LTspice mosfet simulation, what am I doing wrong?

[acehobojoe]

I am attempting to model the circuit shown here with a mosfet. The specific one I am using is a BS170, I found a 2n7002 in the list of actual transistors ltspice has, which should be the same.

I'm only getting about 1db of gain or less, and I know that when I use this in a mosfet gain stage, much like the "super hard on", AMZ mosfet booster, or the  peppermill drive.

I have tested it, and it seems like the actual gain I have on my circuits is 10db or more.

here is the pic. Any help is appreciated.

Also .... I think it went from -6 to + 6 db, so that's the correct amount I think.. anyone see any problems with it though?

http://i.imgur.com/TvsQLHB.png

[samhay]

Try making R1 smaller.

[anotherjim]

Is the DC bias on the output near 4.5v in the sim? If it's offset either way, the input signal x gain could be clipping which might well give a lower gain. The bias in the sim could well be different from a real circuit. Try a lower test signal, 0.2 to 0.3v nearer to real guitar

Have played with this kind of thing myself. I found the best biasing method to get the output to half supply volts was with a voltage divider feeding the gate via 1Meg. 33k top and 100k bottom resistors in the divider gave close to 4.5v DC level on the drain. YMMV of course.

[PRR]

> only getting about 1db of gain

Your plot shows 6.8dB gain.

Agree that DC voltages are a MUST.

The Source resistor R4 may be reduced (without limit) to increase gain (but it will never be monster gain).

Do not believe SPICE's .AC analysis, do a .TRAN (run a sine wave in and see what wave comes out).

[mth5044]

I wish I could help - I ask myself that same question every time I open up the program 

[acehobojoe]

What is the code for a sweep?

[bool]

.STEP param x No-from No-to No-stepsize

fe.:

.STEP param Re 100 1k 100


and change your emitter resistor value to {Re}

... you'll get the hang of it eventually

[mac]

You can add a basic guitar circuit at the input:
a series inductance of about 2H - 3H and coil resistance around 5k - 7k,
a 250k to gnd, the volume pot,
and a 220p - 330p capacitor to gnd to simulate cable capacitance.
See the spice forum for examples. Gus simple model works great.

mac

[mth5044]

^+10000, take a look at Gus's schematics for lots of tips.

[acehobojoe]

Perfect.

[tca]

Try TINA-TI: http://www.ti.com/tool/tina-ti

Much simpler and very simple!