Ideal op-amp

This example uses the non-ideal op-amp model of an LF411C. Since there is only one op-amp in this circuit, we will not reach the component limit of the Lite version. The method described here can be used for circuits with several op-amps. However, the component limit of the Lite version of PSpice limits us to only two non-ideal op-amp models. If more than two op-amps are needed, the Ideal OPAMP model can be used. Since the Ideal OPAMP model has no frequency limitations, it cannot be used to find the bandwidth, but it can be used to find the gain. 

This circuit uses a LF411 op-amp macro model. All op-amp models, except the ideal op-amp model, include bias currents, offset voltages, slew rate limitations, and frequency limitations. Also note that the op-amp model requires DC supplies. The... 

EXEHCI5E fi-U Using feedback, we can reduce the distortion of the push-pull amplifier. Repeat the above drill exercise using the amplifier below. An ideal op-amp is used because a real op-amp cannot drive this push-pull amplifier. 

The ideal operational amplifier is very useful in the Lite version of PSpice. The ideal model has only three components in the subcircuit This small number of components allows many ideal op-amps to be used before the component limit of the Lite version is reached. In the Lite version of PSpice, only two non-ideal op-amp models can be used before reaching the component limit If you have a circuit with a large number of op-amps, you will be forced to use ideal op-amps in the Lite version. 

The drawback of the ideal op-amp model is that none of the non-ideal properties are modeled. In this example, if a non-ideal op-amp model were used in the simulation, the integrator would not work because of bias currents. If this circuit were tested in the laboratory, it also would not work because of bias currents. Thus, the circuit simulation with a non-ideal op-amp matches the results in the lab, but the circuit simulation with an ideal op-amp does not match the lab results. For this example, the ideal model is not a good choice for simulation because it does not match the results in the lab. We will use it here for demonstration purposes only. See EXEHCI5E 6-15 to learn how this integrator performs using non-ideal op-amps. In general, you should always use the non-ideal op-amp models if possible. The only reason you should use the ideal op-amp model is if the circuit is too large for the Lite version of Capture. 

We will now run a circuit with three ideal operational amplifiers. With the Lite version, the component limitation of PSpice limits us to two or three non-ideal operational amplifiers, depending on the complexity of the op-amp model. You may not be able to simulate the circuit of this section depending on the op-amp model you use. The ideal operational amplifier model was created so that a circuit with several operational amplifiers could be simulated using the Lite version. Simulation with ideal op-amps will give you a good idea about what the circuit is supposed to do, but it will not simulate any of the non-ideal properties that may cause your circuit to function improperly, or not meet certain specifications. Always use the non-ideal models when possible. For circuits with lots of op-amps, you will need the professional version of PSpice to accurately simulate the circuit if you want to include the non-ideal properties. Wire the circuit shown below. 

In this section we will use an operational amplifier to create a Schmitt Trigger. A non-ideal operational amplifier must be used because the ideal op-amp model has trouble converging when it is used as a Schmitt Trigger. Wire the circuit ... 

Fig. 17 shows how such an ideal op amp can be configured as a potentiostat and connected to an electrochemical cell to study kinetics. First consider the electrochemical cell in the schematic. Unlike the cells discussed above, this cell has three electrodes. The working elecrode (WE) represents the interface of inter-... 

We can visualize an ideal op-amp as a device that varies its output so as to virtually equalize the voltages at its input pins. Therefore in steady state, the voltage at the node connecting Rf2 and Rfl (see divider block in Figure 7-10) can be assumed to be (almost) equal to Vref-Assuming that no current flows out of (or into) the divider at this node, using Ohm s law... 

Note The lower resistor of the divider, Rfl, does not enter the ac analysis, provided we are considering ideal op-amps. In practice, it does affect the bandwidth of a real op-amp, and therefore may on occasion need to be considered. 

Introduction The Ideal Op-Amp A Collection of Functional Circuits Op Amp Limitations... 

The ideal op-amp is an abstraction that is useful in the design and analysis of active circuits. A design can be formulated in terms of ideal op-amps, and later, if the design shows promise, the circuit can be analyzed in more detail to make sure it works in all circum-... 

FIGtJRE 7.66 A schematic representation of an ideal op amp shown with required power supplies and bypass capacitors. Not shown is the feedback network. The terminal numbers correspond to those used for the 741 op-amp in the minidip configuration. 

FIGURE 7.67 Ideal op-amp models (a) model with note stating that v = 0, (b) model with note that i = 0, (c) model consisting of a nullator and a grounded norator. 

The voltage between the input leads of the ideal op-amp is zero. Thus, v in Fig. 7.66 is zero. 

The output resistance is zero. Thus, the output voltage of an ideal op-amp is independent of the current drawn from the output. 

Three circuit models, which summarize the properties of ideal op-amps, are shown in Fig. 7.67. These models are equivalent, and the choice of which one to use in the analysis of a linear op-amp circuit is merely a matter of convenience. The model shown in Fig. 7.67(a) includes the note y = 0. The connection between terminals 2 and 3 in this figure is sometimes called a virtual connection or, if terminal 3 is connected to ground, a virtual ground. No current flows through this connection, and there is zero voltage between terminals 2 and 3. 

FIGURE 7.68 Inverting amplifier (a) schematic, (b) a model for the ideal op-amp has been substituted into the circuit. 

At low frequencies, the resistances of the switches used to create the analog multiplexers have no effect on the output voltage, but they may affect the results at higher frequencies if parasitic capacitances are not kept to low values. However, the properties of real or tangible op-amps (as opposed to ideal op-amps) will most likely impose tighter limits on the range of frequencies over which this circuit and any of the circuits in this section can be used. Furthermore, real op-amp characteristics will affect gain accuracies. In the next section, we discuss the characteristics of real op-amps and show how these characteristics can be modeled. 

Real, tangible op-amps differ from ideal op-amps in the foUowing ways ... 

An ideal op-amp has infinite-input and zero-output impedances. Figure 7.86 shows a model for an op-amp, which accounts for the finite- (but large) input resistances and the low- (but not zero) output resistance of a real op-amp. For a /uA741, the differential input resistance R is about 2 MS2, the common-mode input resistance Ricu is 100 Mf2, the input capacitance C, is 4-7 pF (including the capacitance of the op-amp package), and the output resistance Ro is about 75 f2. There are other parasitic capacitances such as capacitance between input and output that are not shown, but for many applications, Q is the most significant capacitance. 

Virtual connection A representation of the circuit between the input leads of an ideal op-amp. The voltage across and the current through a virtual connection are both zero. If one input lead of an ideal op-amp is connected to ground, the virtual connection is often termed a virtual ground. 

However, practical op-amps can only approximate these characteristics using feedback arrangements. By using negative feedback, the values of input resistance, output resistance, and bandwidth can be achieved close to an ideal op-amp. 

The closed-loop voltage gain can be obtained from the concept of virtual ground of an ideal op-amp. A virtual ground acts as a short for voltage but an open for current, like a half ground. In such a case as with an ideal op-amp, ip becomes zero for an infinite input resistance and vp becomes zero for an infinite Aql-Therefore,... 

Here the analysis can be simplified by using the concept of virtual short between the input terminals of an ideal op-amp. A virtual short acts as a bootstrap by making... 

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