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4

Customer Design Applications Support

National Semiconductor is committed to design excellence. For sales, literature and technical support, call the

National Semiconductor Customer Response Group at 1-800-272-9959 or fax 1-800-737-7018.

Life Support Policy

National’s products are not authorized for use as critical components in life support devices or systems without the express written approval of

the president of National Semiconductor Corporation. As used herein:

1. Life support devices or systems are devices or systems which, a) are intended for surgical implant into the body, or b) support or

sustain life, and whose failure to perform, when properly used in accordance with instructions for use provided in the labeling, can

be reasonably expected to result in a significant injury to the user.

2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to

cause the failure of the life support device or system, or to affect its safety or effectiveness.

National Semiconductor

National Semiconductor

National Semiconductor

National Semiconductor

Corporation

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Japan Ltd.

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Fax: (+49) 0-180-530 85 86

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Tel: 81-043-299-2309

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Fax: 81-043-299-2408

Tel: 1(800) 272-9959

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Tel: (852) 2737-1600

Italiano Tel: (+49) 0-180-534 16 80

Fax: (852) 2736-9960

National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said

circuitry and specifications.

N

Current Feedback Amplifiers

Some of the key features of current feedback

technology are:

s

Independence of AC bandwidth and voltage gain

s

Inherently stable at unity gain

s

s

High slew rate

s

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.

Equation 1

where:

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

dominates the circuit performance. The value of the

feedback resistor has a large affect on the circuits

s

Decreases loop gain

s

Decreases bandwidth

s

Reduces gain peaking

s

Lowers pulse response overshoot

s

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 CLC5654 (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 frequency

performance. Follow the steps below as a basis for

high frequency layout:

s

Include 6.8

µF tantalum and 0.1µF ceramic

capacitors on both supplies.

s

Place the 6.8

µF capacitors within 0.75 inches of

the power pins.

s

Place the 0.1

µF capacitors less than 0.1 inches

from the power pins.

s

Remove the ground plane under and around the

part, especially near the input and output pins to

reduce parasitic capacitance.

s

Minimize all trace lengths to reduce series

inductances.

s

Use flush-mount printed circuit board pins for

prototyping, 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

V

V

A

1

R

Zj

o

i

v

f

=