Op-amp Circuits

In this section we will use PSpice to determine the bandwidth of an op-amp circuit with varying amounts of negative feedback. For an op-amp circuit, the closed-loop gain times the bandwidth is approximately constant. To observe this property, we will run a simulation that creates a Bode plot for several different closed-loop gains. We will use the circuit below ... 

We have now defined the parameter with the PARAM part and used the parameter as the value for RF. Parameters are used to easily change values in a circuit, pass values to a subcircuit, or change the values of components during a simulation. We will use the parameter RF V3l to change the value of the resistor RF during the simulation. This will allow us to change the gain of the op-amp circuit. 

We would like to see the frequency response of the op-amp circuit for different values of the gain, so we must set up an AC Sweep to run in conjunction with a Parametric Sweep. First we will set up the AC Sweep. Select PSpice and then New Simulation Profile from the Capture menus and then enter a name for the profile and click the Create button. Select the MT Sweep/Nolse Analysts type and fill in the parameters as shown in the AC Sweep dialog box below ... 

This op-amp circuit is a Schmitt Trigger with trigger points at approximately 7.5 V. A sinusoidal voltage source will be used to swing the input from +14 V to -14 V and from -14 V to +14 V a few times. The frequency of the source is 1 Hz. This low frequency is chosen to eliminate the effects of the op-amp slew rate on the Schmitt Trigger performance. If you... 

Next, we will use the op-amp circuit created in the previous section to demonstrate an AC Sweep. We created an op-amp with frequency dependence in the previous section. We will now show how the frequency response varies with feedback. In the circuit below, the op-amp model is used as a non-inverting amplifier with gain 1 + (Rf/R4) ... 

We will start with the op-amp circuit created from ABM parts in Section 6.0.3. Create a new blank project and draw the circuit below ... 

The first circuit we will look at is an op-amp circuit that drives the clock of a J-K flip-flop. Wire the circuit shown below ... 

For a detailed description of op amp circuits, see H. V. Malmstadt. C. G. Enke, and S. R. Crouch. Micmcomimters and Electmnic Instnimenlalion Making the Right Connections, Ch. 5. Washington. DC American Chemical Society. 1994. 

The gain of the inverting op amp circuit (figure 2.15) is a negative of the ratio of the feedback to the input impedance. Thus... 

The sine-wave modulation between 10 Hz and 100 kHz was generated by the internal frequency synthesizer of the lock-in amplifier. The number of data points collected equidistantly in the logarithmic scale between 10 Hz and 100 kHz was 200. The DC bias was added by a simple op amp circuit and the modulation degree was kept at 10%. The driver for the NIR laser diode was WLD3343 (Wavelength Electronics, Inc., Montana) with a 2 MHz bandwidth and 2 A maximum current. 

This operation is useful, for example, in the removal of offsets where through the use of a variable resistor, an adjustable voltage level can be subtracted from the other inputs. We will see a specific example of the use of this op-amp circuit later in this chapter for a piezoelectric force-transducer application. 

The op-amp circuit in Fig. 1.17 implements a bandpass filter without inductors. The input-output transfer function of this circuit can be obtained by using Kirchhoff s current law at the node common to Ri, RijCi, and C2, and by noting that C2 and R3 share a common current. Furthermore, the output voltage appears across R3. The transfer function H(s), between the input voltage source and the output node of the filter is given by... 

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. 

A medley of op-amp circuits is presented in this section. Table 7.3 shows some basic op-amp circuits and lists the chief characteristics of each one. The power supply connections are not shown for these circuits (except for circuits 10 and 11), but it is understood that they are present. Power supply bypass capacitors are also not shown but are assumed to be connected. Circuit 2 in the table is a voltage buffer. It is an example of one of the few applications that require no external components except for a wire connecting the output to the inverting input terminal of the op-amp. 

To analyze an op-amp circuit with resistive feedback, we also need noise models for resistors. Two models for a noisy resistor are shown in Fig. 7.100. The thermal noise (Johnson noise) of the resistor is modeled by a voltage noise source with spectral density as shown in Fig. 7.100(b). The power density of this source is e = 4fcTJ V /Hz where fi is in ohms, k is Boltzmann s constant = 1.38 x 10" J/K, and T is temperature in degrees Kelvin. Thus, thermal noise is white. At 25 C, the noise spectral density is approximately Cr Ay/R nV/ Hz where K is expressed in kilohms. Figure 7.100(c) shows an equivalent resistor noise model that includes a current noise source having power density i = AkT/R A /Hz. 

Mica and plastic film (polycarbonate, polyester, or polystyrene) are most often the types of capacitors used to determine the frequency characteristics or timing behavior of discrete op-amp circuits. Ceramic disc or electrolytic capacitors should generally not be used in these applications. Ceramic disc capacitors are sensitive to temperature and humidity, and electrolytic capacitors are sensitive to temperature and conduct small amounts of DC current. However, ceramic disc and electrolytic capacitors are used for power supply bypassing duties. 

Plastic film capacitors for electronic applications can be obtained with values up to about 10 F, but these capacitors are bulky and expensive, and the use of capacitors this large in value should be avoided if possible. Film capacitors that range in value from nanofarads to microfarads perform nicely in most op-amp circuits. 

From Eq. (7.172) and (7.173), it is seen that tuo can be adjusted by r2, and then Q can be adjusted using r3 without affecting coq. Finally, the gain constant can be adjusted using rj without affecting the previously set variables. This circuit is said to feature orthogonal adjustments. Several three op-amp circuits have this attribute. AH five basic second-order filter sections can be realized with modifications to the low-pass circuit (Schaumann, Ghausi, and Laker, 1990). 

There has been long-standing difficulty in producing fully integrated MOS RC-op amp circuits with precise characteristics. This is because the MOS fabrication process does not result in sufficiently precise control over the absolute values of resistance and capacitance (and, hence, the RC product). In addition, MOS integrated (diffused) resistors have poor temperature and linearity characteristics, as well as requiring a large silicon area. Ordinarily the RC values cannot be controlled to better than 20%. This limitation was soon overcome by the development of circuit techniques wherein the resistor is simulated by a capacitor... 

FIGURE 10.285 The simplest and least expensive active-balanced input op-amp circuit. Performance depends on resistor-matching and the balance of the source impedance. 

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