Structural Design of RF Low Noise Amplifier Circuit

1. RF LNA design requirements

As the first stage of the RF signal transmission link, the low noise amplifier (LNA) has a noise coefficient characteristic that determines the noise performance of the entire RF circuit front end. Therefore, the design of the first stage LNA of the high-performance RF receiving circuit must meet the following requirements:

(1)Higher linearity to suppress interference and prevent sensitivity loss;

(2) A gain high enough to suppress the noise of subsequent modules;

(3) Matching with input and output impedance, usually 50Ω;

(4) Low power consumption as possible, which is required by the development trend of wireless communication equipment.

2. Inductive degenerate cascode structure LNA

The inductive-degenerate cascode structure is one of the most commonly used structures in RF LNA design, because this structure can increase the gain of the LNA, reduce the noise figure, and increase the isolation between the input stage and the output stage, thereby improving stability. The inductive-degenerate cascode structure introduces two inductors, Lg and Ls, at the gate and source of the input stage MOS tube, respectively. By selecting an appropriate inductance value, the input loop resonates near the operating frequency of the circuit, thereby offsetting the imaginary part of the input impedance. In Figure 1, the input impedance of the LNA is:

(1)

When in resonance:

(2)

So:

(3)

The input impedance is purely resistive, and its value is determined by Ls and

From the analysis, we can see that the input impedance of the Inductive-degenerate cascode structure is a 50Ω real part, but this real part is not a real resistance, so it will not generate noise, so it is very suitable as the input pole of the RF LNA.

3. Highly stable LNA

The cascode structure is widely used in RF LNA design, but when the operating frequency is high, the parasitic capacitance Cgd of the MOS tube cannot be ignored, which makes the stability of the entire circuit worse. For a single transistor, the stability can be improved by connecting a small resistor in series at its input or a large resistor in parallel at its output, but since the newly added resistor will deteriorate the noise value, this technology cannot be used for low noise amplifiers.

The literature proposes an improvement to the cascode structure. On the basis of Figure 1, by connecting a small inductor Lg2 to the gate of the M2 tube, the stability of the circuit can be improved while the gain remains unchanged. At the same time, a small resistor is connected to the drain of the M2 tube to adjust the voltage gain as shown in Figure 2(a). (b) shows the small signal equivalent circuit, where Z1 represents the equivalent impedance of the omitted part. It can be seen that since the value of the parasitic capacitance Cgd2 of the M2 tube is relatively small, Lg2 can be almost ignored for the output impedance. Because the gain of the amplifier is equal to the ratio of the output impedance to the input impedance, the addition of Lg2 does not affect the gain of the LNA, and the voltage gain is:

(4)

Where ZLoad=jwLout//(jwCout)-1//Rout, Zs is the impedance of the source inductor LS.

The stability factor of the amplifier is [3]

(5)

Where Δ = S11S22-S12S21 (6)

The stability coefficient K can quickly provide a basis for determining stability. When K>1 and |Δ|<1, the LNA will be unconditionally stable. From formulas (5) and (6), it can be seen that if the reverse gain S12 decreases, the K value will increase, and the LNA will increase in stability. As can be seen from Figure 2(b), the inductor Lg2 and the capacitor Cgd2 of the MOS tube form a low-resistance path that allows the signal fed back from the output end to flow to the ground end, thereby reducing the reverse gain S12 and improving the stability of the LNA.

4. Bias current multiplexing structure

Modern wireless communication equipment requires smaller size, lighter weight, and longer standby time. This requires reducing the power supply voltage of the RF front end, so low voltage and low power consumption technology are urgently needed. From formula (3), we can see that when the input end is in resonance, Ls=RsCgs/gml, where Cgs is the capacitance between the gate and source of M1 tube in Figure 1, and gml is the transconductance of M1 tube. Then the noise coefficient of LNA is [4]:

(7)

From (7), we can see that increasing gml can reduce the noise factor. The cascode structure shown in Figure 1 can obtain a smaller noise factor, but it often requires a relatively large drain current Id, which increases the DC power consumption. Reference [4] proposed a bias current multiplexing technology, the basic idea of ​​which is: in order to save DC power consumption, the PMOS tube and the NMOS tube can be connected in series in the DC bias path. The structure is shown in Figure 3.

The width-to-length ratio and drain current Id of the single NMOS device shown in Figure 3(a) are both twice that of the single NMOS shown in (b), but since the two NMOS are connected in parallel, (a) and (b) have the same transconductance value gm. M2 in (c) is a PMOS tube and has the same width-to-length ratio as the NMOS tube in (b). Since the electron mobility of the PMOS device is slightly lower than that of the NMOS [2], gmc=(gml+gm2)m, that is, its transconductance value is slightly lower, and its input capacitance is similar to Cgs. From equation (7), it can be seen that the noise coefficient of the circuit structure (c) will increase slightly, but since the current is reduced by half, it can effectively reduce the power consumption of the circuit under the condition of a constant power supply voltage, which is conducive to the design of low-power LNA.