Philips PM 5167 - Page 40

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3.1.2
.1. V ia common-base stages 318, 349 th e d riving curre nt s le and le* are applied to switching arr ay 1, U3/3 16 ,
31 9, 346 a nd 348.
At capac i to r e (U2 /5 01 to 510 a nd 512) a time-li near , pos itiv e- or negativ e- going voltage app ears, the value
of which depe nds on the po sition of th e s witching arra y:
I
r*
ue (t ) =C .t
ue (t ) =c .t r es pectivel y
A
bet t er dis tri but ion of dissip at ion is obta ined via comm on-b ase st ages U 3/3 17, 320, 345 and 347.
3
.1 .2. 2. Cu rrents I R a nd I R* fr om t wo cur rent so urce s U3/305 and 332 are app li ed to sw it ch ing array 2, U3/30 6, 308,
331 a nd 334.
The abo ve-me nt ion ed c urr e nt sour ce s ar e ad ju st ed by m ean s of U 3/ 615 a nd U3/657 in such a way th at offset
and ampl it ude of the square-wave voltage corr e spo nd to the values re qu ired. A sq ua re-wave voltage u Ra pp ears
at r esistor Rl (U3/659-660) by reversing pe rio dic ally the switching array.
An inverse s qua r e- wave -u R a pp ears si m ultane ously at resistor U3 /6 48-649.
The co mm on-base stages U3/307, 309, 330 and 333 reduce the square -wave rise ti me .
3. 1.2.3. The co mp a rator (U3/315, 340 to 343) co mpa res the voltages uc and UR .
The co mpa rator switches over as soon as ue reaches U R. As this is a period ical p rocess, a square-wave voltage
appears at Rl and a triangular voltage of equal am pl itude at e .
The rise
times t
r
and tf of the triangular voltage are:
2. e! I
t
r
= iC UR
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 = 4e 1
'I ~ I
At a time-asymmetrical triangular voltage (sawtooth) the oscillator will be fed by two di fferent d riving currents.
As the oscillation frequency must be identical for asym metrical as weil as sy mm etrical operation, the driving
currents le and IC * must behave as follows:
Out y cycle
le
le *
5. I
O, ,
5. I
--
9
0,5
5 .
I
0,9
5 . I
9
3.1.2.4. I mpedance converter 1 is controlled by u e. D ecoupling is performed at very high i mpe dance by a FET-pair
U3/324, which is operated as source follower and as power source. The power so ur ce serves linearizing offset
and temperatur e compensa tion . The co mp lementary emitter-follo wer (U 3 /32 5 and 352) re du ces the ou tp ut
i mpe dance t o 50 n.
3.1.2.5. Imped ance converter 2 con sists of a complementary emitter-foll ow er U3/ 301 a nd 302, which de couples the
square-wave vo lt age - uR at resistors U3/648 and 649 and causes an o ut put i mp e da nce of 50 n.