How to ballpark the high pass filter at the input of the Rangemaster?

[soggybag]

How to ballpark the high pass filter at the input of the Rangemaster?

Would you calculate this as R1 and R2 in parallel with C1? And use the Fc = 1 / 2Pi * R * C?

[R.G.]

There's a good sendup of that at this link:
https://www.electronics-tutorials.ws/amplifier/input-impedance-of-an-amplifier.html

The pass frequency at the input depends on first the input cap, then everything connected to the base node. You are right about the two input bias resistors appearing in parallel to "ground" as part of the filter. The missing part is that the impedance of the base pin itself also appears in parallel to the bias resistors, and this complicates things a little.

The impedance at the base of a bipolar is approximated by the transistor's "Shockley resistance", the effective resistance of the of the base-emitter junction itself, times the current gain of the transistor itself at the operating current. Both are variables, but can be approximated. That's what the article calls "re", and it's approximated as 25mV/Ie. This "resistance" times the current gain at that current looks like a resistor that can never be bypassed in series with the external emitter resistor Re. Re is fully bypassed in the Rangemaster, so you only get effectively re at high frequencies.

The calculation is then that the input high pass resistance term is approximately determined by the iinput cap and the parallel combination of the bias resistors and the current gain and "re" of the transistor. Yep, this varies from transistor to transistor, and this is why a whole lot of circuit design work puts an un-bypassed emitter resistor in bipolar circuits - it stabilizes both the DC bias point and the frequency response against transistor variation somewhat.

[antonis]

No intention to override R.G., by any means , but IMHO, "re" in series with Emitter shunt capacitor impedance times h_FE+1 should be taken into account for Base "reflected" resistance..

P.S.
To be more precise, (h_FE+1) X (re + R_E//0.159/f*C)

[PRR]

....IMHO, "re" in series with Emitter shunt capacitor impedance times h_FE+1 should be taken into account for Base "reflected" resistance..

Your employer is not paying you for 2% precision. (If hFE is over 50, then your correction is 2%.)

[soggybag]

....IMHO, "re" in series with Emitter shunt capacitor impedance times h_FE+1 should be taken into account for Base "reflected" resistance..

Your employer is not paying you for 2% precision. (If hFE is over 50, then your correction is 2%.)

Can you elaborate for us amateurs?

[mac]

IMHO, the input capacitor interacts with the guitar inductance ==> coil inductance -- coil resistance -- input cap --  emitter resistance -- gnd
Kind of bandpass filter if the coil resistance and the emitter resistance are similar, both in the order of 5k to 10k.
When you add some series resistance in the signal path the peak has less Q.
https://www.electronicsteacher.com/alternating-current/filters/resonant-filters.php

f= 1/(2.PI.sqrt(L.C))

for 5nf and 3Hy ==>  f=1,3khz

I think I posted something about this in the Spice simulations zone.

mac

[Rob Strand]

[I could have sworn I pressed quote, it quoted your post but also editted it!]

....IMHO, "re" in series with Emitter shunt capacitor impedance times h_FE+1 should be taken into account for Base "reflected" resistance..

Your employer is not paying you for 2% precision. (If hFE is over 50, then your correction is 2%.)

Can you elaborate for us amateurs?

https://www.youtube.com/watch?v=yP4rEeukgeo

Impedance looking into the base is,   rpi =  VT / IB
Impedance looking into the emitter is, re = VT / IE
We also have the transconductance  gm = Ic/Vbe = IC / VT
where VT = 25.8mV   ;  called the thermal voltage, look it up on wiki, it is a function of temperature.

The impedances of a transistor stage with the emitter resistors bypassed follow rpi and re.
Again look up a web page on a transistor CE amplifier.

We know from transistors that the currents are all related,

IE = (hFE + 1)*IB, and IC = hFE *IB

Anyway,
rpi =  VT / IB
And IE = (hFE + 1)*IB, which can be written as IB = IE / (hFE+1)
so that means we can write rpi = VT / (IE / (hFE+1)) = (hFE + 1) VT / IE
The VT/IE part is re  so rpi = (hFE + 1) * re  ; which is antonis's contention.

The key point is re and rpi are quite different but the relationship between the transistor
currents make re and rpi related.

We often see rpi written in terms of IC, rpi = hFE * VT / IC
From above you can see gm = IC/VT, so that makes  rpi = hFE / gm.

Books and people often approximate hFE+1 ~ hFE and write approximations like re ~ 1/gm.   You might not be saying 1 = 2
but you are saying 200 = 201.   It's important to realize what are approximations and what is exact.   If books
or web pages don't state approximations are made you can get really confusing maths because the maths
never quite adds up.

There's a whole stack of other small errors in the calculations:
- non-ideality factor, can be 1% or more for a transistor.
- Early effect
- hFE dependency on current.

None of these are in the above equations.  You can include them if you wish.  Spice will do all of them.

IMHO, the input capacitor interacts with the guitar inductance ==> coil inductance -- coil resistance -- input cap --  emitter resistance -- gnd
Kind of bandpass filter if the coil resistance and the emitter resistance are similar, both in the order of 5k to 10k.
When you add some series resistance in the signal path the peak has less Q.
https://www.electronicsteacher.com/alternating-current/filters/resonant-filters.php

f= 1/(2.PI.sqrt(L.C))

for 5nf and 3Hy ==>  f=1,3khz

I think I posted something about this in the Spice simulations zone.

mac

The impedance of the pickup will change things for sure.  Generally lower the high-pass cut-off due to the added impedance.

[Gus]

some links that might help you can also search for treble boosters, rangemaster etc. at this forum for others posts

Silicon transistor ones with simple guitar sims before the treble boost

This one changes the input cap value
https://www.diystompboxes.com/smfforum/index.php?topic=93052.msg799216#msg799216

This has an input control
https://www.diystompboxes.com/smfforum/index.php?topic=93926.0
https://www.diystompboxes.com/smfforum/index.php?topic=112679.msg1040866#msg1040866

[PRR]

...Books and people often approximate hFE+1 ~ hFE and write approximations like re ~ 1/gm.   You might not be saying 1 = 2 but you are saying 200 = 201.   It's important to realize what are approximations and what is exact....

I was taught "assume hFE>50". That is, your design must work over a w-i-d-e range of parameters. (And Prof did not want see a blow=up if hFE++50.0 either.) This was also the Silver Age of 10% resistors, and unregulated power supplies.

Yes, +1 is the right math but if your circuit cares it is far too fussy. Your boss (if you were doing this for a job) does not want to know "right math", she wants to know "no unhappy buyers". Making those checks is very different than checking your "+1"s.

[antonis]

 

My quibble didn't concern "+1" on h_FE rather than Emitter resistor bypass capacitor value..

Many people think that it's enough for bypass cap impedance to be much lower than Emitter resistor value for CE amp to be considered "grounded Emitter" hence only re to be taken into account for inpedance & gain calculation but, in reality, that impedance shoud be much lower than re..!!
(or else, re + bypass cap capacitive reactance should be considered in series.. - at least for low frequencies)

e.g.
For 22μF bypass cap and 1mA working current, re = 25R and cap's impedance at 300Hz is also 25R..
By ignoring that cap we result into 50% error both for impedance and gain calculation..

[PRR]

> in reality, that impedance shoud be much lower than re..!!

Yes.

[Rob Strand]

I was taught "assume hFE>50". That is, your design must work over a w-i-d-e range of parameters. (And Prof did not want see a blow=up if hFE++50.0 either.) This was also the Silver Age of 10% resistors, and unregulated power supplies.

Yes, +1 is the right math but if your circuit cares it is far too fussy. Your boss (if you were doing this for a job) does not want to know "right math", she wants to know "no unhappy buyers". Making those checks is very different than checking your "+1"s.

It depends what your goal is, understanding or producing a circuit.   You can produce working circuits without understanding much at all.

My main point is: It's easy to start from a correct result and apply an approximation.  It's not possible to start with an approximation and make it right again.   It doesn't help if you are learning and seeing differences between books.   The differences only make sense if you know the correct version.