Tolerance, resistors

Unfortunately, the lab used in the creation of the circuits in this book closely resembles the lab of other engineering companies around the world. We use 5% tolerance resistors and 10% tolerance capacitors that are either soldered to a vector board or plugged into a solderless breadboard. This introduces various parasitics and inaccuracies in the results. In order to be more precise in showing the accuracy of SPICE simulation software, we frequently run the simulations with the stated values of the resistors or capacitors used in our lab breadboards. The measured values for each resistor and capacitor used in our breadboard configuration which may be different, are shown in Fig. 3.3. 

Note that in the Fourier results, the second harmonic is now greatly attenuated. The hardware circuit used 5% tolerance resistors, causing the simulation frequency to be slightly off. Another simulation was run... 

Calibration cell (shown in Figure 7.1b). An electrical calibration may be performed using a stabilized power supply (current generator), a high-accuracy resistor, a precision voltmeter and an electronic chronometer. Very high absolute tolerance resistors (0.01-0.001%) with low temperature coefficient are available, for example from Vishay-SFERNICE, France. 

When you place a resistor part called R () jn your circuit, you are using an ideal resistor that is, the resistor has no tolerance and no temperature dependence. The resistor is exactly the value you specify and the resistance does not change as the temperature changes. In practice, all resistors have tolerance and temperature variations. If you use a 1 k 2 resistor with 5% tolerance, the resistor will not be exactly equal to 1000 2. Its value could be anywhere between 950 2 and 1050 2. Temperature variations are anywhere from 50 parts per million (ppm) to 400 ppm. In PSpice we can specify both temperature variations and resistor tolerance. Tolerance will be covered in Part 9. 

No matter what the application, the circuit is designed to make the collector current independent of VBE and Hfe-We will assume that the resistors used have no tolerance and have their exact values specified in the circuit. To see how the tolerance of resistors affects the collector current, see Section 9.C. From previous sections we know that the resistance of a resistor, and Hfe and VBe of a BJT, are affected by temperature. The question is, if the temperature changes, how much does the collector current change ... 

The Performance Analysis capabilities of Probe are used to view properties of waveforms that are not easily described. Examples are amplifier bandwidth, rise time, and overshoot. To calculate the bandwidth of a circuit, you must find the maximum gain, and then find the frequency where the gain is down by 3 dB. To calculate rise time, you must find the 10% and 90% points, and then find the time difference between the points. The Performance Analysis gives us the capability to plot these properties versus a parameter or device tolerances. Hie Performance Analysis is used in conjunction with the Parametric Sweep to see how the properties vary versus a parameter. The Performance Analysis is used in conjunction with the Monte Carlo analysis to see how the properties vary with device tolerances. In this section we will plot the bandwidth of an amplifier versus the value of the feedback resistor. See Sections 9.B.3 and 9.E to see how to use the Performance Analysis in conjunction with the Monte Carlo analysis. 

The resistor model is used to specify resistor tolerance and temperature dependence. We will not discuss temperature dependence here. The main use of the resistor model in this manual is to specify resistor tolerance. See Part 9, page 504, for examples of using this model to specify tolerance. 

The uniform distribution specifies that the part is equally likely to have a value anywhere in the specified tolerance range. The first example we will look at is the 5% resistor model included in class.lib. If you look at Appendix E on page 620, you will see the following line in the file class.lib ... 

The model name is R5pcnt. It is a resistor model because the model type is RES. The nominal value of the model is R=1 and it has a 5% tolerance. The distribution is uniform. A uniform distribution means that the model parameter R is equally likely to have a value of 1.05, 1,0.95, or any other value between 1.05 and 0.95. When you use this model, the actual value of the resistor is the value specified in the schematic times the parameter R. Thus, if the value of a 5% resistor is specified as lk in the schematic, it may have a value anywhere between 950 and 1050 when used with the Monte Carlo analyses. An equivalent model is ... 

The Gaussian tolerance distribution generates the distribution shown below. The graph shows a 1 kQ resistor with a 5% tolerance. 

The output is similar to the previous simulation except that the deviations from the nominal value are smaller. Only one run did not pass the specification. The run had a lower deviation of 0.0115, which corresponds to a gain of 0.5 - 0.0115 = 0.4885. Smaller deviations should be expected since, in the Gaussian distribution, the bulk of the resistors were within plus or minus one standard deviation. From the results of the two previous simulations, we conclude that the tolerance distribution has a large impact on the Monte Carlo results. 

Remember that in a Gaussian distribution, the tolerance specified is the standard deviation a, and the full distribution extends 4ct. Change both resistors to RIOgauss ... 

The Performance Analysis can be used in conjunction with the Monte Carlo analysis to view the distribution of a parameter as a function of device tolerances. For this example, we will display how the spread of the gain V(Vo)/V(Vl) varies with resistor tolerances. We will use the voltage divider of the previous section and 5% resistors with a uniform distribution ... 

In amplifier design it is important to know how your bias will change with device tolerances. In this section we will find the minimum and maximum collector current of a BJT when we include variations in the transistor current gain, 0F, and resistor tolerances. The circuit above was previously simulated in the Transient Analysis and AC Sweep parts. We will use the same resistor values as before, but we will change the resistor models to include tolerance. The BJT is also changed to the model QBf. This model allows 0F to have a uniform distribution between 50 and 350. 

This information tells us that to achieve maximum collector current, 0F was increased to 350, RE was decreased by 5%, Rl was decreased by 5%, and R2 was increased by 5%. The other resistors had tolerance, but their values had no effect on the collector current. 

EXERCISE 9-1 Find the maximum and minimum collector current of the circuit in this section if the base resistors have 1% tolerance rather than 5% tolerance. 

This circuit contains 5% resistors, capacitors with +80%, -20% tolerance, and a 2N3904 transistor that has been modified to include tolerance in p. The modified model is shown below. See Section 7.C for instructions on how to modify a BJT model. 

The following models simulate resistors with tolerances... 

The presence or absence of a fourth band represents a margin-of-error factor (commonly called the resistor s tolerance range). 

Note Be sure to look for a fourth band. If there is no fourth band, it means the tolerance level of the resistor is not very good—it s between plus or minus 20% of the value indicated by the other bands. If there is a fourth band, it will be either silver or gold. Silver represents a tolerance of 10%. Gold represents a tolerance of 5%. ... 

---