24
The input impedance of a differential amplifier equals r’e times _______
- Aβ
- BRE
- CRC
- D2β
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Differential input resistance is defined as the equivalent resistance that would be measured at either input terminal with the other terminal grounded.
$({R_{i1}} = {\left( {\frac{{{V_1}}}{{{I_{b1}}}}} \right)_{{V_{2 = 0}}}})$
$(= {\left( {\frac{{{V_1}}}{{\frac{{{i_e}_1}}{\beta }}}} \right)_{{v_2} = 0}})$
Substituting the values
$({R_1}_i = \frac{{\beta r_e'\left( {r_e' + 2{R_E}} \right)}}{{r_e' + {R_E}}})$
Since RE ? re’
$({R_1}_i = \frac{{\beta r_e'\left( {2{R_E}} \right)}}{{{R_E}}})$
$({R_1}_i = 2\beta r_e')$
$({R_{i1}} = {\left( {\frac{{{V_1}}}{{{I_{b1}}}}} \right)_{{V_{2 = 0}}}})$
$(= {\left( {\frac{{{V_1}}}{{\frac{{{i_e}_1}}{\beta }}}} \right)_{{v_2} = 0}})$
Substituting the values
$({R_1}_i = \frac{{\beta r_e'\left( {r_e' + 2{R_E}} \right)}}{{r_e' + {R_E}}})$
Since RE ? re’
$({R_1}_i = \frac{{\beta r_e'\left( {2{R_E}} \right)}}{{{R_E}}})$
$({R_1}_i = 2\beta r_e')$