Not all amplifier designs are created equal, and it’s important to be familiar with common specifications and understand certain concepts when designing with high-speed amplifiers. In the context of this article, high-speed amplifiers refer to operational amplifiers (op amps) with a gain bandwidth product (GBW) greater than or equal to 50 MHz, but these concepts can apply to lower-speed devices as well. Here are some common questions designers have when using high-speed amplifiers.
A: Decompensated op amps have a closed-loop minimum gain stable specification, but offer a wider GBW and lower noise – for the same current consumption – as their unity-gain-stable counterparts.
“Decompensated” simply means that there is a second pole above 0 dB placed in the Aol (open-loop gain) response curve. This second pole also dictates the minimum gain required to ensure the amplifier’s stability. Think of the Aol curve being “shifted up,” as shown in Figure 1. The increased Aol results in a wider bandwidth.
Figure 1: Open-loop gain response curve for decompensated amplifiers
Reducing the size of the degeneration resistors in the input pair of the amplifier increases Aol, as illustrated in Figure 2. Smaller degeneration resistors also help reduce amplifier noise.
Figure 2: Degeneration resistors in an op amp
The OPA858 and OPA859 are two examples of a decompensated and unity-gain-stable amplifier, respectively. For the same current consumption, the OPA858 has wider bandwidth and lower noise, as shown in Table 1.
(Unity gain stable)
Quiescent current (IQ)
Gain Bandwidth (GBW)
Voltage noise (Vn)
Minimum gain (Acl)
Table 1: Comparing decompensated and unity-gain-stable amplifiers
In addition to increased bandwidth and lower noise, the decompensated architecture also enables a higher slew rate. Overall, the minimum gain specification offers a trade-off in performance that you can take advantage of if you can forego unity gain and meet the minimum gain requirement. Examples of applications where it’s possible to easily meet the minimum gain specification include current-sensing circuits that measure a voltage across a shunt resistor, gain stages in a signal chain and transimpedance circuits.
A: Current feedback amplifiers are op amps that feed back a portion of the output signal as current in order to control the amplifier. Current feedback amplifiers differ from voltage feedback amplifiers that rely on feedback in the form of a voltage. Most designers are familiar with voltage feedback architectures because they are more common and emphasized in most electronics curriculums.
Figure 3 offers a basic input-stage comparison of voltage and current feedback amplifier architectures, where the voltage feedback amplifier is modeled as a voltage-controlled voltage source and the current feedback amplifier is modeled as a current-controlled voltage source.
Figure 3: Comparing voltage and current feedback op amp architectures
Both architectures are still used as error amplifiers in a negative feedback circuit, but the type of feedback they require is what differs. You can use either amplifier type in inverting and noninverting gain configurations, for instance. One distinct advantage of the current feedback architecture is that the bandwidth is not dependent on the gain. In the voltage feedback architecture, however, as the gain increases, the bandwidth decreases, as shown in Equation 1:
In the current feedback architecture the bandwidth remains nearly constant regardless of the gain, as shown in Figure 4. This graph appears in the THS3491 data sheet.
Figure 4: Gain and bandwidth relationship of a current feedback op amp
Table 2 compares some of the main differences between voltage and current feedback amplifiers.
Voltage feedback amplifier
Current feedback amplifier
Bandwidth varies with gain
Almost constant bandwidth over gain
Many rail-to-rail output options
Larger headroom needed for output
Better low-frequency distortion
Better high-frequency distortion
Limited slew rate
Very high slew rate facilitating high full-power bandwidth
Restriction on the minimum stable gain for decompensated amplifiers
Stable across gains if feedback transimpedance remains constant
Low input-referred voltage and current noise
Higher input-referred current noise (unequal for inverting and noninverting inputs)
Applications requiring DC precision
High-speed and precise analog-to-digital converter (ADC) interfaces
Digital-to-analog converter interfaces
High-speed ADC interfaces
Table 2: Comparing voltage feedback and current feedback amplifier applications
Note that current feedback amplifiers are not meant to operate without a resistor in the feedback path. A current feedback amplifier data sheet will suggest specified values for RF; these values are important because the value of RF dictates the compensation of the amplifier, even in unity gain. Like Figure 4, Table 3 is from the THS3491 data sheet.
Table 3: Example of recommended values for RF from the THS3491 data sheet
For more details about the differences between these two architectures, check out Understanding Voltage Feedback and Current Feedback Amplifiers. You can also learn more about current feedback architectures by watching the TI Precision Labs online training videos.
A: Generally speaking, it’s likely that the inductance of the package leads as well as the breadboard’s capacitance and inductance are causing your high-speed amplifier to oscillate. Similarly, when designing with high-speed op amps, it’s important to minimize capacitance and inductance on printed circuit boards (PCBs). Even devices on the lower end of the high-speed amplifier GBW spectrum, like the 50-MHz OPA607, require these types of board-level design considerations.
Here are some ways you can optimize your high-speed layout design:
When evaluating the performance of high-speed op amps, it’s best to use the designated evaluation module for a particular device. These boards demonstrate good high-speed board layout design and use SMA connectors to maintain a high-fidelity and impedance-controlled signal path. For more details on high-speed board layout practices, you can read High Speed PCB Layout Techniques.
Overall, high-speed op amps operate much like their lower-speed counterparts. By considering just a few design nuances, you can take advantage of all of the speed and performance benefits that they offer for your system. Which of these questions were most relevant to you? Leave a comment below.
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