Does a cathod follower invert the phase?

[JGR]

Hey all,

Can anyone tell me if a cathode follower in a typical Marshall style circuit inverts the phase of the signal?

Thanks!

JR

[Satamax]

90 degrees out of phase with the input!

[Chris / CMW amps]

The cathodefollower-section (V2B, just before the tonestack) does NOT invert the signal.

[Satamax]

Chris, phase relation in a valve is not constant? +90 wrt the grid for the plate and -90 for the cathode? I thought so!

[Chris / CMW amps]

It's constant but depending on the use/circuit itself. At the plate: out of phase, at the cathode: in phase. (see V2B in a tweed 5E3 Deluxe as example)

[Satamax]

Chris, i don't know what you mean. The signal is 0 degrees on the cathode with relation to the grid?

[Chris / CMW amps]

Chris, i don't know what you mean. The signal is 0 degrees on the cathode with relation to the grid?

Yep!
Out of phase: 180 degrees, in phase 0 degree[s])

[Enzo]

As the signal rises at the grid, it causes more current to flow through the tube, this causes the voltage across the cathode resistor to rise at the same time. And due to the current through the plate load resistor, the voltage drop across the resistor rises, which means the voltage at the plate will decrease.

So the voltage changes at the cathode follow the voltage changes at the grid. Not in the sense of happening after or later, but in the sense of they do what the grid does, as in follow directions.

And the voltages at the plate are the exact opposite (other than amplitude) of those at the grid,

There is no phase lag between elements - we are taling audio here - the cathode is right in step with the grid, and the plate is 180 degrees out.

[Bruce / Mission Amps]

So the voltage changes at the cathode follow the voltage changes at the grid. Not in the sense of happening after or later, but in the sense of they do what the grid does, as in follow directions.

Hence the reason it is called a cathode follower...

[Satamax]

As the signal rises at the grid, it causes more current to flow through the tube, this causes the voltage across the cathode resistor to rise at the same time. And due to the current through the plate load resistor, the voltage drop across the resistor rises, which means the voltage at the plate will decrease.

So the voltage changes at the cathode follow the voltage changes at the grid. Not in the sense of happening after or later, but in the sense of they do what the grid does, as in follow directions.

And the voltages at the plate are the exact opposite (other than amplitude) of those at the grid,

There is no phase lag between elements - we are taling audio here - the cathode is right in step with the grid, and the plate is 180 degrees out.

Hence the reason it is called a cathode follower...

Thanks a lot Enzo and Bruce. Not my original question, but i've opened my big mouth to give a plainly wrong answer and stand corrected I don't know where i picked up that 90° ahead and 90° lag thing, over here of course, but about what???

Bye.

Max.

[Le Basseur]

I don't know where i picked up that 90° ahead and 90° lag thing, over here of course, but about what???

Me knows!Me knows!
It's the straight angle wich reaches the boiling temperature point at 90° Celsius!
(sorry folks,I couldn't resist!

[Arthur B.]

There is no phase lag between elements - we are taling audio here - the cathode is right in step with the grid, and the plate is 180 degrees out.

This is true even outside the audio band, since the grid to cathode capacitor is not charged appreciably in grounded plate operation.

[Enzo]

OK, while I learned electronics in an RF environment, it was 40-50 years ago, so I forgot a lot. Didn't want someone coming back with "No no no, at UHF there is a 4 degree phase shift..."

Thanks

[Satamax]

Me knows!Me knows!
It's the straight angle wich reaches the boiling temperature point at 90° Celsius!
(sorry folks,I couldn't resist!

Yeah all right!

But is it in relation with the rotation of the cow's tail or definately not? Or is there a Π fraction of the rotation of the cow's tail in relation with it???

[Arthur B.]

Since no explanation of grounded cathode and grounded anode operation is given, I'll give my own.

In the grounded cathode configuration, the input is the grid, the anode is the output and the cathode is the reference for both, hence grounded cathode.

The gain of this stage without the degenerative feedback of a cathode resistor is:

dv(a-k)/dv(g-k) = -mu (note the sign) or dv(a-k) = -mu*dv(g-k)

In the grounded anode configuration, the input is the grid, the cathode is the output and the cathode is the reference for both, hence grounded anode.

Before we go on, we need the anode to grid voltage. That is simply:

dv(a-g) = dv(a-k)-dv(g-k) = -mu*dv(g-k)- dv(g-k) = -(mu+1)*dv(g-k)

The gain of this stage is:

dv(k-a)/dv(g-a) = (-mu*dv(g-k))/(-(mu+1)*dv(g-k)) = mu/(mu+1)

Note that there is always a loss with the “cathode follower”.

Questions? Comments? Flames?