Common source

At low frequencies and using a simplified hybrid-pi model (where the output resistance due to channel length modulation is not considered), the following closed-loop small-signal characteristics can be derived.

Bandwidth of common-source amplifier tends to be low, due to high capacitance resulting from the Miller effect.

Figure 4 shows the corresponding small-signal circuit when a load resistor RL is added at the output node and a Thévenin driver of applied voltage VA and series resistance RA is added at the input node.

The limitation on bandwidth in this circuit stems from the coupling of parasitic transistor capacitance Cgd between gate and drain and the series resistance of the source RA.

The size of CM is decided by equating the current in the input circuit of Figure 5 through the Miller capacitance, say iM, which is: to the current drawn from the input by capacitor Cgd in Figure 4, namely jωCgd vGD.

One trick is to add a common-gate (current-follower) stage to make a cascode circuit.

A reduction to 1/ √2 occurs when ωCM RA = 1, making the input signal at this value of ω (call this value ω3 dB, say) vG = VA / (1+j).

[3] Examination of the output side of the circuit in Figure 2 enables the frequency dependence of the gain vD / vG to be found, providing a check that the low-frequency evaluation of the Miller capacitance is adequate for frequencies f even larger than f3 dB.

Figure 1: Basic N-channel JFET common-source circuit (neglecting biasing details).
Figure 2: Basic N-channel JFET common-source circuit with source degeneration.
Figure 3: Basic N-channel MOSFET common-source amplifier with active load I D .
Figure 4: Small-signal circuit for N-channel MOSFET common-source amplifier.
Figure 5: Small-signal circuit for N-channel MOSFET common-source amplifier using Miller's theorem to introduce Miller capacitance C M .