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Designing with ultra-low voltage MOSFET arrays

Posted: 05 Sep 2005 ?? ?Print Version ?Bookmark and Share

Keywords:analog? mosfet? electrically-programmable threshold? epad? mosfet?

continuation...

EPAD MOSFET switch

An EPAD MOSFET acts as a switch when it is turned on with an appropriate gate voltage, where a conducting channel forms between the drain and the source terminals (See Figure 8). The source terminal acts as the input and the drain terminal as the output. The on-resistance of the switch depends on the channel-on current as controlled by the gate voltage. In this case if an enhancement mode device is used, the switch can be turned-on with a positive bias voltage on the gate terminal, with the signal propagating from the source to the drain terminal. The signal can be either digital or analog in nature, as long as the user takes into account the input and output impedance levels relative to the channel on-resistance of the switch.


Figure 8

The switch can be turned-off by grounding the gate or by setting a gate voltage at 0.4V or less below the threshold voltage. When turned-on, a switch can pass a signal voltage up to the gate voltage minus the Vgs(th). When applied using the EPAD MOSFET Array family, the minimum operating voltage of an EPAD MOSFET switch is limited by the off-state leakage current. In this case, using the subthreshold characteristics mentioned previously, an analog or digital switch could be operated at a minimum supply in the range of 0.4V to 0.2V.

EPAD MOSFET normally-on switch

A normally-on switch is a switch that is normally already turned-on when the gate is at ground voltage or when there is no supply voltage present. This function is analogous to a normally closed FORM B (NC) relay with which the contacts are already closed when the relay coil is not energized, and which requires a voltage source to energize the relay coils in order to open the contacts. Depletion mode EPAD MOSFETs are naturally normally-on devices where a conduction channel already exists when there is 0.0V bias on the gate. The resulting conducting channel behaves similarly to a resistor when Vds is at low levels.

However, beware that due to the high input impedance of the gate, the gate voltage can "float" to a value other than zero. In an actual circuit it would be desirable to ground the gate, connect a fixed resistor to the gate or otherwise control the voltage available to the gate.

The key differences between Form B relays and the EPAD MOSFET Array family are that EPAD MOSFETs have higher on-resistances and operate at low voltages (<10V) and low current or power levels. In situations where these EPAD MOSFETs are used as a substitute for the equivalent function of a normally-on switch, they offer significant benefits such as size, density, power consumption, mechanical ruggedness (all solid-state) and cost. Furthermore, the switch channel on-resistance can be modulated and controlled directly by Vgs without using other active circuit elements. Consider the case of building a normally-on switch using a Zero Threshold MOSFET such as the ALD110900. The device is in the on state and conducting a current at about 1uA when the gate is grounded. This is a reliable and dependable current value, and an external sensing circuit can be designed to detect and utilize this current signal.

In the case of using a Zero Threshold MOSFET as a switch, a signal can pass from V+ rail to 0.0V rail with an appropriate circuit configuration. However, a normally-on switch cannot be turned-off unless a negative voltage relative to the source voltage is available to be applied to the gate in order to turn off the EPAD MOSFET.

Re-arranging the circuit configuration, a zero threshold MOSFET can also be used as a high-side switch, which can pass a high level signal that is near or at V+ potential. To turn on such a switch, connect the gate to V+ (Vg = V+). Assuming V+ is at least 0.4V, grounding the gate will turn this switch off.

Likewise, other depletion mode EPAD MOSFETs can also be used either as high-side switches or as normally-on switches, each having a corresponding normally-on on-resistance value and a corresponding turn-off voltage. Tradeoffs can be made between on-resistances desired versus the gate voltages required to modulate and/or to turn on and turn off the switch.

EPAD MOSFET current source and current mirror

A basic current source is shown in Figure 9. Many designs of current sources using conventional MOSFETs can be implemented using the EPAD MOSFET family. The following presents two special notes worthy of discussion.

First, by using a low threshold enhancement mode EPAD MOSFET such as the ALD110902 (dual version with Vgs(th)= 0.20V), the current source starts operating at very low output voltages. This low output voltage extends the useful output voltage range of the current source. The output can start sourcing (sinking) current at an earlier voltage and thus expand the useful signal range of the current source. In addition, if the current is from a low-level current source, the output voltage can be extended all the way to near ground potential.


Figure 9

Second, by using a Zero Threshold MOSFET, such as the ALD110900 (dual with Vgs(th)=0.00V), the current source can start operating with zero supply voltage, where V+ = 0.0V. With a controlling or modulating voltage, this current source can be configured to become a normally-on current source, starting current sink at 0.0V at the output terminal and yet can also be modulated or turned-off with an external applied control signal.

Low voltage EPAD MOSFET oscillator

Another circuit function that is widely used is a RC oscillator. Figure 10 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. V1 can be either at V+ or at a different value, depending on the desired output high level. If V1 is at a different voltage level, then the output buffer also acts as a level shifter.

Using a low threshold enhancement mode EPAD MOSFET such as the ALD110802 (quad with Vgs(th)= 0.20V), an example of this oscillator operates on less than 0.2V supply voltage and less than 70nW of power.


Figure 10

Ultra low voltage and Nanopower EPAD MOSFET differential amp

One of the key circuits used in analog design is a differential amplifier circuit. A simple inverting amplifier using an EPAD MOSFET inverter has been mentioned in the previous section. A basic very low operating voltage differential amplifier using EPAD MOSFETs is shown here in Figure 11. Different versions of this basic differential amplifier using various low voltage EPAD MOSFETs or Zero Threshold EPAD MOSFETs can be designed to reduce operating voltage or to minimize power dissipation. An example is a differential amplifier designed with EPAD MOSFETs that operate at an ultra supply voltage of 0.2V and consume only 570nW.

This basic differential amplifier consists of 3 matched pairs. U5 and U6 are a matched-pair and connected to V+ via bias resistor Rb. The purpose of this matched pair is to provide bias to an input differential pair, U3 and U4. U1 and U2 are biased in the sub-threshold region and are used as active loads. This circuit configuration works with many different matched pair Vgs(th) combinations, in conjunction with various combinations of V+, Rb and input/output ranges.

Tradeoffs of key circuit performance of the differential amplifier include parameters such as:

? V+ nominal value (with max. and min. target values);

? power dissipation goal;

? input voltage range;

? output voltage range;

? output-drive characteristics;

? frequency of operation;

? noise performance; and

? offset voltage

This basic differential amplifier, while appearing not very complicated, is unconventional in that it operates the EPAD MOSFET devices in the subthreshold region. Therefore it requires a different perspective in how these EPAD MOSFET transistors are biased and utilized in the circuit. A wide range of possible performances can be associated with each different circuit configuration using different members of the EPAD MOSFET array family.


Figure 11

For purposes of illustration, the main objectives of this differential amplifier have been focused on ultra low-voltage and ultra low-power versions operating at or near DC. Examples of some key specifications achieved:

Example A

Products used: ALD110800, ALD110902

Single stage: V+ = 0.5V, Rb = 275 kilohms, I+ = 1.9?A, Pd = 960nW, Gain = 24 Dual stage: V+ = 0.5V, Rb = 275 kilohms, I+ = 2.8?A, Pd = 1.4?W, Gain = 525

Example B

Products used: ALD110800, ALD110900

Single stage: V+ = 0.2V, Rb = 184 kilohms, I+ = 2.8?A, Pd = 574nW, Gain = 20 Dual stage: V+ = 0.2V, Rb = 184 kilohms, I+ = 4.8?A Pd = 960nW, Gain = 238

Conclusion

This paper provides the reader with an understanding and some basic concepts on how useful circuits can be implemented using this EPAD MOSFET Array family. To a seasoned designer, there is much that is familiar since the EPAD MOSFETs mentioned here are natural extensions to conventional enhancement mode MOSFETs and all the textbook-based theories and equations still apply.

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.

The ultra low voltage and NanoPower characteristics of the EPAD MOSFET Array family and how they can be biased and used in a circuit design can enable new products that feature novel power sources. Many circuits that one has designed in the past can now be naturally extended to new ranges and uses with ALD EPAD MOSFETs. These family of products begins to offer possibilities in circuit topographies that are quite novel, and in some cases even revolutionary.

About the abuthor

Robert Chao is the president of Advanced Linear Devices Inc.

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