8.1.1 Video Distribution

The wide bandwidth, high slew rate, and high output drive current of the THS309x matches the demands for video distribution for delivering 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 63. Video Distribution Amplifier Application

8.1.2 Driving Capacitive Loads

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

Figure 64 through Figure 69 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’s feedback path. See SLOA013 for recommended resistor values versus capacitive load.

Figure 64. Recommended R_ISO vs Capacitive Load

Figure 65. Driving a Large Capacitive Load Using an Output Series Isolation Resistor

Figure 66. Driving a Large Capacitive Load Using an Output Series Ferrite Bead

Placing a small series resistor, R_ISO, between the amplifier’s output and the capacitive load, as shown in Figure 65, is an easy way of isolating the load capacitance.

Using a ferrite chip in place of R_ISO, as shown in Figure 66, is another approach of isolating the output of the amplifier. The ferrite's 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 67 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 68 gives similar results with reduced DC offset and low-frequency noise. (See the Related Documentation section for expanding the usability of current-feedback amplifiers.)

Figure 67. Driving a Large Capacitive Load Using a Multiple Feedback Loop With Stabilizing Input Resistor (R_IN)

Figure 68. Driving a Large Capacitive Load Using a Multiple Feedback Loop With Stabilizing Input Ferrite Bead (F_IN)

Figure 69 is shown using 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 like when driving large FET transistors.

Figure 69. Driving a Large Capacitive Load Using 2 Parallel Amplifier Channels

Figure 70 shows a push-pull FET driver circuit typical of ultrasound applications with isolation resistors to isolate the gate capacitance from the amplifier.

Figure 70. PowerFET Drive Circuit

8.2.1 Design Requirements

Use two THS3091 amplifiers in a parallel load-sharing circuit to improve distortion performance.

8.2.2 Detailed Design Procedure

In addition to providing higher output current drive to the load, the load sharing configuration can also provide improved distortion performance. In many cases, an operational amplifier shows better distortion performance as the load current decreases (that is, for higher resistive loads) until the feedback resistor starts to dominate the current load. In a load sharing configuration of N amplifiers in parallel, the equivalent current load that each amplifier drives is 1/N times the total load current. For example, in a two-amplifier load sharing configuration with matching resistance (refer to Figure 71) driving a resistive load (RL), each series resistance is 2*RL and each amplifier drives 2*RL . A convenient indicator of whether an op amp will function well in a load sharing configuration is the characteristic performance graph of harmonic distortion versus load resistance. Such graphs can be found in most of TI’s high-speed amplifier data sheets. These graphs can be used to obtain a general sense of whether or not an amplifier will show improved distortion performance in load sharing configurations.

Two test circuits are shown in Figure 71, one for a single THS3091 amplifier driving a double-terminated, 50-Ω cable and one with two THS3091 amplifiers in a load sharing configuration. In the load sharing configuration, the two 100-Ω series output resistors act in parallel to provide 50-Ω back-matching to the 50-Ω cable.

Figure 72 and Figure 73 show the 32-MHz, 18-VPP sine wave output amplitudes for the single THS3091 configuration and the load sharing configuration, respectively, measured using an oscilloscope. An ideal sine wave is also included as a visual reference (the dashed red line). Figure 72 shows visible distortion in the single THS3091 output. In the load sharing configuration of Figure 73, however, no obvious degradation is visible.

Figure 74 and Figure 75 show the 64-MHz sine wave outputs of the two configurations from Figure 8. While the single THS3091 output is clearly distorted in Figure 74, the output of the load sharing configuration in Figure 75 shows only minor deviations from the ideal sine wave.

The improved output waveform as a result of load sharing is quantified in the harmonic distortion versus frequency graphs shown in Figure 76 and Figure 77 for the single amplifier and load sharing configurations, respectively. While second-harmonic distortion remains largely the same between the single and load sharing cases, third-harmonic distortion is improved by approximately 8 dB in the frequency range between 20 MHz to 64 MHz.

8.2.3 Application Curves

Figure 72. 32-MHz Sine Wave Output (Gain = 5 V/V, Signal Amplitude Referred to Amplifier Output), Single THS3091 Circuit Configuration

Figure 74. 64-MHz Sine Wave Output (Gain = 5 V/V, Signal Amplitude Referred to Amplifier Output), Single THS3091 Circuit Configuration

Figure 76. Harmonic Distortion vs Frequency, Single THS3091 Circuit Configuration

Figure 73. 32-MHz Sine Wave Output (Gain = 5 V/V, Signal Amplitude Referred to Amplifier Output), Two THS3091 Amplifiers in Load Sharing Configuration

Figure 75. 64-MHz Sine Wave Output (Gain = 5 V/V, Signal Amplitude Referred to Amplifier Output), Two THS3091 Amplifiers in Load Sharing Configuration

Figure 77. Harmonic Distortion vs Frequency, Two THS3091 Amplifiers in Load Sharing Configuration