JFET buffer

JFET output buffer
JFET output buffer

As mentioned inn the chapter about transistors, JFETs allow for simple low parts number count circuits, due to their depletion mode character.

In the Fig we see two simple single.ended buffer, which could find useage in line-level preamplifiers or filters. They feature very high input impedance and low output impedance. The current drive capability into an attached load is restricted to the value of the idle current (bias).

In praxis the load current should not exceed much more than 1/10 of that, as distortion increases rapidly above.

Rather only high load impedances >2kOhm will be served well.

When using same kind and matched JFETs,same source resistor values and symmetrical dual supply rails, the input and output nodes settle at the same potential, here 0V.

Care should be taken with raised supply rails and large gate resistor values.

The typically tiny gate leakage current doesn´t lead to a considerable offset voltage drop over the gate input resistor.

Under certain conditions, like high drain-source voltage, high temperature (eg, due to high drain current) the gate leakage shoots up.

This on the other hand leads to a self-amplifying effect, as it raises the drain current, following raising temperature, following higher gate leakage, etc. etc.

This effect is known as Latch-Up. A JFET once latched remains defect.

The second circuit expands the first by two same JFETs.

The cascode (see chapter ´Transistors´) has a couple of advantages.

The gate-source voltage of the cascoding JFET is at the same the drain-source voltage of the cascoded JFET.

In other words generates the cascoding JFET the supply voltage for the cascoded JFET and keeps it on a constant and low niveau.

The signal-leading lower JFET can work with very low distortion and low heat power losses even under high drain current conditions.

Higher supply rails as the lower JFET could withstand may be used, only limited by the breakdown voltages of the upper JFET.

Still though the circuit can supply for only rather small load currents and  serve comparably high load impedances.

But this it can do on a very high quality nievau.