Tektronix 494AP service manual O o o

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. The 10 MHz inputsignalfrom the VR circuitsdrives input preamptifiere2075, in th€ tog lmftiner circuits on A62A1. The input preamptifier

-ir"nsier" the S0O input signal to the high_impedance

input of the first amplifier stage. The input signal is ilso apptied to transistor0107S,a common-baseamplifierthat acts as a bufierto supplythe l0 MHz lF signatto the rear_panel connector.

..	Frgm the input preamplitier,
		the signat is apptied
throughsevencascadedstageseach cinsisting'ot
		an
emitter follower driving a common base amptifier.
Resistorsbetweenthe emitterfollower
		andthe common
base sectionsdeterminethe gain. Diodesswitchvari-
ous resitorsin or out of the circuitto varythe gain for
different input levels. All of the
except that the first three stages		"t"g"J are similar,

diodesfor an additionalgainitep."ontaii an extra set of

All stages have maximumgain for low amplitude inputsignals. As the inputamptitudeincreases,the last stageswitchesto a lowergain. For furtherincreasesin input amplitude,additionalstages decreasegain back to -the first stage. Then the second set of gain- switchingdiodes switchesinto the first, second,and third stag€sto reducethe gainfor the larggstsignals.

Typically,whenthe inputlevelto the emitterfollower in th€ last stage is less than 60 mV peak-to-peak,

the transistorconductsenoughto maintainfonrvardbias on serieslimitingdiodes. The 10 MHz signatpath at that levelis throughbothdiodes,a capacitjr, ani a resistor networkto the commonbasesectionof ihe stage. The gain .of the stage under these conditionsis ipproxi- mately10 dB.

-As the input signalvoltageincreases,more current llows throughthe left diod€ to increasethe reversebias of the right diode. This sharplyreducesthe stagegain to unity. The signalcurrentthenflowsonlyin the lower path aroundthe diodes.This changetakesptaceduring the positive-goingportionof each cycte. tne opposite occurs duringthe negative_goingpdrtion of the signal above the minimuminput level. As the input signal increasesbeyondthe pointat whichthe gainof the final stagedecreasesto unity,the samesequinceoccursin the precedingstage.and in succession,backto the first stage.

Theory of Operailon - 4g4Ll4g4ApServtce,Vot. l

Signal levels above this point activatethe second tier of gain reductionin the first three stages. These stages each incorporate a second set of diodes that r€duces the gain by another7 dB. In the first tier of gain reduction. reduction started at the last stage and proceededto the first; in the secondtier, the reduction starts at the first stage and proceedsto the third.

In the first three stages,the lower diodes are for- ward biased untit the second tier of gain reduction. With a further increase in input signai level, limiting occursin the samemanneras previouslydescribedand results in less than unity gain through the stage (approximatety 112r. The one-two-th6e reducrion sequenceis establishedby the valuesof the pull_down resistorsat the cathodesof the diodepairs.

Detector Circuit

This circuit demodulatesthe 10 MHz output of the Log Amplifier,producingthe VIDEOsignalthat drives the VideoAmplifiercircuits. The detectoiconsistsof an operationalamplifierwith a diode detectorin the feed_ back path. A low-passfilter at the output,shown on diagram23,filtersthe RF from the dEtectlOsignal.

The operationalamplifiEris madE up of common emitt€r amplifier01012 and a differentialamplitierthat consistsof Q2010and e20l8. The summingnode for the n€gativeinput is the base of e1012 (the positive inputis at the groundedemitt€rof el012).

The differentialamptifier'shigh impedanceoutput allowsit to rapidlychangeduringthe periodwhen both detectordiodes are effectivelyopen circuited;that is, whenthe outputis near0 V. Whenneitherdiodeis con- ducting,it is necessarythat the output changerapicfly throughthat zone.

Figure 7-11 shows a simptified ac-equivalent schematicdiagramof the detectorcircuit. Two detector diodes are used, but only the negativehalf cycle is takenas the output. Ac couplingis usedon both sides of the detector diodes to prevent ternperature coefficient effects of the operationalamplifier from affectingthe detectoroutput.

The detector output signal is appliedto the Video Amplifier. A low-pass filter shown on the foilowing diagramcompl€testhe detectorby filteringthe remain- ing RF.

7-31