JP2010011220A - Common mode feedback circuit, and differential transmission transmitter, differential transmission receiver and differential amplification circuit with common mode feedback circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a common mode feedback circuit which is capable of securing a wide operating voltage range for a common-mode voltage and further operating with a power supply voltage lower than a conventional common mode feedback circuit. <P>SOLUTION: An attenuator 3 is further provided, and the attenuator 3 attenuates a common-mode voltage vcm and outputs it as a voltage v1. A common mode feedback circuit part 2 compares the voltage v1 with a reference voltage vref and outputs a control voltage v-cmfb corresponding to a comparison result. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a differential transmission circuit, and more particularly, to a common mode feedback circuit and a differential transmission transmitter, a differential transmission receiver, and a differential amplifier circuit including the common mode feedback circuit.

  In recent years, an LVDS (Low Voltage Differential Signaling) interface has been used as a high-speed serial interface such as an LCD controller for reducing power consumption. In the LVDS interface, serial data is transmitted as a differential signal by a transmitter circuit, and data is transferred by receiving the differential signal by a receiver circuit.

  With the miniaturization of transistors in LSI, the power supply voltage is lowered and the circuit speed is increased. However, a circuit at an interface part between chips or between boards requires a withstand voltage of ESD (Electro Static Discharge), so that a fine transistor may not be used in the I / O part. Similarly, in the LVDS interface, when the ESD withstand voltage is necessary, a fine transistor may not be used.

  In general, a transistor having a high ESD withstand voltage has a large threshold voltage and is not suitable for use at a low voltage. However, since the power supply voltage of the chip is lowered in accordance with a fine transistor, an interface part circuit such as LVDS is required to operate a transistor having a high ESD withstand voltage at a low voltage.

  FIG. 9 is a block diagram of a conventional common mode feedback circuit 101. The common mode voltage vcm is detected by dividing the voltage with the differential signal v_p-v_n and the two resistors R0. The common mode voltage vcm is input to the non-inverting input terminal (+) of the common mode feedback circuit unit CMFB, and the predetermined reference voltage vref output from the power supply 102 is input to the inverting input terminal (−) of the common mode feedback circuit unit CMFB. To do. The input of the power supply 102 is electrically grounded. Thereby, the control voltage v_cmfb for adjusting the common mode voltage vcm to the reference voltage vref can be output.

  The common mode voltage vcm = vref is controlled by using the common mode feedback circuit 101 for the low voltage differential transmission transmitter 103 shown in FIG.

  FIG. 10 shows a low voltage differential transmission transmitter (LVDS transmitter) 103 using a conventional common mode feedback circuit. The input data Data is output as a signal v_p and a signal v_n by the transmitter. A difference between the signal v_p and the signal v_n is a differential signal v_p−v_n. Here, the common mode voltage vcm of the differential signal v_p−v_n is controlled by the common mode feedback circuit 101 so that vcm = vref. vref is a predetermined reference voltage as described above.

FIG. 11 is an example of a circuit diagram of the common mode feedback circuit unit CMFB used in the low voltage differential transmission transmitter 103 of FIG. In order to widen the operating voltage range of the common mode voltage vcm, it is desirable for the transmission circuit that the common mode voltage vcm is equal to the midpoint voltage vdd / 2 during normal times. In particular, when the common mode voltage vcm is directly input to the common mode feedback circuit unit CMFB, the common mode feedback circuit unit CMFB needs to operate with an input voltage of vdd / 2. In the common mode feedback circuit unit CMFB shown in FIG. 11, it is necessary to satisfy the equation (1) in order to widen the operating voltage range of the common mode voltage vcm. Here, VGS is the gate-source voltage of the MOS transistor 104, VDS is the drain-source voltage of the MOS transistor 105, and vcm≈vref.
vdd-vcm> VGS + VDS (1)
As a technique for performing differential transmission in the same manner as the low-voltage differential transmission transmitter 103 in FIG. 10, Patent Document 1 discloses a differential that suppresses fluctuations in the common mode voltage during signal transmission and reduces the capacitance value of the phase compensation capacitor. A signal transmission circuit and a differential signal transmission device are disclosed.
JP 2006-340266 A (released on December 14, 2006)

  The gate-source voltage VGS and the drain-source voltage VDS described above vary depending on the threshold voltage Vth of the MOS transistor used, but the normal gate-source voltage VGS is about 0.6 V to 0.9 V, and the drain-source voltage The voltage VDS needs to be about 0.15V to 0.2V.

  Here, ESD withstand voltage is required in the circuit of the interface portion between chips or between boards performing LVDS. For this reason, it may be necessary to use a MOS transistor having a large threshold voltage Vth and a gate-source voltage VGS of 0.8V to 0.9V.

For example, in the equation (1), when the gate-source voltage VGS = 0.9 V and the drain-source voltage VDS = 0.2 V, if the common mode voltage vcm is the midpoint voltage vdd / 2, then the equation (2) Is established.
vdd-vdd / 2> 1.1 (2)
In order to satisfy (2), the power supply voltage vdd needs to be 2.2 V or more. Therefore, in order to operate the common mode feedback circuit unit CMFB in a state where the operating voltage range of the common mode voltage vcm is wide, the power supply voltage vdd needs to be 2.2 V or more.

  The present invention has been made in view of the above-described conventional problems, and the object thereof is to widen the operating voltage range of the common mode voltage, which is lower than that of the conventional common mode feedback circuit. The object is to provide a common mode feedback circuit capable of operating at a power supply voltage.

  In order to solve the above-described problem, the common mode feedback circuit of the present invention includes a first resistor that receives a first input signal at one end and a second resistor that receives a second input signal at one end connected in series. A circuit that divides a differential signal that is a difference between the first input signal and the second input signal and outputs the divided signal as a common mode voltage; and an attenuation unit that attenuates the common mode voltage and outputs the attenuated voltage as an attenuated voltage; And a common mode feedback circuit unit that compares the attenuated voltage with a reference voltage and outputs a control voltage according to the comparison result.

  In the conventional common mode feedback circuit, the operating voltage range of the common mode voltage is widened. Therefore, for a circuit using the common mode feedback circuit, the common mode voltage is usually equal to one half of the power supply voltage. desirable. According to the above invention, the common mode feedback circuit unit receives not the common mode voltage but the attenuated voltage obtained by attenuating the common mode voltage by the attenuating means.

  Therefore, it is possible to widen the operating voltage range of the common mode voltage by lowering the power supply voltage so that the attenuation voltage is equal to one half of the power supply voltage. Operation with a lower power supply voltage is possible compared to the feedback circuit.

  In the common mode feedback circuit, the common mode feedback circuit section outputs a larger control voltage when the attenuation voltage is larger than the reference voltage, and smaller control when the attenuation voltage is smaller than the reference voltage. A voltage may be output.

  As a result, the common mode feedback circuit can compare the attenuated voltage with the reference voltage and output the control voltage so that the attenuated voltage and the reference voltage are equal.

  In any one of the common mode feedback circuits, the attenuation voltage may be larger than zero and smaller than the common mode voltage.

  As a result, the common mode feedback circuit unit receives not the common mode voltage but the attenuation voltage that is larger than zero and smaller than the common mode voltage. Therefore, it is possible to widen the operating voltage range of the common mode voltage by lowering the power supply voltage so that the attenuation voltage is equal to one half of the power supply voltage. Operation with a lower power supply voltage is possible compared to the feedback circuit.

  Since the differential transmission transmitter, the differential transmission receiver, and the differential amplifier circuit of the present invention include any one of the above-described common mode feedback circuits, operation with a lower power supply voltage is possible.

  As described above, the common mode feedback circuit of the present invention further includes an attenuating unit, the attenuating unit attenuates the common mode voltage and outputs the attenuated voltage, and the common mode feedback circuit unit includes the attenuated voltage and the reference voltage. And a control voltage corresponding to the comparison result is output.

  Therefore, it is possible to increase the operating voltage range of the common mode voltage, and to provide a common mode feedback circuit capable of operating at a lower power supply voltage than the conventional common mode feedback circuit. Play.

  One embodiment of the present invention will be described below with reference to Examples 1 to 4 and FIGS.

  FIG. 1 is a block diagram of a common mode feedback circuit 1 according to an embodiment of the present invention. The common mode feedback circuit 1 generally includes a common mode feedback (CMFB) circuit unit 2, an attenuator (ATT) 3, a resistor 4, a resistor 5, and a power source 6. The resistance values of the resistors 4 and 5 are R0.

  A signal v_p is transmitted from the outside of the common mode feedback circuit 1 to the signal line 7. A signal v_n is transmitted from the outside of the common mode feedback circuit 1 to the signal line 8. A difference between the signal v_p and the signal v_n is a differential signal v_p−v_n.

  One end of the resistor 4 is connected to the signal line 7, and the other end of the resistor 4 is connected to one end of the resistor 5 and the input of the attenuator 3. The other end of the resistor 5 is connected to the signal line 8. The output of the attenuator 3 is connected to the non-inverting input terminal (+) of the common mode feedback circuit unit 2. The input of the power source 6 is electrically grounded, and the output of the power source 6 is connected to the inverting input terminal (−) of the common mode feedback circuit unit 2.

  The common mode voltage vcm is obtained by dividing the differential signal v_p−v_n by the resistor 4 and the resistor 5. The attenuator 3 receives the common mode voltage vcm and outputs the voltage v1. The voltage v1 is input to the non-inverting input terminal (+) of the common mode feedback circuit unit 2, and the reference voltage vref is input to the inverting input terminal (−) of the common mode feedback circuit unit 2. The reference voltage vref is output from the power supply 6. The common mode feedback circuit unit 2 outputs a control voltage v_cmfb. The control voltage v_cmfb is a voltage for controlling the common mode voltage vcm to a desired voltage.

The voltage v1 output from the attenuator 3 is obtained when the attenuator 3 attenuates the common mode voltage vcm by a times (0 <a <1). Therefore, equation (3) is established. That is, the voltage v1 is larger than zero and smaller than the common mode voltage vcm.
v1 = a * vcm (3)
Since the common mode feedback circuit unit 2 is controlled so that v1 = vref, the equation (4) is established by substituting the voltage v1 of the equation (3) into this equation.
vref = a * vcm (4)
FIG. 2 is a circuit diagram of the common mode feedback circuit unit 2 according to the embodiment of the present invention. The common mode feedback circuit unit 2 includes an N channel MOSFET M1 to an N channel MOSFET M5, a P channel MOSFET M6 to a P channel MOSFET M8, and a current source I1.

  In the common mode feedback circuit unit 2, the source of the N-channel MOSFET M1, the source of the N-channel MOSFET M2, and the source of the N-channel MOSFET M3 are connected to the power supply line 9, and the power supply voltage vdd is applied. The gate of the N-channel MOSFET M1 is connected to the gate of the N-channel MOSFET M2, the drain of the N-channel MOSFET M1, and the input of the current source I1. The drain of the N channel MOSFET M2 is connected to the source of the N channel MOSFET M4 and the source of the N channel MOSFET M5.

  The gate of the N-channel MOSFET M4 is connected to the input terminal 11. The input terminal 11 is the inverting input terminal (−) in FIG. 1 and receives the reference voltage vref. The gate of the N-channel MOSFET M5 is connected to the input terminal 10. The input terminal 10 is a non-inverting input terminal (+) in FIG. 1 and receives the voltage v1.

  The drain of the N-channel MOSFET M4 is connected to the drain of the P-channel MOSFET M6 and the gate of the P-channel MOSFET M6. The drain of the N-channel MOSFET M5 is connected to the drain of the P-channel MOSFET M7, the gate of the P-channel MOSFET M7, and the gate of the P-channel MOSFET M8.

  The gate of the N-channel MOSFET M3 is connected to the drain of the N-channel MOSFET M3, the drain of the P-channel MOSFET M8, and the output terminal 12. A control voltage v_cmfb is output from the output terminal 12. The output of the current source I1, the source of the P-channel MOSFET M6, the source of the P-channel MOSFET M7, and the source of the P-channel MOSFET M8 are electrically grounded.

In the common mode feedback circuit 1 of FIG. 1, in order to increase the operating voltage range of the common mode voltage vcm, for a circuit using the common mode feedback circuit 1, the voltage v1 is normally equal to the midpoint voltage vdd / 2. desirable. In the common mode feedback circuit unit 2 shown in FIG. 1, in order to widen the operating voltage range of the common mode voltage vcm, it is necessary to satisfy the expression (5). From the expressions (5) and (3), 6) Equation is derived.
vdd-v1> VGS + VDS (5)
vdd-a * vcm> VGS + VDS (6)
Here, VGS is the gate-source voltage of the N-channel MOS transistor M5, VDS is the drain-source voltage of the N-channel MOS transistor M2, and vcm≈ (1 / a) * vref.

For example, when a = 0.5 in the equation (6), the gate-source voltage VGS = 0.9 V, and the drain-source voltage VDS = 0.2 V, the common mode voltage vcm is the midpoint voltage vdd / 2. Then, equation (7) is established.
vdd-vdd / 4> 1.1 (7)
When formula (7) is calculated, formulas (8) to (10) are obtained.
3/4 * vdd> 1.1 (8)
vdd> 1.1 × 4/3 (9)
vdd> 1.467 (10)
As shown in the equation (10), the power supply voltage vdd that satisfies the equation (7) is 1.467V or more. Therefore, by setting a = 0.5, the common mode feedback circuit unit 2 can operate with a power supply voltage vdd of 1.467V or more.

  In the conventional common mode feedback circuit, since the operating voltage range of the common mode voltage vcm is wide, it is desirable for the circuit using the common mode feedback circuit that the common mode voltage vcm is normally equal to the midpoint voltage vdd / 2. . In the common mode feedback circuit 1, the common mode feedback circuit unit 2 receives not the common mode voltage vcm but the voltage v1 obtained by attenuating the common mode voltage vcm by the attenuator 3.

  Therefore, by reducing the power supply voltage vdd so that the voltage v1 and the midpoint voltage vdd / 2 are equal, it is possible to widen the operating voltage range of the common mode voltage vcm, and further to the conventional common mode feedback circuit. Compared to this, operation with a lower power supply voltage is possible.

  In the common mode feedback circuit 1, the common mode feedback circuit unit 2 outputs a larger control voltage v_cmfb when the voltage v1 is larger than the reference voltage vref, and outputs a smaller control voltage v_cmfb when the voltage v1 is smaller than the reference voltage vref. Output. Thereby, the voltage v1 and the reference voltage vref are compared, and the control voltage v_cmfb can be output so that the voltage v1 and the reference voltage vref are equal.

  In the following [Example 1] to [Example 3], usage examples of the common mode feedback circuit according to the embodiment of the present invention will be shown.

[Example 1]
FIG. 3 is a circuit diagram of the low-voltage differential transmission transmitter (LVDS transmitter) 13 according to the first embodiment. LVDS is an abbreviation for Low Voltage Differential Signaling and means low voltage differential transmission. The low voltage differential transmission transmitter 13 generally includes a low voltage differential transmission transmitter unit 14 and a common mode feedback circuit 1.

  The low voltage differential transmission transmitter unit 14 includes an inverter 15, an N channel MOSFET M9 to an N channel MOSFET M11, a P channel MOSFET M12, a P channel MOSFET M13, and a current source I2.

  The input of the inverter 15 is connected to the gate of the N-channel MOSFET M10 and the gate of the P-channel MOSFET M12. The output of the inverter 15 is connected to the gate of the N-channel MOSFET M11 and the gate of the P-channel MOSFET M13.

  The source of the N-channel MOSFET M9 is connected to the power supply line 16, and the power supply voltage vdd is applied. The drain of the N-channel MOSFET M9 is connected to the source of the N-channel MOSFET M10 and the source of the N-channel MOSFET M11.

  The drain of the N-channel MOSFET M11 is connected to the signal line 17 and the drain of the P-channel MOSFET M13. The drain of the N-channel MOSFET M10 is connected to the signal line 18 and the drain of the P-channel MOSFET M12.

  The source of the P-channel MOSFET M12 and the source of the P-channel MOSFET M13 are connected to the input of the current source I2, and the output of the current source I2 is electrically grounded.

  One end of the resistor 4 is connected to the signal line 17, and the other end of the resistor 4 is connected to one end of the resistor 5 and the input of the attenuator 3. The other end of the resistor 5 is connected to the signal line 18. The output of the attenuator 3 is connected to the non-inverting input terminal (+) of the common mode feedback circuit unit 2. The input of the power source 6 is electrically grounded, and the output of the power source 6 is connected to the inverting input terminal (−) of the common mode feedback circuit unit 2. The output of the common mode feedback circuit unit 2 is connected to the gate of an N-channel MOSFET M9.

  Input data Data is input to the input of the inverter 15, the gate of the N-channel MOSFET M10, and the gate of the P-channel MOSFET M12, and the low-voltage differential transmission transmitter unit 14 outputs the signal d_p and the signal d_n. A difference between the signal d_p and the signal d_n is a differential signal d_p−d_n.

Here, the common mode voltage vcm of the differential signal d_p-d_n is controlled by the common mode feedback circuit 1 so as to satisfy (11). Here, 0 <a <1.
vcm = (1 / a) * vref (11)
As described above, in the low-voltage differential transmission transmitter 13, the voltage v1 input to the common mode feedback circuit unit 2 can be reduced by attenuating the common mode voltage vcm by the attenuator 3, so that a lower power supply voltage can be obtained. Is possible.

[Example 2]
FIG. 4 is a circuit diagram of the low-voltage differential transmission receiver (LVDS receiver) 19 according to the second embodiment. The low voltage differential transmission receiver 19 generally includes a receiver 20 and a common mode feedback circuit 21. The common mode feedback circuit 21 is obtained by replacing the common mode feedback circuit unit 2 of the common mode feedback circuit 1 with a common mode feedback circuit unit 22.

  In addition to the receiver 20 and the common mode feedback circuit 21, the low-voltage differential transmission receiver 19 includes a resistor 23, a resistor 24, a capacitor 25, a capacitor 26, a P-channel MOSFET M14, and a P-channel MOSFET M15.

  The resistance values of the resistors 23 and 24 are R1, and the voltage across the resistors 23 and 24 is ΔV. The capacitances of the capacitor 25 and the capacitor 26 are C1. Further, the drain-source current of the P-channel MOSFET M14 and the drain-source current of the P-channel MOSFET M15 are I3.

  In the low-voltage differential transmission receiver 19, one end of the resistor 4 is connected to the signal line 17, and the other end of the resistor 4 is connected to one end of the resistor 5 and the input of the attenuator 3. The other end of the resistor 5 is connected to the signal line 18. The output of the attenuator 3 is connected to the non-inverting input terminal (+) of the common mode feedback circuit unit 22. The input of the power source 6 is electrically grounded, and the output of the power source 6 is connected to the inverting input terminal (−) of the common mode feedback circuit unit 22.

  The output of the common mode feedback circuit unit 2 is connected to the gate of the P-channel MOSFET M14 and the gate of the P-channel MOSFET M15. The source of the P-channel MOSFET M14 and the source of the P-channel MOSFET M15 are electrically grounded.

  The signal line 17 is connected to one end of the resistor 23 and one end of the capacitor 25. The other end of the resistor 23 is connected to the other end of the capacitor 25, the drain of the P-channel MOSFET M14, and the signal line 27. The signal line 27 is connected to the first receiver input terminal 29, and the signal d_p1 is transmitted.

  The signal line 18 is connected to one end of the resistor 24 and one end of the capacitor 26. The other end of the resistor 24 is connected to the other end of the capacitor 26, the drain of the P-channel MOSFET M15, and the signal line 28. The signal line 28 is connected to the second receiver input terminal 30, and the signal d_n1 is transmitted.

  FIG. 5 is a circuit diagram of the common mode feedback circuit unit 22 used in the low voltage differential transmission receiver 19 according to the second embodiment. The common mode feedback circuit unit 22 includes N-channel MOSFET M16 to N-channel MOSFET M20, P-channel MOSFET M21, P-channel MOSFET M22, current source I4, and resistor 31. The resistance value of the resistor 31 is Rgm.

  In the common mode feedback circuit unit 22, the source of the N-channel MOSFET M16, the source of the N-channel MOSFET M17, and the source of the N-channel MOSFET M18 are connected to the power supply line 32, and the power supply voltage vdd is applied. The gate of the N-channel MOSFET M16 is connected to the gate of the N-channel MOSFET M17, the gate of the N-channel MOSFET M18, the drain of the N-channel MOSFET M16, and the input of the current source I4.

  The drain of the N-channel MOSFET M17 is connected to one end of the resistor 31 and the source of the N-channel MOSFET M19. The drain of the N-channel MOSFET M18 is connected to the other end of the resistor 31 and the source of the N-channel MOSFET M20.

  The gate of the N-channel MOSFET M20 is connected to the input terminal 34. The input terminal 34 is the inverting input terminal (−) in FIG. 4 and receives the reference voltage vref. The gate of the N-channel MOSFET M19 is connected to the input terminal 33. The input terminal 33 is a non-inverting input terminal (+) in FIG. 4 and receives the voltage v1.

  The drain of the N-channel MOSFET M19 is connected to the drain of the P-channel MOSFET M21 and the gate of the P-channel MOSFET M21. The drain of the N-channel MOSFET M20 is connected to the drain of the P-channel MOSFET M22, the gate of the P-channel MOSFET M22, and the output terminal 35. A control voltage v_cmfb is output from the output terminal 35. The output of the current source I4, the source of the P channel MOSFET M21, and the source of the P channel MOSFET M22 are electrically grounded.

  The transconductance gm of the common mode feedback circuit unit 22 is determined by gm = 1 / Rgm.

  Returning to FIG. 4, when the differential signal d_p−d_n transmitted by the transmitter is input to the low voltage differential transmission receiver 19, the voltage v <b> 1 is adjusted to v <b> 1 = a * vcm by the attenuator 3. The low voltage differential transmission receiver 19 compares the voltage v1 with the reference voltage vref in the common mode feedback circuit unit 22, and outputs a current I3 corresponding to the control voltage v_cmfb output from the common mode feedback circuit unit 22. As a result, the signal d_p and the signal d_n are level-shifted by dropping the voltage between both ends ΔV = R1 * I3.

The both-end voltage ΔV is obtained by the following equation (12). Here, gm = 1 / Rgm is established between the resistance value Rgm of the resistor 31 and the transconductance gm of the common mode feedback circuit unit 22.
ΔV = R1 * (1 / Rgm) * (a * vcm−vref) (12)
In FIG. 5, since the reference voltage vref is compared and the voltage v1 and the transconductance gm, the following equation (13) is established.
ΔI = (1 / Rgm) * (a * vcm−vref) (13)
As described above, the voltage v1 is adjusted to v1 = a * vcm by the attenuator 3.

Since the current I3 shown in FIG. 4 is equal to ΔI, the following equation (14) is established.
I3 = ΔI = (1 / Rgm) * (a * vcm−vref) (14)
Therefore, ΔV is obtained by the following equation (15). Equation (12) is derived from Equation (15).
ΔV = R1 * I3 = R1 * (1 / Rgm) * (a * vcm−vref) (15)
For example, if vdd = 1.8V, vcm = 0.9V, a = 0.5, vref = 0.3v, gm = 30 μS (Rgm = 33.33 kΩ) and R1 = 100 kΩ, equation (16) is established. When calculated, ΔV = 0.45V.
ΔV = 100 × 10 ³ * (1 / 33.33 × 10 ³ ) * (0.5 * 0.9−0.3) (16)
The common mode voltage vcm_1 of the differential signal d_p1-d_n1 is 0.9V−0.45V = 0.45V by level-shifting the signal d_p and the signal d_n by dropping the voltage between both ends ΔV = 0.45V. It is controlled to become. The differential signal d_p1-d_n1 is a difference between the signal d_p1 and the signal d_n1. The common mode voltage vcm_1 is a midpoint voltage between the signal d_p1 and the signal d_n1, and vcm_1 = (d_p1 + d_n1) / 2.

  As described above, in the low-voltage differential transmission receiver 19, the voltage v1 input to the common mode feedback circuit unit 22 can be lowered by attenuating the common mode voltage vcm by the attenuator 3, so that the lower power supply voltage Is possible.

Example 3
FIG. 6 is a circuit diagram of the differential amplifier circuit 36 according to the third embodiment. The differential amplifier circuit 36 generally includes a transconductance amplifier 37 and a common mode feedback circuit 38. The common mode feedback circuit 38 is obtained by replacing the common mode feedback circuit unit 2 of the common mode feedback circuit 1 with a common mode feedback circuit unit 39.

  The differential amplifier circuit 36 includes a P-channel MOSFET M23 and a P-channel MOSFET M24 in addition to the transconductance amplifier 37 and the common mode feedback circuit 38. The transconductance amplifier 37 has a first input terminal 42, a second input terminal 43, a first output terminal 44, and a second output terminal 45.

  In the differential amplifier circuit 36, one end of the resistor 4 is connected to the signal line 7, and the other end of the resistor 4 is connected to one end of the resistor 5 and the input of the attenuator 3. The other end of the resistor 5 is connected to the signal line 8. The output of the attenuator 3 is connected to the non-inverting input terminal (+) of the common mode feedback circuit unit 39. The input of the power source 6 is electrically grounded, and the output of the power source 6 is connected to the inverting input terminal (−) of the common mode feedback circuit unit 39.

  The output of the common mode feedback circuit unit 39 is connected to the gate of the P-channel MOSFET M23 and the gate of the P-channel MOSFET M24. The drain of the P-channel MOSFET M23 is connected to the signal line 8, and the drain of the P-channel MOSFET M24 is connected to the signal line 7. The source of the P-channel MOSFET M23 and the source of the P-channel MOSFET M24 are electrically grounded.

  The signal line 40 is connected to the first input terminal 42, and the input signal vin_p is transmitted. The signal line 41 is connected to the second input terminal 43, and the input signal vin_n is transmitted. The first output terminal 44 is connected to the signal line 7, and the signal v_p is transmitted to the signal line 7. The second output terminal 45 is connected to the signal line 8, and the signal v_n is transmitted to the signal line 8.

  FIG. 7 is a circuit diagram of the common mode feedback circuit unit 39 used in the differential amplifier circuit 36 according to the third embodiment. The common mode feedback circuit unit 39 includes an N channel MOSFET M25 to an N channel MOSFET M28, a P channel MOSFET M29, a P channel MOSFET M30, and a current source I5.

  In the common mode feedback circuit unit 39, the source of the N-channel MOSFET M25 and the source of the N-channel MOSFET M26 are connected to the power supply line 46, and the power supply voltage vdd is applied. The gate of the N-channel MOSFET M25 is connected to the gate of the N-channel MOSFET M26, the drain of the N-channel MOSFET M25, and the input of the current source I5.

  The drain of the N-channel MOSFET M26 is connected to the source of the N-channel MOSFET M27 and the source of the N-channel MOSFET M28. The drain of the N-channel MOSFET M28 is connected to the drain of the P-channel MOSFET M29 and the gate of the P-channel MOSFET M29.

  The gate of the N-channel MOSFET M28 is connected to the input terminal 48. The input terminal 48 is the inverting input terminal (−) in FIG. 6 and receives the reference voltage vref. The gate of the N-channel MOSFET M27 is connected to the input terminal 47. The input terminal 47 is a non-inverting input terminal (+) in FIG. 4 and receives the voltage v1.

  The drain of the N-channel MOSFET M28 is connected to the drain of the P-channel MOSFET M30, the gate of the P-channel MOSFET M30, and the output terminal 49. A control voltage v_cmfb is output from the output terminal 49. The output of the current source I5, the source of the P channel MOSFET M29, and the source of the P channel MOSFET M30 are electrically grounded.

The differential signal vin_p-vin_n, which is the difference between the input signals vin_p, ¥ and the input signal vin_n, is amplified by the transconductance amplifier 37, the resistor 4 and the resistor 5 to obtain the differential signal v_p-v_n. In this case, the following equation (17) is established between the differential signal v_p-v_n and the differential signal vin_p-vin_n. Here, gt is the gain of the transconductance amplifier 37.
(V_p-v_n) / (vin_p-vin_n) = gt * (2 * R0) (17)
As described above, in the differential amplifier circuit 36, the common mode feedback circuit 38 detects the common mode voltage vcm of the differential signal v_p−v_n with the resistors 4 and 5, and the common mode voltage vcm with the attenuator 3. Attenuated and adjusted to an arbitrary voltage v1, and a control voltage v_cmfb for controlling the common mode voltage vcm to a desired voltage is output.

  By attenuating the common mode voltage vcm by the attenuator 3, the voltage v1 input to the common mode feedback circuit unit 39 can be lowered, so that operation with a lower power supply voltage is possible.

  [Example 4] shown below shows an example of the attenuator 3 according to the embodiment of the present invention.

Example 4
FIG. 8 is a circuit diagram of the attenuator 3 according to the embodiment of the present invention. The attenuator 3 has a resistor 51 and a resistor 52. The resistance value of the resistor 51 is R_a1, and the resistance value of the resistor 52 is R_a2.

  The input terminal 50 to which the common mode voltage vcm is input is connected to one end of the resistor 51. The other end of the resistor 51 is connected to one end of the resistor 52 and the output terminal 53. From the output terminal 53, the voltage v1 is output. The other end of the resistor 52 is electrically grounded.

The attenuator 3 shown in FIG. 8 is a resistance voltage dividing circuit, and the following equation (18) is established. An arbitrary voltage v1 can be set by setting the resistance value R_a1 and the resistance value R_a2.
v1 = vcm * R_a2 / (R_a1 + R_a2) (18)
[Summary of Embodiment]
In the common mode feedback circuit 1 according to the embodiment of the present invention, a resistor 4 to which a signal v_p is input at one end and a resistor 5 to which a signal v_n is input at one end are connected in series, and a signal v_p and a signal v_n are connected. The differential signal v_p-v_n, which is the difference, is divided and output as the common mode voltage vcm, the attenuator 3 that attenuates the common mode voltage vcm and outputs it as the voltage v1, and the voltage v1 and the reference voltage vref are compared. And a common mode feedback circuit unit 2 that outputs a control voltage v_cmfb according to the comparison result.

  In the conventional common mode feedback circuit, since the operating voltage range of the common mode voltage vcm is wide, it is desirable for the circuit using the common mode feedback circuit that the common mode voltage vcm is normally equal to the midpoint voltage vdd / 2. . According to the above configuration, the common mode feedback circuit unit 2 receives not the common mode voltage vcm but the voltage v1 obtained by attenuating the common mode voltage vcm by the attenuator 3.

  Therefore, by reducing the power supply voltage vdd so that the voltage v1 and the midpoint voltage vdd / 2 are equal, it is possible to widen the operating voltage range of the common mode voltage vcm, and further to the conventional common mode feedback circuit. Compared to this, operation with a lower power supply voltage is possible.

  In the common mode feedback circuit 1, the common mode feedback circuit unit 2 outputs a larger control voltage v_cmfb when the voltage v1 is larger than the reference voltage vref, and outputs a smaller control voltage v_cmfb when the voltage v1 is smaller than the reference voltage vref. It may be output.

  Thereby, the common mode feedback circuit 1 compares the voltage v1 with the reference voltage vref, and can output the control voltage v_cmfb so that the voltage v1 and the reference voltage vref are equal.

  In the common mode feedback circuit 1, the voltage v1 may be larger than zero and smaller than the common mode voltage vcm.

  As a result, the common mode feedback circuit unit 2 receives not the common mode voltage vcm but the voltage v1 larger than zero and smaller than the common mode voltage vcm. Therefore, by reducing the power supply voltage vdd so that the voltage v1 and the midpoint voltage vdd / 2 are equal, it is possible to widen the operating voltage range of the common mode voltage vcm, and further to the conventional common mode feedback circuit. Compared to this, operation with a lower power supply voltage is possible.

  Since the differential transmission transmitter, the differential transmission receiver, and the differential amplifier circuit of the present invention include any one of the above-described common mode feedback circuits, operation with a lower power supply voltage is possible.

  The common mode feedback circuit 1 of the present invention can take a wide operating voltage range of the common mode voltage, and can operate at a lower power supply voltage than the conventional common mode feedback circuit. It is suitable for use in a common mode feedback circuit for voltage differential transmission (LVDS).

It is a block diagram of a common mode feedback circuit according to an embodiment of the present invention. It is a circuit diagram of the common mode feedback circuit unit according to the embodiment of the present invention. 1 is a circuit diagram of a low voltage differential transmission transmitter according to an embodiment of the present invention. It is a circuit diagram of the low voltage differential transmission receiver which concerns on the other Example of this invention. FIG. 6 is a circuit diagram of a common mode feedback circuit unit according to another embodiment of the present invention. It is a circuit diagram of the differential amplifier circuit which concerns on another Example of this invention. It is a circuit diagram of the common mode feedback circuit part which concerns on another Example of this invention. It is a circuit diagram of the attenuator which concerns on embodiment of this invention. It is a block diagram of the conventional common mode feedback circuit. This is a low-voltage differential transmission transmitter using a conventional common mode feedback circuit. It is a circuit diagram of the common mode feedback circuit used for the low voltage differential transmission transmitter of FIG.

Explanation of symbols

1, 21, 38 Common mode feedback circuit 2, 22, 39 Common mode feedback circuit section 3 Attenuator (attenuator)
4 Resistance (first resistance)
5 Resistance (second resistance)
6 Power supply 7, 8, 17, 18, 27, 28, 40, 41 Signal line 9, 16, 32, 46 Power supply line 10, 11, 33, 34, 47, 48, 50 Input terminal 12, 35, 49, 53 Output terminal 13 Low voltage differential transmission transmitter 14 Low voltage differential transmission transmitter 15 Inverter 19 Low voltage differential transmission receiver 20 Receiver 23, 24, 31, 51, 52 Resistor 25, 26 Capacitor 29 First receiver input terminal 30 1st 2 receiver input terminal 36 differential amplifier circuit 37 transconductance amplifier 40 signal line 41 signal line 42 first input terminal 43 second input terminal 44 first output terminal 45 second output terminal Data input data I1, I2, I4, I5 Current Source I3 Current M1-M5, M9-M11, M16-M20, M25-M28 N channel OSFET
M6 to M8, M12 to M15, M21 to M24, M29, M30 P-channel MOSFET
R, R0, R1, Rgm, R_a1, R_a2 Resistance value VDS Drain-source voltage VGS Gate-source voltage d_p, d_n signal vin_p, vin_n input signal v_p-v_n, d_p-d_n, vin_p-vin_n Differential signal v_n (Second input signal)
v_p signal (first input signal)
gm transconductance gt gain v1 voltage (damping voltage)
vcm Common mode voltage vdd / 2 Midpoint voltage (1/2 power supply voltage)
vdd power supply voltage vref reference voltage v_cmfb control voltage

Claims (6)

A first resistor to which a first input signal is input at one end and a second resistor to which a second input signal is input at one end are connected in series, and the difference between the first input signal and the second input signal is A circuit that divides a differential signal and outputs it as a common mode voltage;
Attenuating means for attenuating the common mode voltage and outputting the attenuated voltage;
A common mode feedback circuit comprising: a common mode feedback circuit unit that compares the attenuated voltage with a reference voltage and outputs a control voltage according to the comparison result.   The common mode feedback circuit unit outputs a larger control voltage when the attenuation voltage is larger than the reference voltage, and outputs a smaller control voltage when the attenuation voltage is smaller than the reference voltage. The common mode feedback circuit according to claim 1.   The common mode feedback circuit according to claim 1, wherein the attenuation voltage is larger than zero and smaller than the common mode voltage.   A differential transmission transmitter comprising the common mode feedback circuit according to claim 1.   A differential transmission receiver comprising the common mode feedback circuit according to claim 1.   A differential amplifier circuit comprising the common mode feedback circuit according to claim 1.

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