Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Synchronizing
Reexamination Certificate
1999-12-30
2002-09-03
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Synchronizing
C327S134000
Reexamination Certificate
active
06445233
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to low voltage CMOS rectifiers. More particularly, the present invention is directed towards a CMOS rectifier with controlled transient response for use in audio amplifier compression and automatic sound level control circuits.
BACKGROUND OF THE INVENTION
It is desirable in a variety of amplifier applications to have an automatic gain control element that adjusts the gain as a function of input signal level strength. An automatic gain control element may be used by itself or with additional compressor elements to reduce the gain as a function of input signal level strength.
The designers of audio systems describe an “attack” time required for an automatic gain control element to adjust the gain of an audio amplifier in response to a sudden increase in audio signal level and a “release” time for the gain to recover after the audio signal level decreases to a normal or average level. Typically, the attack time is defined as the time that it takes for an audio amplifier to stabilize to within about 1 dB of its final value after a sudden increase in input audio level. The release time is typically defined as the time that it takes for an audio amplifier to decrease to within about 1 dB of its final value after the input audio level has decreased to normal levels. Typically, it is desirable that an audio amplifier have a release time that is much longer than its attack time.
A short attack time is desirable to produce a comfortable reduction in sound level after an abrupt increase in sound level, e.g., the ringing of an alarm bell.
However, an abrupt return to ordinary gain levels when the audio signal level decreases to ordinary levels may sound unnatural to many users. Consequently, it is typically desirable that the release time be much longer than the attack time. As an illustrative example, a preferred attack time for an automatic gain control used in an audio compressor may be less than one millisecond whereas a preferred release time may be ten milliseconds.
More generally, the transient response of an automatic gain control element can also be described in terms of an attack function and a release function. Commonly, the attack and release function is in the form of an exponential response, since a variety of passive circuits comprised of resistors and capacitors may be used to implement an exponential response. For a simple exponential response, the exponential time constant describes the attack/release function.
An automatic gain control element may be used to perform several different functions. One function of an automatic gain control element is as an automatic sound level controller that maintains an average audio output level, or loudness, that is comfortable to the listener. This has the advantage that the user does not need to manually adjust the sound level to achieve a comfortable level of sound amplification as the background sound level varies. Another function of an automatic gain control element is to compress the gain at high input signal levels in order to avoid the generation of distortion which may otherwise occur at such signal levels. As is well-known in the field of amplifier design, amplifiers typically produce a distorted output when the input signal level exceeds a preferred level. For example, when the inputs to an amplifier become too large the output of a “linear” amplifier may become saturated at a constant power-output regardless of further increases in input signal level strength, resulting in non-linear (distorted) amplification. Still another function of an automatic gain control element is as a syllabic compressor. A syllabic compressor automatically compresses sound as a function of amplitude according to a function that is designed to improve the ability of the listener to understand certain sounds, such as distinguishing between consonant and syllabic sounds.
Although a variety of compressors are known in the amplifier art, compressors suitable for miniature hearing aids pose special problems. First, there are severe space limitations in miniature hearing aids. Hearing aid designers are forced by space constraints to severely limit the number of discrete capacitors and resistors in the total hearing aid circuit, since discrete components greatly exacerbate the problems of fitting all of the hearing aid components into a unit sized to fit partially or totally in the ear canal. Thus, an audio compressor for use in a miniature hearing aid should utilize a minimum number of discrete capacitors and resistors. Second, there are severe power supply limitations. Miniature hearing aids are powered by a single miniature hearing aid battery with a nominal voltage of less than 1.5 volts. Consequently, an audio compressor circuit for use in a hearing aid should be consistent with low voltage operation. Third, cost is an important consideration for the low-cost hearing aid market. A hearing aid integrated circuit, including one or more compressors, should be compact in order to reduce the per-unit cost.
Hearing aids are one of the most common examples of miniature audio devices in which it is desirable to incorporate automatic gain control elements, typically as audio compressors. However, a wide variety of other miniature audio devices, such as cellular phones and micro-recorders, may also beneficially use automatic gain control elements as syllabic compressors, gain compressors, or as a sound level controllers to improve the sound quality of transmitted or recorded speech. There is a general need for a miniature control circuit that may be used in combination with a voltage-controlled amplifier as part of a compressor with selectable attack/rise characteristics.
Conventional audio compressors used in hearing aids commonly utilize signal characterization elements requiring at least two discrete capacitors and one discrete resistor to implement an attack/release function in a compressor. For example, U.S. Pat. No. 4,718,099 discloses a voltage controlled compression amplifier with controlled attack and release time constants designed for use as a compressor in a hearing aid. As shown in the prior art block circuit diagram of
FIG. 1
, a voltage controlled amplifier
10
has an output
12
that may be fed into other amplifier elements
26
,
28
that are suitably coupled by connections
27
,
29
to a hearing aid receiver. The gain of voltage controlled amplifier
10
is further regulated by a gain control terminal
13
coupled
15
to a gain control voltage source
14
. Gain control voltage source
14
includes a Shotcky diode (not shown in
FIG. 1
) to produce a rectified current
16
when the output
12
exceeds a predetermined threshold level. The rectified current
16
of gain control voltage source
14
is coupled to node
24
of a signal characterizer
20
. The function of signal characterizer
20
is to convert an input signal
16
into a form suitable for regulating the voltage at gain control terminal
13
, i.e., to decrease the gain of amplifier
10
with an appropriate attack time in response to an abrupt increase in amplifier output
12
and to restore the gain of amplifier
10
with an appropriate release time when the amplifier output
12
decreases to normal levels. Signal characterizer
20
comprises a first capacitor
21
in parallel with a series connection of resistor
23
and second capacitor
22
. The signal characterizer
20
has a short attack time corresponding to the equivalent RC turn on time of first capacitor
24
and a a slow release time corresponding to the equivalent RC turn-off time of a second capacitor
22
and its associated resistor
23
.
One drawback of the automatic gain control circuit of U.S. Pat. No. 4,718,099 is that signal characterizer
20
requires comparatively large discrete resistors and discrete capacitors in order to achieve RC time constants of capacitors
21
,
22
consistent with attack and release times on the order of microseconds. For example, in a preferred embodiment of U.S. Pat. No. 4,718,099, a first time constant means comprise
Becker Anthony J.
Brown Clyde “Kip” M.
Pinai Hoang Minh
Coudert Brothers LLP
Cunningham Terry D.
Dinh Paul
The Engineering Consortium, Inc.
Wright Jerry G.
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