Ultra Low Current TRF Radio

Ultra-low Current Operation and Current Mirroring:

The core of this circuit operates at incredibly low currents, with the base of Q2 drawing just a few nanoamps. Q1 and Q2 form a current mirror at DC, meaning the current entering Q1's collector (around 1 microamp) is replicated at Q2's collector. This translates to a base current for Q2 of roughly 1uA divided by its Hfe, again falling within the nA range.

Current Gain and Component Choices:

This 1uA current from Q2's collector then flows into the base of Q3, where it's amplified by Q3's Hfe. Assuming a typical Hfe of 300, this results in a collector current for Q3 of around 300uA. To adjust this gain, you can play with the value of resistor R2 or even substitute it with an audio transformer for further flexibility.

Resonance and Antenna Coupling:

L1 and C2 form the resonant circuit, crucial for tuning the radio to specific frequencies. This circuit then couples to your chosen antenna, whether that's a simple wire or a more elaborate design.

Input Impedance and Noise Considerations:

While Q2's input impedance is exceptionally high under such low currents, it's worth noting the significant base-emitter junction capacitance. This would introduce substantial phase noise if the circuit were used as a regenerative detector, making it unsuitable for that application. However, this capacitance isn't an issue for a TRF radio, where it doesn't significantly impact performance.

Key Points:

• Extremely low current operation (nA)

• Q1 and Q2 form a DC current mirror

• Current gain adjustable through R2 or audio transformer

• L1 and C2 form the resonant circuit

• High input impedance for Q2

• Base-emitter junction capacitance not an issue for TRF radio