Reference Section • 81
Chapter 5
Feedback Resistor
• The feedback resistor determines the gain of the headstage in V-Clamp mode and
the amount of current that can be passed in I-Clamp mode.
• The value of this resistor is set in the Gains tab under the Options button (
).
• See also External Command Inputs, Headstage, Noise, Overload.
V-Clamp Mode
In V-Clamp mode, changing the value of the feedback resistor (R
f
) in the headstage
provides a method of changing the gain of the headstage. Choice of the appropriate R
f
involves a tradeoff between two competing factors. (See Chapter 5,
HEADSTAGE, for
a technical discussion of these factors.)
• Larger R
f
means smaller current noise due to the headstage circuitry.
• Smaller R
f
means a larger range of membrane currents can be measured without
saturating the headstage circuitry.
Thus, larger values of R
f
are more suited to patch recordings, where the noise is more
critical and the currents are smaller, whereas smaller values of R
f
are more suited to
whole-cell recordings, with their larger currents. The following table summarizes
these properties for different values of R
f
.
Feedback Resistor Experiment Type Range Noise*
50 MΩ Whole Cell 1-200 nA 3.0 pA rms
500 MΩ Whole Cell 0.1-20 nA 1.4 pA rms
5 GΩ Patch 10-2000 pA 0.9 pA rms
50 GΩ Patch 0.2-200 pA 0.28 pA rms
* Bandwidth 10 kHz using an 8-pole Bessel filter. Noise is measured with the headstage open-circuit;
i.e.
it
represents the best possible intrinsic noise of the headstage circuitry.
Note: V
cmd
is limited to 10 V in the MultiClamp 700A, which in turn limits the
maximum amount of current that can be injected throught the headstage resistor into
the electrode. For example, with R
f
= 500 MΩ, the maximum current that can be
injected is 10 V/500 MΩ = 20 nA. These current limits are listed in the Options/Gains
panel of the MultiClamp Commander.
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