18v AMZ MOSFET BOOSTER

[jammybstard]

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.

[amptramp]

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.

[jammybstard]

Like this:


Or like this:


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

[merlinb]

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.

[amptramp]

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.

[jammybstard]

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?

[R.G.]

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  ) 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.   

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.

[jammybstard]

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!

[PRR]

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

[jammybstard]

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?

[spaceace76]

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?

[bool]

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

[spaceace76]

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

[bool]

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.

[spaceace76]

FWIW, had to adjust R2 and R4, I changed R1 (recalculated the ratio as RG suggested) but the voltages stayed the same. The AMZ webpage actually specifies changing R2 for biasing purposes, guess we should have started there. I dropped R2 down to 33k and doubled R4 for 5k6. This gave me 5.5v, just what we need. Can also confirm zener is fine as-is.

I think the disc cap I used wasn't rated for 18v so now the circuit has no sound! I have to find time to test it out but I think it's opened up to ground which is killing my signal.

I knew I would get around to trying this out someday! Thanks for the advice, guys

[amz-fx]

FWIW, had to adjust R2 and R4, I changed R1 (recalculated the ratio as RG suggested) but the voltages stayed the same. The AMZ webpage actually specifies changing R2 for biasing purposes, guess we should have started there. I dropped R2 down to 33k and doubled R4 for 5k6. This gave me 5.5v, just what we need. Can also confirm zener is fine as-is.

I think the disc cap I used wasn't rated for 18v so now the circuit has no sound! I have to find time to test it out but I think it's opened up to ground which is killing my signal.

I knew I would get around to trying this out someday! Thanks for the advice, guys

Use the stock circuit but change R2 to 120k, and that should get you going.

Best regards, Jack

[spaceace76]

Indeed it has. The voltages look good as far as biasing goes however I still have no sound. I don't have an extra BS170 or 2N7000 on hand, so after quadruple checking every component and connection, it has to be my MOSFET.

Audio probe goes dead right after C1, which I replaced and confirmed is blocking DC.

Some readings for those interested:

D: 8.95
G: 8.89
S: 8.89
Supply: 17.68
Vr: 8vdc

I also noticed the circuit seems to be draining power rather quickly, or at least quicker than it did previously. Again leads me to think the MOSFET is the issue.
Then I performed this test: http://www.utm.edu/staff/leeb/mostest.htm
and the reading never goes high as described in step 4. I had one extra BS170 on hand when I began replacing components looking for the fault, and both the old and new ones don't pass this test. The 'new' BS170 was just sitting around in a plastic (not anti static) bag.

I have a few new devices on order from smallbear (and some sockets) so if the MOSFET is the culprit it will be swapped out relatively soon.

[Jubz]

Hi.

Sorry for necrothread.

since a few day Im scratching my head around the mosfet biasing at higher voltage than 9V. I would like to try 25V and the mosfet boost replacing the first gain stage in a big muff (why ? I dont even know probably just because I have putted charge pumps in my test box and I like big muffs and mosfets).

Following RG great article and staring like a dumb at the AMZ mosfet booster article for hours, I ended up trying to get 1V across RSOURCE and the rest of the supply split between Vmosfet and RDRAIN and the mosfet ( so 11,8V for each the supply being measured to 24,5v). I need to get 12,8V measured on the collector of the mosfet. Trying at 1ma and assuming 2V VT, I end up with 12K RDRAIN, 1K RSOURCE and 68k for Rbias  supply and 10 k for RBIAS GROUND (these values were multiplicated to reach some I have in my bags). The mosfet BS170 I have putted on the breadboard was luckily on pair with datasheet values (between 2Vt and 2,1Vt I guess) because I measured these values.

But here the headroom follows the gain. Even without bypass and resistor/pot capacitor the gain is huge (X12 like a traditional input gain stage set by NFB) and the volume painful. I want more headroom but not necessarly more gain than the AMZ mosfet booster (I want to take a clean blend from this stage and I dont want to go in the gated fuzz territory). But if i make RSOURCE bigger I will lose headroom. Same if I lower Rdrain or change bias resistors. Is there a way to go from unity gain to huge clean boost VS huge clean boost to monstruous clean boost without sacrificing headroom ? The sustain knob of the muff could stay a volume pot kind of (and the first stage be a fiwed gain value) but, why dont have fun and test to control the gain of the muff with a gain knob on the AMZ mosfet control and hope for... I dont know but why not (better signal/noise ratio perhaps).


One other option I want to test is to take the clean blend from the mosfet source and the boosted signal from the mosfet drain. But I think in this case I should ideally split equally the supply between mosfet RDRAIN and RSOURCE to allow swing on the the source. And lose a lot of headroom of gain on the drain. But the idea of buffered clean blend and input boost with a single mosfet is tempting.

Other question. At the end of his article RG suggests a 1v across Rsource and 8v equally splitted between mosfet and Rdrain because it helps with Vt variation. I dont understand why it helps with VT variation since with a 3V Max VT in this configuration the mosfet stay asleep. My guess reading the artcle was getting a bigger RSOURCE was swamping VT variation

[PRR]

After many decades in the audio racket, I have no idea what "headroom" is.

Especially in a signal chain with many gain-knobs. Who cares WHAT the signal voltage is at each step of the way? You will always adjust so the final (speaker) level is appropriate.

Input clipping is at-most supply voltage.

Input clipping is at-most supply voltage divided by gain.

[slashandburn]

After many decades in the audio racket, I have no idea what "headroom" is.

I know this one! I think it's about how comfortable a fit your cans are. People with bigger heads clearly need cans with more headroom.   

Edit: pardon my facetiousness, just a little humour based on an incredibly misinformed Amazon review for a pair of over-ear headphones. Dude must've had a big head.