Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – Unwanted signal suppression
Reexamination Certificate
1999-11-22
2001-10-30
Le, Dinh T. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
Unwanted signal suppression
C327S362000, C327S553000, C327S103000, C607S028000
Reexamination Certificate
active
06310512
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to an improved G
m
cell circuit useful for realizing a continuous time band pass filter and in particular to a method and apparatus for realizing a continuous band pass filter based upon a transconductance G
m
cell with bipolar transistors that is useful in an impedance sensor in biomedical applications.
BACKGROUND
The human body has electrical characteristics which can be measured for characterizing organ function and for the application of different therapies. For instance, the heart is a complex network of nerve and muscle tissue which operates in synchrony to pump blood throughout the body. Cardiac function may be monitored by sensing the electrical signals naturally conducted at certain places in the heart.
Sometimes it is convenient to apply signals to the body to determine the function of organs of the body. One way to apply signals is to use an implanted series of electrodes which apply a known current and measure the resulting voltage. The relationship between applied current and measured voltage is known as impedance. Thus, impedance is measured by injecting a known current using electrodes and monitoring the electrical voltage required to pass the known current between electrodes. The higher the magnitude of impedance, the higher the magnitude of voltage measured across the load for a known current magnitude.
If the electrodes are placed such that the impedance is measured across a right ventricular portion of the heart, then the impedance measured is a function of the stroke of the right ventricle. The stroke volume of the right ventricle provides a measure of the blood volume pumped by the heart into the lungs in one stroke.
The change in impedance is due to the conductive nature of blood and its changing volume in the left ventricle between contractions. The measured impedance will vary depending on the placement of the electrodes. For example, as shown in FIG.
1
A and
FIG. 1B
, if a current is conducted between the housing of an implantable device
12
and a tip electrode
13
on the end of a catheter
14
with the tip electrode
13
positioned in the apex of the right ventricle
15
, then the impedance observed between two electrodes,
16
and
17
, located within the right ventricle (and before the tip electrode
13
) will measure an increased impedance for a contracted ventricle (systole—
FIG. 1B
) as opposed to when the ventricle is not contracted (diastole—FIG.
1
A). This is because in diastole, the ventricle is holding more blood and has more conductive volume to transfer current. In systole, the ventricle is contracted and has less blood, leaving less volume for conduction.
A system for indicating the stroke volume of the heart by tracking the impedance changes of the ventricle through contractions is shown in block diagram form in
FIG. 2 through 4
of my commonly assigned copending patent application entitled System for Processing Bursted Amplitude Modulated Signals Using an Impedance Sensor, Ser. No. 09/297,004 filed Feb. 8, 1999. In that application there is shown a low power processing system for processing bursted amplitude modulated signals by performing impedance-related measurements across a load. The system operates by injecting current pulses of constant amplitude across the load using at least a first electrode and a second electrode, the current pulses including bursts of a plurality of pulses at a pulse frequency at which the current pulses are repeated, the bursts transmitted at a burst frequency. It includes detecting voltages across at least a third electrode and a fourth electrode; high pass filtering the voltages to produce filtered voltages; amplifying the filtered voltages to produce amplified voltage signals. It also includes bandpass filtering the amplified voltage signals with a bandpass filter with a center frequency equal to approximately the pulse frequency to generate first filtered signals; rectifying the first filtered signals to produce rectified signals; integrating the rectified signals to produce integrated signals; sampling-and-holding the integrated signals after each burst to capture an integrated pulse value for each burst, creating a plurality of discrete integrated pulse values. It also includes further bandpass filtering of the plurality of discrete integrated pulse values using a filter including an upper cutoff frequency less than the burst frequency to produce the output related to the time-varying impedance of the load.
In the system shown in my prior application referred to above, the realization approach used in the first bandpass filter
42
is continuous time filtering. Because the continuous time filter technique does not use a sampling clock, it is able to process a high frequency signal.
A potential disadvantage for utilization of this type of filter circuit may be the need for tuning circuitry. Tuning may be required because the filter coefficients are determined as a product of two dissimilar elements such as capacitors and resistors (or transconductors). Although the variation of values of capacitors in integrated circuits is small, in the order of ±5% , the variation in resistors may be ±50%. Another characteristic of continuous time filters is the presence of flicker noise and poor linearity. All of these characteristics are addressed by the present invention which provides an improved realization of a transconductance gain cell that is particularly adapted for use in a bandpass filter realized from a continuous time filter.
Thus there is a need in the art for a self-adjustable continuous time band pass filter with a transconductance cell having bipolar transistors and a self adjusting bias circuit to stabilize the overall transconductance of the transconductance cell.
SUMMARY OF THE INVENTION
Those skilled in the art, upon reading and understanding the present specification, will appreciate that the present self adjustable continuous time bandpass based upon an improved transconductance gain cell satisfies the aforementioned needs in the art and several other needs not expressly mentioned herein. An integrated gain-cell differential transconductor having an overall transconductance, G
m
, is provided. The transconductor cell has a fixed transconductor portion coupled to a translinear gain cell. The fixed transconductor has at least one internal bias current I and is characterized by a transconductance determined by the reciprocal of the magnitude of a first linearizing resistor R
G1
. The circuit also has a translinear gain cell operatively coupled to the output of the fixed transconductor and having at least one internal bias current I
2
, the gain multiple of the translinear gain cell being determined by I
2/
I
1
and the overall transconductance G
m
of the integrated gain-cell transconductor being 1/R
G1*
I
2,
/I
1
. The circuit has variable bias current supply operatively coupled to the fixed transconductor which produces a bias control signal from a second resistor R
G2
which replicates R
G1
and varies bias current I
1
of the fixed transconductance portion in inverse proportion to the variation of R
G1
, thereby compensating G
m
for variations in R
G1
.
In one application, the fixed transconductor has a differential pair of input transistors and the linearizing resistor is comprised of a pair of resistors, each resistor having a resistance R
G1
/2 which is connected in series with an emitter of each of the differential pair of input transistors of the fixed transconductor.
In another application the fixed transconductor has a differential pair of input transistors and the linearizing resistor has a resistance R
G1
and is connected between the emitters of the differential pair of input transistors of the fixed transconductor portion.
In one application a bandpass filter is realized using a plurality of transconductor gain cells, a plurality of capacitors connected to the outputs of at least some of the plurality of gain cells, and where the gain cells and the capacitors are connected for realizing a second order f
Briskin Boris
Linder William J.
Cardiac Pacemakers Inc.
Le Dinh T.
Schwegman Lundberg Woessner & Kluth P.A.
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