(See related pages)
|
|
|
| SEC. 8-1 VOLTAGE-DIVIDER BIAS The most famous circuit based on the emitter-bias prototype is called voltagedivider bias. You can recognize it by the voltage divider in the base circuit. SEC. 8-2 ACCURATE VDB ANALYSIS The key idea is for the base current to be much smaller than the current through the voltage divider. When this condition is satisfied, the voltage divider holds the base voltage almost constant and equal to the unloaded voltage out of the voltage divider. This produces a solid Q point under all operating conditions. SEC. 8-3 VDB LOAD LINE AND Q POINT The load line is drawn through saturation and cutoff. The Q point lies on the load line with the exact location determined by the biasing. Large variations in current gain have almost no effect on the Q point because this type of bias sets up a constant value of emitter current. SEC. 8-4 TWO-SUPPLY EMITTER BIAS This design uses two power supplies: one positive and the other negative. The idea is to set up a constant value of emitter current. The circuit is a variation of the emitter-bias prototype discussed earlier. SEC. 8-5 OTHER TYPES OF BIAS This section introduced negative feedback, a phenomenon that exists when an increase in an output quantity produces a decrease in an input quantity. It is a brilliant idea that led to voltage-divider bias. The other types of bias cannot use enough negative feedback, so they fail to attain the performance level of voltagedivider bias. SEC. 8-6 TROUBLESHOOTING Troubleshooting is an art. Because of this, it cannot be reduced to a set of rules. You learn troubleshooting mostly from experience. SEC. 8-7 PNP TRANSISTORS These pnp devices have all currents and voltages reversed from their npn counterparts. They may be used with negative power supplies; more commonly, they are used with positive power supplies in an upside-down configuration. |