Numerous topology of UWB LNA

The noise performance of an LNA is directly dependent on its input matching. The wide-band input matching is intrinsically noisier than narrow-band counterparts as the noise performance can not be optimized for a specific frequency. Thus the designer has to be trade off between

the input matching and noise. The Fig. 3.2.1 shows four basic 50Ω input matching techniques[5][6][7][8][9].

Fig. 3.2.1 Basic input matching topology. (a)Inductive source degeneration.(b)Direct resistor termination.(c)Shunt-series feedback.(d)Common-gate 1/gm termination

The four input matching is only suit for narrow band amplifier. In wireless mobile communications systems, silicon integrated circuits have been widely employed in narrow-band systems, where limited gain and increased parasitic is tolerable due to lower operation frequency and the application of tuned networks.

There are few examples of development of high-frequency wideband amplifiers employing silicon transistors, in particular in CMOS technology.

z Distributed amplifiers [10]

The Fig. 3.2.2 shows a basic four-stage single-ended distributed amplifier.

Fig. 3.2.2 Basic four single-end distributed amplifier

The distributed amplifiers normally provide wide bandwidth characteristics but they consume large dc current due to the distribution of multiple amplifying stages, which make them unsuitable for low-power application. And the distributed amplifiers are not optimized for noise. This bring the challenge of finding a low-power topology that satisfies all the other design requirements, the most stringent one being the input match.

z Resistive shunt-feedback amplifiers

In Fig. 3.2.3 a conceptual schematic of a shunt feedback amplifier is shown[11].

Fig. 3.2.3 Conceptual schematic of a shunt feedback amplifier.

In the resistive shunt-feedback amplifier, input resistance is determined by the feedback

resistance divided by the loop-gain of the feedback amplifier. The input impedance is

resistive shunt-feedback amplifier has better bandwidth than conventional low noise amplifier but it has limited input match at higher frequencies due to the parasitic input capacitance. As consequence, the maximum input capacitance that can be tolerated to achieve an input reflection coefficient equal to -10dB at ƒ = 10 GHz is as low as

(

)

(

)

= R f

C π _s =200fF. To satisfying this requirement with a CMOS amplifier stage while achieving sufficient gain and low noise is difficult. Some paper has been present by using this topology over the low band of UWB(3-5GHz)[12] shown in Fig.3.2.4.

Fig. 3.2.4 UWB LNA topology. (a) Overall schematic. (b) Small-signal equivalent circuit at the input.

In this topology, one of the key role of the feedback resistor Rf is to reduce the Q-factor of the resonating narrowband LNA input circuit. The Q-factor of the circuit can be approximately given by

. According to this equation, the

narrowband LNA input matching can be converted into a wideband amplifier by the proper

selection of Rf. The feedback resistor helps to extend the bandwidth of the amplifier as well as the gain flatness, while contributing a small amount in NF degradation.

z Ultra-wideband low noise amplifier using LC-ladder filter input matching network [11][13]

Recently another topology of wideband LNA has been present. It expands the conventional narrow-band LNA using source degeneration by embedding the input network of the amplifying device in a multisection reactive network so that the overall input reactance is resonated over a wider bandwidth. Fig. 3.2.5 shows a typical narrowband cascade LNA topology and its small-signal equivalent circuit.

Fig. 3.2.5. Narrowband LNA topology. (a) overall schematic. (b) Small-signal equivalent circuit at the input The inductor Ls is added for simultaneous noise and input matching and Lg for the impedance matching between the source resistance Rs and the input of the narrowband LNA[14]. Fig.

3.2.5(b) shows the equivalent small-signal circuit. Assume the gate-drain Cgd can be ignore the impedance of the gate terminal is a series RLC circuit. The reactive part of the input impedance is resonated at the carrier frequency in narrowband design. The basic concept of the LC-ladder input matching is expanded from the input impedance of the narrowband which is a series RLC circuit. Consider a fourth-order bandpass ladder filter, shown as in Fig. 3.2.6.

Fig. 3.2.6 Fourth-order bandpass ladder filter used for impedance matching.

The right part of the bandpass filter looks similar to the equivalent circuit of the inductively degenerated transistor in Fig. 3.2.5(b). Therefore, the bandpass filter can embed the inductively degenerated transistor and obtain the desire input impedance. The LC-ladder filter input matching of wideband LNA has two significant drawbacks. Because the LC-ladder filter at the input mandates a number of reactive elements, which could lead to a larger chip area and noise figure degradation in the case of on-chip implementation.

z Ultra-wideband low noise amplifier using the common-gate as the first stage.[15]

The common-gate architecture that is illustrated in Fig. 3.2.1(d) has highest potential to achieve the wide-band input matching. In traditional narrow-band receiver the common-gate is not used widely due to its relatively lower gain and higher noise figure than a common-source amplifier. The actual configuration of common-gate stage is shown in Fig.

3.2.7(a).

Fig. 3.2.7 (a) Configuration of a common-gate input stage. (b) The small-signal equivalent circuit.

From the Fig. 3.2.7(b),we can derive the input impedance inductors and capacitors and can be regarded as purely reactive within the frequency band of

interest. // // ( )

After some mathematical calculation

))

Since gm1Xo2(ω)<<Ro2+Xo2(ω),the real part in the denominator will remain relatively constant within the 3.1-10.6GHz UWB band. The imperfect matching of the common-gate stage throughout the band arise from the frequency dependent Xs(ω) that dominates the imaginary part in the denominator. To get a good matching over the wide band , the LC tank of Xs(ω) formed by Ls and Cgs should be selected such that they resonate at the center of the 3.1-10.6GHz, leaving only a 50Ω real input impedance. The noise figure of the common-gate input stage UWB LNA can be improved by increasing gm1 but it will degrade the input matching.

z Reactive-feedback wideband amplifier [16][17]

An alternate approach to the modified UWB LNA is a negative-feedback amplifier. Feedback offers numerous benefits for broadband application, including gain, stability over processing and supply variations, lower distortion and the ability to get port impedances for noise and impedance matching. However the simple feedback stage is difficulty to achieve a 50Ω input match, low noise figure and low power consumption in simultaneous. Broadband resistive feedback also meets the bandwidth target, but the reactive-feedback using transformer contributes less noise as losses are relatively small in the transformer windings. The

schematic shown in Fig. 3.2.8 is present recently [16].

Fig. 3.2.8 Reactive feedback UWB LNA.

The transformer in the first stage give a broadband input match without multi-inductor network which will add noise and require more chip area. The current-reuse is also approved in this design for lower DC power consumption. The transformer of second stage provides a broadband output match relatively close to 50Ω so the output buffer is not need for output match.

z Wideband matching using the transistor intrinsic gate-drain capacitor [18]

Recently a novel wideband input match has been present. It considers the gate-drain capacitor has significant effect on the circuit performance. The Fig. 3.2.9 show a simple common source amplifier with source degeneration inductor and the drain loaded an equivalent capacitor and resistor from the next stage.

Fig. 3.2.9 The small signal equivalent circuit of common-source with inductive source degeneration

The Cgd and ro are neglected in conventional analysis of low noise amplifier. It is inaccurate numerically. If both Cgd and ro are considered we will find that the input match at high frequency is depend on the resistive load and at low frequency is depend on the capacitive load. We can achieve wideband match without external input match network. It also can achieve low noise match.