**Bias Circuit Design:**

Bias Circuit Design can be amazingly simple. Usually, it is just a matter of determining the required voltage across each resistor and the appropriate current levels. Then, the resistor values are calculated by application of Ohm’s law.

Designs usually begin with specification of the supply voltage and the required levels of I_{C} and V_{CE}. The resistor values are calculated to meet these requirements, and standard value resistors are selected. Usually, resistors with a tolerance of ±10% are used wherever possible. These are less expensive than ±5% and ±1% components.

**Base Bias Circuit Design:**

A base bias circuit is very easily designed. This is illustrated in Fig. 5-36.

When selecting the standard value resistors, a decision must be made whether to select the next smaller resistance value or the next larger value. In general, it is best to select the resistance value that tends to increase the transistor collector-emitter voltage, thus keeping V_{CE} from approaching zero.

After design, the circuit should be analysed using the selected standard-value components and the maximum and minimum h_{FE} values for the transistor.

**Collector-to-Base Bias Circuit Design:**

The design procedure for a collector-to-base bias circuit is similar to that for base bias, with the exception that the voltage and current levels are different for calculating R_{B} and R_{C}. In collector-to-base bias, the voltage across R_{B} is (V_{C} – V_{BE}) and the current through R_{C} is (I_{B} + I_{C}). The design equations are shown in Fig. 5-38.

Once again decisions must be made about selecting the next larger or the next smaller standard value resistors. As in the case of the base bias circuit , selection of the smaller value for R_{C} and the larger value for R_{B} tends to produce a larger V_{CE} than the specified level. The design should be analysed using the selected standard-value components and the transistor h_{FE(max)} and h_{FE(min) }values.

**Voltage Divider Bias Circuit Design:**

When designing a voltage divider bias circuit the voltage divider current (I_{2} in Fig. 5-40) should be selected much larger than the transistor base current (I_{B}). This makes the base voltage (V_{B}) a stable quantity largely unaffected by the transistor h_{FE} value. However, a high level of I_{2} results in small resistance values for R_{1} and R_{2}, and this gives the circuit an undesirable low input impedance.

A rule-of-thumb approach to selection of I_{2} is to use a voltage divider current approximately equal to one-tenth of the transistor collector current.

As can be easily demonstrated, this gives reasonably large values for R_{1} and R_{2} while still keeping I_{2} much larger than I_{B}.

If V_{E} is not specified, it should be selected much larger than the transistor V_{BE}.

This is because V_{BE} can vary from transistor to transistor, and it can also change with temperature increase or decrease. Making V_{E} very much larger than V_{BE} minimizes the effect of V_{BE} changes on the circuit bias conditions. Typically, as another rule-of-thumb, V_{E} is selected as 5 V regardless of the supply voltage. When V_{CC} is low, V_{E} can be as low as 3 V.

Figure 5-40 shows the equations used for calculating each resistor value.

**Designing with Standard Resistor Values:**

In all circuit designs a suitable standard resistor should normally be selected when each resistor value is calculated, instead of first completing the design. Then, the new resistor voltage drop or current level should be determined before calculating the next component value.

**More Bias Circuits:**

**Base Bias with Emitter Resistor:**

The bias circuit shown in Fig. 5-43 is the usual base bias arrangement with the addition of an emitter resistor. Analysis reveals that this circuit has essentially the same stability characteristics as a similar base bias circuit. The only advantage of the emitter resistor in this case is that it gives the circuit a higher input resistance. An emitter resistor could also be employed with collector-to-base bias, and this would also produce a higher input resistance without significantly altering the circuit stability.

**Voltage Divider and Collector-to-Base Combination:**

Figure 5-44 shows a voltage divider bias circuit with resistor R_{1} connected to the transistor collector instead of to the supply. Thus, the circuit combines collector-to-base bias with voltage divider bias. An analysis of the circuit shows that this combination produces even greater bias stability than voltage divider bias alone.

The design procedure for this circuit is similar to voltage divider bias design, except that for calculating the resistances of R_{1} and R_{3}, the voltage across R_{1} is (V_{C} – V_{B}) instead of (V_{CC} – V_{B}), and the current through R_{3} is (I_{C} + I_{2}).

**Emitter Current Bias:**

The emitter current bias circuit in Fig. 5-46(a) uses a plus and minus voltage supply (+V_{CC} and -V_{EE}) and has the transistor base grounded via resistor R_{3}. This is similar to voltage divider bias, in fact, as illustrated In Fig. 5-46(b), a voltage divider (R_{1} and R_{2}) could be used to provide V_{B} instead of grounding the base via R_{1}.

So long as there is very little voltage drop across base resistor R_{1}, in Fig. 5-46(a), this circuit (like voltage divider bias), has excellent bias stability. Normally, R_{1} is selected to have a voltage drop much smaller than the transistor base-emitter voltage:

A reasonable rule-of-thumb to use is,

This mean that the transistor base voltage can be treated as ground level, and the voltage at the emitter terminal is always V_{BE} below ground, [see Fig. 5-46(a)]. So, the voltage across R_{E} is a constant quantity:

The transistor collector voltage is,

and, as illustrated, the collector-emitter voltage is