TEC-B-01 User Manual
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version 1.1
page 27
the "large" time constant determines the maximum gain which can be achieved without oscillations
and thus, the accuracy of the clamp. With the gain adjusted to this level the integrator time constant
and "small" time constant determine the speed of response of the system.
The clamp performance can be increased considerably if the influence of the current injecting
electrode is excluded as far as possible from the clamp loop since the electrode resistance is nonlinear.
This is achieved if the output of the clamp system is a current source rather than a voltage source. In
this case the clamp transfer function has the magnitude of a conductance (A/V). Other advantages of
this arrangement are that the clamp current can be determined by a differential amplifier (with no need
of virtual ground, see Greeff and Polder, 1997; Polder and Houamed, 1994) and that the bandwidth of
the feedback system can be altered easily (e.g. for noise suppression during simultaneous patch clamp
recordings, see Stühmer, 1992; Stühmer et al. 1992; Stühmer and Parekh, 1995).
This output circuit is equipped with large bandwidth high voltage operational amplifiers. To avoid
deterioration of clamp performance caused by electrode overload the output current has to be limited
by an electronic circuit to a safe level. With electrodes in the range of one M
and a voltage of ±150
V, the maximum current will be 150 µA. With this current a cell with a capacity of 100 nF can be
depolarized by 100 mV in approximately 100 µs, which comes close to the theoretically possible speed
of response, without any detectable deviations from the command level. With an output compliance of
225 V and a x2 or x5 range current injecting headstage, currents up to 500 µA can be injected (see
Greeff and Polder, 1997; Polder and Houamed, 1994).
The accuracy of a two electrode clamp system and the speed of response is determined by the cell
capacity, the resistance of the current injecting microelectrode (that limits the maximum amount of
injected current) and the equivalent time constant and accuracy of the potential recording and feedback
electronic systems. Therefore, the design of the potential recording site is very important. A
differential potential registration with a reference electrode that registers the bath potential minimizes
errors due to resistances outside the cell in series with the cell membrane. Driven shield and capacity
compensation circuits are used to improve the speed of response.
In some cases, a series resistance compensation circuit (for series resistance inside the cell) which adds
a current proportional gain can improve the clamp performance considerably (Greeff and Polder, 1997;
Greeff, 2000; Greeff and Kühn, 2000). The use of such a circuit enhances the speed of response and
improves the accuracy of the clamp system. But the noise level is also increased because both circuits
are positive feedback loops.
In addition to the elements of the clamp loop itself, this oocyte clamp amplifier has some additional
units that facilitate experiments such as electrode resistance test units, oscillation shut-off unit,
adequate output signal amplification, filtering and display units, facility for compensating capacitive
currents, etc.
