電壓和電流反饋放大器應(yīng)用電路

       電壓和電流反饋放大器應(yīng)用電路的結(jié)構(gòu)一般相同，除了幾個(gè)關(guān)鍵點(diǎn)。

　　電流反饋比電壓反饋放大器有更高的轉(zhuǎn)換速率。像這樣，電流反饋比電壓反饋放大器能更好的解決高速問題。“電流反饋放大器”的名字帶有一些神秘色彩，但一般電流反饋和電壓反饋放大器應(yīng)用電路的結(jié)構(gòu)是一致的，除了幾個(gè)關(guān)鍵點(diǎn)。

　　首先，電流反饋放大器電路的反饋電阻值必須保持在小范圍內(nèi)。電阻值越低，電流反饋放大器的穩(wěn)定性越差?？梢栽陔娏鞣答伔糯笃鳟a(chǎn)品手冊中找到規(guī)定的電阻值。電壓反饋放大器的反饋電阻值更寬。放大器的驅(qū)動能力限制了電阻的最小值，全電路噪聲限制電阻最大值。

　　圖1顯示了適于電流或電壓反饋放大器的電路。如果反饋電阻RF等于2RIN，其中RIN為輸入電阻，各通道的閉環(huán)增益為–2V/V。乍看，容易假設(shè)閉環(huán)帶寬與增益帶寬積除以各通道的增益相等，或|–2V/V|。但千萬別做這個(gè)假設(shè)！

　　如果使用如圖1電路中電壓或電流反饋放大器，噪聲增益為：

　　其中，N為輸入通道數(shù)。帶電壓反饋放大器的電路帶寬等于增益帶寬積除以噪聲增益。例如，如果使用180MHz增益帶寬積的電壓反饋放大器，三個(gè)輸入通道（N=3），增益–2V/V，電路閉環(huán)帶寬為25.7 MHz。額外的通道減少閉環(huán)帶寬，即使輸入信號持續(xù)達(dá)到–2V/V的增益。

　　如果使用如圖1電路的電流反饋放大器，放大器閉環(huán)帶寬較少的依靠閉環(huán)增益和輸入通道數(shù)。如果用這樣的放大器設(shè)計(jì)電路，首先應(yīng)該挑選合適的反饋電阻，各廠商的規(guī)格和電路噪聲增益。然后選擇合適的RIN值。從這一點(diǎn)，如果電路增加通道，或許將發(fā)生信號帶寬和增益劇增的微小變化。如果那個(gè)情形出現(xiàn)，退回并確定反饋電阻的選擇。對電流和電壓反饋放大器，噪聲增益通常與公式1的結(jié)果相等，但減少電流反饋放大器電路的反饋電阻值，使電路帶寬增加。

英文原文：

　　Voltage- and current-feedback amps are almost the same

　　The application-circuit configurations for voltage- and current-feedback amps are generally the same, except for a few key points.

　　By Bonnie Baker -- EDN, 10/25/2007

　　Current-feedback amplifiers have a higher slew rate than do voltage-feedback amplifiers. As such, current-feedback amps can better solve high-speed problems than their voltage-feedback counterparts. The name “current-feedback amp” carries some mystique, but, generally, the application-circuit configurations for voltage- and current-feedback amps are the same, except for a few key points.

　　First, the feedback resistor of a current-feedback-amp circuit must stay within a small range of values. Lower value resistors reduce the current-feedback amp’s stability. The feedback resistor’s higher values reduce the current-feedback amp’s bandwidth. You can find the prescribed feedback-resistor value in the current-feedback amp’s product data sheet. The voltage-feedback-amp’s feedback-resistance value is more forgiving. This amplifier’s drive capability limits the resistor’s minimum value, and the overall circuit noise limits the maximum value.

　　Figure 1 sho ws a circuit that is appropriate for either a current- or a voltage-feedback amp. If the feedback resistance, RF, equals 2RIN, where RIN is the input resistance, the closed-loop gain of each channel is –2V/V. At first glance, it is easy to assume that the closed-loop bandwidth equals the gain-bandwidth product divided by each channel’s gain, or |–2V/V|. Don’t make this assumption!

　　If you use a voltage- or current-feedback amp with the circuit in Figure 1, the noise gain is:

　　where N is the number of input channels. This circuit’s bandwidth, with a voltage-feedback amp, equals the gain-bandwidth product divided by the noise gain. For instance, if you have a voltage-feedback amp with a gain-bandwidth product of 180 MHz and there are three input channels (N=3) at a gain of –2V/V, the circuit’s closed-loop bandwidth is 25.7 MHz. Additional channels reduce the closed-loop bandwidth, even though the input signals continue to see a gain of –2V/V.