Common emitter with emitter resistor fixed bias calculation

[Fancy Lime]

Hi there,

I am trying to optimize a Vulcan stage. For that I need to understand how to calculate the bias resistor for a fixed bias scheme for a common emitter stage with an emitter resistor. Curiously, that is about the only biasing variant that I cannot seem to find calculations for. CE fixed bias without RE? Sure. Voltage divider bias with RE? Of course. Self bias with or without RE? No problemo. But fixed bias with RE seems elusive. Any help?

Thanks,
Andy

[Vivek]

Please wait till Rob Strand wakes up

[Rob Strand]

Fixed bias = Voltage divider bias with RB2 (base to gnd) resistor set to infinity
or
Self bias without RE, but replace IB = (VCC-VBE)/RBB with IB = (VCC - VBE - VE)/RBB
ie. The base voltage is shifted up by the emitter voltage so there is less voltage drop across RBB.


FWIW, at least one of the Vulcan's has a diode + resistor to ground on the base.  That's changes things.  It's more like divider bias.

[antonis]

https://electronics.stackexchange.com/questions/380141/some-questions-about-the-emitter-resistor-added-to-the-fixed-bias-circuit

P.S.
Isn't Vulcan an JFET (rather than BJT) overdrive..??

[iainpunk]

isn't the vulcan overdrive a normal voltage divider bias with an extra diode for more compression?

[antonis]

Aren't "Emitters" refered only on BJTs..??

[iainpunk]

Isn't Vulcan an JFET (rather than BJT) overdrive..??

https://www.diystompboxes.com/analogalchemy/sch/vulcan.gif

no, its BJT's and its also resistor divider bias, and the diodes should non-linearise the signal in the opposite way of the transistors.

cheers, Iain

[Fancy Lime]

@Rob
Thanks, that was the info I was after!

@Antonis
There is a BJT version just called "Vulcan Overdrive": https://www.diystompboxes.com/analogalchemy/sch/vulcan.html
And there is a JFET version called "JFET Vulcan Overdrive": https://www.diystompboxes.com/analogalchemy/sch/jfetvulcan.html
I was referring to the BJT version.

About the divider bias comments: Yes, the "actual" bias is set via a divider made up of 4M7 and 1M0 resistors and the resistance of the forward biased diode. But a fixed bias situation occurs when the signal goes high because of the diode. This is what makes the "compression" effect, as it is sometimes called. I'd rather call it soft asymmetrical clipping.

See, when the circuit is idle, the voltage divider is what defines the bias point. When the input signal goes low, the stage still behaves pretty much normal. Things get interesting when the input goes high. There comes a point, when the input signal overpowers the current going through the bias resistors. When that happens, the diode shuts off, effectively disconnecting everything except the 4M7 resistor to supply from the base. At that point, the output of the stage is no longer determined by the input signal but by the current through 4M7. And that is nothing other than a fixed bias that determines the lowest possible state of the collector of the stage, no matter how high the input (cathode of the diode) goes. Since the input is now disconnected, there is no further amplification, the signal is clipped. Because the resistor values are all pretty high, we are operating in a range where the diode acts as a pretty smooth current controlled resistor, so we slip gently in and out of the "fixed bias with no input" vs "divider bias with input" behavior. Really great work by Joe on that one. Super nifty. BTW the same does not happen in the JFET version, which makes this really a completely different effect despite the identical layout. In the JFET Vulcan, the diodes are only used for "delinearizing" the input (which also happens in the BJT version), whereas the actual clipping comes from the JFETs themselves on both halfcycles.

My objective here is to optimize the two biasing points separately, one for the real bias and the other for the "point of saturation", and see what influence that has on the sound. The idea is to have a relatively high headroom in the first stage and run the signal into the diode saturation/fixed bias situation early in the second stage while keeping the other half as far from the positive rail as possible. This should result in a sweeter overdrive, or so I hope. It's all a bit fiddly, since the 4M7 resistor, as well as the emitter and collector resistors influence both parameters and the diode behaves differently depending on how much current goes through it. So changing the bias resistors may necessitate changing the diode for the wanted behavior. But if the voltage drop changes, resistors may need to be adjusted again... Iterative process. Trying different diodes alone makes a big difference to the sound.

Cheers,
Andy

[Rob Strand]

I'd rather call it soft asymmetrical clipping.

Yes it sort of softens the BJT characteristic.    We had discussions on this years ago.  In the limit you end-up with something like a current mirror which is "perfectly" linear.

I have tried to do feedback biasing on those vulcan like stages.   The main idea was the feedback biasing let you define the lower clipping point better.  It doesn't even have to saturate, a bit like the Fetzer.      Another hope was as the collector voltage drops the amount of base drive drops off and that makes the lower clip point softer.     What happens though is you end up needing a fairly low collector-base resistor  and that means more current through the input cap when the input signal swings negative.    I tried to compensate that by putting a lowish value resistor to ground after the input cap, which fixes one problem but makes the input impedance lower.   A lot of juggling anyway, and more the point I was just poking around on spice.   It's something that needs to be built to see how it sounds.   The idea might seem OK on paper but turn out to be a total crock in practice.
       

[antonis]

Just remebered an old Vulcan "tweak" (shorting diodes and/or stages gain setting impedances)..
(J2 signal 1V/800Hz..)

It might be helpful to you, Andy..

No shorts..



Diodes shorted..



Diodes & Feedback-gain resistors shorted..