HP 5245L Service Manual

Section
fI
Paragraphs
2-11
to
2-18
2-II.
THE
TRANSISTOR.
2-t2.
GENERAL.
Transistors
are used
throughout
the counter
in
circuit
configurations
such as
the ampli-
fier,
the flip-flop
or
binary,
the
trigger
circuit, and
the one-shot
multivibrator,
Irl
the folqu/ing
para-
graphs,
basic
transistor
operation
and
a
few
basic
transistor
circuits
are discussed.
These
paragraphs
discuss
the
easily
observed
changes
in
currents
and
voltages
in
transistor
circuits
which help
technicians
Iocate
circuit
faults
butdo
not
attempt
todescribe horv
transistors
work
internally.
2-13.
BIASING
AI.{D
CONDUCTION.
VACUUM
tUbCS
and
transistors
are functionally
similar. Irr
the tube
a
small
grid-to-cathode
voltage
controls
a
larger
plate-to-cathode
current
flow.
In
a
transistor
a small
base-to-emitter
current
controls
a
large
collector-to-
emitter
current.
A
comparison
of
basic
vacuum
tube,
NPN
transistor,
and PNP
transistor
operation is
shown in Figure
2-2A;
indicated
current
represents
conventional
florv
of
positive
charges
external
to the
transistor
and is
not
intended
to indicate
florp
of car-
riers
inside
the
transistor
structure.
Notice
that ilre
effect
of
emitter-base-collector
voltages
is
totally
reversed
between
NPN
and PNp
transistorsl
circuits
which
are
arranged
for
an NpN
transistor
usually
func-
tion
normally
for
a PNP
transistor
if
supply
voltages
are
reversed.
2-t4.
AMPLIFIERS.
As
witl vacuum
tubes,
three
basic
amplifier
types
are
available
(figure-Z-2l)
,
These
amplifiers
maybe
used
alone
orin
combination
to form
complex
circuits.
2-15.
FLIP-FLOP.
The
flip-flop
is
a
bi-stabte
two-
transistor
circuit
in
which
one
transistor
conducts,
holding
the
other
cut
off.
Each
input
pulse
causes
a
reversal
of
states;
that is,
the
cut
off
transistor
is
turned
on
and
the
conducting
transistor
is
cut off.
In
the flip-flop
shorrn
in Figure
2-gA,
e1
is
initially
con-
ducting
heavily;
its
collector
voltage
is
only
slightly
negativel
a near-zero
voltage
is
supplied
to
the base
of
Q2
(junction
of
R2?-28
divider).
-ine
voltage drop
across
R24
produces
a
sufficienily
negative
voltage
at
the
emitter
of
Q2
to hold
e2
cut
off.
With
e2
cut
off
the
R18-R19-R20
divider
delivers
a negative
volt-
age
to the base
of
Q1
to keep
it
conducting.
2-16. At time
t1 the
positive
input
pulsecutsoff
el;
the
Q1
collector
voltage
goes
negative
and
drives
ez
into
conduction
(R2?-R2B
divider
to
e2
base);
the
e2
collector
voltage
and
the
Ql
base
voltage
(R1g-R20
divider)
then become
considerably
less
negative,
per-
mitting
Q1
to
remain
cut
off.
The
R26-R2?-R2B
divider
delivers
a sufficiently
negative
voltage
to
the
base
of
Q2
to drive it
into
conduction.
In
a
similar
manner
the
positive
input
pulse
at
time
t2 cuts
off
e2
and
starts
a
sequence
of
events
which
6nds
with
el
conducting
and
Q2
cut
off. Note
that
a
positive
input
pulse
has
no
effect on
Ql
if
it
is
already
cut off. A
negative
reset pulse
applied
to
the
base
of
et
returns
the flip-flop
to its initial
condition
(e1
conducting,
e2
cutoff).
The
diode
CRg
removes
the negative
pulse
from
the
differentiated
square
wave input.
Wilhout
this
diode,
the negative pulse
would
drive
el
which
is
cut
off
and
the
stage
would
switch from
one
state
2-2
Model
5245L
Figure
2-2. Transistor
Operation
to
the
other
but
would
not
divide
by
two.
The
AC
coupling
through
C2 and C3
insures
fast
switching.
The
DC
coupling
through
R1g and
R2? insures
bi-
stable characteristics.
2-L7.
BINARY
CIRCUIT.
In
this
manual
a flip,flop
which completes
its
operating
cycle
and
produces
an
output
pulse
after
receipt
of
two
similar input
pulses
is
called
a binary
circuit,
since
it is
a counting
device
in
a binary
system.
The
binary
circuit
is
driven
from
a
single
inputwhich
is
connectedeither
through
a
pair
of resistors
or through
a
pairof
gating
diodes
toeach
transistor
base.
2-18.
TRIGGER
CIRCUIT.
The
trigger
circuit is
a
limiter
or
squaring
circuit
which
produces
an
output
waveform
with
very fast
rise
and
fall
times. The
02349-1
ooooo-c-36
A. Amplifier
Characteristics
SHARACTERISTIC
COMMON
BASE
COMMON
EMITTER
COMMON
COLLECTOR
lnpul
lmpedonce
Oulpul
lmpedonce
Volloge
Goin
Currenl
Goin
Power
Goin
300-50J)
300Ko-500Ko
500- 1500
<t
20dB-30d8
5000-1500J}
30KO-50KO
300-tooo
25-50
25dB-40d8
20K0-500Ko
500-loooo
<l
2s-50
todB-20d8
(Emitler
Follower)
B. Transistor
Biasing
TYPE
CUTOFF
CONDUCTION
NPN
COLLECTOR
,orr.d
\5/
EMITTER
+v
.,df.Ht^,,
comnoNtl\
,/
CURRENT
\-Y
L
PNP
COLLECTOR
,O"Q
EMITTER
-V
I
,-^6
lggg$'\y'uarr,r
I
CURRENT
r/7

HP 5245L Specifications

General

Brand	HP
Model	5245L
Category	Cash Counter
Language	English