Monday, 26 September 2016

Dirty Little Secret MOSFET 3.3V version: LTSpice analysis

Why JFET and not MOSFET?

JFET is a rare bird these days, you just have to go to a distributor and check the number of references of each of them, for example at Digikey there are less than 900 references of JFET devices and more than 40K of MOSFET devices. Transistor curves are quite similar, but it's much easier to find a proper MOSFET device that works well at 3.3V, N-JFET devices are biased with Vgs<0, while MOSFET devices are biased with Vgs>0. There are lots of MOSFET devices called "logic level gate" with low Vgs biasing values that are easy to bias from 3.3V power, while the offer of JFET devices that can work at 3.3V is quite limited, since as we saw in a previous post, JFET devices with low pinch-off voltages, and close to 0 Vgs have to be chosen.

These are the typical curves Id vs Vds for different Vgs values of Fairchild  FDV301VN:

The basic MOSFET amplifier only has one transistor compared to the 2 used on JFET circuit, because it is easier to bias the device:
The small signal frequency response shows a maximum gain of 37.5dB at 1kHz and 37.4dB at 10Hz for bias potentiometer set at 0.3

Dirty Little Secret 3.3V MOSFET version

The figure below shows schematics of the DLS MOS 3.3V circuit using Fairchild FDV301N:

The figure below shows the time response for a decaying 600mVpp 440kHz sinewave and gain from 0 to 1 in 0.1 steps:
The figure below shows the frequency response with gain from 0 to 1 in 0.1 steps and tone controls set with bass=0.3, mid=0.6 and treble=0.6:
The figure below shows the frequency response with gain=0.5, bass=0 to 1 (0.1 steps), mid=0.6 and treble=0.6:
The figure below shows the frequency response with gain=0.5, bass=0.3, mid=0 to 1 (0.1 steps),  and treble=0.6:
The figure below shows the frequency response with gain=0.5, bass=0.3, mid=0.6 and treble=0 to 1 (0.1 steps):

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