*2.1. Bulk Driven Buffer: Simulation and Analytical Results*

Figure 2a illustrates a conventional bulk-driven flipped voltage follower, where the input voltage is applied to the bulk of transistor MD, a bias voltage *VBIAS* is applied to its gate, and the output voltage *VOUT* is obtained at the source. A negative feedback loop is established around transistors MF and MD, which forces the current *IB* via the constant voltage *VBN* to flow through the drain of device MD, and ensures a very low output resistance.

**Figure 2.** Bulk-driven FVF cell: (**a**) conventional approach; (**b**) proposed bootstrapped version; and (**c**) small–signal circuit (*gm*,*MD* = 0 for the bootstrapped case).

The proposed circuit is implemented by adding a capacitor *CG* between the gate and source terminals of MD and a cutoff transistor MG acting as a pseudo-resistor between *VBIAS* and the gate of MD, as shown in Figure 2b, in a similar way as in the quasi-floating gate transistor technique [28]. It is worth noting that these elements are the ones usually employed to design a bootstrapping circuit [29,30], but they are used here to cancel out the gate transconductance of transistor MD, i.e., *gm*,*MD* = 0, thus enhancing the voltage gain of the cell.

Figure 2c depicts the equivalent small-signal circuit of Figure 2a and the main parameters of the cell are summarized in the second column of Table 1, where *gm*,*Mi*, *gmb*,*Mi*, and *ro*,*Mi* are the gate transconductance, the bulk transconductance, and the output resistance of transistor M*i*, respectively. In addition, *RD*,*MD* and *RS*,*MD* are the equivalent resistances seen from the drain and source terminals of MD, also respectively. The small-signal equivalent circuit of the buffer in Figure 2b is very similar to the one illustrated in Figure 2c, but due to the bootstrapping effect *gm*,*MD* = 0. As a result, the corresponding small-signal expressions are modified accordingly for the proposed approach, as shown in the third column of Table 1. Note that, for the case of the voltage gain, the proposed circuit avoids the signal attenuation inherent in the bulk-driven technique. In return, the values of *Rout* and *RS*,*MD* are incremented due to the cancellation of *gm*,*MD*. On the other hand, the open loop gain is the same for both circuits, i.e., *gmb*,*MD* · *ro*,*MD*, whereas the loop gain can be expressed as (*gm*,*MD* + *gmb*,*MD*) · *ro*,*MD* and *gmb*,*MD* · *ro*,*MD* for the conventional and the bootstrapped version, respectively [31].


**Table 1.** Small-signal parameter comparison of the conventional and bootstrapped buffers.
