CLC418 datasheet(5/12 Pages) NSC

5 / 12 page

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CLC418 OPERATION

The CLC418 has a current-feedback (CFB) architecture

built in an advanced complementary bipolar process.

The key features of current-feedback are:

AC bandwidth is independent of voltage gain

Inherently unity-gain stability

Frequency response may be adjusted with

feedback resistor (Rf in Figures 1-3)

High slew rate

Low variation in performance for a wide range

of gains, signal levels and loads

Fast settling

Current-feedback operation can be explained with a

simple model. The voltage gain for the circuits in Figures 1

and 2 is approximately:

where:

Av is the DC voltage gain

Rf is the feedback resistor

Z(j

ω) is the CLC418’s open-loop

transimpedance gain

is the loop gain

The denominator of the equation above is approximately

low

frequencies.

Near

the

-3dB

corner

frequency, the interaction between Rf and Z(jω)

dominates the circuit performance. Increasing Rf does

the following:

Decreases loop gain

Decreases bandwidth

Reduces gain peaking

Lowers pulse response overshoot

Affects frequency response phase linearity

CLC418 DESIGN INFORMATION

Standard op amp circuits work with CFB op amps. There

are 3 unique design considerations for CFB:

The feedback resistor (Rf in Figures 1-3) sets

AC performance

Rf cannot be replaced with a short or a capacitor

The output offset voltage is not reduced by

balancing input resistances

The following sub-sections cover:

Design parameters, formulas and techniques

Interfaces

Application circuits

Layout techniques

SPICE model information

DC Gain (non-inverting)

The non-inverting DC voltage gain for the configuration

shown in Figure 1 is:

Figure 1: Non-Inverting Gain

The normalized gain plots in the

Typical Performance

Characteristics section show different feedback

resistors (Rf) for different gains. These values of Rf are

recommended for obtaining the highest bandwidth with

minimal peaking. The resistor Rt provides DC bias for

the non-inverting input.

For Av < 6, use linear interpolation on the nearest Av

values to calculate the recommended value of Rf. For Av

≥ 6, the minimum recommended R

f is 200Ω.

Select Rg to set the DC gain:

DC gain accuracy is usually limited by the tolerance of Rf

and Rg.

DC Gain (unity gain buffer)

The recommended Rf for unity gain buffers is 3kΩ. Rg is

left open.

Parasitic capacitance at the inverting node

may require a slight increase of Rf to maintain a flat

frequency response.

DC Gain (inverting)

The inverting DC voltage gain for the configuration

shown in Figure 2 is:

The normalized gain plots in the

Typical Performance

Characteristics

section

show

different

feedback

resistors (Rf) for different gains. These values of Rf are

recommended for obtaining the highest bandwidth with

minimal peaking. The resistor Rt provides DC bias for

the non-inverting input.

For |Av| < 6, use linear interpolation on the nearest Av

values to calculate the recommended value of Rf. For

|Av| ≥ 6, the minimum recommended Rf is 200Ω.

1/2

CLC418

418 Fig1

0.1

µF

6.8

µF

Vin

VCC

0.1

µF

6.8

µF

VEE

3(5)

2(6)

1(7)

Z j

+ ( )ω

Z j

( )

= +

−

= −

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