Final answer:
To find the base current in a common emitter amplifier with a current gain of 70 and an emitter current of 8.8 mA, divide the emitter current by the sum of 1 plus the current gain, which gives us a base current of approximately 0.123 mA.
Step-by-step explanation:
In a common emitter amplifier, the current gain (often denoted as β or hFE) is the ratio of the collector current to the base current. Given that the current gain is 70 and the emitter current is 8.8 mA, we can find the base current using the relation between the emitter current (IE), the base current (IB), and the collector current (IC). In a bipolar junction transistor (BJT), the emitter current is the sum of the base and collector currents (IE = IB + IC). Since IC = β * IB, we can substitute IC in the equation to get IE = IB + β * IB, giving us IE = IB * (1 + β). Therefore, to find IB we rearrange the equation to IB = IE / (1 + β).
Calculating the base current, we get IB = 8.8 mA / (1 + 70) = 8.8 mA / 71 ≈ 0.1239 mA. So, the closest answer to the calculated base current is 0.123 mA (Option b).