The gate threshold voltage was the key constraint for the design engineers at the basic transistor level. With EPAD MOSFET arrays the gate threshold levels can now be precisely controlled.
EPAD MOSFET matched-pair arrays include monolithic quad/dual N-Channel zero threshold mode, enhancement mode and depletion mode devices. This product family offers tightly-controlled threshold voltages. The tight control of threshold voltages enables predictable transistor operation at very low voltages and currents.
A unique member of this MOSFET Array family is the Zero-Threshold MOSFET. This device, along with other very low threshold voltage members of the product family, constitute a class of MOSFET devices that enable ultra low supply voltage operation and nanopower small signal circuit designs, applicable in either analog or digital circuits. In some instances, a circuit that depends on a single ultra-low voltage and low-power supply source can be implemented.
"Zero Threshold" mode devices
The Zero Threshold MOSFET is a special case in the EPAD MOSFET family where the individual threshold voltage of each MOSFET is fixed at zero. These devices are unique by using electrical threshold settings to offer very low voltage switching with sharp turn-off and very low leakage characteristics similar to that for a conventional MOSFET.
A zero threshold MOSFET reduces or eliminates input to output voltage level shift in circuits where the signal is referenced to GND or V+. This feature can significantly reduce output signal level shift from that of the input and enhances operating signal range, especially for very low operating voltage environments. With zero threshold devices an analog circuit with multiple stages can be constructed to operate at extremely low power supply or bias voltage levels.
Enhancement Mode devices
Which require a positive bias voltage to turn on? No conductive channel exists between the source and drain at zero applied gate voltage.
This precise threshold voltage feature enables designs that simplify or reduce circuit complexity in the design and also in the circuit stages that follow. In some situations, supply voltages and supply currents can also be greatly reduced.
Depletion Mode devices
The depletion mode threshold voltage is at a negative voltage at which the MOSFET device turns off. Without supply voltage and with Vgs = 0.0V these EPAD MOSFET devices are already turned on and exhibit a controlled on-resistance between the source and drain terminals. These depletion modes EPAD MOSFET devices can be modulated and turned-off under the control of the gate voltage. Where the threshold voltage is linearly shifted, the gate control voltages must also shift accordingly relative to the threshold voltage shift in order to turn on, modulate or turn off a specific depletion mode EPAD MOSFET device.
EPAD MOSFET Key Performance Characteristics
- Minimum offset voltage and differential thermal response with excellent temperature coefficient tracking characteristics
- Low input capacitance
- Fast switching speed
Low Voltage EPAD MOSFET Oscillator
EPAD MOSFET is widely used as a RC oscillator. Figure 2 show a low voltage EPAD MOSFET RC oscillator. In this circuit U1A, U1B and U1C form the basic three-stage oscillator with feedback resistor and capacitor network R4, Cosc and R5. The oscillator operates in low frequency ranging from a few hertz to kilohertz. The output is tapped and buffered with U1D as an output buffer stage. Power to the output stage is supplied by VL. VL can be either at V+ or at a different value, depending on the desired output high level. If VL is at a different voltage level, then the output buffer also acts as a level shifter.
EPAD MOSFET RC Oscillator
Conclusion
Many circuits that one has used in the past can also be naturally extended and readily applied here. Due to the extension of voltage and current ranges to lower limits and the precision threshold voltages, a new mindset and a fresh look at many old circuits and their related design configuration issues may be appropriate.
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