JP2012244276A - Source follower circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a source follower circuit that implements an improved noise characteristic.SOLUTION: The source follower circuit includes: a source follower circuit section comprising field effect transistors (M1, M2); and a current mirror circuit section comprising field effect transistors (M3, M4, M5). Capacitances (C1, C2) are connected to gates of the field effect transistors (M4, M5) in a cross coupling manner to make the current mirror circuit section function as an amplifier.

Description

  The present invention relates to a source follower circuit with improved noise characteristics from a current source.

  Conventionally, a source follower circuit is used to connect two circuits having different impedances in an integrated circuit (see, for example, Patent Document 1). FIG. 4 is a diagram illustrating a configuration example of a conventional source follower circuit. This source follower circuit includes a source follower circuit unit 20 configured by differentially input field effect transistors M20 and M21, and a current mirror circuit unit 21 configured by field effect transistors M22, M23, and M24. I have. The field effect transistor M <b> 20 has a gate connected to the input terminal Vin <b> 1 and a drain connected to the voltage source 22. The field effect transistor M <b> 21 has a gate connected to the input terminal Vin <b> 2 and a drain connected to the voltage source 22. The gate and drain of the field effect transistor M22 are connected to the constant current source 23, and the source is connected to the ground. The gate of the field effect transistor M23 is connected to the gate and drain of the field effect transistor M22, the drain is connected to the source and output terminal Vout1 of the field effect transistor M20, and the source is connected to the ground. The gate of the field effect transistor M24 is connected to the gate and drain of the field effect transistor M22, the drain is connected to the source and output terminal Vout2 of the field effect transistor M21, and the source is connected to the ground.

  In the source follower circuit configured as described above, the current mirror circuit unit 21 is driven by supplying a constant drive current to the field effect transistors M20 and M21 of the source follower circuit unit 20, and the input signal is input to the input terminals Vin1 and Vin2. Is differentially input, voltages having the same phase as the input signal are differentially output from the output terminals Vout1 and Vout2 connected to the sources of the field effect transistors M20 and M21.

JP-A-4-267611

  However, in the conventional source follower circuit, there is a problem that noise is superimposed on the output terminals Vout1 and Vout2 from the current mirror circuit unit 21 used as a current source, and the influence of the noise appears greatly in the output signal.

  The present invention has been made in view of such a point, and an object thereof is to provide a source follower circuit with improved noise characteristics.

  In the source follower circuit of the present invention, a differential input terminal composed of first and second input terminals to which a differential signal is supplied, a gate is connected to the first input terminal, and a drain is connected to a voltage source. A first field effect transistor having a gate connected to the second input terminal, a drain connected to a voltage source, a drain connected to a constant current source, and a gate- A third field effect transistor having a drain connected and a source grounded, a gate connected to a drain and a gate of the third field effect transistor, and a drain connected to the source of the first field effect transistor A fourth field effect transistor having a source grounded, a gate connected to the drain and gate of the third field effect transistor, and the drain A fifth field effect transistor connected to the source of the second field effect transistor, the source of which is grounded, a first output terminal connected to the source of the first field effect transistor, and the A differential output terminal composed of a second output terminal connected to the source of the second field effect transistor, and between the gate of the first field effect transistor and the gate of the fifth field effect transistor. A first capacitor connected to the second field effect transistor; a second capacitor connected between the gate of the second field effect transistor and the gate of the fourth field effect transistor; and the third field effect. A first resistor provided between the drain and gate of the type transistor and the gate of the fourth field effect transistor, and the drain and gate of the third field effect transistor. A second resistor provided between the bets and the gate of the fifth field-effect transistor, that provided with the features.

  According to this configuration, the first input terminal is connected to the gate of the fifth field effect transistor constituting the current mirror circuit via the first capacitor, and the second input terminal is connected to the current mirror circuit. Since the gate of the fourth field-effect transistor to be configured is connected via the second capacitor, the fourth and fifth field-effect transistors function as a source-grounded amplifier circuit, whereby the source follower circuit The S / N ratio of the output signal can be improved, and the noise characteristics can be improved.

  In the source follower circuit, the resistance values of the first and second resistors may be variable. A high-pass filter composed of a first resistor and a second capacitor is connected in parallel to the second input terminal, and a high-pass filter composed of the second resistor and the first capacitor is parallel to the first input terminal Since the cut-off frequency can be changed by changing the resistance values of the first and second resistors, the noise characteristics can be further improved. Become.

  In the source follower circuit, each of the first and second resistors includes a plurality of resistance components having different resistance values, and each of the resistance components of the first resistor is selectively used in the fourth field effect transistor. A first switch connected to the gate; and a second switch that selectively connects each resistance component of the second resistor to the gate of the fifth field effect transistor. May be provided.

  According to the present invention, a source follower circuit with improved noise characteristics can be provided.

It is a block diagram of the source follower circuit which concerns on this Embodiment. It is a graph which shows the result of having simulated the amplifier characteristic of the said source follower circuit and the conventional source follower circuit. It is a graph which shows the result of having simulated the noise figure of the said source follower circuit and the conventional source follower circuit. It is a block diagram of the conventional source follower circuit.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a configuration diagram of a source follower circuit according to an embodiment of the present invention. The source follower circuit includes a source follower circuit unit configured by field-effect transistors M1 and M2 that are differentially input, and a current mirror circuit unit configured by a constant current source 2 and field-effect transistors M3, M4, and M5. And. One field effect transistor M1 constituting the source follower circuit section has a gate connected to the input terminal Vin1, a drain connected to the voltage source 1, and a source connected to the output terminal Vout1. The other field effect transistor M2 has a gate connected to the input terminal Vin2, a drain connected to the voltage source 1, and a source connected to the output terminal Vout2.

  The field effect transistor M3 constituting the current mirror circuit section has a gate and a drain connected to the constant current source 2 and a source connected to the ground. The gate of the field effect transistor M4 is connected to the gate and drain of the field effect transistor M3 via the resistor R1, the drain is connected to the source and output terminal Vout1 of the field effect transistor M1, and the source is connected to the ground. ing. The gate of the field effect transistor M5 is connected to the gate and drain of the field effect transistor M3 via the resistor R2, the drain is connected to the source and output terminal Vout2 of the field effect transistor M2, and the source is connected to the ground. ing.

  The resistor portion R1 includes resistance components R11 and R12 having different resistance values, and is selectively connected to the gate of the field effect transistor M4 via the switch SW1. The resistance portion R2 includes resistance components R21 and R22 having different resistance values, and is selectively connected to the gate of the field effect transistor M5 via the switch SW2. Note that the resistance component R11 in the resistance portion R1 and the resistance component R21 in the resistance portion R2, and the resistance component R12 in the resistance portion R1 and the resistance component R22 in the resistance portion R2 are configured with the same resistance value. The resistor units R1 and R2 may be configured by variable resistors, and the switches SW1 and SW2 may be configured by field effect transistors to be switchable.

  The gate of the field effect transistor M1 of the source follower circuit is connected to the gate of the field effect transistor M5 of the current mirror circuit section through the capacitor C2. The gate of the field effect transistor M2 of the source follower circuit is connected to the gate of the field effect transistor M4 of the current mirror circuit section through the capacitor C1.

  That is, the gates of the field effect transistors M1 and M2 of the source follower circuit and the gates of the field effect transistors M4 and M5 of the current mirror circuit section are cross-connected through the capacitors, respectively. As a result, the field effect transistor M4 of the current mirror circuit section forms a source grounded amplifier circuit whose gate is connected to the input terminal Vin2 and whose source is grounded, and amplifies the input signal applied to the input terminal Vin2. The polarity is inverted and output to the output terminal Vout1. Similarly, the field-effect transistor M5 of the current mirror circuit unit constitutes a source grounded amplifier circuit whose gate is connected to the input terminal Vin1 and whose source is grounded, and amplifies the input signal applied to the input terminal Vin1. The polarity is inverted and output to the output terminal Vout2.

  Further, a high-pass filter including a capacitor C2 and a resistor R2 is connected to the input terminal Vin1, and the cutoff frequency of the high-pass filter changes by switching the resistance component of the resistor R2. Similarly, a high-pass filter composed of a capacitor C1 and a resistor R1 is connected to the input terminal Vin2, and the cutoff frequency of the high-pass filter changes by switching the resistance component of the resistor R1.

Next, the operation of the source follower circuit configured as described above will be described.
A high-frequency input signal is differentially input to the input terminals Vin1 and Vin2 of the source follower circuit. A drain current corresponding to the gate voltage of the high-frequency input signal applied to the gate flows through the field effect transistors M1 and M2. At this time, the same current as that of the field effect transistor M3 flows through the field effect transistor M4 constituting one current mirror circuit portion and the field effect transistor M5 constituting the other current mirror circuit portion. A high-frequency input signal input from the input terminal Vin1 to the gate of the field effect transistor M1 is input to the gate of the field effect transistor M5 via the capacitor C2. The high frequency input signal input from the input terminal Vin2 to the gate of the field effect transistor M2 is input to the gate of the field effect transistor M4 via the capacitor C1.

  An output signal having the same phase as the input signal to the gate of the field effect transistor M1 is output from the field effect transistor M1 constituting the source follower circuit unit to the output terminal Vout1. At this time, the field effect transistor M4 outputs a high frequency input signal (input signal to the gate of the field effect transistor M2) applied to the gate from the input terminal Vin2 to the output terminal Vout1.

  Here, since the high-frequency input signal is differentially input to the field effect transistors M1 and M2, the output signal output from the output terminal Vout1 has the same phase. For example, when a positive input signal is input from the input terminal Vin1 to the gate of the field effect transistor M1, an input signal having a phase opposite to that of the input terminal Vin1 (minus) is input from the input terminal Vin2 to the gate of the field effect transistor M2. Is done. At this time, an output signal in phase with the input signal to the gate is output from the field effect transistor M1, and thus a positive output signal is output to the output terminal Vout1. On the other hand, since an output signal having a phase opposite to that of the input signal to the gate is output from the field effect transistor M4, a positive amplified output signal is output to the output terminal Vout1. Therefore, since an output signal having the same phase is output to the output terminal Vout1, an output signal having a gain of 1 or more can be obtained without canceling the phases.

  An output signal in phase with the input signal to the gate of the field effect transistor M2 is output from the field effect transistor M2 constituting the source follower circuit unit to the output terminal Vout2. At this time, the field effect transistor M5 outputs a high frequency input signal (input signal to the gate of the field effect transistor M1) applied to the gate from the input terminal Vin1 to the output terminal Vout2.

  Also at this time, as described above, for example, a positive input signal is input from the input terminal Vin1 to the gate of the field effect transistor M1, and the input terminal Vin2 is input to the gate of the field effect transistor M2 in the opposite phase (minus) to the input terminal Vin1 Assuming that a signal is input, the field effect transistor M2 outputs an output signal in phase with the input signal to the gate, and therefore a negative output signal is output to the output terminal Vout2. On the other hand, since an output signal having a phase opposite to that of the input signal to the gate is output from the field effect transistor M5, a negative amplified output signal is output to the output terminal Vout2. Therefore, since an output signal having the same phase is output to the output terminal Vout2, an output signal having a gain of 1 or more can be obtained without canceling the phases.

  In this source follower circuit, capacitors C1 and C2 are added to one current mirror circuit section and the other current mirror circuit section, respectively, and field effect transistors M4 and M5 of the current mirror circuit section are made into amplifiers. Since the amplified field effect transistors M4 and M5 amplify the high frequency input signal supplied to the input terminals Vin1 and Vin2 and apply the amplified signal to the output terminals Vout1 and Vout2, an output signal having a gain of 1 or more can be obtained. it can. Thereby, the S / N ratio of the output signal S output from the output terminals Vout1 and Vout2 is increased, and the noise characteristics can be improved.

  In addition, in the region where the frequency of the high frequency input signal is higher than the predetermined value, the resistance units R1 and R2 are switched to a small resistance value, and in the region where the frequency of the high frequency input signal is lower than the predetermined value, the resistance units R1 and R2 are set to a large resistance value. Switching. In one current mirror circuit section, an RC high-pass filter is configured by the resistor section R1 and the capacitor C1. The cut-off frequency of the RC high-pass filter is changed by switching the resistance components R11 and R12 of the resistance unit R1 constituting the high-pass filter to the resistance components R11 and R12 having different resistance values by the switch SW1.

  Also in the other current mirror circuit portion, an RC high pass filter is configured by the resistor portion R2 and the capacitor C2. The cut-off frequency of the RC high-pass filter is changed by switching the resistance components R21 and R22 of the resistance unit R2 constituting the high-pass filter to the resistance components R21 and R22 having different resistance values by the switch SW2.

  Switching of the resistance components of the resistance part R1 and the resistance part R2 is performed synchronously. In a region where the frequency of the high-frequency input signal supplied to the input terminals Vin1 and Vin2 is higher than a predetermined value, the cutoff frequency is increased by switching the resistance parts R1 and R2 to small resistance components R11 and R21. In the region where the frequency of the high frequency input signal is lower than a predetermined value, the cutoff frequency is lowered by switching the resistance parts R1, R2 to the large resistance components R12, R22. As the resistance values of the resistance portions R1 and R2 increase, the value of the generated thermal noise increases. However, by reducing the resistance values of the resistance portions R1 and R2 in a high frequency range, it is possible to suppress thermal noise.

  Next, the result of simulating the characteristics of the source follower circuit (FIG. 1) according to the present embodiment and the source follower circuit of the comparative example (FIG. 4) will be described. In the source follower circuit (FIG. 4) of the comparative example, the field effect transistors M23 and M24 constituting the current mirror circuit section do not have an amplifier circuit configuration for the input signal supplied to the input terminals Vin1 and Vin2.

  FIG. 2 is a graph showing a result of simulating gain characteristics of the source follower circuit according to the present embodiment and the source follower circuit of the comparative example. In this graph, the vertical axis represents voltage gain [dB], and the horizontal axis represents input signal frequency [Hz]. In the gain characteristics of the source follower circuit according to the present embodiment, the plot points are represented by squares (□), and the gain characteristics of the source follower circuit of the comparative example are represented by circles (◯). Further, a two-dot chain line between the input signal frequencies of 100 M to 200 M [Hz] indicates a resistance value switching point that is a point where the resistance value is switched in the source follower circuit according to the present embodiment. The region on the left side of the resistance value switching point is a region where the frequency of the high-frequency input signal supplied to the input terminals Vin1 and Vin2 is lower than a predetermined value, and the larger resistance component among the resistance components of the resistor units R1 and R2. R12 and R22 are input. The region on the right side of the resistance value switching point is a region in which the frequency of the high-frequency input signal supplied to the input terminals Vin1 and Vin2 is higher than a predetermined value, and the smaller resistance component among the resistance components of the resistor units R1 and R2. R11 and R21 are input. As a result of simulation, it was confirmed that there was no change in gain characteristics between the source follower circuit according to the present embodiment and the source follower circuit of the comparative example. Therefore, the source follower circuit according to the present embodiment achieves gain characteristics equivalent to those of the conventional one.

  FIG. 3 is a graph showing the result of simulating the noise figure of the source follower circuit according to the present embodiment and the source follower circuit of the comparative example. In this graph, the vertical axis represents noise intensity [dB], and the horizontal axis represents input signal frequency [Hz]. In the noise characteristics of the source follower circuit according to the present embodiment, the plotted points are represented by squares (□), and the noise characteristics of the source follower circuit of the comparative example are represented by circles (◯). Further, a two-dot chain line between the input signal frequencies of 100 M to 200 M [Hz] indicates a resistance value switching point that is a point where the resistance value is switched in the source follower circuit according to the present embodiment. As a result of simulation, it can be confirmed that the noise intensity included in the output signal output from the output terminal of the source follower circuit according to the present embodiment is sufficiently weaker than that of the source follower circuit of the comparative example. Was found to be improved.

  As described above, in the source follower circuit according to the present embodiment, the field effect transistors M4 and M5 constituting the current mirror circuit section are made into amplifiers and the gates of the field effect transistors M4 and M5 are cross-coupled. Since the capacitors C1 and C2 are connected to each other, the noise characteristics can be improved. Further, the gates of the field effect transistors M4 and M5 constituting the current mirror circuit section are connected to the drain and gate of the field effect transistor M3 through the resistors R1 and R2, and high-pass is performed by the capacitors C1 and C2 and the resistors R1 and R2. Since the filter is configured so that the resistance values of the resistors R1 and R2 can be switched in accordance with the input frequency, at least a noise improvement effect in a high frequency range where the resistance value is reduced can be expected.

  In addition, this invention is not limited to the said embodiment, It can implement variously. In the above-described embodiment, the size, shape, and the like illustrated in the accompanying drawings are not limited thereto, and can be appropriately changed within a range in which the effect of the present invention is exhibited. In addition, various modifications can be made without departing from the scope of the object of the present invention.

DESCRIPTION OF SYMBOLS 1 Voltage source 2 Constant current source M1-M5 Field effect type transistor C1, C2 Capacitance R1, R2 Resistance part R11, R12, R21, R22 Resistance component SW1, SW2 Switch 20 Source follower circuit part 21 Current mirror circuit part 22 Voltage source 23 Constant current source M20 to M24 Field effect transistor

Claims (3)

A differential input terminal comprising first and second input terminals to which a differential signal is supplied;
A first field effect transistor having a gate connected to the first input terminal and a drain connected to a voltage source;
A second field effect transistor having a gate connected to the second input terminal and a drain connected to a voltage source;
A third field effect transistor having a drain connected to a constant current source, a gate-drain connected, and a source grounded;
A fourth field effect transistor having a gate connected to the drain and gate of the third field effect transistor, a drain connected to the source of the first field effect transistor, and a source grounded;
A fifth field effect transistor having a gate connected to the drain and gate of the third field effect transistor, a drain connected to the source of the second field effect transistor, and a source grounded;
A differential output terminal comprising a first output terminal connected to a source of the first field effect transistor and a second output terminal connected to a source of the second field effect transistor;
A first capacitor connected between a gate of the first field effect transistor and a gate of the fifth field effect transistor;
A second capacitor connected between the gate of the second field effect transistor and the gate of the fourth field effect transistor;
A first resistor provided between a drain and a gate of the third field effect transistor and a gate of the fourth field effect transistor;
A second resistor provided between the drain and gate of the third field effect transistor and the gate of the fifth field effect transistor;
A source follower circuit comprising:   The source follower circuit according to claim 1, wherein resistance values of the first and second resistors are variable. The first and second resistors are each composed of a plurality of resistance components having different resistance values,
A first switch that selectively connects each resistance component of the first resistor to a gate of the fourth field effect transistor; and a fifth switch that selectively selects each resistance component of the second resistor. A second switch connected to the gate of the effect transistor,
3. The source follower circuit according to claim 2, wherein the source follower circuit is provided in one integrated circuit.

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