Substitutions for the 1N5294 Current Regulator Diode

Learning the Art of Electronics uses the 1N5294 current regulator diode to supply tail current to the BJT differential amplifier in Lab 5 and to generate sawtooth and triangle waves in Lab 8.  In the figure below, the V-I curve shows the 1N5294 limiting forward current to ~0.75mA when biased ≥ 1.5V.

Unfortunately, the 1N5294 is getting hard to find and expensive (~$19) if you do find it.  The J504, also a 0.75mA current regulator diode ($6.78 at DigiKey), appears to be an exact substitute, albeit in a different form factor:

A cheaper alternative, the E-701 ($1.31 at DigiKey, $0.89 at Mouser) rated at 0.70mA nominal, has a V-I curve almost identical to the J504. We have tested it in the  BJT differential amplifier in Lab 5 and it works fine, even though it is rated at a slightly lower current. (updated 2025-06-30)

Since two 1N5294s are used in parallel to supply 1.5mA of tail current to the differential amplifier in Lab 5, you could just replace them with a single E-152 ($1.31 at DigiKey, $1.00 at Mouser) instead.  This is a 1.5mA (nominal) current regulator diode in a leaded axial package.  The only disadvantage of using a single 1.5mA diode in place of the two 0.75mA diodes in parallel is the higher V_K (the “knee” voltage – the voltage at which the diode starts regulating current –  ~2.5V for the E-152 in the figure below vs. ~1.5V for the 0.75mA diodes in the figures above).  This reduces the lower input common mode range of the differential amplifier (which extends down to approximately V_K +0.6V above the negative supply).  This is unlikely to be a problem with the differential amplifier in Lab 5 where the amplifier operates with a gain of 11 (so the input is limited to ~2.7V peak-to-peak), but the larger knee voltage might not work with the 74HC14 sawtooth generator in Section 8L.3.2. Section 8L.5.2 mentions this problem and suggests using higher supply voltages for the triangle oscillator.  (We also successfully tested the differential amplifier with a single J507 ($6.78 at DigiKey), a 1.80mA current regulating diode.)

A low cost solution to provide tail current to the differential amplifier in Lab 5 is to build a current regulating diode out of a JFET (which is what is in the current regulating diodes anyway – see Section 11N.4.3).  The figure below shows the V-I curve of a J113 JFET ($0.46 at DigiKey) with an 820 Ohm current setting resistor between source and gate.  This gave us 1.5mA current regulation for V_DG ≥1.7V with this particular JFET.   Note that I_DSS (the Drain Current with V_GS = zero volts) of a JFET varies significantly, even between devices with the same part number, so you will need to  use the circuit of Section 3L.2.1 or 12L.4.1 to select the gate-to-source resistor, R_SET, to get a drain current of 1.5mA.  We used a resistance substitution box to hone in on the correct value.  This circuit works with a J210 JFET or the 2N5485 JFET (if you can get it) as well.

Finally, you can use one of the two alternate bipolar transistor current sinks shown in Figure 5L.10 on page 224 (reproduced below) in place of the two 1N5294 current-limiting diodes (Figure 5L.9).  The 2N3094 BJTs used in these alternative designs were used in Lab 4, so incur no additional cost. (Note that the bipolar transistor sinks cannot be used in the sawtooth and triangle wave generators in Lab 8 as those circuits require a floating current source.)

All prices as of 3/2025.
V-I plots created with circuit of Figure 3L.5