Reference Section • 91
Chapter 5
What is Clamped During Voltage Clamping?
We were careful to state in the above discussion that it is only the back of the
electrode that is voltage clamped, not the cell membrane. The voltage at the
cell membrane may differ from that at the back of the electrode because of
bandwidth and voltage errors due to uncompensated series resistance (R
s
). For
this reason, it is always important to consider using Rs compensation. (See
Chapter 5,
SERIES RESISTANCE COMPENSATION.)
Intrinsic Headstage Noise
The intrinsic noise of a resistive-feedback I-V converter (i.e. with an open-
circuit input) is determined, in theory, by the resistance of the feedback
resistor. The rms current noise is given approximately by
I
rms
≈ √(4kTf
c
/R
f
)
where fc is the filter cutoff frequency and k and T are constants. Thus, for low
noise, a high value of R
f
is desirable. This was pointed out in Chapter 5,
FEEDBACK RESISTOR.
Current Clamp Circuit
In I-Clamp mode a separate headstage circuit is used, called a voltage follower. The
essential features of a voltage follower are shown in Figure 4.12. A1 is an (effectively)
infinite input resistance, unity-gain op amp, the output of which is the pipette voltage,
V
p
. A2 is a summing amplifier used for injecting current into the cell. The voltage
across the headstage resistor R
f
is equal to V
cmd
regardless of V
p
. Thus the current
through R
f
is given exactly by I = V
cmd
/ R
f
. If stray capacitances are ignored, all of this
current is injected into the cell.
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