How much gain out of a Jack Orman's MOSFET booster?

Stompboxes circuits published in magazines, books or on DIY electronics websites.
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sopapo1
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Post by sopapo1 »

Hello Im not able to put put more than 14 db-15 db out of a jack orman mosfet booster, its biased at 5.3 v and Im using a bs170 ( with less volt the results are the same) and I have try a dozen, with the same results not near the 35 db that theoretically you can pull out of this...
Its for the tranny used? (bs170).. with a nte490 or 2n000 you can get more gain???
Thanks in advance

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graeme
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Post by graeme »

How about a circuit diagram and a list of voltages measured from your circuit? A picture or two would help otherwise it is impossible to help you.

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george giblet
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Post by george giblet »

Have you checked you pot dials down to zero ohms? See if soldering a big cap from the source to ground at the PCB makes the gain go up. Use a different cap than you are using now.

Some people have problems with the circuit oscillating. Try adding a 1k to 10k resistor in series with the input line (located at the PCB).

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graeme
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Post by graeme »

I ran this circuit on a simulator and got about 13dB at maximum gain setting. At this gain with a standard 200mV pp input signal the output was hitting the power rails.
Not sure why you think it would be capable of 35dB, that is the territory of a maxed out overdrive.

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george giblet
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Post by george giblet »

I ran this circuit on a simulator and got about 13dB at maximum gain setting.
FWIW, my sim shows about 30dB. I'm using a 2N7000 model that was tweaked to match AC and DC measurements for typical ID bias points.

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graeme
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Post by graeme »

george giblet wrote:
I ran this circuit on a simulator and got about 13dB at maximum gain setting.
FWIW, my sim shows about 30dB. I'm using a 2N7000 model that was tweaked to match AC and DC measurements for typical ID bias points.
You're right. My mistake. I was reading absolute levels rather than gain.
OK the gain on my simulator is 26dB which is about 25 - 26 times. A gain of 35dB would be 56 times.
As I said before a standard 200mV input signal would clip (using a 9V supply) the power rails at the max gain setting of this simple circuit.

This is only meant to be a simple (clean) booster. If you want more gain than 26dB then you will need to cascade several mosfets. Swapping different mosfet types won't help you much I don't think. They are not that high a gain device.

Cascading several mosfets will create lots of clipping so you have entered the realm of the overdrive. Have a look at the circuit of a zvex box of rock. If I remember rightly this uses a bunch of cascaded bs170's .

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Post by sopapo1 »

Thanks for the answer folks!!! The 35 db is mentioned by the designer: http://www.muzique.com/schem/mosfet.htm
it says:"Depending on the device used, maximum gain should be about 35dB and minimum ~3 dB. The source resistor is bypassed by a large capacitor in series with a variable resistor used as a gain control. The minimum resistance setting yields maximum gain."
I used the layout from this page: http://www.pisotones.com/MB/MB.htm the author of the is getting 17 dbs o f max gain.

George, I will check the pot, do you think that is possible it could be a faulty pot that dont reach zero ohms?? After that, I will try the cap across source and ground.. by large cap do you mean 1000 uf??
Thanks to all

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george giblet
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Post by george giblet »

I used the layout from this page: http://www.pisotones.com/MB/MB.htm the author of the is getting 17 dbs o f max gain.
That guy is getting the same gain as you so maybe the BS170 produces a lower gain.

In the past I have checked the gains of MOSFET stages. From what I recall the BS170 was close to the 2N7000. Perhaps the parts available these days are slightly different.
George, I will check the pot, do you think that is possible it could be a faulty pot that dont reach zero ohms?? After that, I will try the cap across source and ground.. by large cap do you mean 1000 uf??
Thanks to all
Try 100uF just to see if there is any effect. It's worth trying but reading that page I guess you not the only one to see less gain with the BS170.

If you really want more gain you would have to increase the top 2.7k to say 10k then adjust the bottom 2.7k to get 5.3V on the drain. The increase in gain will only be a few dB not 20dB.

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