Low-frequency Characteristics
At low frequencies and under small-signal conditions, the circuit in Figure 1 can be represented by that in Figure 2, where the hybrid-pi model for the BJT has been employed. The input signal is represented by a Thévenin voltage source, vs, with a series resistance Rs and the load is a resistor RL. This circuit can be used to derive the following characteristics of the common-base amplifier.
| Definition | Expression | Approximate expression | Conditions | |
|---|---|---|---|---|
| Open-circuit voltage gain | ||||
| Short-circuit current gain | ||||
| Input resistance | ||||
| Output resistance |
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- Note: Parallel lines (||) indicate components in parallel.
In general the overall voltage/current gain may be substantially less than the open/short circuit gains listed above (depending on the source and load resistances) due to the loading effect.
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