Author Topic: Can anyone help me in getting my feet wet in modding a circuit?  (Read 422 times)

eightsevenzero

https://imgur.com/a/HONwmJ3

Ok so here's a simple boost schematic. I built it with a 2N5458 and it sounds really good. Is there anyone who can tell me the "why" behind the values and placements/functions of the different resistors and caps in this circuit? Reason is, I'm trying to connect the dots in understanding how I can build off of circuits like this more and integrate things such as say another JFET gain stage, a clipping diode section, tone control stage, how you would add a gain control as opposed to just a volume, etc. I understand some of these things on their own (bias resistors, coupling capacitors, etc) but when it comes to integrating them into a circuit like this, I don't really how to start. Thanks!

antonis

Re: Can anyone help me in getting my feet wet in modding a circuit?
« Reply #1 on: March 14, 2021, 06:54:17 AM »
https://www.electronics-tutorials.ws/amplifier/amp_3.html

https://www.electronics-notes.com/articles/analogue_circuits/fet-field-effect-transistor/common-source-amplifier-circuit.php

https://www.theengineeringknowledge.com/common-source-fet-amplifiers-operation/

>I understand some of these things on their own (bias resistors, coupling capacitors, etc) but when it comes to integrating them into a circuit like this, I don't really how to start.<

By taking into account some JFET CS amp "parameters", like Power Supply voltage, FET transconductance, Drain current, Output impedance, Drain quiescent voltage, Configuration gain, In-Out-Bypass HPFs corner frequencies, etc.

Of course, most of the above come out of some combinations..
e.g.
for X10 working current while maintaning same Drain quiescent voltage, Drain resistor value should be 10 times lower automatically resulting into 10 times lower output impedance toghether with 10 times higher transistor power dissipation..
"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

eightsevenzero

Re: Can anyone help me in getting my feet wet in modding a circuit?
« Reply #2 on: March 14, 2021, 09:19:38 AM »
Thank you! This was super helpful.

Mark Hammer

Re: Can anyone help me in getting my feet wet in modding a circuit?
« Reply #3 on: March 16, 2021, 06:21:04 PM »
The basic gain is partly set by the 10k resistor from source to ground.  Make it a lower value, and the gain is increased.  The 4u7 capacitor in parallel with the 10k provides a zero-ohms path to ground, providing for lots of gain, but ONLY for AC; specifically, frequency content above the rolloff set by the value of that capacitor.

Okay, now let's imagine we have a 20k or 25k linear pot.  The wiper of the pot is tied to the source of the FET, along with the 10k resistor.  One of the outside legs of the pot is connected to a 10uf cap, with the negative lead of the cap going to ground.  The other outside leg of the pot goes to a 100nf cap, whose other lead also goes to ground.

The two caps provide for different low-end rolloffs, and the pot provides for varying the resistance in series with each cap, in reciprocal fashion.  When the pot is set to its midpoint, there's 10k (or 12.5k if it's a 25k pot) on each side of the wiper.  That presents no particular advantage for AC, relative to the default 10k resistor.  Rotate the wiper towards the 10uf cap, and you progressively reduce the resistance in series with it, providing a full bandwidth boost, that gets greater as you decrease the resistance between source and cap.  Because decreasing the resistance in one direction increases the resistance on the other side of the wiper, that cap has no impact on gain or tone.

Okay, now rotate the pot past the midpoint in the opposite direction.  As you reduce the resistance from wiper to that cap, you also create increasing gain, but for higher frequencies, rather than the full bandwidth.    If the 10uf cap provided a rolloff at 10hz, the 100nf cap will now move the point where boost is applied to a frequency 100x what the larger cap does.  In this hypothetical instance, our variable boost would now begin at roughly 1khz, making it a treble enhancer.  I differentiate this from "treble booster" because that implies something like a Rangemaster, and this won't sound like that.

But the basic principle of being able to get two different variable boosts with one control in such a simple circuit is a pleasing one.  Try it out!

antonis

Re: Can anyone help me in getting my feet wet in modding a circuit?
« Reply #4 on: March 16, 2021, 06:45:37 PM »
Okay, now let's imagine we have a 20k or 25k linear pot.  The wiper of the pot is tied to the source of the FET, along with the 10k resistor.  One of the outside legs of the pot is connected to a 10uf cap, with the negative lead of the cap going to ground.  The other outside leg of the pot goes to a 100nf cap, whose other lead also goes to ground.

And delete "original" 4.7μF bypass cap, I should dare to subjoin.. :icon_redface:
"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..