Massive Audio DB8000 User Manual download pdf (Page 73)

Fig.

2.

Newly developed by Motorola

Semiconductor

Products,

this

high -

frequency

r.f.

amplifier utilizes an

n- channel

field -effect

transistor and

can

operate in

the 500 -MHz range.

IN

2N3823

OI

source

configuration,

and is

operated

at zero

gate bias.

Capacitor

Cl, and coil L1 tuned

by C2, form

a 50 -ohm input impedance

net-

work.

Coil L2,

tuned by capacitor

C7,

serves

as the drain load. Coil

L2 also pro-

vides

a feedback signal

through

C3 to Ql's

gate for stage

neutralization.

Capacitor

C8, in conjunction

with L2

and

C7,

matches

the stage to

a 50 -ohm load. A

r.f. choke (RFC1)

bypassed by C6 iso-

lates

the drain supply voltage source

from

the

rest of the circuit. Further isolation is

achieved

by C4 and

CS across the battery,

controlled by

Sl.

If you live in a

small

town, you may not

be

able to get all

the

parts required for

this r.f. amplifier; but you can order

them

from one of the

major

mail -order houses.

Coil

L1

is made

from 11/2 turns of No. 18

tinned wire, wound on

a 1 /y "- diameter

form and spread out about

/8 "; L2 con-

sists

of

31/

turns of No. 18 tinned

wire

wound on a 3/8" -diameter form, and spread

out % ". This

coil

is

tapped at 11/4 turns

from the end that goes to the

transistor's

drain.

The r.f. choke is a commercial

0.47 -11H unit, and J1 and

J2 are

standard

VHF

coaxial connectors. Capacitors Cl,

C2, C3, C7, and C8 are good

- quality ce-

ramic

trimmers,

while C4, CS, and

C6 can

be either ceramic or mica types.

A 15 -volt

battery

and

a

s.p.s.t. switch

are required

for the

power supply.

As in all

VHF circuits,

layout

and wir-

ing

are

extremely

critical. Appropriate

layout,

wiring, and construction techniques

should be observed. Keep signal

and

ground leads

short

and direct, and make

all

grounds

to a

common point.

Shield the

areas indicated by the dashed

lines

in

Fig.

2; note

that

one shield cuts across Ql, iso-

lating the drain electrode.

Once the

circuit

wiring has been completed and checked,

the

adjustable

capacitors must

be

set

for

proper tuning and

impedance

matching

with

both

the source and load,

and to

achieve

stage

neutralization.

Transitips.

If reader

mail

is any indica-

tion,

most

readers

find the Darlington

cir-

cuit

especially fascinating. And with good

reason -for

it

is

one

of the simplest and

most

versatile of all multistage

direct -

coupled

transistor circuits. So,

although

this circuit has been discussed briefly

in

February, 1967

_L

Fig. 3.

Packaged in

a single case, this Darling-

ton pair can be treated as

a

single

transistor by

making

the

input

and output

connections

as

shown.

past columns,

a

more

detailed review

might be worthwhile

at this

time.

In its basic form, the Darlington

circuit

consists

of

two

transistors

with

*hP

emitt"r

of the first connected

directly to the

base

of

the second, and having both collectors tied

together, as shown in Fig.

3.

The transistor

pair

(Q1 and Q2)

becomes, in

effect, a

"super" transistor (Q'), for the configura-

tion

has only three active connections:

base (of Ql), collector

(Ql

and Q2 together),

and

emitter

(of Q2). This is indicated in the

diagram

by the dashed circle.

From a practical

standpoint, this

direct -

coupled pair can be treated as a single tran-

sistor,

with appropriate base bias (through

RI), load RL, and input and

output

con

-

(Continued on page

102)

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