Simply bypassed CE losses

[Lino22]

Let's say i want to bypass a simple CE stage like this (blue line)




How do i count the impedance to ground the circuit will become?
I don't know which side to look at it from . I know how to count the input and output impedance, but this case seems to be a bit different to me.
Or do i replace it with the AC analysis equivalent circuit, which is (in this case) split by the transistor and then put the input and output in parallel?

[antonis]

I'm trying to get into your point but..

edit: Impedance on Vout is (roughly) Rc in parallel with whatever comes next..

P.S.
You should ground transistor input when bypassed just in case of (........)??
(fill dots space..)

[Rob Strand]

[please skip, this answer is not relevant.]
See the calculation in your other post.  The transistor has the equivalent of a resistance 're' in series with the emitter.   It's like an emitter resistor you can't get rid of or bypass.

[antonis]

Rob, I think OP wishes to know the bypassed point's impedance (at Vout)..

[Rob Strand]

Rob, I think OP wishes to know the bypassed point's impedance (at Vout)..

I think it's time for Rob to go to bed, up since 5:30am ... not reading the question 

[Lino22]

P.S.
You should ground transistor input when bypassed just in case of (........)??
(fill dots space..)

Yes, i see, i am (by)passing on this one then.

It is a recovery stage at the end of an effect loop (to sort out my DE7 + Rangemaster problem mentioned in my other post), and i thought it would be nice to bypass it when the signal jacks are removed, using just the simple switch on the jack socket.

I will add a DPDT switch to it like a good boy. I don't want any uncontrolled oscillating on stage.

[Lino22]

Rob, I think OP wishes to know the bypassed point's impedance (at Vout)..

I think it's time for Rob to go to bed, up since 5:30am ... not reading the question 

Rob, here is what i meant - how much will the bypassed circuit suck the signal.

[R.G.]

You're probably already aware of this, but for other beginners who might not be:
The amount of signal loss that any loading causes depends on both the loading impedance, as being figured here, and on the impedance of the signal source. If, for instance, you drive the bypassed CE stage with the output of a low-impedance source like an opamp or power amp chip, the impedance of the CE stage almost doesn't matter. The amplifier can put out as much current as needed to make the load carry the undiminished signal voltlage.
If, instead, you drive the bypassed CD stage with something like a guitar pickup that looks like a 4K to 12K resistor and 1-4H inductor in series with the signal, the impedance of the CE stage matters hugely. Perhaps more importantly, the impedance of that Henry-range inductor causes increasing losses as frequency goes up, so the higher the signal frequency, the more loading matters, and the more higher frequencies lost. This is the basis of "tone sucking" as it's entered the jargon of the pedal and guitar amp world.
Any loading AFTER the bypassed CE stage matters too. It adds right on in parallel to the loading on the collector of the CE stage when the CE stage is bypassed around.
You gotta consider the source impedances and loading impedances of everything you connect up.

[antonis]

I think Lino22 needs a DPDT switch, like below:



Rs (100k - 1M) are optional (but recommended)..
Or use a 3PDT switch wired with CE amp IN & OUT grounded when bypassed..

[Rob Strand]

Rob, here is what i meant - how much will the bypassed circuit suck the signal.

RG and antonis have got it covered.

For your circuit, the load is the collector resistor.

For a circuit with feedback bias (and non bypassed feedback resistors) the output impedance can be somewhat lower than the collector resistor value.  To make things more confusing the impedance depends on whether the input is open or shorted and if there is a series resistor in the input line.







The low output impedance of the feedback biased circuit isn't widely appreciated!

[antonis]

Indonesian feedback..??

[Rob Strand]

Indonesian feedback..??

For a wider audience  .

Seriously though,  in the past 3 years or so I'm finding it more difficult to get pics of very basic circuit configurations.    It's like the good stuff is now the minority in a sea of junk information.  Back in 2000 most of the info was quite good.

[Lino22]

Guys thank you for your help .

I know i can use DPDT is and the reason i was asking this weird question was a weird solution in a weird circuit, where i would love to use just a socket switch and get away with it, but it looks like i won't because the output impedance of the pedal before is not set (= any pedal) and there is a threat of uncontrolled oscillations as Antonis mentioned. In short - i just wanted to pull a cable out and get it bypassed.

Rob, thanks for the extensive explanation of the Z that drags the signal down.

I also understand that while feeding an input inpedance Zi from an output impedance Zo, i need Zi << Zo otherwise the signal gets sucked.

[Rob Strand]

i need Zi << Zo otherwise the signal gets sucked.

Zo >> Zi yes?

[antonis]

i need Zi << Zo otherwise the signal gets sucked.

Zo >> Zi yes?

I think Rob should go to bed..!!
(he says the same thing with Lino22..)

@Lino22: Low driver's impedance and high driven's one..

[Rob Strand]

I think Rob should go to bed..!!

Now you mention it I'm not sure if Lino22 is talking about the input and output side.

I'm pretty much running on negative sleep today.  And for other reasons I'm not expecting to be 100% for another 3 to 4 months - so you might have to check my work on occasion (not joking).

[Lino22]

Haha, i mean Zo << Zi. I definitely need more sleep, only we have 12:38 PM here ...