PRINCIPLES OF OPERATION
•
79
Chapter 8
In fact, despite the slow current output of the I-V converter, the voltage clamp of
the pipette is rapid. The pipette is connected to the negative input (summing
junction) of the op amp. The command potential is connected to the positive input
of the op amp. The operation of the op amp in this configuration is to force the
potential at the summing junction to rapidly follow the potential at the positive
input. If the command potential is a step, the potential at the summing junction,
and hence the pipette, is also a step. The current required to achieve this step is
passed through the feedback resistor (R
f
) and the associated stray feedback
capacitance (C
Rf
) of the I-V converter. The output of the I-V converter settles to
the final value with time constant R
f
C
Rf
. This relatively slow settling occurs
despite the fact that the step at the summing junction is fast.
In this discussion, we have carefully referred to the fact that it is the "pipette" that
is rapidly voltage clamped. The membrane potential is voltage clamped to its final
value much more slowly. To a reasonable approximation, the time constant for
voltage clamping the membrane is R
p
C
m
, where R
p
is the pipette resistance and C
m
is the membrane capacitance.
What is Clamped During Voltage Clamping?
Voltage clamping is the intrinsic mode of operation of a patch clamp headstage.
The series combination of the pipette and the patch/cell membrane is voltage
clamped — its voltage remains constant at a user-specified value (V
c
), assuming
that the membrane potential equals the command potential. This is true only if the
current causes a negligible voltage drop across the pipette resistance.
Capacitance Compensation
Pipette Capacitance Compensation
The FAST and SLOW PIPETTE CAPACITANCE COMPENSATION controls are
used to charge the pipette capacitance (C
p
) during a voltage step. A simplified
circuit of the fast and slow compensation circuitry is shown in the Figure 18.
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