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| SEC. 19-1 FOUR TYPES OF
NEGATIVE FEEDBACK There are four ideal types of negative feedback: VCVS, ICVS, VCIS, and ICIS. Two types (VCVS and VCIS) are controlled by an input voltage, and the other two types (ICVS and ICIS) are controlled by an input current. The output sides of VCVS and ICVS act like a voltage source, and the output sides of VCIS and ICIS act like a current source. SEC. 19-2 VCVS VOLTAGE GAIN The loop gain is the voltage gain of the forward and feedback paths. In any practical design, the loop gain is very large. As a result, the closed-loop voltage gain is ultrastable because it no longer depends on the characteristics of the amplifier. Instead, it depends almost entirely on the characteristics of external resistors. SEC. 19-3 OTHER VCVS EQUATIONS VCVS negative feedback has a curative effect on the flaws of an amplifier because it stabilizes the voltage gain, increases the input impedance, decreases the output impedance, and decreases harmonic distortion. SEC. 19-4 THE ICVS AMPLIFIER This is a transresistance amplifier, equivalent to a current-to-voltage converter. Because of the virtual ground, it ideally has zero input impedance. The input current produces a precise value of output voltage. SEC. 19-5 THE VCIS AMPLIFIER This is a transconductance amplifier, equivalent to a voltage-to-current converter. It ideally has infinite input impedance. The input voltage produces a precise value of output current. The output impedance approaches infinity. SEC. 19-6 THE ICIS AMPLIFIER Because of the heavy negative feedback, the ICIS amplifier approaches the perfect current amplifier, one with zero input impedance and infinite output impedance. SEC. 19-7 BANDWIDTH Negative feedback increases the bandwidth of an amplifier because the roll-off in open-loop voltage gain means that less voltage is fed back, which produces more input voltage as a compensation. Because of this, the closed-loop cutoff frequency is higher than the open-loop cutoff frequency. |