Calculation of a Saturated Transistor Circuit

The circuit specifications are: when VIN = 12 V, VOUT 10 V at IOUT = 1 mA. The circuit diagram is shown in Figure 2–14.

Figure 2–14. Saturated Transistor Circuit

The collector resistor must be sized (Equation 2–26) when the transistor is off, because it has to be small enough to allow the output current to flow through it without dropping more than two volts to meet the specification for a 10-V output.

When the transistor is off, 1 mA can be drawn out of the collector resistor without pulling the collector or output voltage to less than ten volts (Equation 2–27). When the transistor is on, the base resistor must be sized (Equation 2–28) to enable the input signal to drive enough base current into the transistor to saturate it. The transistor beta is 50.

Substituting Equation 2–27 into Equation 2–28 yields Equation 2–29.

When the transistor goes on it sinks the load current, and it still goes into saturation. These calculations neglect some minor details, but they are in the 98% accuracy range.