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Wideband Amplifers

For applications such as television, amplifers that will pass very wide requency bands ;such as

rom 30 to 4,500,000 cycles, are required. a pentode resistance!coupled amplifer is

constructed with low plate load resistances, the hi"h!requency response is e#tended. Low-

frequency compensationis achieved by a parallel $!% combination in series with the load

resistor. High-frequency compensationis achieved by a small coil in series with the load

resistor. &hese are called compensated ampliers. 'ther types o wideband amplifers are

possible.(5,(),(*

Fi"ure 33. n the cathode ollower, the

entire +output+ volta"e is ed bac in

series to oppose the impressed si"nal

volta"e -i.

&he input capacitance o the ollowin" tube plus the wirin" capacitance are important actors

limitin" the hi"h!requency response pa"e (/1. For this reason, a circuit called the cathode

follower(/is oten used between two wideband ampliyin" sta"es. &he circuit arran"ement is

shown in Fi". 33. &he cathode ollower has a "rid!input capacitance that is much less than the

actual capacitance between "rid and cathode. &he "rid!input impedance is correspondin"ly hi"h.

&he internal impedance as measured at the output terminals is a low resistance

&he cathode ollower acts lie a step!down transormer and can be used or impedance

matchin". &he output volta"e and the input volta"e are in phase. &he +volta"e "ain+ is less than

unity and is

2 new approach to the desi"n o wideband amplifers is the so!called distributed amplier.29n

this amplifer two identical artifcial transmission lines, composed o sections o inductance and

capacitance

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Fi"ure 34. implifed circuit o a distributed amplifer that will pass a requency band o

many millions o cycles.

which may include the interelectrode capacitances1, are used. &he "rids o several tubes areconnected in +parallel+ across successive sections alon" one line, and the plates are connected in

+parallel+ across correspondin" sections o the other line, the arran"ement bein" shown in Fi".

34. &he lines are, o course, low!pass flters pa"e )/1. &hey are terminated with m!derived

sections. &he cuto requency is made considerably hi"her than the ma#imum requency to be

amplifed, and the input impedance is essentially pure resistance, equalin" the iterative

impedance o the artifcial line. an input si"nal volta"e is impressed, the frst tube amplifes

this si"nal and impresses an output si"nal on the upper line. &his si"nal divides equally, part

passin" to the let, where it is lost, and part "oin" toward the ri"ht. &he input si"nal travels

toward the ri"ht on the "rid line, where the ampliyin" action is repeated, and the output si"nal

impressed on the plate line a"ain divides, part "oin" to the let, where it is lost, and part "oin"toward the ri"ht, where it is 6oined by the si"nal travelin" to the ri"ht rom the frst tube. n this

way, the si"nals rom the successive tubes build up, "ivin" an amplifed output. 2n amplifer,

constructed as shown in Fi". 35, was ound to have a volta"e "ain o about decibels rom 7ero

cycles to 40,000,000 cycles, and the band width can be made wider i desired.

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