CN203193607U - Source follower based on PMOS transistor - Google Patents

Abstract

The utility model provides a source follower based on PMOS transistor belongs to source follower technical field. The source follower comprises a current source and a PMOS transistor serving as an input device, wherein a body terminal of the PMOS transistor is connected with an input terminal of the source follower_in) Is connected so as to make the voltage (V) between the source and the bulk of the N-well NMOS transistor_sb) Remains substantially constant in the event of input signal variations. The source follower has small distortion and good linearity, and is particularly suitable for being applied to high-speed and heavy-load occasions.

Description

Source follower based on PMOS transistor

technical field

The utility model belongs to the technical field of source followers and relates to a source follower based on a PMOS transistor. the

Background technique

Source followers based on MOS devices (that is, MOSFETs, that is, MOS transistors) are widely used in various functional circuits. For example, the source follower can usually be used as a high-speed input buffer, its circuit structure is simple, the source follower can provide high input impedance, low output impedance and wide signal bandwidth; compared with the operational amplifier based on closed-loop drive, the source follower There are no stability issues with the device. Therefore, source followers are well suited for buffer and driver circuits. the

FIG. 1 is a schematic diagram of a cross-sectional structure of a conventional PMOS transistor, and FIG. 2 is a schematic diagram of an equivalent circuit of the conventional PMOS transistor of FIG. 1 . FIG. 3 is a schematic circuit diagram of a conventional source follower formed based on the PMOS transistor shown in FIG. 1 . the

As shown in Figure 1, the traditional PMOS transistor is selected to be formed on a P-type substrate or substrate 111, which includes an N well 113, a source region 121, a drain region 122, a bulk terminal (Bulk, hereinafter referred to as B) 132, a source (Source, hereinafter referred to as S) 133 , gate (Gate, hereinafter referred to as G) 134 , drain (Drain, hereinafter referred to as D) 135 and gate dielectric layer 140 . Wherein, the N well 113 can be of N conductivity type, which can be formed by patterning N-type doping to the P-type substrate 111 to form a PMOS transistor; in the N well 113, the source region 121 and the drain region 122 are formed by patterning doping , the two can be formed by doping at the same time, or can be formed by doping in steps, and both are of the P conductivity type, and the specific doping concentration is not limiting. Between the source region 121 and the drain region 122, the gate bias can be A channel can be formed under the control of the voltage; the source 133 can be formed by drawing a metal electrode from the source region 121, and the drain 135 can be formed by drawing a metal electrode from the drain region 122; the body terminal 132 can be formed by drawing a metal electrode from the N well 113; the gate dielectric layer 140 Specifically, but not limited to, it can be SiO ₂ , which can be formed by patterning and oxidizing the surface of the substrate 111 made of silicon material, and the gate 134 is formed between the source 133 and the drain 135 . In order to avoid the above parasitic diodes from being forward biased, the P-type substrate 111 is biased to the ground signal GND.

While forming the PMOS transistor shown in FIG. 1 according to the above embodiment, parasitic capacitances C _sb , C _db and C _b are usually formed in the PMOS transistor, wherein C _sb is formed between the source region 121 and the N well 113 The junction capacitance of the diode (as shown in Figure 1), _Cdb is the junction capacitance of the diode (as shown in Figure 1) formed between the drain region 122 and the N well 113, and _Cb is the junction capacitance of the N well 113 and the P-type substrate The junction capacitance of the diode (as shown in Figure 1) formed between the bottom 111.

In the existing source follower formed based on the PMOS transistor shown in FIG. 1 , at least three parasitic capacitances C _sb , C _db and C _b inevitably exist. Since the PMOS transistor has the characteristic that the gain Gain will change with the voltage of the output signal, that is, there is a voltage dependence. Therefore, the existing source follower based on the PMOS transistor is prone to large fluctuations when the input signal swing is large. Distortion, thus easy linear distortion, poor linearity.

Moreover, due to the existence of three parasitic capacitances C _sb , C _db and C _b , it is easy to affect the high-frequency performance of the source follower based on the PMOS transistor. Therefore, the high-frequency performance of the source follower of the existing PMOS transistor Or high-speed applications, prone to dynamic distortion.

In view of this, it is necessary to propose a new type of source follower. the

Utility model content

One of the purposes of the present invention is to improve the linearity of the source follower. the

Another object of the present invention is to reduce the linear distortion and dynamic distortion of the output signal of the source follower. the

To achieve the above or other objectives, the utility model provides a source follower based on a PMOS transistor, including a current source and a PMOS transistor used as an input device, wherein the gate of the PMOS transistor is defined as the source The input terminal of the pole follower (V _in ), the source of the PMOS transistor is defined as the output terminal of the source follower (V _out ), the drain of the PMOS transistor is connected to the ground signal (GND), high A level signal (V _DD ) is connected from one end of the current source, and the other end of the current source outputs current to the source;

Wherein, the body terminal (B) of the PMOS transistor is connected to the input terminal (V _in ), so that the voltage (V _sb ) between the source and the body terminal changes under the condition of input signal change remain essentially constant.

According to the source follower of an embodiment of the present utility model, wherein, the PMOS transistor comprises:

P-type substrate;

Patterning an N well formed by doping in the P-type substrate;

the bulk terminal (B) drawn from the N-well;

Patterning a source region and a drain region formed by doping in the N well;

a source (S) derived from said drain region;

a drain (D) derived from said drain region; and

Gate (G).

In an embodiment, in the PMOS transistor, there is a first parasitic capacitance (C _sb ) caused by a diode formed between the source region and the N well, and between the drain region and the N well The second parasitic capacitance (C _db ) caused by the formed diode, and the third parasitic capacitance (C _b ) caused by the diode formed between the N well and the P-type substrate.

Wherein, the first parasitic capacitance (C _sb ) is equivalently placed between the source and the body terminal, and the second parasitic capacitance (C _db ) and the third parasitic capacitance (C _b ) are equivalent The ground is placed in parallel between the body terminal and the drain.

Specifically, the P-type substrate is grounded. the

The technical effect of the utility model is that, by connecting the bulk terminal of the N well with the input terminal _Vin , on the one hand, the gain of the source follower can not be affected by the receptor effect, the linearity is improved and the linear distortion is greatly reduced; On the other hand, the parasitic capacitance at the output terminal of the source follower can be subtly reduced significantly, and when the frequency of the input signal at the input terminal V _in changes, the dynamic distortion is small, which solves the dynamic distortion in the case of high-frequency input signal The problem. Therefore, the source follower of the present invention is very suitable for high-speed and heavy-load applications.

Description of drawings

The above and other objects and advantages of the present invention will be more fully understood from the following detailed description in conjunction with the accompanying drawings, wherein the same or similar elements are denoted by the same reference numerals. the

Fig. 1 is a schematic cross-sectional structure diagram of a traditional PMOS transistor;

FIG. 2 is a schematic diagram of an equivalent circuit of the conventional PMOS transistor of FIG. 1 .

FIG. 3 is a schematic circuit diagram of a source follower formed based on the PMOS transistor shown in FIG. 1 . the

FIG. 4 is a schematic circuit diagram of a source follower formed based on the PMOS transistor shown in FIG. 1 according to an embodiment of the present invention. the

Detailed ways

The following introduces some of the possible embodiments of the present invention, aiming to provide a basic understanding of the present invention, not intended to confirm the key or decisive elements of the present invention or to limit the scope of protection. It is easy to understand that, according to the technical solution of the utility model, those skilled in the art can propose other alternative implementation modes without changing the spirit of the utility model. Therefore, the following specific embodiments and drawings are only exemplary descriptions of the technical solution of the utility model, and should not be regarded as the whole of the utility model or as a limitation or restriction on the technical solution of the utility model. the

It will be understood that when a component is referred to as being "connected" to another component, it can be directly connected to the other component or intervening components may be present. Conversely, when a part is said to be "directly connected" to another part, it means that no intermediate parts exist. the

FIG. 3 is a schematic circuit diagram of a source follower formed based on the PMOS transistor shown in FIG. 1 . Among them, the PMOS transistor is used as an input device, the gate of the PMOS transistor is used as the input terminal V _in , and the input signal (such as the signal output by the previous stage) is input from the gate; the source of the PMOS transistor is used as the output terminal V _out ; the drain The pole biases the ground signal GND, and the body terminal also biases the ground signal GND; and, a current source is placed between the power signal V _DD and the source, which provides a constant bias current for the input device, and the high-level power signal V _DD is biased at the source after passing through the current source. Therefore, C _db is placed between drain D and body terminal B, C _sb is placed between source S and body terminal B, and C _b is also placed between drain D and body terminal B in parallel with C _db . The output signal of the source follower shown in Figure 3 can follow the input signal, and its gain Gain can be calculated by the following formula (1):

是输入器件（在此为PMOS晶体管）的输出阻抗，其与电流源并联；

是PMOS晶体管的跨导。通常

>>1/

, 因此，源极跟随器的增益Gain接近等于1，也即，输出信号基本跟随输入信号。 in,

is the output impedance of the input device (here a PMOS transistor), which is in parallel with the current source;

is the transconductance of the PMOS transistor. usually

>>1/

, Therefore, the gain Gain of the source follower is close to 1, that is, the output signal basically follows the input signal.

In the source follower of the traditional PMOS transistor, the gain is calculated similarly by formula (1). However, due to the fact that the input device's It changes with the voltage level of the output signal, so that its gain Gain also changes with the voltage of the output signal. This characteristic is usually called voltage dependence. This voltage dependence will cause the source follower to exhibit nonlinear characteristics when the input signal swings at a high amplitude, that is, greater distortion is prone to occur when the input signal swings relatively large.

是随源极和衬底（B）之间的偏压而变化，也即随V_sb变化；当衬底接地时，源极为输出信号的电压（约等于输入信号的电压），V_sb随输入信号的电压而变化，

和源极跟随器的增益Gain均随输入信号的电压变化而变化。  The root cause of the linear distortion of the source follower lies in an important factor that causes voltage dependence: the body effect. This is because,

It varies with the bias voltage between the source and the substrate (B), that is, it varies with V _sb ; when the substrate is grounded, the source is the voltage of the output signal (approximately equal to the voltage of the input signal), and V _sb varies with the input signal voltage changes,

Both Gain and the gain of the source follower vary with the voltage of the input signal.

和源极跟随器的增益Gain并不会随源极跟随器的输入端（即栅极）的输入信号的电压变化而变化，也消除了电压依赖性。与传统的源极跟随器相比较，图3所示的源极跟随器大大减小了线性失真，可以提供相对更好的线性度。  Therefore, in the source follower of the embodiment shown in FIG. 3 , generally, the body terminal B of the PMOS transistor is directly connected to its own source S. In this way, the voltage V _sb between the source S and the body terminal B is basically constant to zero, so that the body effect is basically eliminated,

And the gain Gain of the source follower will not change with the voltage change of the input signal at the input terminal (ie gate) of the source follower, and the voltage dependence is also eliminated. Compared with the traditional source follower, the source follower shown in Figure 3 greatly reduces linear distortion and can provide relatively better linearity.

However, the applicant found that in the source follower of the embodiment in Figure 3, the output terminal V _out is connected to parasitic capacitances C _db and C _b (C _sb is short-circuited due to the substrate connection and does not appear at the output terminal); when the input signal When the frequency rises, part of the output current through the PMOS transistor will be shunted to the capacitive channel (load capacitance or parasitic capacitance) at the output. Since this dynamic current changes with the frequency of the input signal, this will cause the output impedance of the input device to It also has voltage dependence. Therefore, the gain Gain of the source follower changes with the signal voltage, causing dynamic distortion. The higher the frequency of the input signal, the larger the capacitance at the output (including parasitic capacitance), and the more severe the dynamic distortion.

FIG. 4 is a schematic circuit diagram of a source follower formed based on the PMOS transistor shown in FIG. 1 according to an embodiment of the present invention. In the embodiment shown in Figure 4, the PMOS transistor of the embodiment shown in Figure 1 is also selected as the input device, the power supply voltage V _DD is connected to the current source, and the current source is placed at the source and the high-level power supply voltage (V _DD ) between, which is used to provide a substantially constant bias current for the input device; the input signal of the source follower is input from the gate G of the PMOS transistor, and the gate G is defined as the input terminal V _in of the source follower; the signal Output from the source S of the PMOS transistor, which is defined as the output terminal V _out of the source follower; further, the drain D of the PMOS transistor and the P-type substrate 111 are biased to the ground signal GND. In particular, the body terminal B is connected to the input terminal _Vin or the gate G, for example, in FIG. 1 , the body terminal 132 can be directly connected to the gate 134 . In this way, the source follower of the embodiment shown in FIG. 4 will have the following two advantages at the same time.

是输入器件的输出阻抗，

是PMOS晶体管的跨导）也不受体效应影响，相比传统的源极跟随器，其线性度好，线性失真大大减小。  First, the body effect of source followers based on PMOS transistors is considered. It can be noted that in the embodiment shown in Fig. 4, since the body terminal B is connected to the input terminal _Vin or the gate G, the body terminal B is equal to the potential of the gate G, so V _sb =V _sg ; and since the source follows In the device, the source S voltage follows the input gate G, so V _sg will be a constant, it will not change with the voltage of the input signal, that is, there is no voltage dependence problem. The gain Gain of the source follower shown in Figure 4 ( Gain can be calculated similarly by formula (1), where,

is the output impedance of the input device,

It is the transconductance of the PMOS transistor) and is not affected by the receptor effect. Compared with the traditional source follower, its linearity is better and the linear distortion is greatly reduced.

Second, the influence of the parasitic capacitance at the output on the dynamic distortion is considered. It can be noted that in the embodiment shown in Fig. 4, by connecting the body terminal B to the input terminal _Vin or the gate G, C _db and C _b are placed in parallel between the body terminal B and the drain D, C _db and C _b no longer appear at the output terminal V _out ; and for C _sb , although it is placed between the source terminal S and the body terminal B, as mentioned earlier, V _sb =V _sg , the V _sb voltage value is also Is a constant (does not change with the voltage of the input signal), so the parasitic capacitance C _sb will not increase the total capacitance of the output terminal V _out . In this way, compared with the structure of the source follower shown in Figure 4, the parasitic capacitance at the output terminal is significantly reduced, and when the frequency of the input signal at the input terminal _Vin changes, the source follower shown in Figure 4 above will not appear Voltage dependence issues and dynamic distortion issues.

In summary, the source follower of the embodiment shown in Figure 4 not only reduces the linear distortion caused by the body effect, but also eliminates the problem of dynamic distortion caused by the parasitic capacitance of the PMOS transistor, so it can maintain good low-frequency linearity In the case of characteristics, the dynamic distortion is reduced, and it has good high-frequency characteristics, which is very suitable for high-speed and large-load applications. the

It should be understood that the input signal at the input terminal V _in may be the output of the previous stage circuit, for example, the previous stage circuit may be an operational amplifier. The specific type of input signal is not limiting.

It should also be understood that the source follower of the embodiment shown in FIG. 4 is not only suitable to be manufactured and formed in an integrated circuit, but also suitable to be formed by PMOS transistor discrete devices through wire connection. the

The above examples mainly illustrate the source follower based on PMOS transistors of the present invention. Although only some embodiments of the utility model have been described, those skilled in the art should understand that the utility model can be implemented in many other forms without departing from the gist and scope thereof. The examples and implementations shown are therefore to be regarded as illustrative and not restrictive, and the present invention may cover various modification and replacement. the

Claims (4)

1. A source follower based on a PMOS transistor, comprising a current source and a PMOS transistor used as an input device, wherein the gate of the PMOS transistor is defined as the input terminal of the source follower, and the PMOS The source of the transistor is defined as the output terminal of the source follower, the drain of the PMOS transistor is connected to the ground signal, the high-level signal is connected from one end of the current source, and the other end of the current source output current to the source; It is characterized in that the bulk terminal of the PMOS transistor is connected to the input terminal, so that the voltage between the source and the bulk terminal remains substantially constant under the condition of input signal changes. 2. The source follower according to claim 1, wherein the PMOS transistor comprises: P-type substrate; Patterning an N well formed by doping in the P-type substrate; a body terminal drawn from the N-well; Patterning a source region and a drain region formed by doping in the N well; a source drawn from the drain region; a drain drawn from the drain region; and grid. 3. The source follower according to claim 2, wherein, in the PMOS transistor, there is a first parasitic capacitance caused by a diode formed between the source region and the N well, the drain The second parasitic capacitance caused by the diode formed between the region and the N well, and the third parasitic capacitance caused by the diode formed between the N well and the P-type substrate. 4. The source follower according to claim 3, wherein the first parasitic capacitance is equivalently placed between the source and the body terminal, and the second parasitic capacitance (C _db ), and the third parasitic capacitance is equivalently connected in parallel between the body terminal and the drain.

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