CLC1605 datasheet(14/21 Pages) EXAR

14 / 21 page

Data Sheet

14/21

Rev 1E

General Information - Current Feedback

Technology

Advantages of CFB Technology

The CLC1605 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

determined by the value of the feedback resistor, Rf. By

using 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

resistors

• 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

feedback 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 impedance

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 employs

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 limitations

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

amplifiers, visit www.exar.com for detailed application

notes, such as AN-3:

The Ins and Outs of Current

Feedback Amplifiers.

OUT

= 1 +

Eq. 1

1 +

o(jω)

VIN

VOUT

Zo*Ierr

Ierr

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