With the development of millimeter-wave （mm-wave） theory and technology，modern testing and measurement instruments should possess higher frequency response and precision to meet the demands of high-frequency signal measurement^［1-4］. The ultra-wideband low noise amplifier （UWB LNA） with the characteristics of wideband，high gain，and low noise，can amplify signals，and improve the accuracy and sensitivity of the measurement，which can play an important role in testing instruments such as oscilloscopes，spectrum analyzers，and vector network analyzers.

Previous works^{［1，3-8］} demonstrated UWB LNA in W-band. However，the bandwidth or the gain flatness is not satisfying in the whole W-band. Due to the low single-stage gain in W-band，cascading multi-stage to achieve appreciable gain is a common solution. In Refs. ［1］ and ［6］，four identical stages are cascaded to achieve high gain in W-band. However，the gain flatness is not good in the whole W-band. In Ref. ［7］，the amplifier is composed of a three-stage input stage and a balanced two-stage output stage to enhance the gain，but the gain flatness is up to 10 dB. Besides，few works discuss the impact of bypass capacitors in the broadband amplifier design.

In this paper，a four-stage UWB LNA covering the whole W-band is proposed for instrument applications. A bypass circuit composed of dual shunt capacitors is proposed to provide a wideband RF grounding，which can reduce the inter-stage crosstalk across the four stages. A dual-resonance input matching network is designed to implement wideband input matching and noise matching. Besides，the gain of each stage is matched at different frequencies in the frequency band to achieve wideband performance.

The proposed UWB LNA is designed with the WIN semiconductor GaAs PP10-20 technology. The schematic of the LNA is shown in Fig. 1. The LNA consists of four common source （CS） stages. The transistor gate width is 2×25 μm in all stages. The drain bias voltage V_dd and the gate bias voltage V_g are 2 V and -0.3 V，respectively. The total current is 44 mA. The gate bias voltage is fed through a large resistor of 2 kΩ to prevent RF loss. 9-Ω resistors R₁ and R₂ are added to the drain bias path in the second and third stages to improve the stability at low-frequency bands.

The proposed UWB LNA is a single-ended topology. In the inter-stage matching network design of a single-ended amplifier，bypass capacitors are necessary to implement RF ground and reduce the inter-stage crosstalk. In the broadband amplifier design，the key basis is the broadband bypass RF ground. The short circuit is provided by the series resonance formed by the capacitor and the parasitic inductance of the ground back hole，as shown in Fig. 2（a）. In general，a single shunt capacitor can provide one resonance，as shown in Fig. 2（b）. However，the bandwidth of the RF isolation is limited when using the single shunt capacitor. In the work，dual shunt capacitors （see Fig. 2（a）） with different capacitances are proposed to provide a wideband RF isolation. As shown in Fig. 2（b），the bypass circuit composed of dual shunt capacitors can provide >30 dB isolation in the whole W-band.

The degeneration inductor is utilized at the first stage to increase the real part of the input impedance，and make the optimum noise and gain impedance closer^［5-6］. The input matching network is designed with dual resonance and provides a good compromise between the noise and gain matching. Terminated at 50-Ω resistor，the optimal noise source impedances and the conjugate values of the input impedances of the LNA at 75-110 GHz after input matching are shown in Fig. 3. The optimal noise source impedances are close to 50 Ω. The input matching network provides the dual resonance to implement the wideband input matching.

The simulated gain of each stage and the whole LNA is shown in Fig. 4. The first stage and the second stage are optimized for noise performance. The third stage and the fourth stage are matched for wideband gain performance. The simulated results show a peak gain of 17.6 dB at 93 GHz and a gain flatness of less than 1.5 dB in the whole W-band.

The die photograph of the proposed LNA is shown in Fig. 5. The LNA occupies an area of 1.85 mm² （2.1 mm×0.88 mm）. The S-parameters are measured via on-wafer probing using a Keysight N5245A vector network analyzer with V-band and W-band extenders. In the large-signal measurement，the input signals are generated by a signal source （Keysight E8257D） with ×6 multiplier module （OML S10MS） and an adjustable attenuator.

The measured and simulated small-signal S-parameters of the LNA are shown in Fig. 6（a）. The LNA exhibits a peak gain of 20.4 dB at 108 GHz. The measured small signal gain is 16.9-20.4 dB across 66-112.5 GHz. The stability factor is larger than 1 in the full frequency bands. The measured results show a good agreement with the simulation. The measured gain is slightly higher than the simulation，which might be caused by a shift of the reference plane at the source of the transistor model by a few micrometers.

In some broadband applications，the group delay performance of the amplifier in the device needs to be considered. The measured group delay shows variations of ±15 ps across the whole W-band，as shown in Fig. 6（b）.

The measured and simulated input 1-dB compression points （IP_1dB） are shown in Fig. 7. The measured IP_1dB is around -12 dBm in the whole W-band.

The noise figure （NF） of the LNA is measured by the Y-factor method^［9-10］. The NF measurement setup is shown in Fig. 8. To use the Y-factor method，an excess noise ratio （ENR） source is needed. A W-band mixer module operating at 80-100 GHz with an NF of around 4 dB and a 2-18 GHz LNA module are utilized to convert the RF signals to an intermediate frequency （IF） signal of 2 GHz. The IF signal is fed into the spectrum analyzer to obtain the output noise power density. Turning the noise source on and off，Y can be obtained，which is the difference between the output noise and power density. The ENR is the number given by the noise source. The NF can be calculated by：

The measured and simulated NFs of the LNA are shown in Fig. 9. The LNA exhibits 3.9 dB NF at 90 GHz. The measured NF is less than 5.1 dB across 75-100 GHz.

The performance comparisons with state-of-the-art V/W-band LNA in GaAs technologies are summarized in Table 1. The proposed UWB LNA exhibits wide bandwidth covering the whole W-band. The noise and linearity performances are also competitive in W-band LNA.

A UWB LNA fabricated by 0.1-μm GaAs pHEMT technology is presented. The dual shunt capacitor bypass circuit and the dual-resonance input matching network are proposed to achieve wideband performance. The UWB LNA exhibits 52.1% percentage bandwidth and the NF is less than 5.1 dB in W-band，which is suitable for instrument applications.