Application Division

Current Feedback Amplifiers

Some of the key features of current feedback technology

are:

•

Independence of AC bandwidth and voltage gain

•

Inherently stable at unity gain

•

Adjustable frequency response with R

f

•

High slew rate

•

Fast settling

Current feedback operation can be described using a simple

equation. The voltage gain for a non-inverting or inverting

current feedback amplifier is approximated by Equation 1.

V

V

A

1

R

Zj

o

i

v

f

=

(1)

where:

A

R

Z(j

ω) is the open loop transimpedance gain

The denominator of Equation 1 is approximately equal to 1 at

low frequencies. Near the −3dB corner frequency, the inter-

action between R

mance. The value of the feedback resistor has a large affect

on the circuits performance. Increasing R

affects:

•

Decreases loop gain

•

Decreases bandwidth

•

Reduces gain peaking

•

Lowers pulse response overshoot

•

Affects frequency response phase linearity

Layout Considerations

A proper printed circuit layout is essential for achieving high

frequency performance. National provides evaluation boards

for the CLC5644 (CLC730024-DIP, CLC730031-SOIC) and

suggests their use as a guide for high frequency layout and

as an aid for device testing and characterization. General

layout and supply bypassing play major roles in high fre-

quency performance. Follow these steps below as a basis

for high frequency layout:

•

Include 6.8µF tantalum and 0.1µF ceramic capacitors on

both supplies.

•

Place the 6.8µF capacitors within 0.75 inches of the

power pins.

•

Place the 0.1µF capacitors less than 0.1 inches from the

power pins.

•

Remove the ground plane under and around the part,

especially near the input and output pins to reduce para-

sitic capacitance.

•

Minimize all trace lengths to reduce series inductances.

•

Use flush-mount printed circuit board pins for prorotyp-

ing, never use high profile DIP sockets.

Active Filter Application Notes

OA-21 Simplified Component Pre-Distortion for High Speed

Active Filters

OA-26 Designing High-Speed Active Filters

OA-27 Low-Sensitivity, Lowpass Filter Design

OA-28 Low-Sensitivity, Bandpass Filter Design with Tuning

Method

OA-29 Low-Sensitivity, Highpass Filter Design with Parasitic

Compensation

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