• Use the recommended feedback resistor value
• Do not use non-linear elements in the direct feedback path
• Follow general high-speed amplifier layout guidelines

Feedback Resistor For CFB amplifiers, pay close attention to the elements in the feedback path. The manufacturer's data sheet will provide a recommended value for the feedback resistance, R_f. An excessively large or small R_f value will compromise stability. Within reason, the feedback resistor can be used to adjust the frequency response and, as a rule of thumb, if the recommended R_f is doubled, the bandwidth will be cut in half.

Common Applications Using a CFB Amplifier

The following sections illustrate possible configurations for common applications using CFB amplifiers, although these topologies can also be used for VFB amplifiers.

Unity Gain

CFB amplifiers are inherently unity-gain stable, although some CFB amplifiers have been optimized for higher gains. A voltage follower circuit cannot be used to implement a unity-gain configuration with a CFB amplifier. A feedback resistor must be included. Fig. 1 illustrates this point.

Fig. 1: For unity-gain configurations using CFB amplifiers, R_fmust be included.

Integrator

Non-linear elements cannot be put in the feedback path of a CFB amplifier. The most simplistic example is an integrator circuit and Fig. 2 illustrates the common integrator topology. A CFB amplifier cannot be used in this configuration because of the capacitive feedback but this doesn't mean that a CFB amplifier cannot be used as an integrator, simply that a different configuration must be used. One implementation is also shown in Fig. 2. In this circuit, feedback capacitance is acceptable because the amplifier relies on R_ffor stability.

Fig. 2: Traditional integrator topology and a possible topology for CFB amplifiers.

Active Filter

Active filters, rectifiers, and other applications where the traditional configurations include a non-linear feedback component can also be implemented with a CFB amplifier. Fig. 3 shows a 2nd Order Sallen-Key Bandpass Filter topology for use with CFB or VFB amplifiers.

Fig. 3: 2nd Order Sallen-Key Bandpass Filter Topology

The center frquency is:

The overall DC gain is determined by:

The Q factor of the filter is related to the gain by:

Layout

General layout and supply-bypass play major roles in the performance of a high-speed amplifier. Parasitic capacitances and inductances on the input and output pins of a high-speed amplifier will compromise stability. Follow the steps below as a basis for high-frequency layout and whenever possible use the manufacturers' evaluation board as a guide.