What is a High-Speed Amplifier and Why Use One?

The propagation delay of a circuit varies drastically based upon the components selected. If a DC/DC converter uses average current control for power factor correction, for example, then a slower amplifier can be used. However, if there is a need to detect peak current, or if the system needs to react and shutdown in less than 1µs, then a high-bandwidth amplifier is required. In Figure 3, the current waveforms of peak detection and average detection are overlaid. The reason using a higher-bandwidth amplifier for peak detection is necessary is the fact that the transient current spikes occur very quickly – when measuring average current, these spikes remain undetected.

Figure 3 Average Current vs Peak Current Waveforms

Newer implementations of onboard chargers, DC/DC converters, and traction inverters take advantage of the reduced power consumption and faster switching frequencies provided with Gallium Nitride and Silicon Carbide switches. Many GaN switches are susceptible to failure from current spikes over 54A and at a duration of as little as 1µs. In Figure 4, the rise times of a 10MHz and a 50MHz op amp are compared, showing that a high-bandwidth 50MHz op amp reduces propagation delay by 0.8µs. This allows for ample time to detect and respond to an overcurrent condition.

Figure 4 10MHz Ideal Rise Time vs 50MHz Ideal Rise Time

To achieve detection and shutdown in less than 1µs, allocate 35% of the total response time to the amplification stage. If the amplifier requires a gain of 1V/V, then the required bandwidth of the amplifier is 1MHz; however, most shunt-based current monitoring circuits require gains in the range of 50V/V to reduce the power consumption of the shunt resistor. To achieve this level of gain, an amplifier with at least 50MHz is required, as Equation 1 demonstrates.

      [1] 0.35 is a constant value used to approximate bandwidth.
       This is related to the time required for a capacitor to charge from 10% to 90%.
       See the Op Amp Slew Rate – TI Precision Labs video for more details.

Texas Instruments (TI) offers a variety of amplifiers for different system requirements. Figure 5 is a graph demonstrating rise times of several amplifiers with increasing bandwidth, and Table 1 is a comparison chart of the rise time.

Figure 5 Rise Times of Several TI Amplifiers

Some designers choose to cascade two or more lower-bandwidth amplifiers with different amplification stages to achieve the same amplification at a reduced cost. Using multiple lower-bandwidth amplifiers adds additional propagation delay and requires more space on the circuit board. High-bandwidth amplifiers historically were perceived to be costlier alternatives; however, with modern manufacturing capabilities at TI, these newer devices are able to be priced more competitively and are comparable to using multiple lower-bandwidth amplifiers.