A differential amplifier system and a method for improving common-mode stability

Abstract

This invention belongs to the field of wireless communication technology. It provides a differential amplifier system and a method for improving common-mode stability. The invention uses a large decoupling capacitor C1 at the transformer center tap to provide AC ground, and a common-mode resistor R1 to improve the circuit's common-mode stability. Since the common-mode resistor R1 is not in the differential signal loop path, the use of common-mode resistor technology does not affect differential-mode performance. Because the required common-mode resistor value is small, this design occupies a small layout area. This invention solves the problem of low common-mode stability in differential amplifiers.

Description

Technical Field

[0001] This invention relates to amplifiers in transceivers of communication systems, and more specifically to a differential amplifier system and a method for improving common-mode stability. Background Technology

[0002] For any transmitter chip, an amplifier module is always needed internally to provide gain for the overall link. Whether it is a low-noise amplifier, a driver amplifier, a variable gain amplifier, or a power amplifier, all of these amplifiers essentially provide gain for the link. Differential amplifiers are widely used due to their various advantages.

[0003] This structure remains widely used today due to its advantages: 1. Differential structures suppress common-mode signals; 2. Differential structures can double the voltage swing at the output port, increasing output power by 3dBm, which is highly attractive for silicon-based processes with lower power supply voltages; 3. Differential structures shield external power supply pads, GNDPADs, bias pads, and other DC pads connected to the PCB from gold wires. Simultaneously, the inductance from the transistor to ground also shields the differential-mode path. This reduces the source stage traces in the differential-mode path, resulting in higher amplifier gain; 4. If a transformer-based balun is used for input-output matching to convert differential signals to single-ended signals, the differential structure inherently provides DC and AC blocking.

[0004] However, when performing simulation analysis on differential amplifiers, designers often assume that the amplifier input signal is an ideal differential-mode signal, that is, they do not consider the scenario where the input signal may be a common-mode signal. However, since the balun designed in reality cannot be ideal, there must be a small amount of common-mode signal in the amplifier input signal. Furthermore, noise in nature itself can also be regarded as a small common-mode signal. Therefore, not considering the scenario where the input signal may be a common-mode signal will lead to the differential amplifier having poor stability. Summary of the Invention

[0005] In view of the above-mentioned shortcomings in the prior art, the present invention provides a differential amplifier system and a method for improving common-mode stability, which solves the problem of poor stability of existing differential amplifiers.

[0006] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: a differential amplifier system, comprising: an input matching network and a differential amplifier; the first output terminal of the input matching network is connected to the first differential input terminal of the differential amplifier, and its second output terminal is connected to the second differential input terminal of the differential amplifier.

[0007] Furthermore, the input matching network includes: inductors L1, L2, L3, L4, capacitor C1, and resistor R1;

[0008] One end of inductor L1 serves as the reference voltage input terminal RFIN, and the other end is connected to grounded inductor L2; inductors L1 and L2 constitute the primary coil; one end of inductor L3 serves as the first output terminal of the input matching network, and the other end is connected to one end of inductor L4 and one end of capacitor C1 respectively; the other end of inductor L4 serves as the second output terminal of the input matching network; inductors L3 and L4 constitute the secondary coil; the other end of capacitor C1 is connected to one end of resistor R1; the other end of resistor R1 is grounded.

[0009] Furthermore, the resistance R1 is greater than R in , where R in This represents the real part of the input impedance of the differential amplifier.

[0010] The beneficial effects of the above-mentioned further solution are as follows: the present invention achieves the destruction of the resonant cavity oscillation condition by connecting a common-mode resistor R1 in series with the decoupling capacitor C1 connected in parallel to ground at the center tap of the differential amplifier, thereby improving the common-mode stability of the amplifier.

[0011] Furthermore, a first neutralizing capacitor C is provided between the second differential input terminal and the first output terminal of the differential amplifier. f A second neutralizing capacitor C is provided between the first differential input terminal and the second output terminal of the differential amplifier. f .

[0012] The beneficial effect of the above further solution is: neutralizing capacitor C f It can cancel out parasitic capacitance C gd This results in a decrease in differential-mode gain and a reduction in stability.

[0013] Further, the differential amplifier includes: NMOS transistor M1 and NMOS transistor M2; the gate of NMOS transistor M1 serves as the first differential input terminal of the differential amplifier; the gate of NMOS transistor M2 serves as the second differential input terminal of the differential amplifier; the source of NMOS transistor M1 is connected to the source of NMOS transistor M2 and grounded; the drain of NMOS transistor M1 serves as the first output terminal of the differential amplifier; the drain of NMOS transistor M2 serves as the second output terminal of the differential amplifier.

[0014] A method for improving the common-mode stability of a differential amplifier system is as follows: a capacitor C1 and a resistor R1 are connected sequentially at the middle tap of the secondary coil of the input matching network. The AC signal is filtered out by the capacitor C1, and the resonant cavity oscillation condition is disrupted by the resistor R1, thereby improving the common-mode stability.

[0015] In summary, the beneficial effects of this invention are as follows:

[0016] 1. In a common-mode resonant cavity (i.e. Figure 7 Adding a common-mode resistor R1 makes the equivalent resistance in the common-mode resonant cavity positive, making the oscillation mode of the resonant cavity damped oscillation, thereby eliminating the oscillation condition of the common-mode resonant cavity and realizing the elimination of common-mode oscillation.

[0017] 2. Since the center tap of the transformer is AC ground, the differential signal of the differential amplifier circulates inside the amplifier. The common-mode resistor R1 is not in the differential signal circulation path. Only the common-mode signal passes through the transformer center tap path. Therefore, the use of common-mode resistor technology will not affect the differential-mode performance.

[0018] 3. Since the real part of the input impedance of a differential amplifier is not large, the required value of the common-mode resistor is extremely small. Therefore, using common-mode resistor technology will not occupy too much layout area. Attached Figure Description

[0019] Figure 1 This is a circuit diagram of a differential amplifier system.

[0020] Figure 2 This is a schematic diagram of the differential-mode feedforward signal of a differential common-source amplifier.

[0021] Figure 3 This is a schematic diagram of a single-transistor common-source amplifier with small-signal characteristics.

[0022] Figure 4 This is a schematic diagram of a differential amplifier using a neutralizing capacitor for cross-coupling.

[0023] Figure 5 This is a schematic diagram of the differential-mode feedforward signal of a differential common-source amplifier with a neutralizing capacitor.

[0024] Figure 6 This is a schematic diagram of the common-mode feedforward signal of a differential common-source amplifier.

[0025] Figure 7 This is a schematic diagram of common-mode signal series oscillation. Detailed Implementation

[0026] The specific embodiments of the present invention are described below to enable those skilled in the art to understand the present invention. However, it should be understood that the present invention is not limited to the scope of the specific embodiments. For those skilled in the art, various changes are obvious as long as they are within the spirit and scope of the present invention as defined and determined by the appended claims. All inventions utilizing the concept of the present invention are protected.

[0027] like Figure 1As shown, a differential amplifier system includes: an input matching network and a differential amplifier; the first output terminal of the input matching network is connected to the first differential input terminal of the differential amplifier, and its second output terminal is connected to the second differential input terminal of the differential amplifier.

[0028] The input matching network includes: inductor L1, inductor L2, inductor L3, inductor L4, capacitor C1, and resistor R1;

[0029] One end of inductor L1 serves as the reference voltage input terminal RFIN, and the other end is connected to grounded inductor L2; inductors L1 and L2 constitute the primary coil; one end of inductor L3 serves as the first output terminal of the input matching network, and the other end is connected to one end of inductor L4 and one end of capacitor C1 respectively; inductors L1 and L3 are coupled to each other, and inductors L2 and L4 are coupled to each other; the other end of inductor L4 serves as the second output terminal of the input matching network; inductors L3 and L4 constitute the secondary coil; the other end of capacitor C1 is connected to one end of resistor R1; the other end of resistor R1 is grounded.

[0030] An input matching network is used to provide input matching and DC blocking / AC passing functions for the differential amplifier. The gate bias voltage of the differential amplifier is provided through the center tap of the secondary coil, i.e., the VB1 terminal. The center tap is connected to AC ground using a large decoupling capacitor C1. A common-mode resistor R1 is used to improve the common-mode stability of the circuit.

[0031] In this embodiment, inductors L1, L2, L3, and L4 are used as equivalent descriptions of a transformer. The capacitance range of the decoupling capacitor C1 is:

[0032]

[0033] Where f is the operating frequency, in When the decoupling capacitor C1 blocks the signal by less than 1Ω, it can be used for signal filtering to achieve the effect of decoupling.

[0034] Resistance R1 is greater than R in , where R in This represents the real part of the input impedance of the differential amplifier.

[0035] A first neutralization capacitor C is provided between the second differential input terminal and the first output terminal of the differential amplifier. f A second neutralizing capacitor C is provided between the first differential input terminal and the second output terminal of the differential amplifier. f .

[0036] The differential amplifier includes: NMOS transistor M1 and NMOS transistor M2; the gate of NMOS transistor M1 serves as the first differential input terminal of the differential amplifier; the gate of NMOS transistor M2 serves as the second differential input terminal of the differential amplifier; the source of NMOS transistor M1 is connected to the source of NMOS transistor M2 and grounded; the drain of NMOS transistor M1 serves as the first output terminal of the differential amplifier; the drain of NMOS transistor M2 serves as the second output terminal of the differential amplifier.

[0037] A method for improving the common-mode stability of a differential amplifier system is as follows: a capacitor C1 and a resistor R1 are connected sequentially at the middle tap of the secondary coil of the input matching network. The AC signal is filtered out by the capacitor C1, and the resonant cavity oscillation condition is disrupted by the resistor R1, thereby improving the common-mode stability.

[0038] Theoretical Analysis:

[0039] For amplifiers without cross-coupling, their differential-mode stability is practically the same as their common-mode stability; however, for amplifiers with cross-coupling, their differential-mode stability differs significantly from their common-mode stability. Let the gate-drain parasitic capacitance of the transistor be C. gd A schematic diagram of the differential common-source amplifier differential-mode feedforward signal is shown below. Figure 2 As shown, the parasitic capacitance C gd It will introduce feedforward current I and parasitic capacitance C gd The resulting feedforward current I will reduce the gain of the common-source amplifier and decrease its stability.

[0040] Figure 3 This is a schematic diagram of a single-transistor common-source amplifier with small-signal characteristics. In the diagram, C... gd C gs C ds These are the gate-drain parasitic capacitance, gate-source parasitic capacitance, and drain-source parasitic capacitance, respectively. gs For the small signal swing of the gate source, r ds For internal resistance, Z L For load, g m Assuming transconductance, let the current flow through capacitor C gd The current is I, and the voltage at the gate node is V. Under the premise of input conjugate matching, the capacitor C... gs It will be input to the matching network for resonance, capacitor C gs It will not affect the stability of the circuit, so the analysis of capacitor C is performed here. gs For the input impedance at the rear, equation (1) holds:

[0041]

[0042] Where r is Figure 3 The real part of the equivalent impedance of the network within the dashed box, where x is... Figure 3The network within the dashed box represents the imaginary part of the equivalent impedance, where j is the imaginary unit, ω = 2πf is the angular frequency, and f is the operating frequency.

[0043] Capacitor C gs The input impedance Z at the rear in It can be expressed as equation (2):

[0044]

[0045] Where x is Figure 3 The imaginary part of the equivalent impedance of the network within the dashed box, r is Figure 3 The real part of the equivalent impedance of the network within the dashed box.

[0046] Therefore, Z can be obtained. in The real part Re(Z) in Equation (3) is given by:

[0047]

[0048] Z in imaginary part im(Z) in Equation (4) is given by:

[0049]

[0050] From equations (4) and (3) above, it can be seen that when x < 0 (the load is capacitive), the real part of the input impedance is always greater than 0, that is, the amplifier will not oscillate; when x > 0 (the load is inductive), the capacitance C gd This introduces a negative term into the real part of the input impedance, potentially causing the amplifier's input impedance to be less than zero. Since the amplifier's output impedance is typically capacitive, and to achieve better matching, the load is usually inductive, the capacitance C can be considered... gd This will cause the real part of the input impedance to decrease, thus leading to oscillation. For example... Figure 2 As shown, when a small AC signal is input to the gate of the common-source amplifier, the signal is amplified by the transistor, generating an amplified inverted signal at its drain. A portion of the signal at the drain will pass through the parasitic capacitance C. gd Feeding forward to the gate, since the two signals are inverted, effectively reduces the swing of the input signal, thus effectively reducing the gain of the common-source amplifier. This is equivalent to the parasitic capacitance C. gd This will reduce the gain of the common-source amplifier.

[0051] Figure 4 This is a differential amplifier using a neutral capacitor cross-coupled configuration. Through capacitor cross-coupling, the differential common-source amplifier can introduce a capacitance C from the two drains of the differential amplifier. gd A current I1, which is opposite in magnitude and equal in magnitude to the current I, is introduced into the gate of the common-source amplifier transistor. The two capacitors used to introduce the current I1 are called neutralizing capacitors C.f Schematic diagram of differential-mode feedforward signal in a differential common-source amplifier with neutralization capacitor. Figure 5 As shown. From the differential signal level, a neutralizing capacitor C of appropriate capacitance value... f The parasitic capacitance C was canceled out. gd This results in a decrease in differential-mode gain and a reduction in stability. This is due to the neutralizing capacitor C. f The introduced current I1 is greater than C gd When a current I is introduced, a new positive feedback is triggered, at which point the differential-mode gain increases and stability decreases. Therefore, the neutralizing capacitor C... f An appropriate value needs to be selected.

[0052] Capacitor C f The formula is:

[0053]

[0054] Where, r g r is the parasitic resistance of the transistor gate. s is the input source impedance of the transistor gate. In this embodiment, r g r is the parasitic resistance of the gate of NMOS transistor M1 or NMOS transistor M2. s This is the input source impedance of the gate of NMOS transistor M1 or NMOS transistor M2.

[0055] For a differential common-source amplifier with an ideal differential input signal, the neutralizing capacitor C f The parasitic capacitance C was canceled out. gd At that time, the neutralizing capacitor C f This can simultaneously improve gain and stability. However, since it's impossible to generate a purely ideal differential signal in a circuit, the signal input to the amplifier can always be split into a higher-energy differential signal and a lower-energy common-mode signal. Because of the common-mode signal input, the amplifier's common-mode stability also needs to be considered in the circuit design. In the case of a common-mode input signal, such as... Figure 6 As shown, the parasitic capacitance C gd The resulting feedforward current I will still reduce the common-mode gain of the common-source amplifier and decrease its common-mode stability. At this time, because the two drain signals of the differential amplifier are in phase but out of phase with the input signal, the neutralizing capacitor C... f The introduced current I1 and C gd The introduced currents I are in the same direction, and the two currents will superimpose, which can also be considered as neutralizing the capacitor C. f In common-mode, it will function in conjunction with parasitic capacitance C. gd The same function. That is, the neutralizing capacitor C in common-mode. fThis can also lead to a decrease in gain and stability. Therefore, circuits using neutralizing capacitors experience a decrease in stability. If common-mode oscillation occurs in the circuit, it will still not function properly. Thus, it is necessary to improve common-mode stability when using neutralizing capacitors.

[0056] Therefore, for the differential amplifier system of the present invention, a common-mode resistor R1 needs to be added to improve the stability of the system in common-mode mode.

[0057] If the differential amplifier in this system has a common-mode oscillation risk, then it can be assumed that... Figure 1 The common-mode signal is input in the direction of the boundary line B. The real and imaginary parts of its common-mode input impedance are both negative (if the real part is positive, it is absolutely stable, so if there is common-mode oscillation, the real part must be negative; and according to formula (4), the imaginary part of its input impedance is negative). Therefore, in the narrow band range, it can be equivalent to a resistance value of -R. in resistor in series with capacitor C in Assume that the inductance values ​​of inductors L3 and L4 are equal and are L, then from Figure 1 The common-mode signal is input in the direction of the boundary line A, and its common-mode input impedance can be equivalent to an inductor with an inductance of L / 2 connected in series with a capacitor C1 and a resistor R1. Therefore, the common-mode series oscillation model is as follows: Figure 7 As shown. It is easy to see that when (R1-R) in When )>0, the link does not meet the oscillation condition, which shows that the common-mode resistor R1 can improve common-mode stability.

[0058] The present invention has the following effects:

[0059] 1. In differential mode: by adding capacitor C f Cancel parasitic capacitance C gd This results in a decrease in differential-mode gain and a reduction in stability.

[0060] 2. Compared to other methods of increasing common-mode stability (such as adding a series resistor in the transistor gate), since the center tap of the transformer is AC ground, the differential-mode signal of the differential amplifier circulates inside the amplifier. The common-mode resistor R1 is not in the circulation path of the differential-mode signal. Only the common-mode signal passes through the path of the transformer center tap. Therefore, the use of common-mode resistor technology will not affect the differential-mode performance.

[0061] 3. R1 only needs to be greater than R in That is, but due to the influence of parasitic capacitance, R in The value of R is generally small, and for power amplifiers, which have the most serious stability problems, the transistor size is relatively large. in The value of the common-mode resistor is smaller than that of other amplifiers, so the required resistance value is extremely small. Therefore, using common-mode resistor technology will not occupy too much layout area.

Claims

1. A differential amplifier system, characterized by The application relates to an input matching network and a differential amplifier; a first output end of the input matching network is connected with a first differential input end of the differential amplifier, and a second output end of the input matching network is connected with a second differential input end of the differential amplifier. The input matching network comprises an inductor L1, an inductor L2, an inductor L3, an inductor L4, a capacitor C1 and a resistor R1. One end of the inductor L1 is used as a reference voltage input end RFIN, and the other end of the inductor L1 is connected with the inductor L2; the inductor L1 and the inductor L2 constitute a primary coil; one end of the inductor L3 is used as a first output end of the input matching network, and the other end of the inductor L3 is respectively connected with one end of the inductor L4 and one end of the capacitor C1; the other end of the inductor L4 is used as a second output end of the input matching network; the inductor L3 and the inductor L4 constitute a secondary coil; the other end of the capacitor C1 is connected with one end of the resistor R1; the other end of the resistor R1 is grounded. The differential amplifier comprises an NMOS transistor M1 and an NMOS transistor M2; a gate of the NMOS transistor M1 is used as a first differential input end of the differential amplifier; a gate of the NMOS transistor M2 is used as a second differential input end of the differential amplifier; a source of the NMOS transistor M1 is connected with a source of the NMOS transistor M2 and is grounded; a drain of the NMOS transistor M1 is used as a first output end of the differential amplifier; and a drain of the NMOS transistor M2 is used as a second output end of the differential amplifier. The resistance R1 is greater than R in where R in is the input impedance real part of the differential amplifier.

2. The differential amplifier system of claim 1, wherein, A first neutralizing capacitor C is provided between the second differential input of the differential amplifier and the first output of the differential amplifier f A second neutralizing capacitor C is provided between the first differential input of the differential amplifier and the second output of the differential amplifier f .

3. The differential amplifier system of claim 2, wherein, Specifically, the capacitor C1 and the resistor R1 are connected in sequence at a middle tap of the secondary coil of the input matching network, AC signals are filtered through the capacitor C1, and a resonance cavity starting condition is destroyed through the resistor R1, so that common mode stability is improved.

4. A method of improving common mode stability of a differential amplifier system according to claim 1, characterized by, ​

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