6

Resistive Loads

The DC gain of the EL2003 and EL2033 is the product of the

unloaded gain (0.995) and the voltage divider formed by the

device output resistance and the load resistance.

The high frequency response of the EL2003 and EL2033

varies with the value of the load resistance as shown in the

characteristic curves. If the 100MHz peaking is undesirable

when driving load resistors greater than 50

Ω, an RC snubber

circuit can be used from the output to ground. The snubber

circuit works by presenting a high frequency load resistance

of less than 50

Ω while having no loading effect at low

frequencies.

Capacitive Loads

The EL2003 and EL2033 are stable driving any type of

capacitive load. However, when driving a pure capacitance

of less than a thousand picofarads the frequency response

has excessive peaking as shown in the characteristic curves.

The squarewave response will have large overshoots and

will ring for several hundred nanoseconds.

If the peaking and ringing cause system problems they can

be eliminated with an RC snubber circuit from the output to

ground. The values can be found empirically by observing a

squarewave or the frequency response. First just put the

resistor alone from output to ground until the desired

response is obtained. Of course the gain will be reduced due

restore the gain at low frequencies. Start with a small

capacitor and increase until the response is optimum. Too

large a capacitor will roll the gain off prematurely and result

in a longer settling time. The figure below shows an example

of an EL2003 driving a 330pF load, which is similar to the

input of a flash converter.

FET INPUT BUFFER WITH HIGH OUTPUT CURRENTS

Parallel Operation

BW 100MHz

SR = 1000V/µs

J1, J2 2N5911 Dual FET

R1, R2 Offset Adjust

A

V

0.995

R

L

R

L

R

OUT

+

------------------------------

×

=

SMALL SIGNAL RESPONSE

LARGE SIGNAL RESPONSE

EL2003, EL2033