EL2082C

Current-Mode Multiplier

Applications Information

Contd

Current-feedback op-amps again solve the summing-junction capacitance problem in this connection

The loop bandwidth here becomes a matter of transimpedance over frequency and its phase characteris-

tics Unfortunately this is generally poorly documented in amplifier data sheets A rule of thumb is

that the transimpedance falls to the value of the recommended feedback resistor at a frequency of

phase margin of the op-amp is usually close to 90 degrees at this frequency

In general Rf is initially the recommended value for the particular amplifier and is then empirically

quired Sometimes a very small Cf can be used to improve loop stability but it often must be in series

with another resistor of value around Rf2

A virtue of placing the EL2082 in feedback is that the input-referred noise will drop as gain increases

This is ideal for level controls that are used to set the output to a constant level for a variety of inputs

as well as AGC loops Furthermore the EL2082 has a relatively constant input signal amplitude for a

variety of input levels and its distortion will be relatively constant and controllable by setting Rf Note

that placing the EL2082 in the feedback path causes the circuit bandwidth to vary inversely with gain

The next circuit shows use of the EL2082 in the feedback path of a non-inverting op-amp

1V

J

EL2082 in feedback

non-inverting gain

2082 – 16

This example has the same virtues with regards to noise and distortion as the preceding circuit and its

bandwidth shrinks with increasing gain as well The typical 12 pF sum of EL2082 output capacitance in

parallel with stray capacitance necessitates the inclusion of Cf to prevent a feedback pole Because of

this 12 pF capacitance at the op-amp -input current-feedback op-amps will generally not be useable As

before the loop bandwidth and phase margin must accommodate the extra phase lag of the EL2082

10