To find the overall current gain $A_i$ of the circuit in Figure 6–56, we need to understand the contributions of each transistor stage. The current gain is primarily determined by the small-signal current gain of the transistors and the coupling between the stages.

Steps to Determine the Overall Current Gain $A_i$:

1. Identify the Small-Signal Parameters:

   • Transistor $Q_1$ and $Q_2$ have current gains $\beta_{ac1}$ and $\beta_{ac2}$.
   • For $Q_1$: $\beta_{ac1} = 150$
   • For $Q_2$: $\beta_{ac2} = 100$

2. Determine the Current Gain for Each Stage:

   • The current gain of a common-emitter amplifier stage is approximately $\beta$.

Overall Current Gain:

The overall current gain $A_i$ of a two-stage amplifier is the product of the current gains of the individual stages.

$A_i = \beta_{ac1} \times \beta_{ac2}$

Substituting the given values:

$A_i = 150 \times 100 = 15000$

Conclusion:

The overall current gain $A_i$ for the circuit in Figure 6–56 is:

$A_i = 15000$

Would you like further details or have any questions about this calculation?

Here are five questions you might consider asking:

1. How do different stages in an amplifier affect the overall gain?
2. What is the role of $\beta$ in determining the current gain?
3. How do we ensure the stability of the amplifier with high current gain?
4. What are the effects of coupling capacitors on the overall current gain?
5. How does the biasing of transistors influence the current gain?

Tip: When calculating overall gain in multi-stage amplifiers, always verify each stage's individual gain and ensure proper biasing to achieve accurate results.