Philips PM 5167 Instruction Manual page 43

42
[ ual transistor U5/303 is located in the feedback path to reduce the load effects of the inverting inputs of
a nplifier U/ 5 351 and 352. By means of resistor U5/60511606 the offset voltage at the output of the voltage­
c ment converter is adjusted to zero between the emitter connections of U5/301 and 305.
[iode U5/401 limits the voltage at the input SWEEP IN to 12 V maximum.
F or time-asymmetrical operation (duty cycle 0.1 or 0.9) two different charging currents are required. For this,
t lle controi unit contains another pair of voltage-current converters (U5/353, 354, 312, 314, 316), which is
c >ntrolled by the same differential voltage. Amplifier pair U5/ 353, 354 is electrically identical to pair
L 5/351 / 352 and adjusted correspondingly.
1 he resistors on wafer 4, switch U2, which determine charging time and frequency, have twice the value of the
c mesponding resistors on wafer 2, switch U2. The output currents ±0,5. I obtained are amplified by 10 in two
c ment mirrors U5/317-320 and made ±5 I.
For asymmetrical operation voltage-current converters U5/351, 352 supply currents ±5 1/9 via the corresponding
c Irrent mirrors.
III this case resistors U2/617 to 620 are used whose values are 9/5 higher than those of the corresponding
rl sistors U2/608 to 611.
3.1.3.3. 1"e output currents of the voltage-current converters are "reflected" by each a current-mirror pair in ratio 1: 1
O'
10: 1 and supplied to the oscillator. The current mirrors are mainly made of a series diode (e.g. U5/306)
-r ~sistor; see example in fig. 8.
11e collector current liN of voltage-current converter U5/301 flows through the current mirror. The resulting
v' Jltage drop controls a current back fed emitter-stage, e.g. U5/305, which supplies the output current IOUT .
jj
ssume the current-voltage characteristics of e.g. diode U5/306 (transistor used as diode) and the emitter-base
d ode of the output transistor (e.g. U5/308) are identical as weil as their relevant resistors_ The ratio II N/IOUT
tl len is 1: 1; otherwise, the current ratio IOUT/11 N is inversely proportional to the resistance ratio.
T1e practical differences of the input diodes and the differences in resistances are compensated by potentiometer
(I .g. fig. 8, U5/620). The built-in potentiometer supplies a correcting voltage, causing a correcting current
sl ,perimposition.
T le base cu rrents which interfer at low values (II N .;;; lOmA) are every time neutral ized by potentiometers
L 5/628, 629, 672, 673.
3.1.3.3.1. rhe current mirror U5/308, 307 is used for time-symmetrical operation. The collector current of transistor
L 5/308 operates as frequency-determining charging current le for the oscillator from the positive direction.
Z ,ner diodes Ul/423, 424 reduce the power dissipation.
T le relevant charging current le* for the oscillator is supplied by voltage-current converter U5/352, 305 via
L 5/307.
T lis current mirror forms the reference for current mirror U5/ 308, which is adjusted by symmetry controi
U5/310, 327, 328 to I e
=
I e * (see 3.1.3.4.). The accuracy of the current mirror for I e * determines the
fr
~quency
accuracy for time-symmetrical operation; maximum adjustment is obtained by means of U5/625
a ld 629.
3.1.3.3.2.
~urrent-mirror
pair U5/319, 320 amplifies the input current 10 times, which corresponds to the ratio of
rE
sistances U5/660 to 675 or U5/661 to 676. These current mirrors are adjusted by potentiometers
U5/665, 670. The Zener diode reduce the power dissipation.
3.1.3.4_
~
ymmetry controi U5/31 O adjusts the charging current automatically to I e * for time-symmetrical operation.
T le symmetry controi is driven by the voltage applied between zero potential and the smallest potential to
ei rth of the oscillator capacitor, whose galvanic zero is disconnected by switch Ul/VI for symmetrical
o leration. The controi loop keeps this potential difference at a constant value near zero.
T lis means that I e and I e * are equal.
3.1.3.5. T le single-shot logic contains a flip-flop (U5/325 and 326) .
T le latter can be set to OFF or START by means of switch SINGLE SHOT MODE; it can be set by input
sljnal eONTROL INPUT (at the rear) and reset by the square-wave signal from the oscillator.
'1\ hen switch SINGLE SHOT MODE is set to SET PHASE, the static blockade at point U5/7 is cancelled, so
H at the flip-flop is reset by the first positive-going square-wave at point U5/e and U5/ 321-322.
I r the oscillator u R is kept to -5 V (see 3.1.2.8.) via point U5/X, the potential of wh ich jumps from O to -5 V
w len switching-over. The potentials at points U5/6 and 4 jump simultaneously from negative va lues to -5 V
ar d +5 V respectively. The start potential of the triangular voltage is adjustable within this voltage range by
m ,ans of potentiometer SET PHASE.
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