Editing Two-port network (section)

===Example: common-base amplifier===
[[Image:Common base hybrid pi current follower.PNG|thumbnail|200px|Figure&nbsp;7: Common-base amplifier with AC current source {{math|''I''{{sub|1}}}} as signal input and unspecified load supporting voltage {{math|''V''{{sub|2}}}} and a dependent current {{math|''I''{{sub|2}}}}.]]
'''Note:''' Tabulated formulas in Table 2 make the {{mvar|h}}-equivalent circuit of the transistor from Figure&nbsp;6 agree with its small-signal low-frequency [[hybrid-pi model]] in Figure&nbsp;7. Notation: {{math|''r''<sub>&pi;</sub>}} is base resistance of transistor, {{math|''r''<sub>O</sub>}} is output resistance, and {{math|''g''<sub>m</sub>}} is mutual transconductance. The negative sign for {{math|''h''<sub>21</sub>}} reflects the convention that {{math|''I''{{sub|1}}, ''I''{{sub|2}}}} are positive when directed ''into'' the two-port. A non-zero value for {{math|''h''<sub>12</sub>}} means the output voltage affects the input voltage, that is, this amplifier is '''bilateral'''. If {{math|1=''h''<sub>12</sub> = 0}}, the amplifier is '''unilateral'''.

{| class="wikitable" style="text-align:center; vertical-align:center; margin: 1em auto 1em auto"
|+ Table 2
! !! Expression !! Approximation
|-
| <math>h_{21} = \left. \frac{ I_{2} }{ I_1 } \right|_{V_2=0} </math>
| <math> -\frac{ \frac{\beta}{\beta + 1} r_\mathrm{O} + r_\pi }{ r_\mathrm{O} + r_\pi} </math>
| <math> -\frac{ \beta }{ \beta + 1 } </math>
|-
| <math>h_{11} = \left. \frac{V_1}{I_1} \right|_{V_2=0}  </math> 
| <math> r_\pi \mathbin{\|} r_\mathrm{O} </math>
| <math>r_\pi</math>
|-
| <math>  h_{22} = \left. \frac{I_2}{V_2} \right|_{I_1=0} </math>
| <math> \frac{1}{(\beta + 1)(r_\mathrm{O} + r_\pi)} </math>
| <math> \frac{1}{(\beta + 1)r_\mathrm{O} } </math>
|-
| <math> h_{12} = \left. \frac{V_1}{V_2} \right|_{I_1=0} </math>
| <math> \frac{r_\pi}{r_\mathrm{O} + r_\pi}</math> 
| <math> \frac{r_\pi}{r_\mathrm{O}} \ll 1</math>
|}