7

Unfortunately the isolation resistor is not inside the op amp

feedback loop and cannot be neglected when computing the

DC voltage gain into a resistive load. If load dependent DC

gain is not tolerable then additional high frequency feedback

from the op amp output (the EL2008 input) and an isolation

resistor from the buffer output can be used to stabilize the

loop. This configuration requires the op amp to be unity gain

stable. This feedback method will allow the EL2008 to boost

the output of the EHA2505 amplifier below and serve as a

variable, bipolar 1A voltage supply with short circuit

protection.

Video Distribution Amplifier

The EL2008 can drive 15 double matched 75

Ω cables. If the

EL2008 is used within an op amp feedback loop the output

levels are independent of loading. The circuit below accepts

1 of 2 inputs and drives 15 cables. Pin 8 of the EL2020

(Disable) is used to multiplex between the inputs and can be

easily expanded to accept more inputs. The circuit as shown

when fully loaded has differential phase < 0.1° and

differential gain < 0.1%. The 100

Ω resistor at the EL2008

input (R1) is necessary to stabilize the loop. The 100

Ω

resistor at the EL2008 output (R2) to the -12V supply,

insures that the EL2008 sources current even when the

output voltage is at 0V. This is necessary to achieve the

excellent differential gain and phase values. More

information about driving cables can be found in the EL2003

data sheet. See the EL2020 data sheet to learn more about

using it as a multiplexer.

O.S.

13pF

470pF

1000pF

3300pF

0.1µF

1µF

5µF

45ns

50ns

55ns

60ns

350ns

4µs

20µs

20%

20%

30%

30%

0%

0%

0%

INPUT (TOP TRACE) AND OUTPUT (BOTTOM TRACE)

OF EHA2505 OP AMP BOOSTED BY EL2008.

EL2008