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To determine the small signal open loop gain we break the
circuit up into stages. Looking at Fig. 5.6, we realize that
the circuit is composed of a gain stage that is composed of the
differential amplifier, and the emitter follower output stage, which
acts to lower the output resistance of the circuit.
The overall gain of the circuit is then the product of the
gain of the two stages:
| |
(123) |
where The approximate result of this calculation can be obtained almost
immediately by inspection to be .Now, let's perform a more detailed analysis to show that this is indeed
the case.
First of all the gain of the first stage is:
| |
(124) |
Where
is the input resistance looking into the base
of Q4.
Now, the gain of the second stage is given by
| |
(125) |
If we multiply the gain of the two stages together, we arrive at
the following expression for the total differential mode gain of the circuit Adm.
| |
(126) |
Since RE4 is usually much greater than 1/gm4,
in most cases the gain of the second stage can often be approximated
by .So, the gain of the overall amp is usually given by
the gain of the first stage alone described by
equation (5.25).
Finally, we it is often the case that we can neglect
because it is usually much greater than RC2.
Under these conditions, the total gain of the circuit can be
given by
, which was mentioned above.
Next: Small Signal Output and
Up: Op-Amp Basic Concepts
Previous: Open Loop Analysis
Neil Goldsman
10/23/1998