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November 26, 2014, 05:07:14 AM
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DIYstompboxes.com  |  DIY Stompboxes  |  Building your own stompbox  |  18v AMZ MOSFET BOOSTER 0 Members and 1 Guest are viewing this topic. « previous next »
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Author Topic: 18v AMZ MOSFET BOOSTER  (Read 2865 times)
jammybstard
Posts: 51



18v AMZ MOSFET BOOSTER
« on: June 10, 2011, 03:53:26 PM »

I thought I'd try an 18v version of the http://www.muzique.com/schem/mosfet.htm I breadboarded the following:



But at 18V. I doubled the values of R4, R5 and R6. Is this the right approach? I thought this would keep the current and the gain the same as the 9v version but double the headroom.
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amptramp
Posts: 1969


Ron R.


Re: 18v AMZ MOSFET BOOSTER
« Reply #1 on: June 10, 2011, 06:56:32 PM »

I would be a little careful of the zener diode.  Some of them have a large junction capacitance.  A Zener in series with two back-to-back 1N4148 signal diodes will top out at 4 pF and is the preferred configuration.
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jammybstard
Posts: 51



Re: 18v AMZ MOSFET BOOSTER
« Reply #2 on: June 11, 2011, 03:40:45 AM »

Like this:


Or like this:


I assume the second; I dont really see how the first could work.

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merlinb
Posts: 1221


Merlin, Lancashire UK


WWW
Re: 18v AMZ MOSFET BOOSTER
« Reply #3 on: June 11, 2011, 05:22:35 AM »

I would be a little careful of the zener diode.  Some of them have a large junction capacitance. 
Why would that matter? C2 is already swamping it.
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amptramp
Posts: 1969


Ron R.


Re: 18v AMZ MOSFET BOOSTER
« Reply #4 on: June 11, 2011, 09:08:13 PM »

Use the second image.  There are some zener diodes with over 15,000 pF junction capacitance that have been used as variable capacitances for tuning audio L-C filters.
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jammybstard
Posts: 51



Re: 18v AMZ MOSFET BOOSTER
« Reply #5 on: June 12, 2011, 01:36:02 AM »

I would be a little careful of the zener diode.  Some of them have a large junction capacitance. 
Why would that matter? C2 is already swamping it.
Is it because C2 is a path to ground but Czd would creat some sort of feedback?

I'm still unsure if I've done the right thing in doubling the resistor values (see OP) is it correct? Same gain but more headroom?
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R.G.
more
Posts: 16308


WWW
Re: 18v AMZ MOSFET BOOSTER
« Reply #6 on: June 12, 2011, 06:45:59 AM »

I doubled the values of R4, R5 and R6. Is this the right approach? I thought this would keep the current and the gain the same as the 9v version but double the headroom.
The current in the MOSFET is set by the voltage across the gate-source terminals. In turn, this is set by the voltage on the gate and the resistance on the source to ground.

You've doubled the gate voltage by doubling the supply voltage and leaving the resistances - and more importantly the ratio - of R1 and R2 the same. This ratio needs to be changed to get the current down.

This is all fairly well (I think  icon_biggrin ) explained in "Designing MOSFET Boosters", here: http://geofex.com/Article_Folders/mosboost/mosboost.htm That's why I wrote the article - for people who want to know why instead of painting by the numbers.   icon_biggrin

The bottom line is that if you want the same current but higher voltage, leave the source resistor the same, and correct the ratio of R1 and R2 to get the gate voltage back down to where it was with the lower supply voltage.

At that point, you have a choice. If you leave R4 at the same value, with the same old current it will drop the same voltage and you will not have changed headroom at all. If you double R4, it will move the drain voltage lower, back towards the middle of the new, higher supply voltage. By adjusting the value of R4, you can put it in the middle of the available voltage (supply minus source resistor voltage) and get the most headroom from the new supply voltage. This will probably double the gain as well.

The drain current is set by the voltage across the source resistor. That's set by the gate voltage and the gate threshold voltage, which varies from MOSFET to MOSFET. The DC position of the drain voltage is set by the drain current and the value of the drain resistor, that being subtracted from the power supply. And that determines headroom.

Notice that an opamp running from +/- 18V - as most of the common ones will - will yet again double headroom, and give you all the gain, and more, of a MOSFET.
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R.G.

Every single NASA manned mission starting with the Gemini series has carried a roll of duck tape.
Yes, really. Look it up.
jammybstard
Posts: 51



Re: 18v AMZ MOSFET BOOSTER
« Reply #7 on: June 18, 2011, 11:35:16 AM »

Hi RG; thanks, that helped a lot; I had actully read your article but obviosly not quite got it (my fault I'm sure!).
I'm actually trying to build this to replace my 18V op-amp booster that has to go and I thought I'd try somthing different.



I re-built the original design; replacing R1 and R2 for a trim pot and adjusing it untill the Point A was Half way between V+ and VSource ((V+)-(VR5))/2

And measured About 0.5V between Gate and source.

Then I doubled the voltage and changed R4 for 5K6 and repeated the above and measured About 0.6V between Gate and source.
So I thik that's about there if I've understood you.  It seems to work ok anyway; I need the GF to leave the livingroom so I can give it a blast without getting scowled at!
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PRR
Posts: 6151


Paul R. - Maine USA


Re: 18v AMZ MOSFET BOOSTER
« Reply #8 on: June 18, 2011, 11:38:24 PM »

You could probably just jack the supply voltage, no circuit change, and get insignificant gain change. Gain here is all about resistor ratios, until R6 hits the end-stop. Current may increase, but so what? Its still small. Whether headroom goes up or down depends on bias, and despite being very stable biasing this plan is a bit marginal against device variation: biasing FETs under low-volt supplies is problematic.

> measured About 0.5V between Gate and source.

I believe R.G. is telling you to get the Gate to GROUND voltage the same. Not Gate to Source.

Jack shows 9V*(100K/(100K+62K))= 5.55V Gate to Ground.

Ooops.... you can't measure that directly, your meter will load-down the 10Meg. Since you know the MOSFET Gate is "ZERO!" current, you could measure at C3. Or momentarily replace R3 with some much smaller value.

> measured About 0.5V between Gate and source.

Then 5.5V Gate to ground is 5V left across the 2.7K R5. Also 5V across 2.7K R4. Which does not add up. Maybe Jack's BS170 had much higher threshold voltage. BS170's min/typical/max 1mA bias is 0.8V/2V/3V. The published plan seems to run near 1mA. 0.5V-0.6V seems low. 5.5V applied bias, minus 2V typical, is 3.5V in R5 and R4, leaving a few volts to swing.
_________________

I think the Zener is a non-issue. Few Zeners have huge capacitance, and in this case it is largely bootstrapped away. I suspect it will never come close to the capacitance of a guitar cord. True a couple small plain diodes have lower intrinsic C, may be a penny cheaper, but twice as many legs to solder. s
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jammybstard
Posts: 51



Re: 18v AMZ MOSFET BOOSTER
« Reply #9 on: June 19, 2011, 08:43:37 AM »

I believe R.G. is telling you to get the Gate to GROUND voltage the same. Not Gate to Source.
Jack shows 9V*(100K/(100K+62K))= 5.55V Gate to Ground.

Well this is the Circuit I have built:

Q1 = 2N7000
R1 & R2 = 500K Pot
R3 = 4M7 (needs replacing with a 10M at some point)
R4 = 5K6
R5 = 2K7

Supply Voltage: 17.68v
Drain Voltage: 10.69v
Gate Voltage: 3.81v (on the meter) C3: 5.42v
Source Voltage: 3.28v


The bias set almost exactly halfway between Source and Supply 10.69v ((17.68-3.28)/2)+3.28 = 10.48v; The voltage accross C3 is 5.42, close to as you calculated Jacks.

by ear It sounds good and has oodles of gain but I should try it along side the 9v version.

How do I calculate theoretical max gain when R6 = Zero?



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spaceace76
Posts: 119


Chris Valent


Re: 18v AMZ MOSFET BOOSTER
« Reply #10 on: October 16, 2014, 01:41:33 AM »

I have this circuit installed in a guitar with active pickups.

I just added another battery in series for extra headroom in the active preamp, but ran into biasing problems with this circuit.

If I understand this correctly, I have to change the R1/R2 ratio to achieve the same bias voltage at Vr, so I can up the R2 value to about 230k, resulting in: 18v(100k/(100k+230k))=5.46v

Couldn't care less about the extra headroom in the boost circuit, the pickups sound much more dynamic and lively now, and the boost is just for clean gain. As long as it's biased up properly, I'll be happy.

Will I also have to ditch the zener diode?
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bool
Posts: 222


Re: 18v AMZ MOSFET BOOSTER
« Reply #11 on: October 16, 2014, 03:59:00 AM »

Personally, if you increase the V+ supply voltage, all other things equal, I would only change the R4 and R5 values - to f.e. something like 4K7. ymmv
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spaceace76
Posts: 119


Chris Valent


Re: 18v AMZ MOSFET BOOSTER
« Reply #12 on: October 16, 2014, 01:13:33 PM »

I imagine that would work too, but I'd have to get rid of the zener as well, right?
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bool
Posts: 222


Re: 18v AMZ MOSFET BOOSTER
« Reply #13 on: October 16, 2014, 01:22:11 PM »

You don't have to get rid of the zener. Just leave it as-is. No need to change anything there.

Zeners often serve as gate protection for mosfets. This could be done in a couple of different ways, but a zener is effective.
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