Biological Sciences 300/301, Smith College | Neurophysiology

Checklist for Lab 2: Circuits and Amplifiers

UPDATED: February 5, 2007

How this lab
is structured

Lab 2 is structured as a series of "trios" with three components: a concept, an experiment that illustrates the concept, and an application in neurophysiology.

As you prepare for this lab, try to understand each concept. When you are in lab, make sure that you understand why the experiment works as it does. Don't just generate numbers -- ask yourself why the numbers make sense. This is your chance to ask questions (of yourself, your lab partner, and the instructor). The goal is to develop solid intuitions about electrical resistance and capacitance.

1. Ohm's Law

Concept: In a circuit, the total voltage divides up across a series of resistors.

Experimental data:

measure voltage:

volts (measured)

current (calculated)

across both resistors together
(= across the battery)



across the small resistor



across the large resistor



Application: amplifiers for microelectrodes must have a very high resistance.

Problem: Estimate the proportion of the "real" potential that will be lost across a 10-megohm electrode if the amplifier's resistance is 1012 ohms.

2. Resistors

Concept: Capacitors hold charges. The rate at which they "fill up" depends on the magnitude of the voltage pushing in charges, and how much resistance the charges meet as they are pushed through the circuit. You can observe a capacitor charging by watching the voltage across the capacitor or by watching the current flow through the resistor.

Experiment 1a: Observe charging and discharging of a capacitor by recording the voltage across the capacitor. Then measure the capacitor's time constant by charging and discharging with two different resistor to limit the rate of charge movement. Use T = R C to calculate C.

resistor in circuit

time to charge 66%

capacitance (calculated)

1000 ohms



2200 ohms



Application: The cell membrane and cytoplasm have capacitance and resistance. Pulses of charges injected into cells act as though they are charging a capacitor.

Experiment 1b: Observe the current during the capacitor's charging by recording the voltage across the resistor.

Application: Amplifiers with coupling capacitors (like the oscilloscope on its AC setting) show changes in voltage but not steady-state values.

Experiment 2: Observe the effect of AC coupling in the oscilloscope itself.

3. Preamplifiers

Concepts: very large amplification, differential input for noise reduction, filtering high and low frequencies to "clean up" the signal.

Experiment: Measure the amplification of the preamplifer by comparing the signal going in and the signal coming out. Observe the effects of filtering out high frequencies.

Application: The signal that emerges from the preamplifier will depend heavily on how the filters are set.