D.R. Webster

Control of circuit distortion by the derivative superposition method [MMIC amplifier]

The derivative structure of the characteristics of GaAs FETs naturally gives rise to changes in magnitude and reversals of phase of intermodulation distortion components. An MMIC design method that exploits the phase reversal to achieve control of distortion in an amplifier is presented. An example circuit is designed and its measured performance is compared with that of a conventional amplifier.

Low-distortion MMIC power amplifier using a new form of derivative superposition

Reduction of interchannel interference produced by a power amplifier near 1-dB compression is a key concern for the wireless communications industry. In this paper, we present a 100-mW monolithic-microwave integrated-circuit (MMIC) power amplifier designed using a novel form of the derivative superposition method. The measured results of the MMIC power amplifier showed a two-tone carrier-to-interference (C/I) ratio of 45 dBc with an efficiency of 22.5% when backed off by 4.5 dB from the 1-dB compression point. We demonstrate that the MMIC power amplifier represents a good compromise between C/I ratio, output power, efficiency, and gain at the cost of an increase in total gate width, by comparing it to class-A, class-AB, and class-B single field-effect-transistor amplifiers.

Novel circuit synthesis technique using short channel GaAs FETs giving reduced intermodulation distortion

We present graphically a novel FET synthesis technique based on the derivative structure observed in real GaAs MESFETs. The synthesis allows the generation of continuously differentiable linear or nonlinear transfer characteristics.

High-pass lumped-element transmission lines

In this work the high-pass lumped-element transmission line is shown to have a number of useful advantages over the classic low-pass line including DC isolation between sections, lower loss per section, and a much higher maximum frequency of operation, far beyond the natural self-resonant frequencies of the passive components used.

Effect of model derivative discontinuities on cold FET distortion simulations

This work presents a description of the nonlinear behaviour of the cold FET. A characterisation technique is described together with typical measurements. The work describes key weaknesses in popular simulator models that prevent realistic simulation of 3rd order intermodulation distortion performance of cold FET attenuators, mixers and switches. This includes a previously undescribed derivative discontinuity peculiar to the cold FET condition.

Factors influencing intermodulation distortion performance of a FET

This work describes some of the factors which influence the intermodulation distortion performance of a FET. These factors include bias point, voltage gain, sample to sample variation and gate length. The work also shows how it is sometimes possible to exploit these factors to minimise intermodulation distortion for a given device whilst maintaining output matching.

Total charge capacitor model for short-channel MESFETs

Total charge models for short-channel metal-semiconductor field-effect transistors (MESFETs) currently available in large-signal simulator packages such as SPICE give a poor description of the bias dependence of the gate capacitance. The authors describe a novel empirical model whose structure is deduced from the reconstructed total charge function derived from measured S-parameters. The new model gives a good overall fit to the observed gate capacitance behavior of a commercial monolithic microwave/millimeter-wave integrated circuit (MMIC) MESFET over a wide range of bias conditions and enables large-signal MESFET models to have a superior fit to measured S-parameters, giving greater confidence in design.

Observations on the nonlinear behaviour of single and double InGaP/GaAs HBTs

This paper explores the bias and voltage gain dependence of the small signal intermodulation distortion performance of the single and double InGaP HBT. It was found that the HBT has a zero in its 3rd order intermodulation distortion at low collector currents caused by the emitter resistance. The collector current at which this occurs can be approximately calculated with a simple formula. The small signal intermodulation distortion was little affected by collector voltage or voltage gain. There was very little difference between the small signal intermodulation distortion performance of the single and double HBT. Through a Volterra analysis it is demonstrated that the zero in 3rd order intermodulation distortion can be moved to a higher collector current by reducing emitter resistance.

Distortion compensation of multi-MESFET circuits

We present an analysis of a 4-FET Transconductor at medium frequencies, using a more realistic GaAs MESFET model, to calculate optimum device widths to minimise 2nd order distortion. Provisional analytical results are compared with SPICE simulations using the Parker Skellern model. A multi-FET Frequency Doubler is similarly analysed to calculate optimum device widths for high fundamental rejection and optimum load for 3rd order distortion nulling. The high frequency degradation of the 4-FET Transconductor is discussed. Simulated performance of two high frequency distortion compensated Linearised Isolator designs, together with a variation of the Frequency Doubler, show good performance at high frequencies.<<ETX>>

Effect of circuit parameters and topology on intermodulation in MESFET circuits

Third-order intermodulation intercept is a very important criterion in communication circuits and the authors present a preliminary investigation of the mechanisms responsible. It is shown that interaction of the intrinsic MESFET second-order nonlinearity with linear circuit and parasitic components can cause significant distortion. MESFET output conductance is shown to also cause an interaction effect. Hence, intercept points quite different from those predicted by standard device characterization techniques can be obtained. Some regions of enhanced performance have been identified. Circuit topology also plays a key role in determining distortion. Some experimental results are given.<<ETX>>