TEC-B-01 User Manual
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version 1.1
page 31
9.3.
Speed of Response and Linearity of the Capacitive Transients
For the investigation of voltage activated channels with voltage clamp instruments, some special
techniques for eliminating the capacitive and leak currents have been introduced, such as the P/4 or
more general P/N protocol (see Rudy and Iverson, 1992 for overview). For these protocols the speed
and linearity of response of the clamp system is of great importance.
As outlined in chapter 9.1 the TEC systems are designed following a control theory procedure called
"modulus hugging" (see Froehr, 1985; Polder, 1984;Polder and Swandulla, 1990, Polder and
Swandulla, 2001). The procedure requires a PI (proportional-integral) controller. This procedure is
applicable to control systems composed of an element with one "large" time constant T
m
and many
"small" time constants T
i
. These "small" time constants can be added to an "equivalent" time constant
T
e
.
In case of the TEC control chain the “large” time constant is formed by the time constant of the cell
membrane (several hundred of milliseconds) and the sum of “small” time constants resulting from the
microelectrodes and the electronics (a few ten microseconds).
Note
: Here only the proportional part of the PI controller is considered. Possible improvement of
clamp performance due to series resistance compensation (see Ogden, 1994; Smith et al., 1990, Greeff,
2000; Greeff and Kühn, 2000 for details) is not considered.
General Considerations
For the TEC systems the "small" time constants are at least two orders of magnitude below the "large"
time constant: The "large" time constant is the time constant of the membrane and the equivalent time
constant is composed of the time constants of the electrodes, amplifiers etc.
T
m
= R
m
* C
m
, T
e
=
T
i
with
T
m
= “large time constant
R
m
= membrane resistance
C
m
= membrane capacity
T
e
= “equivalent” time constant
T
i
= “small” time constant
The performance of a clamp system can be improved if a voltage controlled current source is used for
the current injecting electrode. In this case, the very large time constant (hundreds of milliseconds)
formed by the electrode resistance and the cell capacity can be ignored, because the output of the
clamp circuit is a current that flows regardless of the resistance of the injecting microelectrode (Smith
et al., 1990). Thus, the performance of the clamp is no longer dependent on the electrode resistance (as
long as the current source is not saturated). In this case the clamp gain has the magnitude of a
conductance [A/V].
