4. Results

4.1 Large gate area FET

The equivalent input noise voltage at 100 kHz (spot noise) and at 4.2 K for various bias points for MESFET A- is shown in Fig. 4-1 . Trends can clearly be seen. The noise goes up with increasing drain-source voltage Vds but goes down with increasing drain current Id. By analyzing the complete noise spectra it is found that the total lowest noise was achieved at an bias of Id = 4 mA and Vds = 1 V yielding a spot noise at 100 kHz of 1.2 nV/(Hz)^½.

Fig. 4-1. Spot noise at 100 kHz for various MESFET bias for the large MESFET. Although the spot noise has a minimum at 2 V and 2 mA the lowest noise spectrum was found at a bias of 1 V and 4 mA.

In Fig. 4-2 the pass band gain (defined as the gain between the lower and upper frequency cut-offs) is shown in a bias map. The gain goes up with increasing drain current as well as with increasing drain-source voltage. The peak gain is about 11000, and the gain at the bias point with lowest noise is 9000. The transconductance g_m of a FET usually goes up with increasing drain current. Since the gain of the cascode is proportional to the MESFET g_m (see section 2.2 ) it will therefore rise with increasing drain bias. The op-amp stages had a total gain of 400 giving the cascode a gain about 27.5 peak gain, and 22.5 at lowest noise bias (see below).

Fig. 4-2. Pass band gain at 50 kHz as a function of MESFET bias for the large MESFET.

Fig. 4-3(a). Noise spectra at the lowest noise bias for different temperatures for the large MESFET. The peaks below 1 kHz at 50 Hz and odd multiples thereof, comes from magnetic noise pick-up from the power lines via circuit ground loops.

Fig. 4-3(b). Frequency response at lowest noise bias for different temperatures for the large MESFET. A little dip was found above 30 kHz at room temperature.

A complete frequency response is shown in Fig. 4-4 from which the upper cut-off frequency (defined in the same manner as the lower cut-off) can be determined to 700 kHz.

Fig. 4-4. Total frequency response curve at 4.2 K for the large MESFET. The horizontal line shows the -3 dB level from the maximum gain. The lower and upper cut-off frequencies are found at the intersection of the -3 dB line and the frequency response and are 390 Hz and 700 kHz respectively. The dip seen at 700 kHz is of unknown origin.

4.2 Small Gate Area FET

The small MESFET showed spurious noise bursts for a large number of bias points. The bursts made the amplifier unusable and were found at drain currents larger than one mA. Therefore the MESFET had a very limited operational bias range and only three points could be measured. The spot noise at 100 kHz and at 4.2 K for these points are shown together with in Fig. 4-5 . The lowest noise was 2.8 nV/(Hz)^½ obtained for the bias Id = 1 mA, Vds = 0.7 V. The noise goes up with increasing Vds. The gain is shown in Fig. 4-6 . The pass band gain at lowest noise bias point is 5700. The gain goes up for increased Vds. Overall, the amplifier performed poorly with this MESFET.

Fig. 4-5. The spot noise at 100 kHz and 4.2 K for the small MESFET. Only bias points without bursts were measured.

Fig. 4-6. Pass band gain for various bias points at 4.2 K for the small MESFET.

The anomalous noise fluctuations in the output voltage were observed for the smaller MESFET for drain currents greater than 1 mA. This resulted in a jumping noise spectrum. The fluctuations came randomly with an intermittencity ranging form 15 to 30 seconds. In Fig. 4-7 , a burst-like character of the noise fluctuation is seen clearly. Such fluctuations were also reported by Lee [5] .

Fig 4-7. A typical noise fluctuation observed at the output for large MESFET drain currents in the small MESFET. The noise background was otherwise low as can be seen in the figure. The bursts had large low frequency components making the amplifier unusable.

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