Massive Audio DB8000 User Manual download pdf (Page 72)

Fig. 1. Three-

transistor

TRF radio receiver

cir-

cuit submitted

by

reader

Ronald Cook features

a

novel tuner /diode detector

arrangement.

Reader's Circuit.

If there's any

one cir-

cuit

that stands out

as the all

-time favor-

ite among our

readers,

it is the simple AM

broadcast

-band receiver.

Typical of

such

receiver circuits

is the

TRF receiver

illu-

strated

in Fig. 1. Submitted

by reader

Ron-

ald

Cook (7 Montgomery

St., Saugus, Mass.

01906),

the circuit features a

combination

tuner /diode detector

arrangement

followed

by a three -stage

audio amplifier.

Radio -

frequency

signals picked

up

by the anten-

na are

selected by tuned circuit

LI -Cl and

detected

by Dl. The

signal

across

RI, the

volume

control,

is applied through coupling

capacitor

C2 to Ql's base.

Transistor

Ql's base is biased by

voltage

divider R2

-R3, while R4

serves

as

its col-

lector load. The

amplified signal at the col-

lector

is

coupled

through C3 to the base of

Q2,

biased by voltage divider

R5 -R6. Both

QI and Q2

are hooked

up in a common -

emitter

configuration.

The output

of

Q2 is

developed

across R7,

the collector load, and

direct

-coupled to power amplifier

Q3, whose

base is biased

by R7. Transistor

Q3, wired

as an emitter

follower,

furnishes

direct drive

to its

PM loudspeaker

voice

coil

Ioad.

Op-

erating

power

is

supplied by

BI, a 6- or

12

-volt

battery, through

s.p.s.t. switch

SI.

Ronald has used

standard components in

his

design.

Coil LI

is

a

variable

-type

loop -

stick

antenna,

Cl a small fixed ceramic or

mica capacitor, and C2

and C3 are 15- to

25

-volt

electrolytics.

All resistors are half -

watters; volume control potentiometer RI is

a

transistor circuit type. For diode Dl,

Ronald has

chosen the familiar

1N34A.

Transistors QI, Q2, and Q3

are general -

purpose

pnp

types similar to GE's 2N107.

On

-off switch

SI

can be

combined with

78

TC

B-

RI, or it

can be a separate s.p.s.t.

unit.

Battery

B1 is

a

regular

9 -volt transistor

type, but

you could wire from four to

eight

penlight cells in

series. Ronald suggests the

use of a

PM

speaker with a high- impedance

voice

coil winding, and

a

10'

-long antenna.

The circuit

can be assembled on a small

metal

chassis, on a breadboard,

or on a

properly designed etched

circuit board, as

preferred. Depending on the

characteristics

of the transistors used, you may have

to ex-

periment

with

different bias resistor

values

until you come up with the right bias

volt-

age.

Manufacturer's

Circuit. Although the

super- high-

frequency

transistors

described

in our introductory paragraphs

will not be

available

for

some time, you can learn

a

good

deal about

high- frequency

transistor

operation

by experimenting with circuits

like the 200 -MHz r.f. amplifier in Fig. 2.

The circuit

can be modified

for

use as a

buffer stage in a VHF transmitter

or re-

ceiver, be adapted

for

use as an r.f. stage

in an FM receiver, or -if

desired -used "as

is" for

laboratory

experiments.

Designed by engineers at Motorola Semi-

conductor

Products, Inc. (Phoenix,

Ariz.

85008),

the circuit

features

a new moder-

ately priced n- channel junction field

-effect

transistor (or JFET)

with

exceptional VHF

characteristics.

This JFET,

a 2N3823, has

a noise figure of only 2.5 dB at 100 MHz.

an

input capacitance of only 6 pF, a re-

verse transfer capacitance of just 2 pF,

and a

minimum forward transfer

admit-

tance of 3500 pmhos.

It is

useful at fre-

quencies

of

up to

500 MHz.

Transistor

Ql is used in a common-

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