CN103199848A - Source follower based on PMOS transistor - Google Patents

Abstract

The invention providesA source follower based on a PMOS transistor belongs to the technical field of source followers. 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 invention belongs to the technical field of source followers, and relates to source followers based on PMOS transistors.

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.

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

As shown in Figure 1(a), a 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, and a bulk terminal (Bulk, hereinafter referred to as B) 132 , a source (Source, hereinafter referred to as S) 133 , a gate (Gate, hereinafter referred to as G) 134 , a drain (Drain, hereinafter referred to as D) 135 and a 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 as _{shown in} FIG _. The junction capacitance of the diode formed between (as shown in Figure 1 (a)), C _db is the junction capacitance of the diode (as shown in Figure 1 (a)) formed between the drain region 122 and the N well 113, C _b is the junction capacitance of a diode (as shown in FIG. 1( a )) formed between the N well 113 and the P-type substrate 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.

Contents of the invention

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

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

To achieve the above or other objectives, the present invention 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 (V _in ) of the follower, the source of the PMOS transistor is defined as the output terminal (V _out ) of the source follower, the drain of the PMOS transistor is connected to the ground signal (GND), and the high voltage A flat 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.

In the source follower according to an embodiment of the present invention, the PMOS transistor includes:

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 bulk terminal and the drain terminal.

Specifically, the P-type substrate is grounded.

The technical effect of the present invention 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, On the one hand, the parasitic capacitance at the output end of the source follower can be subtly reduced, and when the frequency of the input signal at the input end V _in changes, the dynamic distortion is small, which solves the problem of dynamic distortion in the case of high-frequency input signals question. 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 become more complete and clear from the following detailed description in conjunction with the accompanying drawings, wherein the same or similar elements are denoted by the same reference numerals.

Fig. 1 is a schematic diagram of a traditional PMOS transistor, wherein (a) is a schematic diagram of its cross-sectional structure, and (b) is a schematic diagram of its equivalent circuit.

FIG. 2 is a schematic circuit diagram of a source follower formed based on the PMOS transistor shown in FIG. 1 .

FIG. 3 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.

Detailed ways

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

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.

FIG. 2 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 the 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 2 can follow the input signal, and its gain Gain can be calculated by the following formula (1):

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.

是随输出信号的电压电平的变化而有所变化的，从而导致其增益Gain亦随输出信号的电压的变化而变化，这种特性通常称为电压依赖性。这种电压依赖性会导致输入信号在高幅度摆动时，源极跟随器呈现非线性特性，也即，在输入信号摆幅较大时容易发生较大失真。 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2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. 2 , 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 does not change with the voltage change of the input signal at the input terminal of the source follower (that is, the gate), and also eliminates the voltage dependence. Compared with the traditional source follower, the source follower shown in Figure 2 greatly reduces linear distortion and can provide relatively better linearity.

也具有电压依赖性，因此，源极跟随器的增益Gain随信号电压变化, 引起动态失真。输入信号的频率越高，输出端的电容（包括寄生电容）越大，动态失真越严重。 However, the applicant found that in the source follower of the embodiment in Fig. 2, the output terminal V _out is connected with 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. 3 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 3, 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. 3 will have the following two advantages at the same time.

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. 3, 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 3 ( 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. 3, 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 terminal 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 3 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.

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. 3 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 above examples mainly illustrate the source follower based on PMOS transistors of the present invention. Although only some of the embodiments of the present invention have been described, those skilled in the art should appreciate that the present invention can be implemented in many other forms without departing from the spirit and scope thereof. The examples and embodiments shown are therefore to be regarded as illustrative and not restrictive, and the invention may cover various modifications without departing from the spirit and scope of the invention as defined in the appended claims with replace.

Claims (5)

1. A PMOS transistor based source follower comprising a current source and a PMOS transistor acting as an input device, wherein the gate of the PMOS transistor is defined as the input (V) of the source follower_in) The source of the PMOS transistor is defined as the output (V) of the source follower_out) The drain electrode of the PMOS transistor is connected with a grounding signal (GND) and a high-level signal (V)_DD) The current source is connected from one end of the current source, and the other end of the current source outputs current to the source electrode;

it is characterized in thatIn that the body terminal (B) of the PMOS transistor and the input terminal (V)_in) Is connected so as to make the voltage (V) between the source and the bulk terminal_sb) Remains substantially constant in the event of input signal variations.

2. The source follower of claim 1, wherein the PMOS transistor comprises:

a P-type substrate;

patterning and doping an N well in the P-type substrate;

a body terminal (B) leading out from the N-well;

patterning and doping a source region and a drain region in the N well;

a source (S) led out from the drain region;

a drain electrode (D) led out from the drain region; and

a gate (G).

3. The source follower of claim 2, wherein in the PMOS transistor there is a first parasitic capacitance (C) caused by a diode formed between the source region and the N-well_sb) A second parasitic capacitance (C) caused by a diode formed between the drain region and the N well_db) A third parasitic capacitance (C) caused by a diode formed between the N-well and the P-type substrate_b）。

4. A source follower as claimed in claim 3, characterized in that the first parasitic capacitance (C)_sb) Equivalently between the source and the bulk terminal, the second parasitic capacitance (C)_db) A third parasitic capacitance (C)_b) Equivalently, between the body terminal and the drain terminal.

5. The source follower of claim 2, wherein the P-type substrate is grounded.

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