CLC1606ISO8 datasheet(12/18 Pages) CADEKA

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Data Sheet

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General Information - Current Feedback

Technology

Advantages of CFB Technology

The CLC1606 Family of amplifiers utilize current feedback

(CFB) technology to achieve superior performance. The

primary advantage of CFB technology is higher slew rate

performance when compared to voltage feedback (VFB)

architecture. High slew rate contributes directly to better

large signal pulse response, full power bandwidth, and

distortion.

CFB also alleviates the traditional trade-off between

closed loop gain and usable bandwidth that is seen with

a VFB amplifier. With CFB, the bandwidth is primarily de-

termined by the value of the feedback resistor, Rf. By us-

ing optimum feedback resistor values, the bandwidth of a

CFB amplifier remains nearly constant with different gain

configurations.

When designing with CFB amplifiers always abide by these

basic rules:

• Use the recommended feedback resistor value

• Do not use reactive (capacitors, diodes, inductors, etc.)

elements in the direct feedback path

• Avoid stray or parasitic capacitance across feedback re-

sistors

• Follow general high-speed amplifier layout guidelines

• Ensure proper precautions have been made for driving

capacitive loads

Figure 1. Non-Inverting Gain Configuration with First

Order Transfer Function

OUT

= −

Eq. 2

1 +

o(jω)

VIN

VOUT

Zo*Ierr

Ierr

Figure 2. Inverting Gain Configuration with First Order

Transfer Function

CFB Technology - Theory of Operation

Figure 1 shows a simple representation of a current feed-

back amplifier that is configured in the traditional non-

inverting gain configuration.

Instead of having two high-impedance inputs similar to a

VFB amplifier, the inputs of a CFB amplifier are connected

across a unity gain buffer. This buffer has a high imped-

ance input and a low impedance output. It can source or

sink current (Ierr) as needed to force the non-inverting

input to track the value of Vin. The CFB architecture em-

ploys a high gain trans-impedance stage that senses Ierr

and drives the output to a value of (Zo(jω) * Ierr) volts.

With the application of negative feedback, the amplifier

will drive the output to a voltage in a manner which tries

to drive Ierr to zero. In practice, primarily due to limita-

tions on the value of Zo(jω), Ierr remains a small but

finite value.

A closer look at the closed loop transfer function (Eq.1)

shows the effect of the trans-impedance, Zo(jω) on the

gain of the circuit. At low frequencies where Zo(jω) is very

large with respect to Rf, the second term of the equation

approaches unity, allowing Rf and Rg to set the gain. At

higher frequencies, the value of Zo(jω) will roll off, and

the effect of the secondary term will begin to dominate.

The -3dB small signal parameter specifies the frequency

where the value Zo(jω) equals the value of Rf causing the

gain to drop by 0.707 of the value at DC.

For more information regarding current feedback ampli-

fiers, visit www.cadeka.com for detailed application notes,

such as AN-3:

The Ins and Outs of Current Feedback Am-

plifiers.

OUT

= 1 +

Eq. 1

1 +

o(jω)

VIN

VOUT

Zo*Ierr

Ierr

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