Georgia Institute of Tec hnologySc hool of Electrical and Computer E ngineeringECE 4435 O p A m p Design Laboratory Fall 2005Design Project 1An Effects B ox for Electric G uitarsObjectThe object of this project is to design, assem ble, and evaluate an effects box for an electricguitar. The bo x is to generate a distortion signal that can be added in varying amounts tothe signal output from the guitar pickup to modify the sound of the guitar. The distortionis to be generated in two independent circuits. The first circuit is to generate second orderharmonic distortion. The second circuit is to generate clipping distortion. The outputs fromthese circuits are to be added independently in v arying amoun ts to the signal output fromthe pickup.Many m u sician s sa y that second-order harmonic distortion adds a richn ess to a guitarsignal. ThisdistortionistobegeneratedbyaJFETdifferential amplifier in which theoutpu t signal is taken as a common-mode signal. Clipping of the pickup signal adds a roughrasp y effect to the sound of the guitar. If it is used in moderation, it causes a note to ha veaslightlydifferent ton ality. If it is used heavily, it give s rise to the term “heavy metal.”Description of the CircuitThe circuit is to ha ve an inpu t resistan ce of 10 kΩ. The input stage is to have a voltagegain that can be varied from 0 to 10. The nominal signal lev el at the output of this stagecanbeconsideredtohaveanrmsvoltageof1V. The signal output from the input stageis to be applied to both the second harmonic generator and the clipper. These circuits areto be follow ed by a summing amplifier which allo w s the outputs from the effects circuits tobe summ ed in varying amounts and fed to the output stage. W ith a 1V rms lev el from theinput stage, the gain of the output stage is to be variable such that the rms output lev el canbe adjusted to be in the range of 0 to 4V. The output signal is to be ac coupled with anoutput resistance of 100 Ω.Second Harmonic Generator: The second harm on ic generator is to consist of a JFETdifferential amplifier. The common-m ode output current of the diff ampistobeconvertedinto a v olta ge that drives one input to the sum ming am p lifier. The basic circuit of the JFETdiff amp is show n in Figure 1. The signal which driv es the diff ampmustbeadifferentialsignal that is derived from an un balanced signal to balanced signal conv erter. The resistorslabeled RSand the potentiometer labeled RPsetthesourcebiascurrentineachJFET.The JFETs should be biased at approximately IDSS/2,whereIDSSis the drain current withVGS=0. ItispreferabletomatchthetwoJFETs. Becauseitisimpossibletoobtainanexact matc h , the poten tiom eter can be used to offset the dc bias currents in the two JFETsto optimize the operation of the circuit. The capacitor C is to be large enough so that itis essentially a short circuit for frequencies above 20 Hz. This will be approxim a tely true ifC>2.5/rsπf,wherers=1/gmand gmis the transconductance of each JFET at the dc biaspoint.1Figure 1: Second harmonic generator circuit.The JFET that is a vailable in the laboratory is the 2N5457 n-channel JFET. Its draincurren t can be written in t wo waysiD= IDSSµ1 −vGSVTO¶2= β (vGS− VTO)2where β = IDSS/V2TOand VTOis negative. Typical range values for IDSSand VTOare2mA≤ IDSS≤ 3mA and −3V≤ VTO≤−2V. The transconductance is given bygm=−2VTOpIDIDSS=2pβIDIf the 2N5457 is biased at IDSS/2, its transconductan ce has the value gm= −√2IDSS/VTO.The worst case value for rsin calculatin g the value of C is the smallest value. This translatesinto the largest value of gm.ThisoccurswithIDSS=3mAand VTO= −2V to obta inrs=√2/0.003 = 471.4 Ω.Forf =20Hz, it follo ws that the v alue of the capacitor C in thecircuit above should satisfyC>2.5471.4 × π × 20=84µFThe capacitor should be a non-polar unit. In this case it is realized as the series connectionof two capacitors of value 168 µF. The nearest larger value capacitor in the lab is a 220 µFcapacitor.Let R0S= RS+ RP/2.IfVGSis small compared to V−, the drain current in eac h JFETis given byID=−V−R0SThis equation can be used to calculate a firstcutvalueforR0S. A more exact equation forIDisID=µ−b +√b2− 4ac2a¶22wherea = R0Sb =1√βc = V−+ VTOLet the differen tial input voltage to the diff amp be given b y vd= Vpcos (ωt).Youshouldshow that the ac componen t of the output current is giv en b yio=IDSSV2p4V2TOcos (2ωt)which is at the second harmon ic frequency. This current must be conv erted into a voltagewhich becomes the second harmon ic distortion term. This signal is to be ac coupled fromthe outpu t of the second-harm onic generator.Clipping Generator: The clipping of the guitar signal can be easily generated with adiode clipping circuit. The diodes can be connected in para llel with the feedbac k resistor ofan in verting am p lifier or as the shunt elemen t of a vo ltage divider. A means of varying theamount of clipping on the w av eform b y varying a poten tiometer is to be part of the circuit. Adesirable feature wo uld be to adjust the “hardness” of clipping b y varying a potent iome ter.You are free to implement the clipping in any way that you c hoose. The basic objectiv e isto be able to vary the am ou nt of clipping on the signal.Suggested Design Methodology: You should begin b y drawing a block diagram ofthe circuit. After that, the individual circuits for each block can be designed, assem bled, andtested. After each individual circuit is operational, the circuits can be connected togetherto com plete the