FF theory question

[snoof]

Riddle me this; referring to the schem below, would there be voltage on the ground bus??  It seems as the supposed 1/2 battery voltage could leak backwards from Q2's collector through R9, R5, and R8 to ground??  There is no cap to block this from happening.  Or am I just looney, and this is the way it is supposed to work .


http://www.generalguitargadgets.com/diagrams/fuzzface_b_sc_pn.gif

[Sir H C]

This one uses the dreaded negative ground.  This will work because that ground node is the - terminal of the battery, which is used for the voltage for the collectors.  Problem is that this method of doing the ground often gets oscillating.

[snoof]

Hmm.  I'm trying to help a buddy troubleshoot this crkt, and he's getting 5-6v on the ground trace.  That seemed way wrong to me   And looking at the schem, it looked like voltage has a path to ground through those resistors.

[slacker]

How are you measuring to get 5 volts on the ground trace?
If he's using the ggg board then the first thing I'd do is check that he's followed the correct layout diagram, because the same board can be used for positive ground or negative ground. If you got the jumpers in the wrong places you could get some voltage at the juction of R2 and R8.

[snoof]

yep, we went through that together and it seems that the layout is correct.  So we'll just keep on debugging...

[jaytee]

Maybe it's a bit confusing the way it's drawn. It's a negative ground circuit but it's kind of 'upside down'. The ground connection coming off the 33k resistor should be connected to the battery negative. And all other ground connections. The positive 9v supply is at the bottom. If you put your negative meter lead to ground and measure the voltage to ground you will get zero. There's definitley a path to ground from Q2 collector, otherwise it wouldn't work. Try and imagine the circuit flipped the other way up so the ground connection is at the bottom.

[brett]

Hi
it looks fine to me.  There's nothing wrong with voltage going to ground via resistors - it just results in 100% voltage drop across the resistor(s).  Almost every conventional neg ground NPN circuit has some transistor bases and emitters tied to ground via resistors.  For PNP circuits, that becomes collectors tied to ground.
Keep digging for that bug - your ground should be at zero volts and supply at 9V with this circuit.
cheers

[snoof]

It is drawn rather confusing for sure!  I meant to say above that his wiring matches the layout, not that JD's layout itself was suspect .