el2008 Intersil Corporation, el2008 Datasheet - Page 6

• Manufacturer Part Number: el2008
• Description: 55mhz 1 Amp Buffer Amplifier
• Manufacturer: Intersil Corporation

bias current. But there is no increase in offset voltage. Three
units in parallel can drive a 3: load ±10V at 2500V/µs. The
output impedance will be about 0.33:.
Resistive Loads
The DC gain of the EL2008 is the product of the unloaded
gain (0.999) and the voltage divider formed by the device
output resistance and the load resistance.
The high frequency response varies with the load resistance
as shown by the characteristic curves. Both gain and phase
are shown. If the 80MHz peaking is undesirable when driving
load resistors greater than 50:, an RC snubber circuit can
be used from output to ground. The capacitive load section
discusses snubber usage in more detail.
Capacitive Loads
The EL2008 is not stable driving purely capacitive loads
between 100pF and 500pF. Purely capacitive loads from
500pF to 1000pF will also have excessive peaking as shown
in the characteristic curves. The squarewave response will
have large overshoots and ring for hundreds of
nanoseconds.
When driving capacitive loads, stability can be achieved and
peaking and ringing can be minimized either by adding a
50: (or less) load in parallel with the capacitive load or by an
RC snubber circuit from output to ground. The snubber
values can be found empirically by observing a squarewave
or the frequency response. First just put a resistor alone
from the output to ground until the desired response is
achieved. The gain will be reduced due to the output
resistance of the EL2008 and power consumption will be
high. Then put a capacitor in series with the resistor to
restore gain at low frequencies and eliminate the DC current.
Start with a small capacitor and increase until the response
is optimum. The figure below shows an example of an
EL2008 driving a 100pF load.
A
V
= 0.999* (R
L
/R
6
L
+ R
OUT
)
EL2008
Inductive Loads
The EL2008 with its 1A output current can drive small
motors and other inductive loads. The EL2008’s current
limiting into inductive loads does NOT in and of itself cause
spikes and kickbacks. However, if the EL2008 is in current
limit and the input voltage is changing very quickly (i.e., a
squarewave) the inductive load can kick the output beyond
the supply voltages. Motors are also able to generate
kickback voltages when the EL2008 is in current limit.
To prevent damage to the EL2008 when the output kicks
beyond the supplies, it is recommended that catch diodes be
placed from each supply to the output.
Op Amp Booster
The EL2008 can boost the output drive of almost any
monolithic op amp. If the phase shift in the EL2008 is low at
the op amp's unity gain frequency, no additional frequency
compensation is required. An op amp followed with the
EL2008 can drive loads as low as 10: to ±10V.
Driving capacitive loads with any closed loop system creates
special problems. The open loop output impedance works
into the load capacitance to generate phase lag which can
make the loop unstable. The EL2008 output impedance is
less than 10: from DC to 30MHz. But a capacitive load of
1000pF will generate about 45 degrees of phase shift at
30MHz. More capacitance will cause the problem at lower
frequency.
With enough capacitance even slow op amps will become
unstable. The simplest way to drive capacitive loads is to
isolate them from the feedback with a series resistor. 1: to
5: is usually enough but the final value will depend on the
op amp used and the range of load capacitance.
DRIVING A PURE CAPACITIVE LOAD. TOP TRACE IS
WITHOUT A SNUBBER. BOTTOM TRACE IS WITH A
SNUBBER CIRCUIT.