Download Trade-Offs in Analog Circuit Design - The Designer's by Chris Toumazou, George S. Moschytz, Barrie Gilbert PDF

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In turn, the current density depends on the actual emitter area (thus, on lithography) and is invariably dependent on some on-chip voltage source and at least one resistor. Since the is a diminishing function of temperature, 7 Few op-amp data sheets are forthcoming about this spread, often stating only a typical value. Similar vagueness is often found in the specifications for RF products. Some of this imprecision can be traced to the cost of testing ICs to allow these aspects of performance to be fully guaranteed; some of it has arisen as a kind of tradition, with concerns that the explicit revelation of the magnitude of such spreads would put a more completely-specified part in a “bad light”.

Thus, the resistance alone may vary over a 6 The question of whether the term worst-case always has a definite meaning is discussed later in the chapter. 18 Chapter 2 60% range, in a high-volume, robust design context. Adding to this estimate all the similar variations in the capacitor value, particularly those due to variations in the dielectric layer, and for junction and MOS capacitors their varactor behavior, it is easy to understand why the frequency of our basic oscillator can be predicted in only approximate terms: it already has process, temperature and possibly supply sensitivities even before considering the effect of the active elements.

In a diffused or polysilicon resistor this will arise from variations in doping concentration and the depth of the diffusion or film, and can easily be as high as ±15%, a 30% spread. Conductance in any resistive layer is also a function of temperature, sometimes a strong function. For example, the sheet resistance of a diffused resistor may typically vary by 1,500 ppm/K at T = 300 K, which extrapolates to variation of about 20% over the 130 K range from 230 to 360 K (–43°C to 87°C). This raises the tolerance band to about 50%.