87TP01-ER1210 87TP01 87TP01-E GJR2392700R1210 模块力

87TP01-ER1210 87TP01 87TP01-E GJR2392700R1210 模块

单位增益稳定被称为失补偿放大器。与单位增益稳定放大器相比，失补偿放大器通常具有更高的增益带宽积、更高的转换速率和更低的电压噪声
静态功耗相同。
图50显示了作为温度函数的OPA858的开环幅度（AOL）。结果表明：
温度变化小。OPA858的相位裕度配置为7 V/V的噪声增益（16.9
dB）在温度范围内接近55°。类似地，图51显示了OPA858的AOL幅度为
工艺变化的函数。结果显示了标称工艺角和变化的AOL曲线
标称值的一个标准偏差。模拟结果表明，相位裕度差小于1°
当放大器配置为7V/V的增益时，在工艺变化的标准偏差内。
OPA858的主要应用之一是用作高速跨阻抗放大器（TIA），如图59所示
显示。TIA的低频噪声增益为0dB（1V/V）。在高频下，总输入的比率
电容和反馈电容设置噪声增益。为了大化TIA闭环带宽
反馈电容通常小于输入电容，这意味着高频噪声
增益大于0 dB。因此，配置为TIA的运算放大器不需要单位增益稳定，这
使失补偿放大器成为TIA的可行选择。关于跨阻抗，你需要知道什么
放大器–1部分和您需要了解的跨阻抗放大器–2部分描述
更详细地描述跨阻抗放大器补偿。
图50开环增益与温度图51开环增益和工艺变化
9.3.4转换速率和输出级
除了宽带宽外，OPA858还具有2000 V/µs的高转换速率。转换速率是一个关键
具有窄亚10 ns脉冲的高速脉冲应用中的参数，如光时域
反射计（OTDR）和激光雷达。OPA858的高转换速率意味着该设备准确
再现如图20所示的2-V、亚ns脉冲边缘。OPA858的宽带宽和转换速率
使其成为高速信号链前端的理想放大器。
图52显示了作为频率函数的OPA858的开环输出阻抗。达到高
转换速率和跨频率的低输出阻抗，OPA858的输出摆幅限制为
大约3V。OPA858通常与高速流水线ADC和闪存ADC结合使用
其具有有限的输入范围。因此，OPA858输出摆幅范围与类电压耦合
噪声规格使信号链的整体动态范围大化。
频率（Hz）
电流噪声（A/O Hz）
开环输出阻抗（欧姆
SBOS629A–2018年4月–2018年7月修订www.ti.com
产品文件夹链接：OPA858
提交文件反馈版权©2018，德克萨斯仪器公司
功能描述（续）
图52.开环输出阻抗（ZOL）与频率
9.3.5电流噪声
CMOS和JFET输入放大器在低频时的输入阻抗超过几GΩs。然而，在
在较高频率下，晶体管对漏极、源极和衬底的寄生电容降低了
阻抗。低频时的高阻抗消除了任何偏置电流和相关的散粒噪声。在
频率越高，输入电流噪声越大（见图53），这是由于
CMOS栅极氧化物和下面的晶体管沟道。这种现象是
晶体管的构造是不可避免的

87TP01-ER1210 87TP01 87TP01-E GJR2392700R1210 模块

87TP01-ER1210 87TP01 87TP01-E GJR2392700R1210 模块

unity-gain stable are known as decompensated amplifiers. Decompensated amplifiers typically have higher gainbandwidth product, higher slew rate, and lower voltage noise, compared to a unity-gain stable amplifier with the
same amount of quiescent power consumption.
Figure 50 shows the open-loop magnitude (AOL) of the OPA858 as a function of temperature. The results show
minimal variation over temperature. The phase margin of the OPA858 configured in a noise gain of 7 V/V (16.9
dB) is close to 55° across temperature. Similarly Figure 51 shows the AOL magnitude of the OPA858 as a
function of process variation. The results show the AOL curve for the nominal process corner and the variation
one standard deviation from the nominal. The simulated results suggest less than 1° of phase margin difference
within a standard deviation of process variation when the amplifier is configured in a gain of 7 V/V.
One of the primary applications for the OPA858 is as a high-speed transimpedance amplifier (TIA), as Figure 59
shows. The low-frequency noise gain of a TIA is 0 dB (1 V/V). At high frequencies the ratio of the total input
capacitance and the feedback capacitance set the noise gain. To maximize the TIA closed-loop bandwidth, the
feedback capacitance is typically smaller than the input capacitance, which implies that the high-frequency noise
gain is greater than 0 dB. As a result, op amps configured as TIAs are not required to be unity-gain stable, which
makes a decompensated amplifier a viable option for a TIA. What You Need To Know About Transimpedance
Amplifiers – Part 1 and What You Need To Know About Transimpedance Amplifiers – Part 2 describe
transimpedance amplifier compensation in greater detail.
Figure 50. Open-Loop Gain vs Temperature Figure 51. Open-Loop Gain vs Process Variation
9.3.4 Slew Rate and Output Stage
In addition to wide bandwidth, the OPA858 features a high slew rate of 2000 V/µs . The slew rate is a critical
parameter in high-speed pulse applications with narrow sub 10-ns pulses such as Optical Time-Domain
Reflectometry (OTDR) and LIDAR. The high slew rate of the OPA858 implies that the device accurately
reproduces a 2-V, sub-ns pulse edge as seen in Figure 20. The wide bandwidth and slew rate of the OPA858
make it an ideal amplifier for high-speed, signal-chain front ends.
Figure 52 shows the open-loop output impedance of the OPA858 as a function of frequency. To achieve high
slew rates and low output impedance across frequency, the output swing of the OPA858 is limited to
approximately 3 V. The OPA858 is typically used in conjunction with high-speed pipeline ADCs and flash ADCs
that have limited input ranges. Therefore, the OPA858 output swing range coupled with the class-leading voltage
noise specification maximizes the overall dynamic range of the signal chain.
Frequency (Hz)
Current Noise (A/óHz)
Open-Loop Output Impedance (ohms
SBOS629A –APRIL 2018–REVISED JULY 2018 www.ti.com
Product Folder Links: OPA858
Submit Documentation Feedback Copyright © 2018, Texas Instruments Incorporated
Feature Description (continued)
Figure 52. Open-Loop Output Impedance (ZOL) vs Frequency
9.3.5 Current Noise
The input impedance of CMOS and JFET input amplifiers at low frequencies exceed several GΩs. However, at
higher frequencies, the transistors parasitic capacitance to the drain, source, and substrate reduces the
impedance. The high impedance at low frequencies eliminates any bias current and the associated shot noise. At
higher frequencies, the input current noise increases (see Figure 53) as a result of capacitive coupling between
the CMOS gate oxide and the underlying transistor channel. This phenomenon is a natural artifact of the
construction of the transistor and is unavoidable