Differential Input Resistance:

Differential input resistance is defined as the equivalent resistance that would be measured at either input terminal with the other terminal grounded. This means that the input resistance R_i1 seen from the input signal source v₁ is determined with the signal source v₂ set at zero. Similarly, the input signal v₁ is set at zero to determine the input resistance R_i2 seen from the input signal source v₂. Resistance R_S1 and R_S2 are ignored because they are very small.

Substituting i_e1,

Similarly,

The factor of 2 arises because the r_e' of each transistor is in series.

To get very high input impedance with differential amplifier is to use Darlington transistors. Another ways is to use FET.

Output Resistance:

Output resistance is defined as the equivalent resistance that would be measured at output terminal with respect to ground. Therefore, the output resistance R_O1 measured between collector C₁ and ground is equal to that of the collector resistance R_C. Similarly the output resistance R_O2 measured at C₂ with respect to ground is equal to that of the collector resistor R_C.

R_O1 = R_O2 = R_C        (E-5)

The current gain of the differential amplifier is undefined. Like CE amplifier the differential amplifier is a small signal amplifier. It is generally used as a voltage amplifier and not as current or power amplifier.

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