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The input resistance can readily found by recalling
Lab 2, where we showed that the input resistance into
a common emitter amplifier was
.Well, for a diff-amp, the resistance looking into the base
of Q1 is
, where *R*_{E}' is the effective emitter
resistance of Q1. However, this resistance is actually the resistance
looking into the emitter of Q2. Recall that the resistance looking
into the emitter is
.
So, the input resistance
looking into the base of Q1 is
| |
(117) |

So far in these labs we have considered the BJT collector to
be a perfect current source. For now let's continue this
approximation.
So, if we try to drive the output, which is the collector of Q2,
with an independent source, all the current will go through *R*_{C2}.
Therefore, the output resistance of the diff-amp is just given
by^{}

*Neil Goldsman*

*10/23/1998*