EL2004 EL2004C
350 MHz FET Buffer
Circuit Operation
Contd
Current Limiting
Using Current Sources
Offset Zero Adjust
2004 – 10
Capacitive Loading
The EL2004 is designed to drive capacitive loads
up to several thousand picofarads without oscil-
lation However peak current resulting from
charging currents on fast edges should be limited
below the absolute maximum peak current rating
of 250 mA In some cases it may be necessary to
employ one of the current limit schemes shown
below
2004 – 12
Short Circuit Protection
Dynamic response of the EL2004 was preserved
by excluding current limit circuits which are not
needed in most applications However in situa-
tions where operating conditions are not con-
trolled short circuit protection can be added by
inserting resistors between the output device col-
lectors and supplies as illustrated
Using Resistor
Current Limiting
The inclusion of limiting resistors in the collec-
tors of the output devices will reduce the output
voltage swing and speed Decoupling V
C
a
and
V
C
b
pins with capacitors to ground will retain
full output swing for transient pulses
An alternate active current limit technique that
retains full DC output swing is shown above
Here the current sources are saturated during
normal operation thus applying full supply volt-
age to the V
C
pins Under fault conditions the
voltage decreases as the current source reaches its
limit
R
LIM
e
V
BE
0 6V
e
e
6X
I
SC
100 mA
Power Supplies
The EL2004 has been characterized for both
g
15
and
g
5V dual supply operation but other combi-
nations can also be useful For example in many
video applications it is only necessary for the out-
put to swing
g
2V or less but speed and distor-
tion are important In this situation the input
stage can be operated at the full
g
15V supply
while the output collectors are returned to
g
5V
The speed and distortion will be almost as good
as if the whole circuit was operating at
g
15V
but the dissipation is substantially reduced and
higher load currents can be safely accommodat-
ed
2004 – 11
Suitable resistor values can be calculated as fol-
lows
V
a
V
b
e
I
SC
I
SC
where I
SC
s
100 mA for EL2004
R
SC
e
8
ELANTEC [ ELANTEC SEMICONDUCTOR ]
