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| SEC. 17-1 THE DIFFERENTIAL
AMPLIFIER A diff amp is the typical input stage of an op amp. It has no coupling or bypass capacitors. Because of this, it has no lower cutoff frequency. The input may be differential, noninverting, or inverting. The output may be single-ended or differential. SEC. 17-2 DC ANALYSIS OF A DIFF AMP The diff amp uses two-supply emitter bias to produce the tail current. When a diff amp is perfectly symmetrical, each emitter current is half the tail current. Ideally, the voltage across the emitter resistor equals the negative supply voltage. SEC. 17-3 AC ANALYSIS OF A DIFF AMP Because the tail current is ideally constant, an increase in the emitter current of one transistor produces a decrease in the emitter current of the other transistor. With a differential output, the voltage gain is RC/re.. With a single-ended output, the voltage gain is half as much. Three important input characteristics of an op amp are the input bias current, input offset current, and input offset voltage. The input bias and offset currents produce unwanted input error voltages when they flow through the base resistors. The input offset voltage is an equivalent input error produced by differences in RC and VBE. SEC. 17-5 COMMON-MODE GAIN Most static, interference, and other kinds of electromagnetic pickup are commonmode signals. The diff amp discriminates against common-mode signals. The CMRR is the voltage gain divided by the common-mode gain. The higher the CMRR, the better. SEC. 17-6 INTEGRATED CIRCUITS Monolithic ICs are complete circuit functions on a single chip such as amplifiers, voltage regulators, and computer circuits. For high-power applications, thin-film, thick-film, and hybrid ICs may be used. SSI refers to fewer than 12 components, MSI to between 12 and 100 components, LSI to more than 100 components, VLSI to more than 1000 components, and ULSI to more than 1 million components. SEC. 17-7 THE CURRENT MIRROR The current mirror is used in ICs because it is a convenient way to create current sources and active loads. The advantages of using current mirrors are increases in voltage gain and CMRR. SEC. 17-8 THE LOADED DIFF AMP When a load resistance is used with a diff amp, the best approach is to use Thevenin’s theorem. Calculate the ac output voltage vout as discussed in earlier sections. This voltage is equal to the Thevenin voltage. Use a Thevenin resistance of 2RC with a differential output and RC with a single-ended output. |