Philips PM 5167 Instruction Manual page 40

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40
3.1.2.1. V ia common-base stages 318, 349 the d r iving currents le and le* are applied to switching array 1, U3/3 16,
31 9, 346 and 348.
At capacito r
e (U2/501
t o
510 and 512)
a time- linear, pos itiv e- or negativ e-go ing voltage appears, t he value
of which depe nds on t he position of th e switching array:
I
r*
ue
(t )
=C .
t
ue (t )
= c .
t resp ecti vel y
A bett er distri bution of dissipat io n is obta ined via common-base stages
U3/3 17, 320, 345 and 34 7.
3.1 .2.2. Cu rrents I R and I R* f rom t wo c u rrent so urces U3/305 and 332 are app lied to swit ch ing array 2, U3/306, 308,
331 and 334.
T he above-me ntioned curre nt sources are adjusted by means of U3/615 and U3/657 in such a way th at offset
a nd ampl it ude of t h e square-wave voltage correspond to the values requ ired. A sq ua re-wave voltage u Rappears
at resistor Rl
(U3/659-660)
by reversi ng pe riodically the switc h ing ar ray .
An inverse squa re- wav e -u R appears si m ultaneo us ly at resistor
U3/648-649.
The co mm
on-base stages U3/307, 309, 330
and
333
reduce the square-wave rise ti me.
3. 1.2.3.
T he comparator
(U3/315, 340
t
o 343)
c ompa res the voltages uc and UR .
The co mpa rator switches over as soon as ue reaches UR. As this is a peri od ical p rocess, a square-wave voltage
appears at
Rl
and a triangular voltage of equal am pl itude at e .
T he rise times t r and tf of the triangular voltage are:
2 .e!
I
t r
=iC
U R
The frequency of the triangular voltage is
f = - ' ­
t r
+
tf
1e
At a time-symmetrical triangular voltage, t r
=
tf; le
=
le* and f
=
4 e 1
'I~I
At a time-asymmetrical triangular voltage (sawtooth) the oscillator will be fed by two different d riving currents.
As the oscillation frequency must be identical for asym m etrical as weil as sy mmetrical operation, the driving
currents le and IC* must behave as follows:
Outy cycle
le
le *
5. I
O, ,
5. I - ­
9
0,5
5 . I
0,9 5 . I
9
3.1.2.4.
Impedance
converter 1 is controlled by
ue. Decoupling is performed at very high impedance by a FET-pair
U3/324, which
is operated as source follower and as power source. The power so urc e serves linearizing offset
and temperature compensation . The comp lementary emitter-followe r
(U3/325
and
352)
redu ces the output
impedance t o
50
n.
3.1.2.5.
Impedance converter
2
consists of a complementary emitter-foll ower
U3/ 301 and 302,
which d ecouples the
sq uare-wave vo ltage - uR at resistors U3/648 an d 649 and causes an o utput impeda nce o f 50
n.