Using Vacuum Tubes to learn JFETs

Tubes are infrequently seen in modern designs but can be a useful analogy for learning to implement JFETs.

JFETs continue to be a useful design element, many years on. They allow sensitive measurement and work as protection and amplification elements in modern circuits. There is the assumption that JFETs are difficult to learn. More generally, depletion mode devices (such as JFETs) have an aura of mystery.

To learn JFETs and depletion mode devices, I recommend you learn tubes first. For those from a different era or who never tinkered with a tube application, let me assure you: tubes are easy to understand. Let’s start with this science project analogy:

The glass vacuum chamber has metal at the bottom. It is a metal whose electrons will boil into the vacuum without causing the metal to vaporize or to react with anything in the vacuum chamber. High vacuum keeps out oxygen, which would otherwise ruin the metal.

The heat photons speed up the electrons until they fly right off the metal. This is thermionic emission.

There are so many electrons that it is better to draw them as a cloud rather than as points.

The cloud of electrons in the science project is in equilibrium:

The positive charge left behind in the cathode pulls the electrons back.

Let’s add a metal plate to the top of the vacuum chamber, and attach a wire to it:

This science project is now a primitive tube rectifier.

Tubes use an electric filament instead of a torch to heat up the cathode. The heater is usually not drawn in tube schematics.

Most tube rectifiers have two plates, each with their own connection to a pin. It was easier to draw the 6W4GT, because it has just one plate:

To control the flow of current through the tube, add a control grid:

This type of tube is called a triode. The grid repels some fraction of the electrons and keeps them from flowing to the plate. The grid voltage needs to be negative relative to the cathode. If the grid was positive, it would be like having another plate, and the electrons would flow to the grid instead of the cathode. The amount of current that flows through the tube decreases as the grid voltage becomes more negative. Just a few volts on the grid can control large changes in current.

Very few of the electrons collide with the grid. The grid current is so low that it is hard to measure.

Here is a common circuit to get the right currents to flow to make a useful amplifier circuit:

The triode begins conducting like a regular rectifier tube. As current flows through the cathode, the 2K resistor causes the cathode voltage to rise. This raises the cathode voltage above the grid voltage, and begins to turn off the tube. In this example, the tube reaches a stable operating point when the cathode reaches 2V. The grid voltage is at zero volts, because the grid current is so small that there is no voltage drop across the 1MΩ resistor. The plate current of 1mA causes a 100V drop across the 100K resistor.

Different tube part numbers have different operating currents and voltages that determine the stable operating point. Detailed specifications and curves are in the tube datasheets.

Triodes are usually dual devices, with two triodes inside one glass package. Each half of the device is drawn as one symbol. Here is an example of an audio preamplifier:

This circuit provides a gain of about 20. JFET circuits work in a similar way.

The names of the components map from tubes to JFETs:

  • The grid on a tube triode is akin to the gate on the FET.
  • The plate on a tube triode is akin to the drain on the FET.
  • The cathode on a tube triode is akin to the source on the FET.

The JFET preamp shown above has a gain of about 5. The depletion mode FET works just like the triode, except that it doesn’t require a heater and the drain usually operates at a much lower voltage than the plate of a tube. The JFET gate operates at a negative voltage with respect to the source, just like the grid of a triode.

I was surprised by how easy it is to work with tube circuits. The devices are electrically rugged, forgiving, and they “just work.”

When working with JFETs, keep spares handy. They are easy to destroy! Keep circuit leads short to avoid creating high-frequency oscillations.