Reverse Power Gain in the Grounded Collector Circuit
Author: Leonard Krugman
The grounded collector connection also has the unique ability to furnish power gain in the reverse direction. This characteristic might be anticipated on the basis of the equivalent circuit, since the internal generator r_{m}i_{e} is common to both the input and output circuits, and the values of r_{b} and r_{e} are approximately equal. The equivalent circuit for the reverse connection is illustrated in Fig. 415. The resulting fourterminal parameters for this connection can be evaluated in terms of the internal transistor parameters as before:
Fig. 415. Equivalent "T" for reverse operation of grounded collector connection.
A.
The input loop equation is:
e_{1} + r_{m}i_{e} = i_{1} (r_{e} + r_{c}) + i_{2}r_{c}
Substituting i_{e} = i_{l},
e_{1} = i_{1} (r_{e} + r_{c}  r_{m}) + i_{2}r_{c}
when
i_{2} = 0, e_{1} = i_{1} (r_{e} + r_{c}  r_{m});
then
r_{11} = r_{e} + r_{c}  r_{m},
which is equal to r_{22} in the forward direction.
B.
Using the same input loop equation, when
i_{1} = 0, e_{1} = i_{2}r_{c},
then
,
which is equal to r_{21} in the forward direction.
C.
The output loop equation is
e_{2} + r_{m}i_{e} = i_{1}r_{c} + i_{2} (r_{b} + r_{c});
Since
i_{e} = i_{1}, e_{2} = i_{1} (r_{c}  r_{m}) + i_{2} (r_{b} + r_{c})
when
i_{2} = 0, e_{2} = i_{x} (r_{c}  r_{m});
then
; which is
equal to r_{12} in the forward direction.
D.
Using the same output loop equation, when
i_{1} = 0, e_{2} = i_{2} (r_{b} + r_{c});
then
r_{22} = e_{2}/i_{1} = r_{b} + r_{c},
which is equal to r_{11} in the forward direction.
Therefore, it can be seen that any of the equations derived for operation in the forward direction can be revised for use in the reverse direction by substituting r_{11} for r_{22}, r_{12} for r_{21}, r_{21} for r_{12}, and r_{22} for r_{11}. For example, the maximum available power gain in the forward direction, , becomes in the reverse direction.
