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EL400C

200MHz Current Feedback Amplifier

Applications Information

Theory of Operation

The EL400C has a unity gain buffer from the non-invert-

ing input to the inverting input. The error signal of the

EL400C is a current flowing into (or out of) the inverting

input. A very small change in current flowing through the

inverting input will cause a large change in the output

voltage. This current amplification is called the trans-

inverting input in the steady-state (non-slewing) condi-

tion is very small.

Therefore we can still use op-amp assumptions as a first-

order approximation for circuit analysis, namely that:

1. The voltage across the inputs is approximately 0V.

2. The current into the inputs is approximately 0mA.

Resistor Value Selection and Optimization

The value of the feedback resistor (and an internal

capacitor) sets the AC dynamics of the EL400C. The

nominal value for the feedback resistor is 250

Ω, which

is the value used for production testing. This value guar-

antees stability. For a given closed-loop gain the

bandwidth may be increased by decreasing the feedback

resistor and, conversely, the bandwidth may be

decreased by increasing the feedback resistor.

Reducing the feedback resistor too much will result in

overshoot and ringing, and eventually oscillations.

Increasing the feedback resistor results in a lower -3dB

frequency. Attenuation at high frequency is limited by a

zero in the closed-loop transfer function which results

from stray capacitance between the inverting input and

ground. Consequently, it is very important to keep stray

capacitance to a minimum at the inverting input.

Differential Gain/Phase

An industry-standard method of measuring the distor-

tion of a video component is to measure the amount of

differential gain and phase error it introduces. To mea-

device with 0V DC offset (0IRE) at 3.58MHz for NTSC,

4.43MHz for PAL, and 30MHz for HDTV. A second

measurement is then made with a 0.714V DC offset

(100IRE). 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. Typically, the maxi-

mum positive and negative deviations are summed to

give peak values.

In general, a back terminated cable (75

Ω in series at the

drive end and 75

Ω to ground at the receiving end) is pre-

ferred since the impedance match at both ends will

absorb any reflections. However, when double-termina-

tion is used, the received signal is reduced by half;

therefore a gain of 2 configuration is typically used to

compensate for the attenuation. In a gain of 2 configura-

terminated load at 150

Ω. The EL400C is capable of

driving up to 4 back-terminated loads with excellent

video performance. Please refer to the typical curves for

more information on video performance with respect to

frequency, gain, and loading.

Capacitive Feedback

The EL400C relies on its feedback resistor for proper

compensation. A reduction of the impedance of the feed-

back element results in less stability, eventually

resulting in oscillation. Therefore, circuit implementa-

tions which have capacitive feedback should not be used

because of the capacitor's impedance reduction with fre-

quency. Similarly, oscillations can occur when using the

technique of placing a capacitor in parallel with the feed-

back resistor to compensate for shunt capacitances from

the inverting input to ground.

Offset Adjustment Pin

Output offset voltage of the EL400C can be nulled by

slider attached to pin 1. A full-range variation of the

voltage at pin 1 to ±5V results in an offset voltage

adjustment of at least ±10mV. For best settling perfor-

mance pin 1 should be bypassed to ground with a

ceramic capacitor located near to the package, even if

the offset voltage adjustment feature is not being used.