10

Current Limit

The EL2020 has internal current limits that protect the output

transistors. The current limit goes down with junction

temperature rise. At a junction temperature of +175°C the

current limits are at about 50mA. If the EL2020 output is

shorted to ground when operating on ±15V supplies, the

power dissipation could be as great as 1.1W. A heat sink is

required in order for the EL2020 to survive an indefinite

short. Recovery time to come out of current limit is about

50ns.

Using the EL2020 with Output Buffers

When more output current is required, a wideband buffer

amplifier can be included in the feedback loop of the

EL2020. With the EL2003 the subsystem overshoots about

10% due to the phase lag of the EL2003. With the EL2004 in

the loop, the overshoot is less than 2%. For even more

output current, several buffers can be paralleled.

Capacitive Loads

The EL2020 is like most high speed feedback amplifiers in

that it does not like capacitive loads between 50pF and

1000pF. The output resistance works with the capacitive load

to form a second non-dominate pole in the loop. This results

in excessive peaking and overshoot and can lead to

oscillations. Standard resistive isolation techniques used

with other op amps work well to isolate capacitive loads from

the EL2020.

Offset Adjust

To calculate the amplifier system offset voltage from input to

output we use the equation:

The EL2020 output offset can be nulled by using a 10k

Ω

potentiometer from pins 1 to 5 with the slider tied to pin 7

input bias current. The typical adjustment range is ±80mV at

the output.

Compensation

The EL2020 is internally compensated to work with external

feedback resistors for optimum bandwidth over a wide range

of closed loop gain. The part is designed for a nominal 1k

Ω

of feedback resistance, although it is possible to get more

bandwidth by decreasing the feedback resistance.

The EL2020 becomes less stable by adding capacitance in

parallel with the feedback resistor, so feedback capacitance

is not recommended.

The EL2020 is also sensitive to stray capacitance from the

inverting input to ground, so the board should be laid out to

keep the physical size of this node small, with ground plane

kept away from this node.

Active Filters

The EL2020’s low phase lag at high frequencies makes it an

excellent choice for high performance active filters. The filter

response more closely approaches the theoretical response

than with conventional op amps due to the EL2020’s smaller

propagation delay. Because the internal compensation of the

EL2020 depends on resistive feedback, the EL2020 should

be set up as a gain block.

Driving Cables

The EL2020 was designed with driving coaxial cables in

mind. With 30mA of output drive and low output impedance,

driving one to three 75

Ω double terminated coax cables with

one EL2020 is practical. Since it is easy to set up a gain of

+2, the double matched method is the best way to drive coax

cables, because the impedance match on both ends of the

cable will suppress reflections. For a discussion on some of

the other ways to drive cables, see the section on driving

cables in the EL2003 data sheet.

Video Performance Characteristics

The EL2020 makes an excellent gain block for video

systems, both RS-170 (NTSC) and faster. It is capable of

driving 3 double terminated 75

Ω cables with distortion levels

acceptable to broadcasters. A common video application is

to drive a 75

Ω double terminated coax with a gain of 2.

To measure the video performance of the EL2020 in the non-

inverting gain of 2 configuration, 5 identical gain-of-two

circuits were cascaded (with a divide by two 75

Ω attenuator

between each stage) to increase the errors.

The results, shown in the photos, indicate the entire system

of 5 gain-of-two stages has a differential gain of 0.5% and a

differential phase of 0.5°. This implies each device has a

EL2020 BUFFERED WITH AN EL2004

EL2020