An Improved Circuit of Super Source Follower Based on gm-boosting Technology

An improved circuit of super source follower (SSF) based on gm-boosting technology is proposed in this paper. The traditional SSF circuit is used to reduce the output impedance of the source follower, to provide stronger load capacity as the output stage of the whole circuit, or to push the non-dominant poles in the multi-stage amplifier circuit to a higher frequency position. In the paper, based on the above SSF circuit, an operational amplifier is connected in parallel between the gate and drain of the Nmos as the feedback loop, which takes the drain as the positive input and the gate as the output to form the gm-boosting structure. The advantage of this structure is to further reduce the output resistance through the voltage-voltage feedback characteristics of the gm-boosting structure. Because two loops are formed after adding the structure, as long as the open-loop gain of the operational amplifier is high enough, the loop formed by the operational amplifier will occupy a dominant position, thus shielding the influence of the source follower on the overall output resistance. The output resistance can change with the operational amplifier gain. The simulation results show that the output impedance of the circuit is only about 92 Ω, which is much lower than 1 kΩ of SSF structure.


Introduction
With the continuous optimization and improvement of the chip manufacturing process, the requirements for chip area, power consumption, and performance are getting higher and higher, and the target of high-frequency performance of the circuit is becoming more and more strict, which is also the requirement for the overall stability of the circuit.Therefore, most circuits should achieve single-pole approximation to get a relatively good phase margin.Reducing the equivalent resistance or equivalent capacitance of the non-dominant pole to push its frequency to a high-frequency position is the main method of frequency compensation [1] , but the MOS transistors used as power transistors are generally of large size, thus having a considerable gate capacitance, and amplifier stage will generally provide a large output resistance.At this time, it is a good choice to add a buffer stage [2][3] in the front stage of the power transistor or after the amplifier circuit.
The simplest structure of the buffer level is the source follower (SF) [4] , which is shown in Figure 1.Under the premise of negligent bulk effect, it can provide the output impedance of 1/g m [5]   , which is relatively small compared with the drain-source resistance r o of MOS transistors, but with the further improvement of integration requirements, the size of MOS transistors is decreasing as well.Under the existing 3.3 V application conditions, the transconductance energy of most non-input MOS transistors is as low as 100 uS, so the value of 1/g m1 is also up to 10 kΩ.In many applications, it can not meet the requirements of the target, so it is necessary to increase the value of g m , which brings the increased circuit area.Therefore, a new structure is needed to replace SF.In this case, the configuration of the super source follower (SSF) [6-7-8] is proposed, and its structure is shown in Figure 2. SSF parallels a Nmos based on SF to form a feedback loop and utilizes the negative feedback of the M2 transistor to reduce the output resistance.From a qualitative point of view, when the input DC level does not change but the output DC level increases, the drain current of M1 increases, and the gate-source voltage of M2 increases too, which increases the drain current of M2 and the total current flowing into the output node, leading to the decrease of the total output resistance.
It can be that the output resistance of the SSF structure is: where g m1 and g m2 are the transconductance of M1 and M2, r o1 and r o2 are the equivalent internal resistance of M1 and M2, respectively, and R 1 and R 2 are the impedance of the two current sources.
It is assumed that I 1 and I 2 are ideal current sources while R1→∞ and R2→∞. 1 This value is g m2 r o1 times lower than the output resistance 1/g m1 of the source follower.In practical applications, the value of g m2 r o1 can usually reach the order of several hundred, so the reduction of output resistance of SSF is very considerable.In addition, the Nmos transistor in the feedback loop can also be replaced by an npn transistor [9] , in which case the output resistance is as follows: ( ) where β is the current gain of the npn transistor.

The proposed improved circuit structure and analysis
In some demanding circuit applications, such as the power transistor in 35 V or even more high voltage environment or the current mirror mosfet in high current transmission circuit, because of its very large gate length, the gate capacitance is also very large, and the output resistance provided by the structure of SSF is still difficult to meet the requirements.Figure 3 shows the improved circuit proposed in this paper, in which an operational amplifier is connected in parallel between the gate and drain of the Nmos as the feedback loop, which uses the drain as the positive input and the gate as the output to form the g m -boosting structure [10] .The voltage-voltage feedback characteristic of the g mboosting structure is used to further reduce the output resistance.
As shown in Figure 4, the g m -boosting structure essentially detects the voltage from the output and establishes a negative feedback structure around the MOS transistor.From the point of view of qualitative analysis, the feedback loop detects the output voltage and returns a proportional current to the input summation node, which is essentially voltage-voltage feedback, so it will reduce the original output resistance of the feedforward path.The reduced output resistance is as follows:  3.Because the operational amplifier directly samples the output signal and amplifies it to the drain terminal of M1, the role of M1 in feedback is almost replaced by the operational amplifier, which also means that the W and L of M1 can be very small without affecting the overall performance of the circuit, further reducing the circuit area.When I 1 and I 2 are regarded as ideal current sources, the output resistance of the circuit can be obtained as follows: Then, as long as the gain of the operational amplifier is large enough, the overall output impedance of the circuit will be very small.At the same time, the inversely proportional relationship between the output resistance and the operational amplifier gain allows designers to flexibly choose different kinds of operational amplifiers to meet different application needs.

Results and discussion
In this paper, the open-loop gain of the operational amplifier unit and the output resistance of the existing technology SSF and the improved circuit is tested.

The open-loop gain of the operational amplifier
In the calculation formula of the output resistance described above, the open-loop gain of the operational amplifier directly affects the final result, so to meet the requirements of this paper, the gain of the operational amplifier is one of the important indicators to be considered.After comprehensive consideration of power consumption and area, it is decided to use the basic five-transistor OTA operational amplifier and its open-loop gain is shown in Figure 5.Its low-frequency gain is 36.2382dB.The bandwidth of 3 dB is about 88, 443 Hz, and its parameters can meet the requirements.

Output resistance
Under the condition that the input common-mode level is 1.6 V, the supply voltage is 3.3 V, I 1 current source is supplied to 150 uA, I 2 current source is supplied to 10 uA, and the output resistance of SSF and the output resistance of the improved circuit are shown in Figures 6 and 7, respectively.The output resistance of SSF is about 1.41 kΩ, while the output resistance of the improved circuit is about 91.4 Ω, achieving the design goal of greatly reducing the output resistance.

Conclusion
In the paper, an improved circuit of super source followers based on g m -boosting technology is proposed.This technology is applied to some application environments where the existing SSF structure is difficult to meet.The voltage-voltage feedback characteristic of the g m -boosting structure further reduces the output impedance of the overall circuit and shields the influence of the original SSF loop on the overall output resistance so that the output resistance can vary with the operational amplifier gain.Under the condition of using a 36 dB low-frequency gain operational amplifier, the output resistance of the improved circuit is only 91.4 Ω, which achieves the design goal.

Figure 1 .
Figure 1.The basic structure of SF Figure 2. The basic structure of SSF

Figure 3 .Figure 4 .
Figure 3. Improved circuit of super source follower by using g m -boosting technology

Figure 5 .
Figure 5. AC simulation results of the operational amplifier unit

Figure 6 .Figure 7 .
Figure 6.The output resistance of the SSF circuit