EL2082CN 데이터 시트보기 (PDF) - Elantec -> Intersil
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EL2082CN Datasheet PDF : 16 Pages
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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
Fb3 dB 4 to Fb3 dB 2 where F b3 dB is the unity-gain closed-loop bandwidth of the amplifier The
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
adjusted for amplifier stability at maximum VGAIN then RIN is set for the overall circuit gain re-
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 Rf 2
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
EL2082 in feedback
non-inverting gain
J 1V
Gain e
RF a 95X
Vg
Rg
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
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