Common Drain Amplifier

This configuration is also known as the source-follower because the source voltage follows the gate voltage.

Fig. 39 Large-signal behavior.

Small signal gain

Fig. 40 Small-signal circuit model.

Eliminating body effect

Fig. 41 Eliminating body effect by tying the bulk to source.

Fig. 42 The triple well.

Notes about the triple well:

• ✅ The gain is approximately one.

• ❌ It requires a lot of area on the chip (large footprint).

• ❌ The process needs to support a triple well.

• ❌ Capacitance between n-p regions means more source capacitance.

Fig. 43 Solving for the output resistance.

Full equation

Fig. 44 Full equation.

The gain is given by

\[ A = \frac{ \frac{1}{g_{mb_{1}}} || r_{o1} || r_{o2} || R_L }{ \left( \frac{1}{g_{mb_{1}}} || r_{o1} || r_{o2} || R_L \right) + \frac{1}{g_{m_{1}}} } \]

You want to eliminate the \(1/g_{m_{1}}\) term in the denominator, if possible. It is easy to drop if the preceding term is large.

In this case, to maximize gain, you must maximize \(r_o\).

Summary

The common drain amplifier is good for driving large loads off-chip. For example, you’ve done your amplification in previous stages and can afford to attenuate a bit in exchange for low output resistance.