\[\text{Z}_{\text{i}}=\text{h}_{\text{ie}}+\left(1+\text{h}_{\text{fe}}\right) 2 \text{R}_{\text{e}}=2 \text{h}_{\text{fe}} \text{R}_{\text{e}} \approx 2 \beta \text{R}_{\text{e}}\]
\[\text{Z}_{\text{i}}=\text{h}_{\text{ie}}+\left(1+\text{h}_{\text{fe}}\right) 2 \text{R}_{\text{e}}=2 \text{h}_{\text{fe}} \text{R}_{\text{e}} \approx 2 \beta \text{R}_{\text{e}}\]
\[\text{Z}_{\text{i}}=\text{h}_{\text{ie}}+\left(1+\text{h}_{\text{fe}}\right) 2 \text{R}_{\text{e}}=2 \text{h}_{\text{fe}} \text{R}_{\text{e}} \approx 2 \beta \text{R}_{\text{e}}\]