Data Sheet
General Information - Current Feedback
Technology
Advantages of CFB Technology
The CLCx600 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, R
f
. 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
Ierr
x1
Z
o*Ierr
R
f
V
OUT
Comlinear CLC2600, CLC3600, CLC4600
Dual, Triple, and Quad 300MHz Amplifiers
V
IN
R
g
R
L
V
OUT
V
IN
= −
R
f
R
g
+
1
+
1
R
f
Z
o(jω)
Eq. 2
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 (I
err
) 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 (Z
o
(jω) * I
err
) 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 Z
o
(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, Z
o
(jω) on the
gain of the circuit. At low frequencies where Z
o
(jω) is very
large with respect to R
f
, the second term of the equation
approaches unity, allowing R
f
and R
g
to set the gain. At
higher frequencies, the value of Z
o
(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 Z
o
(jω) equals the value of R
f
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
.
www.cadeka.com
V
IN
Ierr
x1
Z
o*Ierr
R
f
V
OUT
R
L
Rg
V
OUT
V
IN
=
1
+
R
f
R
g
+
1
+
1
R
f
Z
o(jω)
Rev 1A
Eq. 1
Figure 1. Non-Inverting Gain Configuration with First
Order Transfer Function
©2004-2008 CADEKA Microcircuits LLC
9
CADEKA [ CADEKA MICROCIRCUITS LLC. ]
