6

bias current. But there is no increase in offset voltage. Three

units in parallel can drive a 3

Ω load ±10V at 2500V/µs. The

output impedance will be about 0.33

Ω.

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

Ω, 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

Ω (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.

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

Ω 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

Ω 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

Ω to

5

Ω 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.

EL2008