8

Parasitic Capacitances and Stability

When used in positive-gain configurations, the EL2074 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 con-

junction with the approximately 3pF of board-related

parasitic capacitance from the inverting input to ground.

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 additional

capacitor from the inverting input to the output. This capaci-

tor, in conjunction with the parasitic capacitance, maintains a

constant voltage-divider between the output and the invert-

ing input. This technique is used for AC testing of the

EL2074. A 3pF capacitor is placed in parallel with the feed-

back resistor for all AC tests. When this capacitor is used, it

is also possible to increase the resistance values of the feed-

back and gain resistors without loss of stability, resulting in

less loading of the EL2074 from the feedback network.

Video Performance

An industry-standard method of measuring the video distor-

tion of a component such as the EL2074 is to measure the

amount of differential gain (dG) and differential phase (dP)

that it introduces. To make these measurements, a

offset (0IRE) 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 (100IRE). Differential gain is a measure of

the change in amplitude of the sine wave, and is measured in

percent. Differential phase is a measure of the change in

phase, and is measured in degrees.

For signal transmission and distribution, a back-terminated

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 reflections. However, when double

termination is used, the received signal is halved; therefore a

gain of 2 configuration is typically used to compensate for the

attenuation.

The EL2074 has been designed to be among the best video

amplifiers in the marketplace today. It has been thoroughly

characterized for video performance in the topology

described above, and the results have been included as min-

imum dG and dP specifications and as typical performance

curves. In a gain of +2, driving 150

Ω, with standard video test

levels at the input, the EL2074 exhibits dG and dP of only

0.01% and 0.015° at NTSC and PAL. Because dG and dP

vary with different DC offsets, the superior video perfor-

mance of the EL2074 has been characterized over the entire

DC offset range from -0.714V to +0.714V. For more informa-

tion, refer to the curves of dG and dP vs DC Input Offset.

The excellent output drive capability of the EL2074 allows it

to drive up to 4 back-terminated loads with excellent video

performance. With 4, 150

Ω loads, dG and dP are only 0.15%

and 0.08° at NTSC and PAL. For more information, refer to

the curves for Video Performance vs Number of 150

Ω

Loads.

Output Drive Capability

The EL2074 has been optimized to drive 50

Ω and 75Ω

put drive capability makes the EL2074 an ideal choice for

RF, IF and video applications. Furthermore, the current drive

of the EL2074 remains a minimum of 50mA at low tempera-

tures. The EL2074 is current-limited at the output, allowing it

to withstand momentary shorts to ground. However, power

dissipation with the output shorted can be in excess of the

power-dissipation capabilities of the package.

Capacitive Loads

Although the EL2074 has been optimized to drive resistive

loads as low as 50

Ω, capacitive loads will decrease the

amplifier's phase margin which may result in peaking, over-

shoot, and possible oscillation. For optimum AC

performance, capacitive loads should be reduced as much

as possible or isolated via a series output resistor. Coax lines

can be driven, as long as they are terminated with their char-

acteristic impedance. When properly terminated, the

capacitance of coaxial cable will not add to the capacitive

load seen by the amplifier. Capacitive loads greater than

the load capacitance from the amplifier output. A curve of

without peaking.

Printed-Circuit Layout

As with any high-frequency device, good PCB layout is nec-

essary for optimum performance. Ground-plane construction

is highly recommended, as is good power supply bypassing.

A 1µF–10µF tantalum capacitor is recommended in parallel

with a 0.01µF ceramic capacitor. All pin lengths should be as

short as possible, and all bypass capacitors should be as

close to the device pins as possible. Parasitic capacitances

should be kept to an absolute minimum at both inputs and at

the output. Resistor values should be kept under 1000

Ω to

2000

Ω because of the RC time constants associated with the

parasitic capacitance. Metal-film and carbon resistors are

both acceptable, use of wire-wound resistors is not recom-

mended because of parasitic inductance. Similarly,

capacitors should be low-inductance for best performance. If

possible, solder the EL2074 directly to the PC board without

a socket. Even high quality sockets add parasitic capaci-

tance and inductance which can potentially degrade

performance. Because of the degradation of AC perfor-

mance due to parasitics, the use of surface-mount

components (resistors, capacitors, etc.) is also

recommended.

EL2074