Battery polarity insensitive integrated circuit amplifier

Details Battery polarity insensitive integrated circuit amplifier Battery polarity insensitive integrated circuit amplifier

2000-11-29

2002-09-10

Pascal, Robert (Department: 2817)

Amplifiers

With semiconductor amplifying device

Including class d amplifier

33, C257S532000

Reexamination Certificate

active

06448852

ABSTRACT:

FIELD OF THE INVENTION
The present invention is generally directed to compact integrated circuit amplifiers designed to be powered by a single low-voltage battery and more specifically to compact high performance hearing aids designed to fit in the ear canal.
BACKGROUND OF THE INVENTION
There is a general trend towards the miniaturization of a variety of battery-powered electronic devices. Extreme miniaturization of battery powered electronic devices forces designers to use a single miniature battery to achieve a reduction in the size of the entire device. Currently, hearing aids constitute a class of consumer electronic devices in which extreme miniaturization requires the use of a single miniature battery. However, there is a general trend to miniaturize many consumer electronic devices, such as portable stereo listening devices and pagers. Also, it is desirable to miniaturize many health-related electronic devices, such as on-patient medical monitoring devices which monitor heart rate or other life functions.
However, miniature batteries present numerous technical problems. The total stored energy in a battery decreases as the battery volume is decreased. The series resistance of the battery also increases as the battery size (diameter) is reduced. Also, miniature batteries typically contain only one electrolytic cell. A single electrolytic cell has a characteristic maximum source voltage. Typically, the voltage of a miniature battery during its useful lifetime is only slightly more than one volt depending upon the “freshness” of the battery.
The high series resistance and low source voltage of a miniature battery confronts circuit designers with difficult problems in designing efficient circuits which operate properly under these conditions. Commonly, complimentary metal oxide semiconductor (CMOS) integrated circuits, such as microprocessors, are designed to operate from a five volt power supply of constant polarity. Some high performance microprocessors are designed to operate from a three volt power supply. However, a miniature battery with a single electrolytic cell has a source voltage which is never substantially more than about 1.6 volts even when the battery is fresh. Consequently, many common CMOS circuits cannot be directly powered from a miniature battery. Generally, low voltage CMOS circuit design requires different design considerations compared with conventional CMOS circuit design.
Hearing aids are one example of an electronic device in which miniaturization of the total device size requires circuit designers to use a single miniature battery. A new generation of so-called “cosmetically-pleasing” miniature hearing aids is extremely popular. These miniature hearing aids include in-the-ear, in-the-canal, and completely in-the-canal hearing aids. Miniature hearing aids are largely or completely unobservable by others. Moreover, the placement of a miniature hearing aid within the ear canal provides several potential performance advantages, such as a reduction in distortion and noise, improved fidelity, and greater user comfort. Miniature hearing aids are one of the fastest growing areas of the hearing aid market.
Miniature hearing aid designers place a great emphasis in circuit design in allocating the use of limited space and battery power to efficient sound amplification. Many circuit designs which would be practical with a source voltage of 2 V to 3 V are not possible to achieve with a 1.1 V battery. For example, common diode protection circuits to guard against improper insertion of the battery are not feasible with miniature hearing aid batteries. Diode protection circuits are commonly used in many CMOS applications to protect against improper battery insertion. A diode typically has a 0.5-to-0.6 Volt turn-on voltage in the forward direction and is substantially non-conducting in the reverse direction up to a reverse-bias breakdown voltage (e.g., ten Volts). A diode protection circuit permits current flow when the battery is inserted with proper polarity but prevents current flow if the battery is incorrectly inserted. However, with a miniature hearing aid the 0.6 V voltage drop created by a diode protection circuit may render the rest of the CMOS circuit inoperable because the net operating voltage (e.g., 1.1 V−0.6 V=0.5 V) at best barely exceeds the threshold voltage of a single CMOS transistor. In the worst case, a diode protection circuit would cause such a severe voltage drop that the net operating voltage would be below the threshold voltage of a single transistor, making it impossible to design functional transistor circuits directly powered from the battery.
Low-voltage CMOS circuit design involves numerous tradeoffs related to the fact that the available supply voltage is too low to drive MOSFET transistors well into the saturation region. MOSFET transistor switches can only be driven into a high-conductivity ohmic regime if the effective supply voltage is significantly higher than the threshold voltage. As is well known, MOSFET transistors have several distinct operating regimes. There is a linear regime, corresponding to low drain-source voltages and/or low gate voltages. The drain current in the linear regime is commonly expressed by the mathematical equation: I
d
=2 k[(V
GS
−V
T
)V
DS
−0.5 V
DS
2
], where I
d
is the drain current, V
GS
is the gate source voltage, V
T
is the threshold voltage, V
DS
is the drain-source voltage, and k is a constant. In the saturation regime, generally corresponding to higher drain-source voltages and gate-source voltages, the drain current is expressed by the mathematical equation: I
d
=k[(V
GS
−V
T
)
2
]. This saturation regime is also sometimes referred to as an ohmic region. Generally, it is difficult to drive a CMOS transistor into the ohmic region with a voltage source of less than about two volts. In particular, a gate-source voltage less than 1.1 volts is not consistent with a strong enhancement mode of operation because the quantity V
GS
−V
T
is too small.
Circuit designers designing CMOS circuits powered by 1.1 V hearing aid batteries must substantially redesign traditional CMOS circuits to minimize voltage drops between the battery and power amplification circuits. Low voltage CMOS transistors typically have a threshold voltage of approximately 0.4 to 0.6 volts. The drain-source voltage is preferably greater than 100 mV above the threshold voltage to achieve a strongly inverted channel. Consequently, it is difficult to operate more than two strongly inverted transistors in series with a 1.1 V battery because the available battery voltage (1.1 V) is just barely enough to bias two transistors to the strongly inverted channel regime (because 2×0.5 V=1.0 V). In some cases, there is insufficient voltage to drive even two transistors in series to the strongly inverted channel regime. Generally, for a low battery voltage it is difficult to achieve a large enough gate-source voltage to switch MOSFET transistors from fully-on to fully-off states.
Miniature hearing aids are also expensive, typically costing up to two-thousand dollars, which limits their widespread use. Part of the high cost of state-of-the-art miniature hearing aids is a result of the fact that the various components of typical behind-the-ear hearing aid circuits, such as input filters, analog amplifiers, digital electronic elements, filter circuits, and power supply protection circuits, cannot be straightforwardly integrated together because of fundamental incompatibilities (e.g., size limitations, voltage drops, and the problem of achieving a self-consistent (CMOS design architecture). As a result, hybrid manufacturing processes are typically used to combine the function of different chips and discrete circuit elements together. Hybrid techniques, such as wirebonding or soldering different discrete components such as discrete capacitors, filters, amplifiers, and power supply protection circuits together requires labor intensive and space consuming procedures

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