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±5V to ±12V levels, especially if extra dissipation occurs

when driving a load.

The EL4094 as a Level Adjust

A common use for gain controls is as an input signal

leveller—a circuit that scales too-large or too-small signals to

a standard amplitude. A typical situation would be to scale a

variable video input by +6dB to -6dB to obtain a standard

amplitude. The EL4094 cannot provide more than 0dB gain,

but it can span the range of 0dB to -12dB with another

amplifier gaining the output up by 6dB. The simplest way to

obtain the range is to simply ground the B input and vary the

gain of the signal applied to the A input. The disadvantage of

this approach is that linearity degrades at low gains. By

connecting the signal to the A input of the EL4094 and the

signal attenuated by 12dB to the B input, the gain control

offers the highest linearity possible at 0dB and -12dB

extremes, and good performance between. The circuit is

shown on the following page.

The EL4095 can be used to provide the required gains

without the extra amplifier. In practice, the gain control is

adjusted to set a standard video level regardless of the input

level. The EL4583 sync-separator has a recovered

amplitude output that can be used to servo the gain control

voltage. Here is the curve of differential gain and phase

distortion for varying inputs, with the output set to standard

video level:

The differential gain error is kept to 0.3% and the differential

phase to 0.15° or better over the entire input range.

The EL4094 as an Adjustable Filter

Equalizers are used to adjust the delay or frequency

response of systems. A typical use is to compensate for the

high-frequency loss of a cable system ahead of the cable so

as to create a flat response at the far end. A generalized

scheme with the EL4094 is shown below.

FIGURE 2. DIFFERENTIAL GAIN AND PHASE OF

LINEARIZED LEVEL CONTROL

FIGURE 3. +6dB to -6dB LINEARIZED LEVEL CONTROL

EL4094