PRINCIPLES OF OPERATION
•
95
Chapter 8
to help the membrane to charge faster, but to keep the succeeding circuitry
(differentiator, gain blocks, etc.) from saturating and erroneously indicating a
slowly charging membrane.
If you are not sure whether you need to apply an external signal, it is best to just
include it. There is no conflict in having a reset initiated by both an external and
internal signal.
Noise vs. Access Resistance
The Axopatch 200B is quite comfortable with loads of up to 1000 pF of pure
capacitance (the maximum bandwidth decreases to about 20 kHz, no overshoot).
This can be used to great advantage when doing bilayer experiments; the lower the
access resistance, the lower the noise. While there will be some lower limit on the
value of the access resistance, it will not be set by stability criteria of the
instrument.
In bilayer applications, one is typically working with bandwidths below 1 kHz. In
this region, the e
n
C
in
noise has not yet become the major contributor to the overall
noise (where e
n
is the voltage noise of the probe input FETs
1
and C
in
is the
capacitance of the input of the headstage, which is primarily the bilayer membrane
capacitance). However, a resistance in series with the bilayer membrane
capacitance produces voltage noise just as though the headstage had high intrinsic
noise. If this resistance is large enough, then it becomes the major noise
contributor.
Figure 22 shows peak-to-peak noise versus series resistance in a 1 kHz bandwidth,
for a given bilayer membrane capacitance.
1
FET - Field Effect Transistor
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