Reference Section
•
69
Chapter 5
A simplified description of the fast and slow compensation circuitry is shown in Figure
4.6. When the pipette command potential (V
p
) changes, current I
p
flows into C
p
to
charge it to the new potential. If no compensation is used, I
p
is supplied by the
feedback element (R
f
) resulting in a large transient signal on the output (I). By
properly setting the fast and slow magnitude and
τ
controls, a current (I
C1
) can be
induced in capacitor C1 (connected to the headstage input) to exactly equal I
p
. In this
case R
f
needs to supply no current and there is no transient on the output.
Whole-Cell Capacitance Compensation
When in whole-cell mode, a voltage-clamp step must charge not only the electrode
capacitance, but also the capacitance of the cell (C
m
). The decay time constant of the
whole-cell capacitance transient is determined by the product of C
m
and the resistance
in series (R
s
) with C
m
. If R
s
and C
m
are both reasonably large, the resultant capacitance
transient can last for several milliseconds, perhaps distorting the rising phase of
biologically interesting currents. Furthermore, as in the case of the electrode
capacitance transient, the whole-cell transient may saturate the circuitry of the
MultiClamp 700A or downstream instruments if left uncompensated. Finally, whole-
cell capacitance compensation is necessary for series resistance compensation. For all
of these reasons, it is desirable to electronically compensate the capacitance of the cell.
Like electrode capacitance compensation, whole-cell compensation uses a circuit to
inject current directly into the input of the headstage. Figure 4.7 shows a simplified
schematic of this circuit.
