畢業(yè)設(shè)計(jì)論文外文運(yùn)算放大器理想及實(shí)際模型(編輯修改稿)
2025-09-02 17:27 本頁(yè)面
【文章內(nèi)容簡(jiǎn)介】 c and V–dc , from their nominal values, while VCM is the voltage mon to both inputs of the op amp. The openloop gain of the op amp is no longer infinite but is modeled by a network of the output impedance Zout (which may be merely a resistor but could also be a series RL network) in series with a source A ( s ), which includes all the openloop poles and zeroes of the op amp aswhereAOLis the finite dc openloop gain, while poles are at frequencies wp1, w p2, . . . and zeroes are at wZ1, etc. The differential input resistance is ZIN , which is typically a resistance R IN parallel with a capacitor CIN . Similarly, the monmode input impedance ZCM is established by placing an impedance 2ZCM in parallel with each input terminal. Normally, ZCM is best represented by a parallel resistance and capacitance of 2RCM (which is RIN) and CCM/2. The dc bias currents at the input are represented by IB+ and IB– current sources that would equal the input base currents if a differential bipolar transistor were used as the input stage of the op amp, or the input gate currents if FETs were used. The fact that the two transistors of the input stage of the op amp may not be perfectly balanced is represented by an equivalent input offset voltage source, VOS , in series with the input. The smallest signal that can be amplified is always limited by the inherent random noise internal to the op amp itself. In Fig. 3 the noise effects are represented by an equivalent input voltage source (ENV), which when multiplied by the gain of the op amp would equal the total output noise present if the inputs to the op ilar fashion, if the inputs to the op amp were open circuited, the total output noise would equal the sum of the noise due to the equivalent input current sources (ENI+ and ENI–), each multiplied by their respective current gain to the output. Because noise is a random variable, this summation must be acplished in a squared fashion, .,Typically, the correlation (C) between the ENV and ENI sources is low, so the assumption of C 187。 0 can be made. For the basic circuits of Fig. (a) or (b), if the signal source vI is shorted then the output voltage due to the nonideal effects would be (using the model of Fig. 3)provided that the loop gain (also called loop transmission in many texts) is related by the inequalityInherent in Eq. () is the usual condition that R1 ZIN and ZCM. If a resistor R2 were in series with the noninverting input terminal, then a corresponding term must be added to the right hand side of Eq. () of value –IB+ R2 (R1 + RF )/R1. On manufacturers’ data sheets the individual values of IB+ and IB– are not stated。 instead the average input bias current and offset current are specified asThe output noise effects can be obtained using the model of Fig. 3 along with the circuits of aswhere it is assumed that a resistor R2 is also in series with the noninverting input of either Fig. 2(a) or (b). The thermal noise (often called Johnson or Nyquist noise) due to the resistors R1 , R2 , and RF is given by (in rms volt2/Hz)where k is Boltzmann’s constant and T is absolute temperature (176。Kelvin). To obtain the total output noise, one must multiply the E2 out expression of Eq. () by the noise bandwidth of the circuit, which typically is equal to p/2 times the –3 dB signal bandwidth, for a singlepole response system [Kennedy, 1988].SPICE Computer ModelsThe use of op amps can
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