Research Department. REPORT No. F.OlO 15th January, 1934.

Serial No. 1934/4

Investigation by C. H. SMITH.

Drawing Nos. F.OIO.l

APERIODIC AMPLIFIER FROM 50 CYCLES TO 1000 KILOCYCLES.

(Interim Report)

SUMMARY. A board model of a two-stage aperiodic amplifi~r has been constructed which gives a response curve flat to within one decibel between 50 cycles and 1000 kilo­cycles with a gain of about 52 db •

. INTRODUCTION.

to F.OlO.5.

The problem of designing an aperiodic amplifier to have a flat

response between 50 cycles and 1000 kilocycles was undertaken in antici-

pation of television amplifier requirements and in order to provide data

for the design of an aperiodic amplifier-detector for measuring purposes.

DESIGN PRINCIPLES.

The decrease in amplification at the higher frequencies which is a

characteristic of resistance-coupled amplifiers is due to the presence of

undesirable capacities, either from grid and anode to earth, or directly

o >< ----~ from grid to anode in the amplifier stages.

all "'"""""'" IX) -IX)

a:----IX) o g The effect of grid to anode capacity can be reduced substantially m-o m g to zero by the use of screened grid valves, and the effect of capacity to

- earth can be reduced by a reduction in the values of the anode. resistances

\ ,'-------._..//

-2-

and a consequent increase in the number of stages to restore the gain to

the original level. There is, however, a limit to the extent to which

loss of amplification at the higher frequencies may be prevented by this

means. When constant amplification is required over a very wide frequency

band the resulting amplification per stage becomes inconveniently small,

and other devices must be adopted.

There are two simple methods by which the attenuation of high

frequencies in a resistance-coupled amplifier can be reduced without using

exceptionally low values of anode resistance. One method consists of

connecting between the anode of one valve and the grid of the succeeding

stage an inductance which, in conjunction with the anode-cathode and grid-

cathode capacities of the valves, and the anode and grid resistances, forms

a low-pass filter of suitable constants correctly terminated. A second

method consists of connecting, in series with the anode resistance, an

2 inductance whose value is scmewhere in the neighbourhood of CR , where R

is the anode resistance and C is the sum of the grid and anode capacities.

The first method has the disadvantnge that a progressive phase change per

stage from 1f to 2lf, increasing with frequency, occurs. In the second

method, however, the p~ase change is subs~antially constant at~ per stage

over the greater portion of the useful fr~quency band.

RESULTS.

A series of response curves have been taken with a single stage

AC/S2 valve with 300 volts H.T., 80' volts screen potential and 1.5 volts

grid bias. Tests were made only above 15 k.c. as this is the important

frequency ~and.

I'

Fig. 1 shows a set of curves in which an anode resistance of 10,000

ohms has been used in series with inductances of 3000, 2000, 1000 and 500

microhenries. (The variatiornin level between the various curves at the

low frequency end of the scale are probably due to battery variations since

these curves were taken at intervals of several days).

It may be concluded from these curves that the upper limit of

frequency to which a flat response can be obtained, using a 10,000 ohms

anode resistance, is about 800 kilocycles, and the best value of anode

inductance lies between 1000 and 2000 microhenries.

A second series of curves shown in Fig. 2 were taken with an anode

resistance of 5000 ohms and inductances of 1000, 500 and 300 microhenries.

The most promising curve of this group is No. 3, in which a 500 microhenry

coil was used. The increase in amplification at 1000 kilocycles indicates

that a slightly higher anode resistance could be used. The curve of Fig.

3 was therefore taken, with 6000 ohms anode resistance and 500 microhenries

inductance. The slight loss in amplification at 1000 koc. indicates that

the anode resistance is a little high.

In view of these figures, 5,500 ohms anode resistance and 500 micro-

henries was decided upon as being the best compromise, arid a board model

two-stage amplifier was built using components of these values.

The response curve obtained is shown in Fig. 4, and Fig. 5 shows the

actual circuit used to measure the output pf the amplifier stages.

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FIG.5.

S.B.C. sces.EA~CH DEPT.

TWO STAGE APERIODIC AMPLIFIEJ;2. t---"---:":"1rl "EPOS2T F.OlO.5 ..