# Build the Crystal Set Diode Detector Bias Box: a simple and easy way to determine if one's diode is optimum for weak signal reception, or it should have a higher or lower axis-crossing resistance (0.026*n/Is ohms)

By Ben H. Tongue

Quick Summary:  The 'Diode Detector Bias Box' enables one to check whether the diode being used in a crystal radio set has optimum characteristics for that set.  The optimum detector will deliver the greatest low-signal sensitivity.

A detector diode, in order to deliver the highest sensitivity and lowest audio distortion, must be properly impedance matched to its RF source.  It must also be matched to the correct (for that diode) audio and DC load resistances.  See Articles # 0, 1, 5 and 15a for more info on this subject.  How can one know for sure that the diode used in one's own crystal radio set is the best one for it?  Another way of putting it is:  Does my diode have the Saturation Current (Isopt) that the optimum diode, for my set, would have?   An easy way to find the answer is to build and use the diode Detector Bias Box.

A detector diode having particular saturation current (Is) can be biased to perform almost exactly the same as a diode having a different Is.  This statement assumes that the diode conforms to the classic Shockley diode voltage/current relationship:  Id = Is*{exp[(Vd-Id*Rs)/(0.0257*n)]-1)}, at room temperature.  Id is the diode current in Amps.  Is is the diode Saturation Current.  "exp" means:  raise the base of the natural logarithms (2.718...) to the power of the expression following.  Vd is the voltage applied to the diode in volts.  Rs is the fixed series parasitic resistance of the diode in ohms.  n is the Diode Ideality Factor (Emission coefficient) and is dimensionless.  It is usually between 1.05 and 1.2.  The lower the value of n, the higher will be the very weak signal sensitivity.  At low signal levels, the Id*Rs expression is small and can be neglected.

To change the detector performance of a diode of Is = Isor (original) to the performance of a diode of Is = Isop (optimum), a DC bias voltage must be inserted in series with the diode.  The required bias voltage is:  Vbias = 0.0257*n*[ln (Isop/Isor)].  ln represents the natural logarithm of the expression following it.  This equation is accurate if the values of Is and n do not change as a function of diode current.  This assumption is correct for the Schottky diodes I have checked. Some germanium ones I have checked do not accurately follow the Shockley equation.  They tend to have high values of Is such as 500 nA or more.  Germaniums having Is values in the 100-200 nA range do seem to follow the Schockley equation well.  At high currents, Is increases from its value at low currents.  The Vbias equation is given for information only and is not used in the following experimental procedures.  Whether a Schottky, germanium or other diode is used, a convenient way to 'tune' the Is of a diode is to use the "Diode Bias Box".  It effectively enables one to change a diodes' effective Is (and therefore its operating impedance) by merely turning a knob on a box.  The Diode Bias Box also enables one to determine the best diode DC and AC load impedance.

Here is an interesting relationship that applies to most Schottky diodes: A Schottky diode detector having a saturation current of (Is1) that has no external DC current bled into it will perform, as a diode detector, identically to that of another diode having a saturation current of (Is2) if a DC current (Ib) equal to (Is1-Is2) is bled into it.

To use the Bias Box, connect the terminals labeled T1 to the crystal radio set ground and the cold end of the audio transformer primary, if one is used. If no transformer is used, connect the terminals of T1 to the crystal radio set ground and the cold end of the headphone headset.  Also make sure that the connection where the Bias Box is inserted is well bypassed for RF and audio.