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2034 J. Opt. Soc. Am. A/Vol. 22, No. 10 /October 2005 Albrecht et al.

Visual neurophysiology: a field-effect amplifierdesigned and built by R. L. De Valois

Duane G. Albrecht, Carl P. Creeger, and Alison M. Crane

Department of Psychology, Center for Perceptual Systems, University of Texas, Austin, Texas 78712

Received January 21, 2005; revised manuscript received April 11, 2005; accepted April 14, 2005

In the middle of the last century, R. L. De Valois designed and built a unique and effective amplifier based onthe newly developed field-effect transistor (FET). This amplifier has many beneficial qualities for amplifyingthe signals of neurons with minimal disturbance. We have used this amplifier successfully for more than threedecades. We describe the circuitry of the De Valois amplifier and provide performance specifications. The FETamplifier is one of De Valois’s contributions to visual neurophysiology; we share the design in his honor, withthe hope that it might prove useful to others. © 2005 Optical Society of America

OCIS codes: 330, 4270, 330.5380.

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. INTRODUCTIONt this point in time, the beginning of the 21st century,ommercial amplifiers that are specifically designed foringle-neuron electrophysiology are readily available.owever, in the middle of the last century, when the earlyioneers of visual neuroscience (R. L. De Valois andthers)1–4 were beginning to measure the responses ofeurons in the visual system (the retina, lateral genicu-

ate nucleus, and visual cortex), reliable commercial am-lifiers for this specific purpose were not readily avail-ble. As a consequence, each laboratory generallyesigned and built unique amplifiers. De Valois took ad-antage of a newly developed transistor, a field-effectransistor (FET), to design and construct a very effectivemplifier. The design is simple. The device is compact (itts in the palm of the hand); it is battery powered

thereby avoiding the introduction of 60 Hz within thearaday cage); it is relatively easy to construct (one needot be an electrical engineer); and the components are in-xpensive and easy to obtain (from most electronic com-onent sources). Most important of all, this FET amplifierrovides excellent signal-to-noise amplification of theery small signals from individual neurons while drawingery little current from the neuronal electrochemicalvents. Further, because the FET isolates the neuronrom the high-gain amplification, it prevents the amplifierrom affecting the behavior of the neuron.

We have been successfully using this FET amplifier,ith high-impedance electrodes �5–20 M��, for nearly

hree decades.5,6 We have also purchased many of theommercially available amplifiers. In our experience, theerformance of this FET amplifier is superior to the per-ormance of any of the others that we have tried.

In this short report, we provide a brief history of theET. We then describe the basic components of De Valois’sriginal amplifier design (which is based on the FET) asell as a more recent modification of the circuitry. We alsorovide circuit diagrams of both models and measure-ents of their performance. Finally, on the basis of our

bservations, we compare the performance of this ampli-

1084-7529/05/102034-5/$15.00 © 2

er with the performance of other amplifiers that are spe-ifically designed for neurophysiological experiments.

This FET amplifier is one of R. L. De Valois’s valuable,ut little-known, contributions to the field of neurophysi-logy. Here we share the design of this remarkable andnique amplifier with the greater scientific community. Ino doing, we honor the memory of his long and distin-uished research career.

. FIELD-EFFECT TRANSISTORSETs were first conceived in 1926, when Julius Edgar Lil-

enfield filed for a patent on a junction transistor thatould be used as an amplifier. Two decades later, such de-ices became commercially available. In 1945, Williamhockley pursued an idea for making a solid-state device

rom semiconductors. This new idea was based on theremise that an electrical field could cause the flow oflectricity within a nearby semiconductor. Shockleyorked with John Bardeen and Walter Brattain to con-

truct the first semiconductor device that could amplify,he transistor (transfer plus resistor).7 The three physi-ists shared the 1956 Nobel Prize in Physics for the devel-pment of the transistor. John Atalla later developed aew design that used voltage to control an electrostaticeld within the transistor: the FET.In comparison with earlier devices (such as the vacuum

ube or the bipolar transistor), the FET has a variety ofeneficial qualities when used to amplify small signals:igh input impedance, minimal noise, no feedback at lowrequencies, and compact size. These and other qualitiesre ideal for single-neuron electrophysiology. The high in-ut impedance of the FET minimizes the potential effectsf the amplification process on the behavior of the cell be-ause it draws very little current from the cell. The FETan be powered by a small battery within the Faradayage because it requires only a small amount of direct cur-ent (dc). The battery is preferable to dc supplied by anlternating current (ac) power source because the latteran potentially introduce large 60 Hz noise within the

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Albrecht et al. Vol. 22, No. 10 /October 2005 /J. Opt. Soc. Am. A 2035

araday cage. In the De Valois amplifier, the FET, themall battery, and the required circuitry are all quite com-act, and thus the device can be placed very close to thelectrode, using very short leads between the electrodend the amplifier. Longer leads increase the likelihood oficking up noise. Although shielding the longer leadsould potentially reduce the noise problem, it is an unac-eptable solution in this case because shielding wouldause a huge increase in input capacitance; this would de-rade the spike signal.

It is worth noting that the use of a Faraday cage, withdedicated ground that is independent of all other elec-

rical and electronic devices in the facility, circumventsany of the problems associated with single-neuron elec-

rophysiology. The cage and the ground make up a veryimple solution to potentially complex interference prob-ems, including those that are introduced by shielding theeads (which would require a driven shield to be includedn the circuit).

The basic problems associated with designing an am-lifier for visual neurophysiology have been described.8

owever, there is no scientific literature documenting these of FETs in this type of application. Presumably, this isue to the proprietary nature of each design. Thus, weave limited the scope of this short report to the design ofhe De Valois amplifier.

. DESIGN OF THE ORIGINAL AMPLIFIERn this section, we describe the basic design of the ampli-er developed by De Valois in the last century. We thenescribe a newer version of the amplifier, with improvedircuitry following the same FET. Figure 1 shows thechematic wiring diagram for the original amplifier. Inhis schematic, R indicates a resistor, C indicates a ca-acitor, and Q indicates a transistor. The precise descrip-ion for each of the various components appears in Table.The basic design of the amplifier includes two func-

ional components: the input stage and the gain stage.he input stage is composed of R1, R2, C1, and the FET

ig. 1. Circuit diagram of the original FET amplifier designed aion for each of the components identified with symbols.

nd constructed by R. L. De Valois. Table 1 provides a detailed descrip-

Table 1. Component List for De ValoisPreamplifiers

omponent Original Design Updated Design

ransistorsQ1 (FET) 2N4220A 2N4220Aa

Q2 2N3906Q3 2N3904Q4 2N3904Q5 2N3904Q6 2N3906

esistorsb

R1 2.7 k� (1%) 2.7 k�

R2 100 M� 22 k�

R4 270 � 124 k�

R5 15 k�

R6 5 k�

R7 150 �

R8 6.5 k� 124 k�

R9 1 k� (1%)R10 124 k� (1%) 124 k� (1%)R11 22 k� 1 k� (1%)R12 1.5 k� 1 k�

R13 10 k� 10 k�

R14 1 k�

R15 100 M� (1%)apacitorsc

C1 0.001 �F 0.001 �FC2 100 pf 15 �FC3 0.001 �F 150 pfC4 1.5 �F 1.5 �FC5 150 pf 0.005 �FC6 15 �F 15 �FC7 0.005 �F

ntegrated circuitIC1 LM6152

aThere are many types of FET; various others �e.g., the NTE133� may performearly as well as the 2N4220A.

bResistors are metal film or carbon film, 14 W, 5%, unless otherwise specified.

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2036 J. Opt. Soc. Am. A/Vol. 22, No. 10 /October 2005 Albrecht et al.

omponent, Q1. The input stage shifts the dc operatingoint upward, as required for the gain stage. C1 blocksny dc component of the input signal. R1 protects theET (to some extent) from excessively large input volt-ges (e.g., 60 Hz noise or an inadvertent discharge oftatic electricity from the experimenter). The input stagerovides very high impedance (100 M�, in parallel with aapacitance of less than 7 pF).

Ironically, the FET (which can perform as an amplifiern other applications) does not provide any voltage ampli-cation of the signal from the neuron in this application;

n fact, the gain of this stage is slightly less than 1. In thispplication, the FET (Q1) and the other components ofhe input stage function as a buffer, known as a unityain buffer, between the neuron and the high gain of theext stage. The FET prevents the gain stage from inter-ering with the neuron. The very low capacitance load tohe neuron, combined with the high impedance, mini-izes the effects of the amplifier on the behavior of the

euron (e.g., it minimizes the possibility of drawing exces-ive current from the cell, injecting current or ions intohe brain near the cell, and so forth). The FET and relatedomponents of the input stage provide this amplifier withts unique and efficacious properties. In sum, the inputtage drives the gain stage with little or no disturbance ofhe neuron.

It is worth noting that this configuration of the FETrevents this amplifier from being used as (a) a stimula-or, (b) a lesion maker, or (c) a resistance meter. However,here are many commercially available devices that areedicated to performing these specific functions, and theyan easily be connected to the microelectrode in place ofhe FET. This strategy maintains the simplicity of themplifier’s circuitry and allows the device to be dedicatedolely to amplifying action potentials, thus avoiding addi-ional circuitry that could introduce undesirable events.

All the remaining functional components (the gaintage) are designed for high gain and filtering. Specifi-ally, the gain stage is a dc-coupled amplifier with a gainf about 100. Q2 is a high-gain current amplifier; it is

ig. 2. Circuit diagram of a newer design of the original FET amtage. Table 1 provides a detailed description for each of the com

oupled with R5 to transform current into voltage. Q5 is aigh-gain voltage amplifier. Q3 is a voltage input currentmplifier, which provides a second input to Q2. This sec-ndary input inverts the signal to apply negative feed-ack. The negative feedback reduces the otherwise higholtage gain of the circuit; further, it extends the fre-uency response, reduces distortion, and stabilizes the dcperating point of the circuit. Q4 is a unity gain amplifierhat prevents Q5 from loading Q2. Q6 isolates Q5 fromny external load. Finally, C2 and R4 reduce the gain ofigh frequencies to eliminate oscillation caused by theegative feedback.A frequency-dependent negative feedback path from

he output of Q6 to Q3 is introduced to adjust the finalain to approximately 100. Specifically, because gain is in-ersely related to negative feedback, R9 and R10 are usedo form a voltage divider that attenuates the negativeeedback. This feedback path is also used to control theandwidth of the amplifier. C4 reduces the attenuation ofhe feedback divider at low frequencies such that the low-requency response of the amplifier is approximately3 dB at 90 Hz. This circuit also reduces the gain of thec to 1 in order to stabilize the dc operating point. C5 re-uces the feedback divider attenuation at high frequen-ies such that the high-frequency response of the ampli-er is approximately −3 dB at 8 kHz.Finally, further attenuation of the response at high and

ow frequencies is introduced just prior to the final out-ut. C6 and R13 form a simple high-pass filter and blockhe dc component; R14 and C7 form a simple low-pass fil-er.

. DESIGN OF THE NEWER AMPLIFIERhe major difference between the original design and theewer design occurs within the gain stage, with only aery minor modification to the input stage (see below andig. 2). Specifically, the bipolar junction transistor ampli-ers, and the many related components, are replaced by aingle integrated circuit operational amplifier (op amp).

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Albrecht et al. Vol. 22, No. 10 /October 2005 /J. Opt. Soc. Am. A 2037

lthough the basic function of the op amp is the same (tomplify the small signal), the design is simpler, and theverall performance is superior: The gain is higher andore linear, the integrated circuit is more stable than the

ransistors, the power consumption is lower, and therere fewer components. The op amp is ac coupled to the in-ut stage by C2, thus isolating its operating point fromhat of the FET. The op amp establishes its own ideal dcperating point at the midpoint of the battery voltage (via4 and R8). The feedback circuitry and output filtering

ollowing the amplification remain unchanged.The input stage in the newer design is nearly identical

o that of the original design. However, because the oper-ting point of the gain stage is not dependent on the inputtage, the operating point of the input stage can be set toptimize the performance of the FET. This is accom-lished by increasing the value of the source resistor from0 k� (R5 in the original design) to 22 k� (R2 in theewer design). It is worth noting that the specific op amptilized here can accommodate a second channel.

. PERFORMANCE SPECIFICATIONSigure 3 shows the gain of both amplifiers (left verticalxis) as a function of frequency. The right vertical axis ex-resses the same measurements in terms of attenuation.he lower curve plots the gain for the original design, andhe upper curve plots the gain for the newer design. Asan be seen, the overall band of frequencies that are am-lified is very similar for both amplifiers; the only sub-tantive difference is the magnitude of the gain (125 ver-us 100). The measured bandwidth at 3 dB attenuationhalf-power) is approximately 90–8000 Hz. Although thiss a somewhat limited bandwidth (which would not be ap-ropriate for recording some other types of neurophysi-logical signal, e.g., dc recordings), this bandwidth waspecifically chosen because it is a near-optimal band ofrequencies for amplifying action potentials while elimi-ating unwanted signals and extraneous noise.The measured noise of both amplifiers (when measured

nside a properly grounded Faraday cage) is less than�V across the full band of frequencies, less than 2.5 �V

bove 50 Hz, and less than 2.0 �V above 100 Hz. The in-

ig. 3. Measured gain (left vertical axis) and attenuation (rightertical axis) for the original FET amplifier (lower curve) and theewer design of the FET amplifier (upper curve) as a function of

requency.

ut impedance of both amplifiers is 100 M�, in parallelith an input capacitance of less than 7 pF.

. PERFORMANCE OBSERVATIONSELATIVE TO OTHER AMPLIFIERSs noted in Section 1, we have been utilizing this FETmplifier to successfully collect neurophysiological data inhe visual cortex for nearly three decades.5,6 During thisime interval, we have purchased and utilized several dif-erent well-known commercially available amplifiers thatre specifically designed to measure action potentials. Al-hough these amplifiers perform the basic function in-ended by the manufacturer, in our laboratory, eacheems to have its own unique set of disadvantages, andone of them seems to perform as consistently as the FETmplifier designed by De Valois. Interestingly, we havebserved that, ceteris paribus (specifically, the same elec-rode recording action potentials from the same neuron inhe same animal at the same time), the various amplifiersrovide very different signals: Some show no discerniblection potentials, and others show action potentials em-edded within overwhelming noise. Some amplifiers re-eatedly kill neurons. It is our hypothesis that, on theseccasions, the amplifier injected an unwanted current (be-ause the input impedance was so low, 1 M�, according tohe user’s manual) that resulted in cell death. Finally,ver this long time interval, it is our overall assessmenthat the FET consistently outperformed every commer-ially available amplifier that we have ever used.

Comparison of the quantitative measurements of theerformance across these amplifiers reveals that the mea-urements of the De Valois amplifier are more desirable.or example, the input impedance of one such amplifier

as stated in the user’s manual) is only 1 M�, whereashe input impedance of the FET amplifier is 100 M�. Theigh input impedance of the FET unity gain buffer ampli-er ensures that the high-gain stage of the amplifier doesot disturb the neurons. As another example, the inputapacitance of one such amplifier is quite high (35 pF, ac-ording to the user’s manual) in comparison with that ofhe FET (less than 7 pF). The benefit of the low input ca-acitance is that it takes less current from the neuron tonitiate the amplification process. Unfortunately, many ofhe manufacturers do not provide these vital statistics.inally, the manufacturers of the commercially availablemplifiers generally do not provide schematics that de-cribe the circuitry (presumably for proprietary reasons);herefore, it is impossible to assess whether the specificomponents are appropriate for this specific application.

In sum, we believe that the De Valois FET amplifier isuperior to all other amplifiers that we have tried, and weope that others will find this unique device as useful ase have.

CKNOWLEDGMENTShis research was supported by National Eye Instituterant EY-02688 and the University of Texas.

Duane G. Albrecht, the corresponding author, can beeached by e-mail at albrecht@psy.utexas.edu.

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2038 J. Opt. Soc. Am. A/Vol. 22, No. 10 /October 2005 Albrecht et al.

EFERENCES1. R. L. De Valois, C. J. Smith, S. T. Kitai, and A. J. Karoly,

“Response of single cells in monkey lateral geniculatenucleus to monochromatic light,” Science 127, 238–239(1958).

2. H. B. Barlow, “Summation and inhibition in the frog’sretina,” J. Physiol. (London) 119, 69–88 (1953).

3. S. W. Kuffler, “Discharge patterns and functionalorganization of mammalian retina,” J. Neurophysiol. 16,37–68 (1953).

4. D. H. Hubel and T. N. Wiesel, “Receptive fields, bin-ocular interaction and functional architecture in thecat’s visual cortex,” J. Physiol. (London) 160, 106–154(1962).

5. D. G. Albrecht, “Analysis of visual form: anelectrophysiological investigation of visual informationprocessing,” Ph.D. dissertation, (University of California,Berkeley, 1978).

6. R. A. Frazor, D. G. Albrecht, W. S. Geisler, and A. M.Crane, “Visual cortex neurons of monkeys and cats:temporal dynamics of the spatial frequency responsefunction,” J. Neurophysiol. 91, 2607–2627 (2004).

7. W. Schockley, “A uni-polar field-effect transistor,” Proc. IRE40, 1365–1376 (1952).

8. D. M. Snodderly, Jr., “Extracellular single unit recording,”in Bioelectric Recording Techniques, Part A: CellularProcesses and Brain Potentials, R. F. Thompson and M. M.Patterson, eds. (Academic, 1973), pp. 139–141.