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:<math>r_\pi = \left.\frac{v_\text{be}}{i_\text{b}}\right\vert_{v_\text{ce} = 0} = \frac{V_\text{T}}{I_\text{B}} = \frac{\beta_0}{g_\text{m}}</math>
where:▼
▲where:
* <math>\scriptstyle \beta_0 ~=~ \frac{I_\text{C}}{I_\text{B}} \,</math> is the current gain at low frequencies (generally quoted as ''h''<sub>fe</sub> from the [[Bipolar junction transistor#h-parameter model|h-parameter model]]). <math>\scriptstyle I_\text{B}</math> is the DC (bias) base current. This is a parameter specific to each transistor, and can be found on a datasheet.▼
* <math>\scriptstyle I_\text{B}</math> is the DC (bias) base current.
▲* <math>\scriptstyle \beta_0 ~=~ \frac{I_\text{C}}{I_\text{B}} \,</math> is the current gain at low frequencies (generally quoted as ''h''<sub>fe</sub> from the [[Bipolar junction transistor#h-parameter model|h-parameter model]]).
* <math>\scriptstyle r_\text{o} ~=~ \left.\frac{v_\text{ce}}{i_\text{c}}\right\vert_{v_\text{be} = 0} ~=~ \frac{1}{I_\text{C}}\left(V_\text{A} \,+\, V_\text{CE}\right) ~\approx~ \frac{V_\text{A}}{I_\text{C}}</math> is the output resistance due to the [[Early effect]] (<math>\scriptstyle V_\text{A}</math> is the Early voltage).
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