Introduction.
This assignment will cover some of the ways you can build circuits using operational amplifiers. The emphisis in this lab will be on practical circuits used in neurobiology.
Download the file opamp2.ewb by right-clicking over the link and saving it to your directory. There are several separate circuits in this file. Each has a specific use in biological electronics. The schematics are split into two parts: part 1, part 2.
Play with the circuits in the file to see how they work. Vary the controls or substitute different component values. Note that the electrometer amplifier will oscillate (just like a real one) if the negative capacitance feedback is set too high. For the filters shown, vary the frequency and observe the response. Be sure to connect the Bode-plotter to the filters to plot their frequency response.
All circuits will be built using LM358 operational amplifiers except the electrometer amplifier, which will use a higher input resistance operational amplifier.
Note that in this and every lab assignment the verb build means to construct a circuit on the protoype board whith actual parts. The verb simulate means use Electronics Workbench mathematically simulate the circuit.
The differential amplifier shown in opamp2.ewb is the basic
circuit used in almost all physiological a.c. preampllifiers used
to record extracellular events. Simulate the circuit given. Measure
the gain of the differential amplifier for the common mode 60 Hz
noise and for differential mode 100 Hz sources. The ratio of the two gains
is called the common mode rejection. Now build the differential
amplifier and measure the differential gain, common mode gain and common
mode rejection of your circuit at about 60 Hz and about 1000 Hz. At what
frequency does the gain start to roll off. Put differently, what is the
lowpass frequency of your circuit?
opamp2.ewb depends on having
well matched resistor and capacitor values for high attenuation. Simulate
the effect of a 5% variation in the 100k resistors by measuring the attenuation
at 60 Hz relative to, say, 600 Hz. Build a notch filter and measure
its attenuation, then disassemble the circuit and measure the actual values
of each component.
opamp2.ewb is a simplified
version of a common circuit used in intracellular recording amplifiers. This
head stage has a high input resistance relative to the electrode resistance
and supplies a gain of 10. The large time constant of the electrode is reduced
by a small amount of positive feedback through the 5 pf capacitor. Simulate
the frequency response and rise time of the amplifer as you vary the negative
capacitance feedback control. Build the circuit and measure its rise
time as a function of the negative capacitance feedback control.
Your lab notes should include the results of the simulations and the actual circuits you built.