Proyecto estándar

a ] i m a A r e a = a r e a p m T OM Area=areafim n R e g i o n V d s = 2 . 0 0 V - I n p u t R e f e r r e d 3rd H a r moni.on R e g i o n V d a = 2 , 0 0 V - I n p u t R e f e r r e d 3 rd Harmor R L - 2 0 0 v g s (lin) - 1 . 6 5 10 : -MGF1400 C u r t i c e A r e a = a r e a p m 0,5 -150 - 2 0 0 v g s (lin) - 1 . 6 5 10 0.5 r e apm M o d S a t u r a t i o n R e g i o n V d s - 2 .O O V - I n p u t R e f e r r e a i o n V d s = 2 . 0 0 V - I n o u t R e f e r r e d 3 rd H a r m o n i c 1 0 0 0 0 . - 2 0 0 -5 a %00 -50 *^00 -150 -0.5'"^ v g s (lin) -1. 6 5 100.5 - 2 0 0 v g s ( i1n ) -150

Fig 5.7 Sim ulated variation o f the 3rd Harm onic w ith gate bias and load resistance o f the M G F1400 Saturated Region (V ds=2V) for (a) T ajim a (b) TOM (c) C urtice (d) M aterka.

5 . 7 Sum m ary

A wide range of distortion nulling phenomena are possible with a 2D transconductance nonlinearity which are a strong function o f both bias and load resistance. These include no nulls, narrow deep null, shallow null, wide deep null, and double narrow nulls.

The observed distortion behaviour o f a device provides a stringent assessment o f model quality. The Parker Skellem MESFET model reliably predicts the form o f observed distortion nulling phenomena, but does not always correctly predict the position, particularly at high bias and load resistance in the saturated region. Other popular simulator

models appear considerably less reliable, giving tolerable performance only in the triode region.

In the measured distortion o f the MGF1400 operating in its saturated region, it was noted that one value o f load resistance led to simultaneous minimisation o f 2nd and 3rd order intermodulation distortion in the bias region associated with minimum noise. This feature is important for Low N oise Amplifier design.

5 . 8 R e f e r e n c e s

1 A.M.Crossmim, S.A.Maas, Minimisation of Intermodulation Distortion in GaAs MESFET Small Signal Amplifiers, IEEE MTT Vol. 37, No. 9, September 1989.

2 A.E.Parker, Implementing high-order continuity and rate dependence in SPICE models, lEE Proc.-Ccts Dev Syst,, Vol 141, No. 4, Aug 1994.

3 A. E. Parker and J. B. Scott, “Intermodulation Nulling in GaAs MESFETs”, lEE Electronics Letters,

6

A Derivative Based MESFET Synthesis

Technique Exploitine Real Device Behaviour

In this chapter we look at how we can use the observed nonlinearity o f a MESFET presented in Chapter 5 section 5.5 to design a new synthesis technique. The synthesis is unique in as much it does not use an explicit equation to describe the device behaviour, but the derivative structure. In Chapter 5 section 5.6 w e have shown that all the common simulator models except the Parker Skellem model have incorrect derivative structures due to the omission o f the soft pinchoff behaviour. In Chapter 5 section 5.5 w e have shown the Parker Skellem model has the correct form to the derivative structure, but does not give a perfect match to actual device behaviour. The derivatives can be obtained from measured device behaviour. Hence derivatives derived from measured MESFET behaviour would be the ideal route, with the Parker Skellem model providing an acceptable altemative route.

W e begin by considering what the ideal derivative stmcture o f a selected range o f functions would be, given that a real device must pinch off. W e then demonstrate how the realistic derivative stmcture obtained from the Parker Skellem model can be used to synthesise approximations to the ideal derivative stmctures. Finally w e show how derivative

Siibthreshold Ideal Linear Subthreshold Ideal Mixer

S3 A

82 A

8 l / \

''o ff ''gs ''o ff ''gs

Fig 6.1 (a) Derivatives of an Ideal Amplifier (b) Derivatives of an Ideal