+

–

C1

RF

RG

VO

VI

THS6012

V

O

V

I

+

R

F

R

G

S

)

1

R

F

C1

S

VI

C2

R2

R1

C1

RF

RG

R1 = R2 = R

C1 = C2 = C

Q = Peaking Factor

(Butterworth Q = 0.707)

(

=

1

Q

2 –

)

RG

RF

_

+

f

–3dB +

1

2pRC

VI

VO

C1

+

–

RG

RF

R1

f

–3dB +

1

2pR1C1

V

O

V

I

+

1 )

R

F

R

G

1

1 ) sR1C1

THS6012

www.ti.com

SLOS226F – SEPTEMBER 1998 – REVISED JUNE 2012

GENERAL CONFIGURATIONS

A common error for the first-time CFB user is to create a unity gain buffer amplifier by shorting the output directly

to the inverting input. A CFB amplifier in this configuration oscillates and is not recommended. The THS6012,

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 very 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 48).

Figure 48. Single-Pole Low-Pass Filter

If a multiple pole filter is required, the use of a Sallen-Key filter can work very 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 very accurate signals and help minimize

distortion. An example is shown in Figure 49.

Figure 49. 2-Pole Low-Pass Sallen-Key Filter

There are two simple ways to create an integrator with a CFB amplifier. The first one shown in Figure 50 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 one shown in Figure 51 uses positive

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

feedback.

Figure 50. Inverting CFB Integrator

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