Tektronix CURVE-TRACER 576 Instruction Manual page 56

Circuit Description-Type 576
2 KV Bridge•
115 AC
VARIABLE
COLLECTOR
SUPPLY VOLTS
T300
T301
....
,.
....
...
....
...
1500
,
350 ...
...
15 & 75
...
,
/
,
,
SERIES
RESISTORS
NORMT
DC
LEAKAGE
I
MODEl
CURRENT
RETURN
INPUT
COLLECTOR
SWEEP
OUT
CURRENT
SENSING
RESISTOR
From
Device
Under
Test
500V Bridge•
*Bridges drawn from POLARITY switch set to+ (NPN)
Fig. 3-1. Simplified schematic of collector supply circuit.
rectifier assembly is used for rectification. Operation in the
1500 volt range is similar to operation in the 350 volt range
except that the 2 kilovolt bridge is used for rectification.
The POLARITY switch (see the Collector Supply schematic)
allows the choice of three different sweep outputs from the
collector supply by changing the output connections on the
rectifier bridges. The possible outputs are positive-going
+
(NPN) or negative-going - (PNP) full-wave rectifed sine
waves or unrectified sine-waves (AC). In all cases the peak
amplitude of the collector sweep is controlled by the VARI-
ABLE COLLECTOR SUPPLY control and the MAX PEAK
VOLTS switch.
The MODE switch allows the choice of two different Collec-
tor Supply outputs: the normal collector sweep as has been
previously mentioned and a DC collector voltage output. When
the MODE switch is set to DC (ANTILOOP) or LEAKAGE (EMIT-
TER CURRENT) the MAX PEAK VOLTS switch picks one of
four resistor-capacitor combinations which is connected be-
tween the collector sweep output and the current return input.
The purpose of these capacitors is to hold the collector sweep
voltage at a constant DC level set by the VARIABLE COLLEC-
3-2
TOR SUPPLY control. This holding is done by charging the
capacitor up to maximum peak voltage as set by the VARI-
ABLE COLLECTOR SUPPLY control and keeping them
charged with the repetitive collector sweep. The result of
charging these holding capacitors is a dot on the CRT rather
than the normal sweep.
In series with the collector sweep are series resitors R345
through R355. The interconnected MAX PEAK VOLTS and
PEAK POWER WATIS switches add these resistors in series
according to the amount of peak collector current desired. The
amount of this current is determined by the maximum power
dissipation rating of the device under test.
Looping
There is a certain amount of non-discrete capacitance asso-
ciated with the collector supply which causes an effect known
as looping. Part of this undesired capacitance is stray capaci-
tance, which provides an AC current path between the collec-
tor supply and chassis ground. The transformer and the guard
box also exhibit some undesired capacitance between the
guard box potential (common return point connected to guard
REV MAR 1985
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