9.1.1 Video Distribution

The wide bandwidth, high slew rate, and high output drive current of the THS3125 and THS3122 match the demands for video distribution to deliver video signals down multiple cables. To ensure high signal quality with minimal degradation of performance, a 0.1-dB gain flatness should be at least 7x the passband frequency to minimize group delay variations from the amplifier. A high slew rate minimizes distortion of the video signal, and supports component video and RGB video signals that require fast transition times and fast settling times for high signal quality. Figure 40 illustrates a typical video distribution amplifier application configuration.

Figure 40. Video Distribution Amplifier Application

9.1.2 Driving Capacitive Loads

Applications such as FET drivers and line drivers can be highly capacitive and cause stability problems for high-speed amplifiers.

Figure 41 through Figure 47 show recommended methods for driving capacitive loads. The basic idea is to use a resistor or ferrite chip to isolate the phase shift at high frequency caused by the capacitive load from the amplifier feedback path. See Figure 41 for recommended resistor values versus capacitive load.

Figure 41. Recommended R_ISO vs Capacitive Load

Placing a small series resistor, R_ISO, between the amplifier output and the capacitive load, as shown in Figure 42, is an easy way of isolating the load capacitance.

Figure 42. Resistor To Isolate Capacitive Load

Using a ferrite chip in place of R_ISO, as Figure 43 shows, is another approach of isolating the output of the amplifier. The ferrite impedance characteristic versus frequency is useful to maintain the low frequency load independence of the amplifier while isolating the phase shift caused by the capacitance at high frequency. Use a ferrite with similar impedance to R_ISO, 20 Ω to 50 Ω, at 100 MHz and low impedance at dc.

Figure 43. Ferrite Bead To Isolate Capacitive Load

Figure 44 shows another method used to maintain the low-frequency load independence of the amplifier while isolating the phase shift caused by the capacitance at high frequency. At low frequency, feedback is mainly from the load side of R_ISO. At high frequency, the feedback is mainly via the 27-pF capacitor. The resistor R_IN in series with the negative input is used to stabilize the amplifier and should be equal to the recommended value of R_F at unity gain. Replacing R_IN with a ferrite of similar impedance at about 100 MHz as shown in Figure 45 gives similar results with reduced dc offset and low frequency noise.

Figure 44. Feedback Technique With Input Resistor For Capacitive Load

Figure 45. Feedback Technique With Input Ferrite Bead For Capacitive Load

Figure 46 shows a configuration that uses two amplifiers in parallel to double the output drive current to larger capacitive loads. This technique is used when more output current is needed to charge and discharge the load faster as when driving large FET transistors.

Figure 46. Parallel Amplifiers For Higher Output Drive

Figure 47 shows a push-pull FET driver circuit typical of ultrasound applications with isolation resistors to isolate the gate capacitance from the amplifier.

Figure 47. Powerfet Drive Circuit