Amplifiers – With semiconductor amplifying device – Including signal feedback means
Reexamination Certificate
2002-05-10
2003-08-26
Choe, Henry (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including signal feedback means
C330S294000, C330S085000
Reexamination Certificate
active
06611174
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of the Invention
This invention relates to the measurement of small currents using a transimpedance amplifier. The present invention teaches a method to create a transimpedance amplifier that has wide bandwidth and very high linearity.
2. Description of the Prior Art
The IV Converter
FIG. 1
shows a common transimpedance amplifier circuit used to measure small currents know as an IV converter. Referring to
FIG. 1
, an IV converter
10
consists of an operational amplifier U
1
, a feedback resistor R
o
, and a feedback capacitance C
f
U
1
has a non-inverting terminal V+, an inverting terminal V−, and an output terminal Vout. R
o
is connected between Vout and V− so that R
o
provides negative feedback around U
1
. C
f
is connected in parallels with R
o
and represents the sum of the stray capacitance across R
o
together with any additional feedback capacitance that parallels R
o
. An input current Isrc
5
that is to be measured is connected to V−. Thus V− serves as the transimpedance input. V+ is connected to ground. Since U
1
has a very large open loop gain, negative feedback forces V− to act as a low impedance virtual ground, and all of Isrc
5
flows across R
o
. IV converter
10
produces an output voltage Vout given by:
V
out
=−I
src
R
o
(E1)
when C
f
is small enough to be ignored. Thus Vout serves as the transimpedance output. Therefore IV converter
10
measures an input current by producing an output voltage proportional to the input current. The constant of proportionality is called the transimpedance, and IV converter
10
the transimpedance is equal to the feedback resistance R
o
.
An important property of IV converter
10
is the low input impedance it presents to Isrc
5
. This low input impedance isolates Isrc
5
from voltage changes that would otherwise occur due to changing input current. An application requiring such a low input impedance is the linear measurement of photodiode current. Most photodiodes exhibit a nonlinear response with changes in bias voltage. The low input impedance of IV converter
10
ensures that the photodiode bias voltage remains constant, regardless of the value of photodiode current flowing. Another application requiring low input impedance is voltage clamping ionic currents in biological preparations. The low input impedance of IV converter
10
enables it to be conveniently used as biological voltage clamp (see
Electronic Design of the Patch Clamp
by F. J. Sigworth, 1983, found in
Single
-
Channel Recording
, edited by B. Sakmann and E. Neher, 1983). When used as a biological voltage clamp, V+ of U
1
is driven by a command voltage instead of being grounded as in
FIG. 1
, and Isrc
5
is commonly a single cell under patch clamp conditions. The low input impedance of IV converter
10
ensures the cell is voltage clamped at the command voltage.
Signal-to-noise Ratio and Bandwidth of the IV Converter
As with any amplifier, IV converter
10
has an inherent signal-to-noise ratio (SNR) that limits the minimum size input signal that can be detected; to measure small currents it is necessary to maximize the SNR. The dominant noise source that degrades the SNR of IV converter
10
is the thermal Johnson noise of the feedback resistor R
o
. The root-mean-square SNR for a given input signal Isrc in the presence of the thermal noise of R
o
is given by
SNR
rms
=
I
src
c
1
R
o
R
o
(
E2
)
where c
1
is a function of the measurement bandwidth and temperature.
As can be seen from Eq. E2, larger values of R
o
improve the SNR since the numerator of Eq. E2 grows faster than the denominator as R
o
is increased. When IV converter
10
is used to measure small currents such as those produced by a photodiode or a biological preparation, large values of R
o
must be used to ensure adequate SNR; typical values of R
o
range from tens of Megaohms to GigaOhms. When such high R
o
values are used, the effects of C
f
can no longer be ignored. These effects are quantified by the transfer function of Isrc
5
to Vout, given by
V
out
I
src
=
-
R
o
R
o
C
f
s
+
1
(
E3
)
where s is the Laplace transform frequency variable. As shown in Eq. E3, C
f
in parallel with R
o
combine to create a pole with time constant &tgr;
uncomp
=R
o
C
f
which rolls off the frequency response. When R
o
is large, &tgr;
uncomp
becomes large enough to degrade the measurement bandwidth. While &tgr;
uncomp
can be decreased by physically lowering C
f
, stability requirements preclude lowering C
f
much below 5 pF. To ensure stability of IV converter
10
, it is generally necessary that C
f
be on the order of the input capacitance at the inverting terminal V− to avoid gain peaking (see Designing Photodiode Amplifier Circuits with OPA 128, Burr Brown Application Bulletin, 2000) Therefore, IV converter
10
usually has a signal bandwidth limit of ~1kH or less when measuring small currents, which is undesirable.
Prior Art Techniques to Increase Bandwidth
In order to increase the bandwidth of IV converter
10
, it is common practice to use a post-amplification equalizer, as shown in FIG.
1
. Referring to
FIG. 1
, a post-amplification equalizer
20
takes as input Vout and produces an output voltage Vout_equ
25
. Equalizer
20
serves to cancel the pole in equation E3 by introducing a real left-hand plane zero with time constant &tgr;
zero
=&tgr;
uncomp
, In this way, equalizer
20
increases the output bandwidth of IV converter
10
.
While equalizer
20
does increase bandwidth, it has several practical limitations stemming from the need to achieve and then to maintain accurate pole-zero cancellation. To achieve cancellation, equalizer
20
must be manually tuned for each IV converter produced because &tgr;
uncomp
varies with the parameter spread of R
o
and C
f
. This tuning adds significant expense to large volume productions. More significantly, maintaining accurate cancellation is difficult since &tgr;
uncomp
and &tgr;
zero
drift unequally with changes in ambient temperature and humidity. The result of incomplete cancellation is the introduction of nonlinearities in the frequency response of IV converter
10
within the signal measurement bandwidth. Even with stable components, the accuracy of cancellation is degraded at higher frequencies owing to the well-know difficulties in making broadband analog equalizers.
A linear frequency response of IV converter
10
is especially important when voltage clamping ionic currents from single cell preparations using the whole-cell patch clamp technique. Under these conditions Vout is used as positive feedback to compensate for the high impedance of the measuring electrode using a technique called series resistance compensation (see
Sigworth
pages 28 to 32). As outlined by Sigworth, nonlinearities in the frequency response of IV converter
10
destabilizes series resistance compensation. Linear frequency response is also required when implementing series resistance compensation using a membrane state estimator, as taught by Sherman in U.S. Pat. No. 6,163,719 (2000). (See also Sherman et. al. 1999. Series Resistance Compensation for Whole-Cell Patch-Clamp Studies Using a Membrane State Estimator,
Biophys. J
. 77:2590-2601.). Consequently, it is extremely difficult to implement wideband series resistance compensation when using an IV converter equipped with a post-amplification equalizer.
High Input Impedance Transimpedance Amplifiers
In order to increase bandwidth, other transimpedance amplifier architectures are used which, unlike the IV converter, present a relatively high input impedance to the input current source. Rodgers (U.S. Pat. No. 5,982,232 (1999)) teaches a technique wherein the bandwidth of a high input impedance transimpedance amplifier is increased by using positive capacitive feedback to reduce the effects of input capacitance loading. While high bandwidths can be achieved using this technique, the high input impedance precludes its use in the many applications that require a low
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