1 has a nominal impedance of zero at DC.

At RF the output of the cascode is directed into the base of Q2 where transistor non-linear behavior results in AM detection.  L1 is a RFC suitable for the frequency range, for example 220uH for 4 to 30 Mhz, 1 mH for lower frequencies.

Since the exact currents flowing in the circuit are not known you can use a low value for R2 and make up for any gain loss with extra audio amplification. Or you can select a value of R2 that results in a 1 or 2 volt potential difference across it. An audio transformer could be used instead.

An older version of the circuit is here. An improvement is a decoupling capacitor across the first current mirror transistor.