JP2012054685A - Wideband amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a feedback type wideband amplifier of single end output operating at RF(Radio Frequency), especially a feedback type wideband amplifier having excellent secondary strain tolerance.SOLUTION: An output current from an MOS transistor Q1 which outputs an input signal after performing voltage-current conversion is converted into an output voltage by a first resistive element R1, and that output voltage is fed back to the input of the MOS transistor Q1 by a second resistive element R2. Furthermore, a bias is supplied from a first bias circuit (VG1+R3) to the input of the MOS transistor Q1. The bias value is set so that drain current of the MOS transistor Q1 flows, and the absolute value of a component obtained by differentiating the drain current of the MOS transistor Q1 by the gate voltage twice becomes minimal.

Description

  The present invention relates to a broadband amplifier.

Various broadband amplifiers that can be applied to various frequency bands have been proposed.
FIG. 5 shows an example of a conventional broadband amplifier using resistance feedback (see Non-Patent Document 1).
This wideband amplifier includes a MOS transistor Q1 that converts an input signal into voltage-current and outputs it, a first resistance element R1 that converts a current flowing from the MOS transistor Q1 into an output voltage, and an output signal as an input to the MOS transistor Q1. A second resistance element R2 that feeds back, a capacitor C1 for DC-cutting the bias before and after the feedback node, a bias circuit that includes the resistance element R3 and the voltage source VG1, and variably sets the bias to the gate of the MOS transistor; It has.

  In FIG. 5, the input signal is voltage-current converted by the MOS transistor Q1. The current signal output from the MOS transistor Q1 is converted into a voltage signal by the load resistor R1 and output. A part of the output signal is fed back to the input of the transistor Q1 through the feedback resistor R2 and the capacitor C1 for DC cut.

IEEE Transactions on Microwave theory and techniques, vol. 56, no. 5, May 2008 “Resistive-Feedback CMOS Low-Noise Amplifiers for Multiband Applications”

The feedback type broadband amplifier in FIG. 5 has a single-ended output.
In the case of a differential output circuit, an unnecessary output generated by the secondary distortion appears as an in-phase component and is canceled when viewed from the differential output, so that the secondary distortion resistance is relatively good.
However, in the case of a single-ended output, since there is no canceling action due to the differential, it is vulnerable to secondary distortion. In general, the MOS transistor Q1 is biased with emphasis on the current efficiency of the mutual conductance gm. That is, a high gm is obtained with as little drain current as possible. For this reason, the secondary distortion resistance of the circuit as shown in FIG. 5 is not good.

  The present invention has been made in view of the above situation, and is a feedback type broadband amplifier that operates at RF (Radio Frequency) and is a single-ended output, and is particularly excellent in resistance to secondary distortion. The purpose is to realize a broadband amplifier.

In order to achieve the above object, the following techniques are proposed here.
(1) a MOS transistor for converting an input signal into voltage-current and outputting it;
A first resistance element that converts an output current of the MOS transistor into an output voltage; a second resistance element that feeds back the output voltage to an input of the MOS transistor;
A first bias circuit for setting a bias to the input of the MOS transistor;
With
The bias value by the first bias circuit is set so that the drain current of the MOS transistor flows and the absolute value of the component obtained by differentiating the drain current of the MOS transistor twice with respect to the gate voltage is minimized. A wideband amplifier characterized by comprising:

  In the broadband amplifier of (1) above, the bias is set so that the absolute value of the component obtained by differentiating the drain current twice with the bias voltage is minimized, that is, the absolute value is extremely small and substantially zero. The second order distortion is greatly reduced while the gain is not changed by the feedback.

(2) a cascode transistor connected between the MOS transistor and the first resistance element and having an output current of the MOS transistor as a tail current;
A second bias circuit for setting a bias to the input of the cascode transistor;
Further comprising
The wideband amplifier according to (1), wherein the first resistance element converts a current flowing through the cascode transistor into an output voltage.

  In the broadband amplifier of (2) above, the gain is complemented because the influence of the cascode transistor on the capacitive load is suppressed by the cascode transistor, and the fluctuation of VDS of the MOS transistor is further suppressed. This makes it easy to secure the operating point.

(3) The broadband amplifier according to (1) or (2), wherein the first bias circuit variably sets the bias.
In the wideband amplifier of (3), particularly, in the wideband amplifier of (1) or (2), the optimum value of the bias is selected while monitoring the change of the second-order distortion resistance (ipp2) while variably setting the bias. Can do.

  By setting the bias so that the component obtained by differentiating the drain current twice with the bias voltage becomes zero, it is possible to realize a wideband amplifier with greatly improved second-order distortion resistance without changing the gain by feedback. it can.

This is a basic bias circuit of a MOS transistor. It is the figure which showed the change of the 1st differential component (gm) and 2nd differential component (gm2) of drain current ID and drain current ID when changing the bias voltage VG supplied to a gate in FIG. It is a feedback type broadband amplifier of the present invention. 6 is a diagram showing a change in second-order distortion resistance (ipp2) when the bias voltage VG supplied to the gate is changed in FIGS. 3 and 5. FIG. This is a conventional feedback type broadband amplifier.

Hereinafter, the present invention will be clarified by describing embodiments of the present invention in detail with reference to the drawings.
(First embodiment)
The broadband amplifier according to the present invention is configured using a basic bias circuit of a MOS transistor. Therefore, a basic bias circuit of a MOS transistor which is a premise of the present invention will be described below.

FIG. 1 shows a basic bias circuit of a MOS transistor.
Arbitrary gate voltage VG and drain voltage VD are applied to the MOS transistor.
In this circuit, a drain current ID flows for an arbitrary VG.
ID = aW / L (VG−Vp) ²
Here, aW / L is a constant determined by the channel width and channel length of the MOS transistor, and Vp is a pinch-off voltage.

FIG. 2 is a diagram showing changes in the drain current ID and the first derivative component gm and the second derivative component gm2 when the bias voltage VG supplied to the gate in FIG. 1 is changed.
gm≡dID / dVG
gm2≡d ² ID / dVG ²
gm is a single differential component of the drain current ID, and gm2 is a double differential component of the drain current ID. When these coefficients are used, the drain current ID is expressed as follows.
ID≡gm (VG) + gm2 (VG) ² + gm3 (VG) ³ +...

  Referring to FIG. 2, when the gate voltage VG is gradually increased, the drain current starts to flow and gradually increases. However, gm2 decreases rapidly after a certain peak, and finally reaches zero. The point that becomes. If the MOS transistor is biased with this gate voltage VG, it is expected that there is no secondary term in the ID, and as a result, there is no secondary distortion or it is minimized. Although it is most effective when gm2 is 0, if the absolute value of gm2 is extremely small and substantially zero, the secondary distortion can be sufficiently suppressed.

  However, in general, the current efficiency of gm is more important, that is, since a high gm is obtained with as little drain current as possible, the gate voltage VG is a lower value that would increase gm2. Biased at.

  FIG. 3 shows a feedback type broadband amplifier according to the present invention. A MOS transistor Q1 that converts the input signal from voltage to current and outputs it, a cascode transistor Q2 that uses the output current of the MOS transistor Q1 as a tail current, and a first resistance element R1 that converts the current flowing through the cascode transistor Q2 into an output voltage A second resistance element R2 that feeds back an output signal to the input of the MOS transistor Q1, a capacitor C1 for DC-cutting the bias before and after the feedback node, a resistance element R3, and a variable voltage source VG1, and a MOS transistor Q1 And a second bias circuit having a variable voltage source VG2 and variably setting the bias to the cascode transistor Q2.

  In FIG. 3, the input signal is voltage-current converted by the MOS transistor Q1. The current signal output from the MOS transistor Q1 flows through the load resistor R1 as a tail current of the MOS transistor Q2 connected to the MOS transistor Q1 in a cascode. The output current signal is converted into a voltage signal by the load resistor R1 and output. A part of the output signal is fed back to the input of the MOS transistor Q1 through the feedback resistor R2 and the capacitor C1 for DC cut.

  Here, the gate voltage VG of the MOS transistor Q1 is set to gm2≈0 in advance, that is, the absolute value of gm2 is minimized and fine adjustment is effected, and the change in the secondary distortion resistance (ip2) of the output is changed. By adjusting the gate voltage VG while monitoring, it is possible to obtain much better second-order distortion resistance (ip2) than before.

However, biasing with VG in the vicinity of gm2≈0 requires caution when setting the operating point of the circuit because VG becomes relatively high. Also,
ID = aW / L (VG−Vp) ²
GM≡dID / dVG = 2aW / L (VG−Vp)
∴GM≡2ID / (VG-Vp)
Therefore, in order to realize a high VG under a constant drain current ID, W / L of the transistor size must be reduced, and the gm obtained is also reduced.

Therefore, in the present embodiment, the cascode transistor Q2 is installed to suppress the influence on the capacitive load on the MOS transistor Q1 and the gain is supplemented. It is easy to secure.
FIG. 4 is a diagram showing a change in second-order distortion resistance (ipp2) when the bias voltage VG is changed in FIGS.

The feedback type broadband amplifier of FIG. 3 is optimized in advance so that the absolute value of gm2 is minimized, that is, in the vicinity of gm2≈0, while monitoring the change in second-order distortion resistance (ip2), Furthermore, if the gate voltage VG is finely adjusted, good secondary distortion resistance (ip2) can be obtained as shown in FIG.
The gain at this time is kept almost constant by feedback.

Q1 ... MOS transistor Q2 ... cascode transistor

Claims (3)

A MOS transistor that converts an input signal into a voltage-current converter and outputs it;
A first resistance element that converts an output current of the MOS transistor into an output voltage; a second resistance element that feeds back the output voltage to an input of the MOS transistor;
A first bias circuit for setting a bias to the input of the MOS transistor;
With
The bias value by the first bias circuit is set so that the drain current of the MOS transistor flows and the absolute value of the component obtained by differentiating the drain current of the MOS transistor twice with respect to the gate voltage is minimized. A wideband amplifier characterized by comprising: A cascode transistor connected between the MOS transistor and the first resistance element and having an output current of the MOS transistor as a tail current;
A second bias circuit for setting a bias to the input of the cascode transistor;
Further comprising
2. The broadband amplifier according to claim 1, wherein the first resistance element converts a current flowing through the cascode transistor into an output voltage.   The broadband amplifier according to claim 1 or 2, wherein the first bias circuit variably sets the bias.

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