The STAN tool developed by AMCAD Engineering is a patented solution capable of assessing the stability of RF & Microwave circuits, which is a critical step in the design flow. In comparison with other methodologies (Rollet & Nyquist Criterion, NDF etc.), STAN is Fast, Rigourous, User Friendly, and ready to use with commercial CAD software to assess stability. The STAN approach calculates a single-input, single-output (SISO) transfer function for a circuit of interest linearized around a given steady state. A simulated frequency response of the linearized circuit is fitted to a rational polynomial transfer function by means of frequency-domain identification algorithm. If no poles on the right-half plane (RHP) are found, it is considered stable.
The fast evolution of wireless communication technologies requiring higher data transmission rates has forced a change in the way wireless communications systems are designed. Systems nowadays are expected to work with more complex modulated signals and wider bandwidths. Working towards 5G and beyond, and considering an already saturated spectrum in the sub 6-GHz frequencies, systems now need to operate at higher frequencies.
In the case of power amplifiers, more complex structures and technologies have to be used in the design flow to attain the specifications in terms of efficiency and linearity at high frequencies. Monolithic Microwave Integrated Circuits (MMICs) are one of the solutions to achieve the challenging specifications of new applications while allowing smaller sizes and higher frequencies.
Moreover, the Gallium Nitride (GaN) technology provides high-frequency and high-power performance at a lower cost in a still maturing but already established process.
When designing MMICs, stability analysis becomes more relevant because circuits cannot be adjusted after fabrication. Therefore, a thorough and reliable stability analysis is necessary to successfully achieve circuit design specifications. The efficient detection of instabilities in wideband GaN MMIC Power Amplifiers is illustrated by AMCAD Engineering via the PHD dissertation of Dr. Iban Barrutia Inza, University of Cantabria, Spain. Thanks to the stability analysis with the STAN Tool under linear and nonlinear conditions and its factor from the SISO method, instabilities can be easily detected and stabilization can be achieved when other methods fail to provide accurate solutions.
A challenge to solve
Designing PAs able to work from a few GHz up to mmWave frequencies and provide high values of output power can be very demanding; complex structures and amplifying cells must be used to achieve the specifications. The wideband GaN MMIC PA presented in the PhD dissertation had to fulfill the following requirements (Table 1) using the technology D01GH from OMMIC:
Therefore, to attain >30 dBm of saturated output power in the 39 GHz carrier bandwidth, a distributed configuration has been selected, and the design will contain two stages: a gain stage with a power stage connected at its output (Fig.1).
Fig. 1
Because of the high gain and output power and the working frequencies expected, the active devices will form the amplifying cell in a cascode configuration. The power stage will have four cascode amplifiers that will present 8 dB of gain and 31 dBm of output power. The gain stage will have 6 cascode amplifiers presenting a gain of 12 dB and 23 dBm of output power (Fig. 2).
Fig. 2
Different tools for the stability analysis of RF and microwave circuits are available in commercial simulators, with the Rollett factor, K, and the factor being the most used. However, these methods are only valid in linear conditions for a two-port network that is intrinsically stable and exhibits negative resistance at its ports for any value of passive source or load impedance. Moreover, they are not appropriate for a reliable stability analysis of multistage PAs with complex structures where internal stability is more difficult to ensure. To cope with the stability analysis of complex multistage power amplifiers, the STAN Tool proposes a more powerful approach.
Thanks to its unique pole-zero analysis in linear and nonlinear conditions, the STAN Tool provides more insight into the circuit dynamics, giving information about the nature of the oscillation and where in the circuit it is happening. The parametric and multi-node analysis available in the STAN Tool enables efficient detection of instabilities in wideband MMIC power amplifiers.
Stability Analysis
First, the stability analysis of the wideband GaN MMIC power amplifier was carried out at each stage separately (Fig. 3). For the small-signal stability analysis, both the Rollett factor and the STAN Tool analysis have been used, and results will be compared.
Stability analysis of the power stage
The small-signal stability analysis of the power stage is performed over the whole 0.01 GHz–60 GHz operation bandwidth. The amplifying cell in cascode configuration is connected as shown in Fig. 4.
Results from the small-signal stability analyses available in the commercial simulator, the Rollett factor K and the factor, provide unconditional stability throughout the whole bandwidth (K in red and in blue in Fig. 5 (a)). A small-signal stability analysis using the STAN Tool was carried out under the same conditions. However, the pole-zero map from the stability analysis with STAN Tool clearly detects an instability: analyzing the value of the factor obtained from the residue analysis of the SISO method [3], an instability around 400 MHz is detected with > 1 (Fig. 5 (b)).
Large-signal stability analysis
Large-signal stability analysis is not usually performed in the literature of MMIC design, and it is a key step in the design process; instabilities may arise from the new steady state given by the increase in the input power.
The stability of the power stage has been fully verified through parametric and multi-node analyses. Thus, different input power values from 5 dBm up to 20 dBm have been considered, and the perturbation source has been connected to different nodes of the circuit. As can be seen in Fig. 6, the power stage is stable in large signals once the Cstab has been connected.
Fig. 6
The stability analysis in nonlinear conditions can be performed in STAN Tool following the same procedure as in the small signal but using harmonic balance simulations to extract the frequency response of the circuit.
Once the circuit has been stabilized in small and large-signal conditions, the power stage is simulated to analyze its performance. As shown in Fig. 7, circuit specifications have been achieved with PSAT > 31 dBm and G > 8 dB throughout the whole operation bandwidth.
Fig. 7
Stability Analysis of the Gain Stage
The process described above for the stability analysis of the power stage has also been applied to the gain stage. Thus, when performing the small-signal stability analysis with the STAN Tool, a low-frequency instability is clearly detected around 600 MHz at the equivalent node of that of the power stage. This instability is not detected by the Rolett factor as in the power stage (Fig. 8).
Fig. 8
Stability analysis of the wideband GaN MMIC power amplifier Once the stability of the two stages has been analyzed individually, the stability analysis of the wideband GaN MMIC power amplifier is performed. Thus, small signal and parametric, multi-node, large-signal stability analyses have been carried out; the circuit is stable (Fig. 9). The stable gain stage has also attained its specifications in simulation: PSAT > 23 dBm and G = 12 dB in the whole band as shown in Fig. 10.
Fig. 9
Stability analysis of the wideband GaN MMIC power amplifier
Once the stability of the two stages has been analyzed individually, the stability analysis of the wideband GaN MMIC power amplifier is performed. Thus, small signal and parametric, multi-node, large-signal stability analyses have been carried out; the circuit is stable (Fig. 11).
Fig. 10
Fig. 11
As shown in Fig. 12, the stable wideband GaN MMIC power amplifier presents the desired 20 dB of gain and a PSAT > 30 dBm.
The wideband GaN MMIC power amplifier has been manufactured and measured. There is a good agreement between simulation and measurement results: the gain of the amplifier is around 20 dB and the ripple is smaller than 2 dB. Moreover, the mean value of the output power PSAT is above specifications up to 37 GHz, where it falls to 28 dBm for higher frequencies.
Fig. 12
Conclusion
The methodology for the efficient and reliable detection of instabilities and stabilization of wideband GaN MMIC power amplifiers has been described. Thus, thanks to the STAN Tool, instabilities have been easily detected, even those that other widely used methods fail to provide. The STAN Tool enables small-signal and large-signal stability analysis in a rigorous and intuitive manner. Thanks to the factors extracted from the residue analysis of the SISO method and the parametric analysis available in the STAN Tool, the origin of the oscillation can be easily detected, helping the designer to decide about stabilization techniques and to choose the best combination of values to achieve the expected circuit performance. STAN Tool capabilities have been illustrated through the MMIC power amplifier in GaN technology working in the band 1 GHz–40 GHz with an output power of > 30 dBm, as presented in the PhD dissertation of Dr. Iban Barrutia Inza, University of Cantabria, Spain. The stability analysis of such complex designs is a real challenge. The STAN Tool has been proven to be a unique solution that helps designers get the best performance from stable circuits.
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