The FND15, from Solitron, is the integration of an N channel Junction Field Effect Transistor, or NJFET, with back to back Twin Diodes onto a single die. With the use of the FND15, it is possible to build a buffer amplifier small enough to fit into the casing of an ultra-small subminiature microphone while providing an extremely quiet front end for further amplification. What sets the FND15 apart from standard JFETs is the incorporation of miniature, Twin Diodes directly onto the 0.025" by 0.025" die, without the overhead of a monolithic substrate or the need for two or even three discrete die. A schematic of the FND15 die is shown below:

As you can see, it has the standard Drain, Gate and Source contacts. In addition there is a third contact, labeled "Common" which provides access to the diodes along with the gate contact itself.

N channel JFETs are the quietest semiconductors and are most often chosen for the buffer amp between subminiature transducers and their gain amplification stages. For the high impedance transducers, such as subminiature microphones and low noise detectors, it is important that the buffer be as close to the transducer as possible. Using JFET technology it is possible to make an amplifier so small that it will actually fit within the microphone cartridge.

The requirement for the JFET are that it have low self noise, a high transconductance (gm) and a low input capacitance. Subminiature microphones are used in both hearing aids, which operate off of a single 1.3 volt cell, and studio microphones which can operate at much higher voltages. It is therefore necessary for the JFET to function well under both conditions with minimum distortion and noise. The FND15's NJFET meets these requirements as the specifications show.

Microphones and other high impedance sensors often have a source capacitance as low as 5 pfd. To prevent low frequency signal loss, the input stage must have an impedance in the multi-GW range, i.e 109 W. For example with a 5 pfd source impedance and a 1010 W input impedance the signal will be rolled off 3 dB at 30 Hz. Most FET satisfy this requirement, but they often have excessive leakage of gate current. The leakage of this current through the gate biasing element, usually a transistor, limits the input impedance. A value of 100 pa gate leakage will produce a gate bias of 1 volt if a 10 GW (1010 W resistor) is used. The shot noise because of this gate leakage will seriously compromise any claim of a low noise device. Fortunately, the FND15's JFET has a leakage current of less than 1 pa when used in a single cell operation. This would allow a 100GW (1012 W) to be used yet result in only a 0.1volt bias.

A comparison of the shot noise with 100pa of gate leakage and 1pa of gate leakage into a 1 pfd capacitor is shown on the following chart. Also shown is a comparison of the thermal noise of a 10GW resistor, a 100GW resistor and a 10KW, again into 1 pfd. Notice the benefits of low leakage and a high input resistance.

Unfortunately while a 100GW resistor would be ideal for use, the manufacture of such a resistor is difficult and often results in a very noisy device compared to the theoretical. Even if it a quiet resistor could be fabricated, its size would be prohibitive.

The FND15 does not use resistors for gate bias. Instead it uses twin diodes, back to back. Because the diodes are in parallel, but of opposite polarity, one is always biased in the forward direction for any polarity of signal. Such diodes, if properly made will present a high input impedance, on the order of 100GW, when biased by currents of 1pa or less. They will not cause appreciable distortion for signals in the audio range whose level correspond to sound pressure levels of up to 110 SPL for a single cell operation and up to 140 SPL for higher voltage supplies.

For higher level signals, which might overload the first stage and either overload or damage succeeding stages, the twin diodes provide a form of overload suppression because of their clipping effects, something simple resistors can not do.

Some examples of the application of the FND15 follow. The first circuit as shown in schematic #1, is a simple buffer amplifier. It is operated as a simple unity voltage gain amplifier, and the FND15 is complete except for the choice of the Rload, the load resistor. In this application, the supply is only 1.3 volts. The 5pfd capacitor represents the source capacitance of the microphone cartridge.

If the overload is expected to be severe, it might be a wise idea to use a protection resistor between the diodes and ground to limit the overload current. This is shown in schematic #2.

The follower circuits just shown draw on the order of 25 mamps and have gains with in 1 or2 dB of unity with an output impedance of about 4kW. If a lower output impedance is required, reducing the load resistor so that more current is drawn will help. However this will degrade the gain even further. An alternative is the use of a JFET as a current source. Here both the gate and source are tied to ground and the second JFET is operated at Idss. This will provide both a low output impedance and actually improve the gain to nearly exact unity. It is necessary that this second JFET have a lower pinch off voltage that the FND15. This schematic is shown next.

By using a resistor in the Source leg of J2, it is possible to use low current yet near unity gain as seen on schematic #4.

If a higher voltage power supply is available, it may be possible to extend the range of signal amplitude before overload occurs. To do this a second resistor is used in the source leg of the basic circuit and the gate diodes are attached to this tap point through the diode protection resistor as shown on schematic #5.

Again, if a higher voltage power supply is available, it is also possible to provide a dual ended output with the addition of a single resistor to the basic circuit. See schematic #6.

If instead of the dual output, an amplifier with gain is needed, the addition of a capacitor will increase the level of the inverted output. See schematic #7.

Finally, a very low output impedance can be accomplished by using a bipolar transistor along with the FND. See schematic #8.