EL2082C

Current-Mode Multiplier

Applications Information

Contd

The circuit sums two inputs A and B such that the sum of their respective path gains is unity as

controlled by the potentiometer When the potentiometer’s wiper is fully down the slightly negative

voltage at the Vg of the B-side EL2082 cuts off the B signal to better than 70 dB attenuation at

358 MHz The A-side EL2082 is at unity gain so the only (error) signal presented to the op-amp’s -in-

error signal required by the op-amp is very small even at video frequencies the current through the A-

side EL2082 is small and distortion is minimized

At 50% potentiometer setting equal error output signals flow from the EL2082’s since the op-amp still

requires little net -input current The EL2082’s essentially buck each other to establish an output and

50% gain occurs for both the A and B inputs The EL2082’s now contribute distortion but less than in

previous connections The op-amp sees a constant 1k feedback resistor regardless of potentiometer

setting so frequency response is stable for all gain settings

Distortion can be improved by increasing the input resistors to lower signal currents This will lower

the overall gain accordingly but will not affect bandwidth which is dependent upon the feedback

resistors Reducing the signal input amplitude is an analagous tactic but the noise floor will effectively

rise

Another strategy to reduce distortion in video systems is to use DC restoration circuitry such as the

span

The EL2020 EL2030 and EL2120 (at reduced bandwidth since it is compensated for 300

X feedback

resistors) all give the same video performance at NTSC operation

Variable Filters

This circuit is the familiar state-variable configuration similar to the bi-quad

Voltage Tuneable Bi-Quad Filter

Vg

1V



1

2

J

2082 – 23

13