A novel interpretation of transistor S-parameters by poles and zeros for RF IC circuit design

Abstract

In this paper, we have developed an interpretation of transistor S-parameters by poles and zeros. The results from our proposed method agreed well with experimental data from GaAs FETs and Si MOSFET's. The concept of source-series feedback was employed to analyze a transistor circuit set up for the measurement of the S-parameters. Our method can describe the frequency responses of all transistor S-parameters very easily and the calculated S-parameters are scalable with device sizes. It was also found that the long-puzzled kink phenomenon of S-22 observed in a Smith chart can be explained by the poles and zeros of S-22

An analysis of the kink phenomenon of scattering parameter S22 in RF power mosfets for system‐on‐chip (SOC) applications

“…Up to now, two different origins of the S 22 kink phenomenon in deep sub-micron RF MOSFETs have been reported [5][6]. Hjelmgren, et al attributed the phenomenon to the small signal-substrate resistance [5], while Lu, et al concluded that it resulted from the transconductance and, consequently, the size of the transistor [6].…”

“…A comparable full wave simulation using Ansoft's HFSS requires more than 100 hours to accurately model the frequency domain behavior of the whole structure. [6]. In this paper, based on the results of RF power n-MOSFETs for system on chip (SOC) applications, the third origin of the S 22 kink phenomenon in deep sub-micron RF MOSFETs, that is, small-signal gate-drain resistance, is reported and explained quantitatively for the first time.…”

“…The formula shows that the output impedance of the RF power n-MOSFETs follows a "shifted" constant resistance r circle at low frequencies, and then a "shifted" constant conductance g circle at high frequencies, which is in agreement with the experimental results. In addition, the kink effect in scattering parameter S 22 of RF power n-MOSFETs can also be interpreted in terms of poles and zeros [6]. That is to say, according to the two-pole approximation, S 22 have two poles and two zeros.…”

“…For simplicity, all the inductors in Figure 3(a) are temporarily neglected. From local series-series feedback theory, the circuit of Figure 3(a) can be transformed into that of Figure 3(b) with some necessary circuit element modifications [6], which are also shown in Figure 3(b). The expression of intrinsic output impedances Z out,i can easily be obtained from local shunt-shunt theory and is given by…”

“…(4)-(9) r increases and g decreases and, hence, the kink point moves to a new intercept point with a larger r, a smaller g, Figure 4 Equivalent circuits of the output impedance: (a) at low frequencies, the output impedance can be approximated by a "shifted" series RC circuit; (b) at high frequencies, with RЈ gs and RЈ gd negligibly small compared with 1/(sCЈ gs ) and 1/(sCЈ gd ), respectively, the output impedance approaches a "shifted" parallel RC circuit; (c) at high frequencies, with 1/(sCЈ gs ) ϩ RЈ gs ϩ 1/(sCЈ gd ) negligibly small compared with RЈ gd , the output impedance approaches another "shifted" parallel RC circuit (Figure continued and a smaller frequency, which makes the kink phenomenon more prominent, as shown in Figure 2. The kink phenomenon in S 22 of RF power n-MOSFETs can also be interpreted in terms of poles and zeros [6]. At dc frequency, S 22 is equal to…”