C1

+

−

R1

f

*3 dB +

1

2pR1C1

V

O

V

I

+

1 )

R

F

R

G

1

1 ) sR1C1

C2

R2

R1

C1

R1 = R2 = R

C1 = C2 = C

Q = Peaking Factor

(Butterworth Q = 0.707)

(

=

1

Q

2 −

)

_

+

f

*3 dB

+

1

2pRC

+

−

C1

THS3001

V

O

V

I

+

R

F

R

G

S

)

1

R

F

C1

S

THS3001

GENERAL CONFIGURATIONS

A common error for the first-time CFB user is the creation of a unity gain buffer amplifier by shorting the output

directly to the inverting input. A CFB amplifier in this configuration will oscillate and is not recommended. The

THS3001, like all CFB amplifiers, must have a feedback resistor for stable operation. Additionally, placing

capacitors directly from the output to the inverting input is not recommended. This is because, at high

frequencies, a capacitor has a low impedance. This results in an unstable amplifier and should not be considered

when using a current-feedback amplifier. Because of this, integrators and simple low-pass filters, which are

easily implemented on a VFB amplifier, have to be designed slightly differently. If filtering is required, simply

place an RC-filter at the noninverting terminal of the operational-amplifier (see Figure 56).

Figure 56. Single-Pole Low-Pass Filter

If a multiple-pole filter is required, the use of a Sallen-Key filter can work well with CFB amplifiers. This is

because the filtering elements are not in the negative feedback loop and stability is not compromised. Because of

their high slew rates and high bandwidths, CFB amplifiers can create accurate signals and help minimize

distortion. An example is shown in Figure 57.

Figure 57. 2-Pole Low-Pass Sallen-Key Filter

There are two simple ways to create an integrator with a CFB amplifier. The first, shown in Figure 58, adds a

resistor in series with the capacitor. This is acceptable because at high frequencies, the resistor is dominant and

the feedback impedance never drops below the resistor value. The second, shown in Figure 59, uses positive

feedback to create the integration. Caution is advised because oscillations can occur due to the positive

feedback.

Figure 58. Inverting CFB Integrator

Copyright © 1998–2009, Texas Instruments Incorporated

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