干货 | 大功率信号发生器输出级设计

<p><span>作者：Thomas Brand，ADI现场应用工程师</span></p>

<p><span>信号发生器</span><span>用来</span><span>产生</span><span>确定性</span><span>电信号，其特性随时间推移而变化。如果这些信号表现为简单的周期</span><span>性</span><span>波形，如正弦波、方波或三角波，那么</span><span>这种信号</span><span>发生器</span><span>就</span><span>称为函数发生器。它们通常用于检查电路或</span><span>P</span><span>CBA的功能。将</span><span>确定性</span><span>信号</span><span>加到被测电路的</span><span>输入端，</span><span>将</span><span>输出端连接至相应的测量设备（例如示波器）</span><span>，</span><span>用户</span><span>就可以对其</span><span>进行评估。过去，挑战通常包括如何设计</span><span>信号发生器的</span><span>输出级。本文</span><span>将</span><span>介绍如何</span><span>利用</span><span>电压增益放大器</span><span>（</span><span>VGA</span><span>）</span><span>和电流反馈放大器</span><span>（</span><span>CFA</span><span>）</span><span>设计小型经济的输出级。</span></p>

<p><span>典型的信号发生器</span><span>可</span><span>提供</span><span>25mV</span><span>至</span><span>5V</span><span>输出电压。为了驱动</span><span>50</span><span>Ω或更高的负载</span><span>，</span><span>一般会在输出端使用</span><span>大功率</span><span>分立</span><span>器件</span><span>、多个并行</span><span>器件</span><span>，</span><span>或者成本高昂的</span><span>ASIC</span><span>。其内部通常</span><span>具有</span><span>继电器，</span><span>可以使</span><span>设备在不同的放大或衰减</span><span>等级</span><span>之间</span><span>进行</span><span>切换，从而调节输出电平。根据</span><span>需要，在对</span><span>继电器开关</span><span>而</span><span>实现各种增益时，在一定程度上会导致</span><span>工作不连续</span><span>。简化</span><span>方</span><span>框图如</span><strong>图1</strong><span>所示。</span></p>

<p><img alt="图1：典型信号发生器输出级的简化方框图。" data-entity-type="file" data-entity-uuid="735917af-b9d1-49d1-908a-5dfa4d07dfd6" src="http://new.eetrend.com/files/2020-11/wen_zhang_/100059154-113844-1.jpg&…; /></p>

<p><em>图1</em><em>：</em><em>典型信号发生器输出级的简化</em><em>方</em><em>框图</em><em>。</em></p>

<p><span>使用新款放大器IC</span><span>作为输出级功放</span><span>，可</span><span>以在没有任何</span><span>内部继电器</span><span>的情况下</span><span>直接驱动负载</span><span>，</span><span>因此</span><span>可</span><span>简化</span><span>信号</span><span>发生器的输出</span><span>级</span><span>设计，</span><span>并</span><span>降低复杂度和成本。这种输出的两个主要</span><span>器件</span><span>构成一个</span><span>大功率</span><span>输出级，</span><span>可</span><span>提供高速、高</span><span>电</span><span>压</span><span>和大</span><span>电流，以及具有</span><span>连续</span><span>线性微调功能的可变放大器。</span></p>

<p><img alt="图2.带VGA的信号发生器输出级的简化框图" data-entity-type="file" data-entity-uuid="3a55926a-0214-4ecf-9c6b-8c42fbcd8fb8" src="http://new.eetrend.com/files/2020-11/wen_zhang_/100059154-113845-2.jpg&…; /></p>

<p><em>图2.带VGA的信号发生器输出级的简化框图</em></p>

<p><span>首先</span><span>，</span><span>初始</span><span>输入信号必须通过</span><span>VGA</span><span>放大或衰减。</span><span>VGA</span><span>的输出信号可以设置为所需的幅度</span><span>，</span><span>而</span><span>与输入信号无关。例如</span><span>，</span><span>对于</span><span>增益为</span><span>10</span><span>、</span><span>输出幅度</span><span>V</span><span>OUT</span><span>为</span><span>2V</span><span>的情况</span><span>，VGA</span><span>的输出幅度必须调整至</span><span>0.2V</span><span>。遗憾的是，许多VGA</span><span>都会</span><span>因为增益范围有限而产生瓶颈</span><span>——</span><span>增益范围大于45dB</span><span>的情况</span><span>很少。</span></p>

<p><span>ADI公司</span><span>在低功耗VGA</span><span><span>&nbsp;</span></span><span>AD8338上</span><span>实现了0dB至80dB可编程增益范围。因此，在理想条件下，可以将信号发生器的输出幅度</span><span>连续</span><span>设置在0.5mV和5V之间，</span><span>而</span><span>无需使用额外</span><span>的</span><span>继电器或开关网络。通过去除这些机械</span><span>元件</span><span>，可以避免不连续</span><span>的输出</span><span>。因为数模转换器（DAC）和直接数字频率合成器（DDS）通常具有差分输出，所以AD8338提供全差分接口。此外，通过灵活的输入级，</span><span>输入电流有</span><span>任何</span><span>的</span><span>不对称</span><span>，</span><span>都可以通过内部反馈</span><span>回路</span><span>得到补偿。同时，内部节点保持在1.5V。在正常情况下，最大1.5V输入信号在500Ω输入电阻时</span><span>会产生</span><span>3mA电流。在</span><span>更高</span><span>输入幅度（例如15V）</span><span>的情况下</span><span>，可能需要在输入引脚</span><span>串联一个更大的电阻——其阻值要确保所产生的电流同样为3mA大小</span><span>。</span></p>

<p><span>许多商用信号发生器在50Ω（正弦波）负载下提供最大250mW（24dBm）的有效输出功率。但是，这对于具有较</span><span>大</span><span>输出功率的应用通常不够用，例如测试HF放大器或生成超声</span><span>波</span><span>脉冲</span><span>之所需</span><span>。因此，还需要使用电流反馈放大器。</span><a href="https://www.analog.com/en/products/ADA4870.html"><span>ADA4870</span></…±20V电源电压</span><span>下，可以在</span><span>输出</span><span>端以</span><span>17V</span><span>的幅度提供</span><span>1A</span><span>的驱动电流</span><span>。</span><span>它</span><span>可以在</span><span>满</span><span>载</span><span>情况下生成高达2</span><span>3MHz</span><span>的正弦波</span><span>，</span><span>因此</span><span>成为</span><span>了</span><span>通用任意波形发生器的理想前端驱动器。为了优化输出信号摆幅，ADA4870的增益配置</span><span>成</span><span>10，因此所需的输入幅度为1.6V。但是，由于ADA4870具有地参考输入，而上游的AD8338具有差分输出，</span><span>因此</span><span>在两个</span><span>器件</span><span>之间</span><span>应</span><span>连接差分接收器放大器，</span><span>而实现</span><span>差分</span><span>到地参考的</span><span>转换。</span><a href="https://www.analog.com/en/products/AD8130.html"><span>AD8130</span></a>…（GBWP），压摆率为1090V/µs，非常适合</span><span>这种</span><span>应用。AD8338的输出限制在±1V，</span><span>因此</span><span>AD8130的中间增益应设计为1.6V/V。整体电路配置如</span><strong>图3</strong><span>所示</span><span>，其可</span><span>在22.4V（39dBm）幅度和50Ω负载下</span><span>实现</span><span>20MHz带宽。</span></p>

<p><img alt="图3：采用分立设计的信号发生器输出级的简化电路。" data-entity-type="file" data-entity-uuid="73525d5f-e46f-439e-a06c-e1b50b86cd36" src="http://new.eetrend.com/files/2020-11/wen_zhang_/100059154-113846-3.jpg&…; /></p>

<p><em>图3</em><em>：</em><em>采用分立设计的信号发生器输出级的简化电路</em><em>。</em></p>

<p><span>通过</span><span>大</span><span>功率</span><span>的</span><span>VGA（AD8338）</span><span>、</span><span>大功率</span><span>的</span><span>CFA（ADA4870）</span><span>和差分接收器放大器</span><span>（AD8130）</span><span>的</span><span>组合</span><span>，</span><span>就</span><span>可以相对轻松地</span><span>设计出小尺寸大</span><span>功率</span><span>的</span><span>信号发生器输出级。它具有更高的系统可靠性、更长的服务寿命和更低的成本，因此</span><span>比</span><span>传统输出级</span><span>更优</span><span>。</span></p>

<p><span>参考文献</span></p>

<p><span>Hunter, David. “</span><a href="https://www.analog.com/media/en/analog-dialogue/volume-48/number-4/arti… New Devices Help Reinvent the Signal Generator</span></a><span>.”<span>&nbsp;</span></span><em>Analog Dialogue, October 2014.</em></p>