90
•
Reference Section
MultiClamp 700A Theory and Operation, Copyright 2000, 2001 Axon Instruments, Inc.
•
An op amp has infinite input resistance. Therefore, the current flowing out of the
electrode (I
e
) must equal the current (I
f
) flowing through the feedback resistor (R
f
)
because no current is allowed to flow into the ‘–’ input of the op amp.
•
An op amp does all it can to keep the voltage at its two inputs equal. Thus,
because the voltage at the ‘+’ input is V
p
(= the command voltage), the voltage at
its ‘–’ input is also V
p
. The voltage across R
f
must therefore be V
p
– V
o
= I
f
·R
f
by
Ohm’s Law.
Combining both of these pieces of information, the electrode current (which is what we
want) is given by I
e
= I
f
= (V
p
– V
o
)/R
f
. In practice R
f
is a very large resistor (G
Ω
) so
this circuit can measure very small currents (pA). The differential amplifier in the
BOOST CIRCUIT does this calculation of I
e
. Subsequent amplifiers are used to scale
the gain and remove voltage offsets.
High Frequency Boost
A fundamental problem of this circuit when used for patch clamping is that the
output bandwidth of the probe is inherently low. To a first approximation, the
product of R
f
and the stray capacitance sets the bandwidth across it. For
example, if R
f
is 500 M
Ω
and the stray capacitance is 0.5 pF, the bandwidth is
about 600 Hz. To overcome this limitation, the probe output is passed through
a high-frequency boost circuit. The gain of this circuit is proportional to the
frequency.
The high-frequency boost is applied to the output of the I-V converter and
cannot influence the events at the electrode. Thus, one might conclude that the
voltage clamp of the electrode must also be slow. This is not the case, for the
following reason. The PROBE op amp does everything it can to keep the
voltage at its ‘–’ input equal to the command voltage at its ‘+’ input. If the
command is a rapid step, then the voltage at the ‘–’ input (
i.e.
at the back of the
electrode) is also a rapid step. This means the voltage clamp of the electrode is
fast. The RC filtering effect mentioned above applies only to the
output
of the
I-V converter, which can therefore be subjected to
post hoc
boosting.
