Voltage regulator is another fundamental analog building block, based on an error amplifier and a power transistor which drives the loads. In low voltage and low power applications, it presents the major challenge of achieving a good dynamic response with a low quiescent current. Therefore, the adaptive biasing scheme is used, because its the quiescent current is very small at low-load condition and gradually increases to a high value at high-load condition. Hence, when the load current switches from the high-to-low load condition, the higher quiescent current at high-load condition initially provides a fast charging of the gate node of the power transistor connected to the error amplifier, resulting in a small overshoot at the output. However, for the low-to-high load transient edge, the low quiescent current provides a larger undershoot due to the slow discharging of the gate node of the power transistor. For this purpose, a hybrid-mode operational transconductance amplifier (HM-OTA) has been proposed, that does not occupy extra space in silicon or consume additional power to realize, while presenting a fast discharging slew-rate and achieving a good dynamic response. The hydbrid-mode amplifier is depicted in [
8] and it is shown in
Figure 9c, where its topology is compared to the common-mode amplifier (CM-OTA) in (a) and to the differential-mode (DM-OTA) in (b).
The first one (
Figure 9a) is the common-mode amplifer: it is a possible solution for implementing the error amplifier in the voltage regulator due to its single pole behavior, which eases frequency compensation. However, it has very limited dc gain as all the internal nodes have low impedance. Moreover, under large signal operation, the charging/discharging slew rate is also symmetric and quite limited. For the sake of completeness, the gain
of the CM-OTA is approximately given by:
and it is similar to a one-stage amplifier. A conventional differential-mode OTA (DM-OTA) (
Figure 9b) is a more preferred option, as it provides a high dc gain as well as asymmetric slewing operation in large signal. The gain
is indeed:
and it is the two-stage amplifier gain. However, the high impedance from the internal node creates a stability issue in the regulator with a small value of output compensation capacitor and a low quiescent current. Essentially, the value of the output capacitor has to be increased to restore the stability. The HM-OTA is constructed by combining both the CM-OTA and the DM-OTA as shown in
Figure 9c and exploits the advantages of both the structures. Unlike the CM-OTA, the transistor
is segmented into two parts namely,
and
. Also, the gate of
is connected to V1 instead of the node V2 in the proposed HM-OTA. This modification forms a localized differential stage, which forces the delta/difference current of
and
through
. The ratios of
and
are chosen as 1:
and (1-
):
, respectively, for maintaining dc current balancing in the HM-OTA. So, the HM-OTA becomes a combination of
xCM-OTA and (1−
)xDM-OTA for
. The differential gain is in this case:
and it is the gain of an improved one-stage amplifier. The slew-rate performance of the proposed HM-OTA is discussed now. During large signal operation, the charging slew rate SR+ is similar to the conventional CM-OTA/DM-OTA. During discharging operation, the transistors
,
,
completely shut off and the whole tail current flows through
and
. Due to the width ratio of
and
the negative SR is higher than the conventional CM-OTA by a factor of 1/(1−
). Of course, the value of SR- of HM-OTA is still less than the conventional DM-OTA; however the latter causes a stability issue in the regulator as mentioned before. The main advantage of the HM-OTA over the CM-OTA and the DM-OTA is that the dc gain, loop bandwidth, slew rates, and loop stability can be easily controlled with the value of
. Also, this modification does not require any additional space and quiescent current.