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 MOSFET has been employed.
The amplifier characteristics are summarized below in Table 1. The approximate expressions use the assumptions (usually accurate) rO >> RL and gmrO >> 1.
| Table 1 | Definition | Expression | Approximate expression |
|---|---|---|---|
| Short-circuit current gain | |||
| Open-circuit voltage gain | |||
| Input resistance | |||
| Output resistance |
- 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.
Read more about this topic: Common Gate