EL2004/EL2004C
350 MHz FET Buffer
Current Limiting
Using Current Sources
Circuit Operation Ð Contd.
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
The inclusion of limiting resistors in the collec-
tors of the output devices will reduce the output
a
pins with capacitors to ground will retain
voltage swing and speed. Decoupling V
b
full output swing for transient pulses.
and
C
V
C
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.
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 pins. Under fault conditions, the
C
voltage decreases as the current source reaches its
limit.
Using Resistor
Current Limiting
V
0.6V
BE
e
e
e
6X
R
LIM
I
100 mA
SC
Power Supplies
The EL2004 has been characterized for both 15
g
and 5V dual supply operation, but other combi-
g
nations can also be useful. For example, in many
video applications it is only necessary for the out-
g
put to swing 2V or less, but speed and distor-
tion are important. In this situation, the input
g
stage can be operated at the full 15V supply
while the output collectors are returned to 5V.
g
The speed and distortion will be almost as good
g
as if the whole circuit was operating at 15V,
2004–11
but the dissipation is substantially reduced and
higher load currents can be safely accommodat-
ed.
Suitable resistor values can be calculated as fol-
lows:
a
b
V
V
e
e
R
SC
I
I
SC
SC
s
where I
100 mA for EL2004.
SC
8
ELANTEC [ ELANTEC SEMICONDUCTOR ]
