EL2074C datasheet(8/12 Pages) ELANTEC

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EL2074C

400MHz GBWP Gain-of-2 Stable Operational Amplifier

Applications Information

Product Description

The EL2074C is a wideband monolithic operational

amplifier built on a high-speed complementary bipolar

process. The EL2074C uses a classical voltage-feedback

topology which allows it to be used in a variety of appli-

cations requiring a noise gain

≥2 where current-feedback

amplifiers are not appropriate because of restrictions

placed upon the feedback element used with the ampli-

fier. The conventional topology of the EL2074C allows,

for example, a capacitor to be placed in the feedback

path, making it an excellent choice for applications such

as active filters, sample-and-holds, or integrators. Simi-

larly, because of the ability to use diodes in the feedback

network, the EL2074C is an excellent choice for appli-

cations such as log amplifiers.

The EL2074C also has excellent DC specifications:

200µV, VOS, 2µA IB, 0.1µA IOS, and 90dB of CMRR.

These specifications allow the EL2074C to be used in

DC-sensitive applications such as difference amplifiers.

Furthermore, the current noise of the EL2074C is only

3.2pA/

√Hz, making it an excellent choice for high-sen-

sitivity transimpedance amplifier configurations.

Gain-Bandwidth Product

The EL2074C has a gain-bandwidth product of

400MHz. For gains greater than 8, its closed-loop -3dB

bandwidth is approximately equal to the gain-bandwidth

product divided by the noise gain of the circuit. For

gains less than 8, higher-order poles in the amplifier's

transfer function contribute to even higher closed loop

bandwidths. For example, the EL2074C has a -3dB

bandwidth of 400MHz at a gain of +2, dropping to

200MHz at a gain of +4. It is important to note that the

EL2074C has been designed so that this “extra” band-

width in low-gain applications does not come at the

expense of stability. As seen in the typical performance

curves, the EL2074C in a gain of +2 only exhibits 1dB

of peaking with a 100

Ω load.

Parasitic Capacitances and Stability

When used in positive-gain configurations, the

EL2074C can be quite sensitive to parasitic capacitances

at the inverting input, especially with values

≥250Ω for

the gain resistor. The problem stems from the feedback

and gain resistance in conjunction with the approxi-

mately 3pF of board-related parasitic capacitance from

the inverting input to ground. Assuming a gain-of-2 con-

figuration with RF = RG = 250Ω, a feedback pole occurs

at 424MHz, which is equivalent to a zero in the forward

path at the same frequency. This zero reduces stability

by reducing the effective phase-margin from about 50°

to about 30°.

A common solution to this problem is to add an addi-

tional capacitor from the inverting input to the output.

This capacitor, in conjunction with the parasitic capaci-

tance, maintains a constant voltage-divider between the

output and the inverting input. This technique is used for

AC testing of the EL2074. A 3pF capacitor is placed in

parallel with the feedback resistor for all AC tests. When

this capacitor is used, it is also possible to increase the

resistance values of the feedback and gain resistors with-

out loss of stability, resulting in less loading of the

EL2074C from the feedback network.

Video Performance

An industry-standard method of measuring the video

distortion of a component such as the EL2074C is to

measure the amount of differential gain (dG) and differ-

ential phase (dP) that it introduces. To make these

measurements, a 0.286VPP (40 IRE) signal is applied to

the device with 0V DC offset (0 IRE) at either 3.58MHz

for NTSC, 4.43MHz for PAL, or 30MHz for HDTV. A

second measurement is then made at 0.714V DC offset

(100 IRE). Differential gain is a measure of the change

in amplitude of the sine wave, and is measured in per-

cent. Differential phase is a measure of the change in

phase, and is measured in degrees.

For signal transmission and distribution, a back-termi-

nated cable (75

Ω in series at the drive end, and 75Ω to

ground at the receiving end) is preferred since the

impedance match at both ends will absorb any reflec-

tions. However, when double termination is used, the

received signal is halved; therefore a gain of 2 configu-

ration is typically used to compensate for the

attenuation.

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