Design Ideas: November 9, 1995

Many an engineer designing with wideband op amps has battled parasitic capacitance at the inverting input. What’s more, if you design a discrete gain-select circuit, you must deal with declining amplifier performance at higher gain levels. The circuit in Figure 1, using a CLC404 wideband current-feedback amplifier, controls the parasitic capacitance to maintain performance in a discrete gain-select configuration.

Usually, when you use a current-feedback op amp at a fixed gain, you’d choose an optimum feedback resistor (R_F) for that gain. For a fixed gain, R_F provides the closed-loop frequency compensation that takes parasitics at the inverting node into account. In a discrete gain-select configuration, however, R_F remains fixed while you switch in different values of gain-setting resistors (R_G). As R_G gets smaller and gain gets larger, performance can degrade, as shown in Figure 2.

You can significantly reduce this bandwidth degradation by putting a capacitor (C_COMP) in parallel with each R_G (Figure 1). As Figure 3 shows, the modified circuit greatly extends high-frequency performance at higher gains. Bandwidth at gains of four, 10, and 20 virtually doubles, and gain flatness also improves. The improvement comes about because the impedance of R_G in parallel with C_COMP decreases as frequency increases. The result is higher gains as frequency increases and an extension of the bandwidth beyond its “normal” range.

To determine C_COMP, use the formula C= 1/2[pi]R_Gf, where f is the frequency at which the uncompensated response starts to roll off. For example, note the gain-of-10 response in Figure 2—the response begins to roll off at about 15 or 20 MHz. So, if you plug 17.5 MHz into the formula, you obtain a value of 15 pF for C_COMP. The circuit in Figure 1 essentially adds a zero in the frequency response to extend the bandwidth. You could perhaps obtain better frequency control by using a series RC network, but the math then becomes more complicated.

Table 1 gives the value of C_COMP for each gain setting, along with the resulting 3-dB bandwidth. Table 1 also includes differential-gain and -phase figures for video applications. Surprisingly, the differential gain and phase do not exceed 0.08% and 0.05°, respectively, even at a gain of 20. This result indicates that the circuit is appropriate for video switching, in which you need higher gains to account for signal loss over varying cable lengths. The values of C_COMP may differ from those in Table 1, depending on how much parasitic capacitance exists in a layout. In this case, the layout capacitance was approximately 1.5 pF. (DI #1785)