Inductor biased LPB-1

[idsnowdog]

I recently saw a video showing inductor biasing so I would like to breadboard it.  The formulas mean nothing to me so what value inductor would I use for the collector?

[PRR]

To cut to the finish: we don't use inductors in small audio. It takes WAY too many turns to get a useful impedance at 100Hz. Gain might ~~double, but audio gain is very cheap, add a transistor.

Exceptions: 1950s hearing aids, pocket radios with reed speakers into 1962, and guitar-heads hoping for "iron distortion".

[idsnowdog]

The answer to poor gas mileage isn't always a larger gas tank or add another engine.  I would like to get more clean volume.

[Fancy Lime]

The answer to poor gas mileage isn't always a larger gas tank or add another engine.  I would like to get more clean volume.

Then you want more supply voltage, not an inductor. Practically speaking.

But I do like the idea of using an impractically large inductor fir this purpose, simply because it is different. Sometimes, not often but sometimes, doing something that seems a bit stupid on paper can yield interesting results. So I say go for it. Have a spare guitar pickup kicking abour in the "random stuff" drawer? Try that! It's an inductor of about the right value.

[PRR]

more clean volume

More gain? More maximum volume?

How much? Gain of 3? Gain of 300? Max 1 Volt, 10 Volts, 100 Volts?

[Rob Strand]

It's not quite right.

An apples to apples conversion needs to address a few details.

I'd call apples to apples.  Same AC gain.  Same frequency response.   More swing.

The original has an DC coupled output load of 10k and an AC coupled output load of 100k so the combined AC collector load is 10k//100k = 9.09k.

In order for the inductor based circuit to have the same signal gain it needs an AC load of 9.09k.  To do that you could simple place a 10k resistor across the 100k pot.   But now 100nF cap has no high-pass effect because it is being fed by the collector and inductor which present a high impedance.

So you would need to move the the 100nF cap between the 10k and 100k.  So really what you have done is moved the 10k collector resistor from +Vcc to ground, then add the inductor to +Vcc.   You could in fact leave the 10k connected to +Vcc then add the inductor across the 10k.

As far as the inductor value goes.   The inductor will shunt the low frequencies, essentially forming a high-pass filter,  so it needs to be high enough that the low frequencies aren't effected.   The L + R high pass filter sees the 9.09k collector load (since the 100nF passes the lows).  If choose a cut-off of 20Hz the inductor would need to be L = 9.09k / (2*pi*20) = 72H.

The high-pass cut-off formed by the 100nF cap is f3 = 1/(2*pi*(10k+100k)+100n) = 14Hz, so we could use a much larger output cap (for DC blocking) then use the inductor for the high-pass filter, L = 9.09k / (2*pi*14) = 103H.   

We are using a larger inductor than before.   The 72H case has more roll-off than the original circuit ie. it's not quite apples to apples.   We have two cut-offs one from the cap and one from the inductor whereas in the second case the inductor does the main roll-off.

At the end of the day it's a crazy high inductor value.

[idsnowdog]

It sounds like an inductor that value would be impractically large. 

[PRR]

Merlin has a project with a bootstrapped opamp to over-over-drive an amp input.

[Rob Strand]

The other way around trying to get more output swing is to reduce the gain so it doesn't clip so early.

Perhaps change the 390R emitter resistor to 820R to reduce the gain by 6dB.  Then adjust the 430k base resistor so the collector biases to the right voltage.  (You can go further and match the original input impedance by reducing the 43k base resistor, then tweaking the 430k base resistor to set the collector bias voltage.)

[amptramp]

Early tube radios and amplifiers had transformer coupling but on occasion, just an inductor load feeding the next grid through an R-C network.

Inductors do limit both high and low frequency response due to distributed capacitance causing the inductor to look like a parallel L-C tuned circuit causing low and high frequencies to be "out-of-band".

Inductors are bulky and expensive, so you don't see them often.

I have one radio that used a transformer for phase splitting to the push-pull output stage, but it was connected to the triode driver plate via a capacitor and the plate went to a resistor to B+.  The idea here was to prevent saturation of the magnetics.

[R.G.]

You're getting good advice.

To get more clean headroom within the same power supply voltage you have to lower the total gain, or find earlier pinch points that distort before the output stage does to keep things inside the existing power supply voltage.

To get more clean headroom as a bigger output signal, you need a higher power supply voltage. To do that you can use an external higher power supply maker, or you can use an inductor load to make up the higher power supply voltage on a cycle-by-cycle basis as you go along.

Doing this well puts requirements on the inductor's value (it has to be big enough, that's what some of those equations are for), and it has to be quality-enough to not have the internal parasitics eating up the signal you're trying to get out. That's what others of those equations tell you.

If the inductance is not big enough, bass response will suck. If the inductor has internal capacitance too big, the high frequencies will be limited. If the inductor core is not well made, or the wrong material, it will  both distort from the magnetic core losses and possibly damage the transistor if the trannie tries to turn it off too fast.

What inductance? Can't tell you a sane answer without equations. Best choice is to look at what you have on hand and try it to see if it is worth pursuing. Small AC mains tranformers generally have primary inductances in the half to several Henry range. Try that. Hook a small (i.e., smaller than your fist, just so it's easy to move around) trannie's primary as the inductor, make sure all the other wires are open circuited, and see what you get. You'll get something, surely.

Doing a good job, with good frequency response and small size and low power use will be an equations kind of thing.

[drdn0]

Merlin has a project with a bootstrapped opamp to over-over-drive an amp input.

A phenomenal design (that I only suggested to somebody a couple of hours ago), but given the cost of dc-dc converters/inverters now, it's barely $15 of components to create an onboard bipolar, isolated, 9-15v power supply now. Switching noise is in the hundreds of khz range. Going to have double (or more) the output of the Glassblower; not that you'd ever need that much in practicality.

If you want to go cheaper, you can even use a LT1054 + a couple of 7* series regs to get bipolar 12v for half that price but you need to pay a little more attention to not create noise.

Bipolar power supply makes the layout fairly simple too.

[idsnowdog]

I also saw this which I thought looked interesting.  Possibly a way to counteract temperature fluctuations for Germanium transistors?  I tried replacing the resistor between the base and emitter with a few different diodes but that didn't work.  I suspect there's some math that I don't know about to determine the value of the other components and it isn't as simple as replacing a resistor with a diode.

[PRR]

...the Glassblower

Thank you.  https://www.valvewizard.co.uk/glassblower.html

Switching noise is in the hundreds of khz range.

In US (and probably EU too) commercial products, that opens up a bunch of interference regulations. If you can pretend it is all "in sight of" the audio band, or at least not spattering in a police, ship(*), or broadcast band, you may get overlooked. Yes yes we have dozens of these switching converters in every corner of life, several per cellphone, "all my friends are doing it why can't I??"

(*) I live under an airport approach so aircraft band would be bad, but I thought all that moved above 108MHz a long time ago? Even the B-17 I rode in clearly had modern VHF radio wedged in the panel.

[drdn0]

...the Glassblower

Thank you.  https://www.valvewizard.co.uk/glassblower.html

Switching noise is in the hundreds of khz range.

In US (and probably EU too) commercial products, that opens up a bunch of interference regulations. If you can pretend it is all "in sight of" the audio band, or at least not spattering in a police, ship(*), or broadcast band, you may get overlooked. Yes yes we have dozens of these switching converters in every corner of life, several per cellphone, "all my friends are doing it why can't I??"

(*) I live under an airport approach so aircraft band would be bad, but I thought all that moved above 108MHz a long time ago? Even the B-17 I rode in clearly had modern VHF radio wedged in the panel.

If you are selling products it may be a concern.

If you aren't, then I can't see it ever being something you'll get in trouble for. The only time I've ever heard of it being an issue was when EHX did it, and then there are literally millions of available products with cheap imported power supplies which don't meet any standards whatsoever - they're going to be a much bigger target than somebody noodling around with a few pedals will be.

Either way, almost all have some very basic layout/supportive components that reduce propagated noise to below emissions standards for a few dollars extra  - some of the newer Mean Well don't require anything additional to get there

[antonis]

I also saw this which I thought looked interesting.  Possibly a way to counteract temperature fluctuations for Germanium transistors?  I tried replacing the resistor between the base and emitter with a few different diodes but that didn't work.  I suspect there's some math that I don't know about to determine the value of the other components and it isn't as simple as replacing a resistor with a diode.

I don't want to disappoint you but there is nothing constant there..

You can go better by adding another diode in series and an Emitter resistor (in a form of NFB) of value according to desirable Collector current..


P.S.
What bias a transistor in CE amp configuration is its Collector current caused by its Base voltage..
(BJTs - like FETs - are transconductance devices, despite what is described in the vast majority of textbooks..)
If you want constant Collector current you must have both constant Base voltage and constant Base-Emitter drop OR constant ΔVBase / ΔVBase-Emitter
In other words, considering infinite Early voltage, you have ideal current source (sink here due to load placed on Collector)