diode clamping on input will clamp input. this may or may not do the trick for the output - that can work if the amplifier gain is low and if there is no shorted output. adding protection that directly protects output is always better.

 

I was only addressing protection of the input circuit, since that appeared to be what was being discussed. the 50 ohm series resistor does limit the output current a bit. Given what we are shown in any of the figures, a direct short to "ground" would not damage what we are shown of the output circuit. AND, that was also true for the circuits in post #4 and post#10. A direct to common will not cause any overload.
(comment deleted)

 

Why are any of these schemes better than diode clamping at the input??

Hi,

Where would it get clamped to?

I don't think that's the reason for the limiting action in this circuit, but again I don't like to guess when I see one-tenth of a circuit I like to see the whole thing. The thread starter still has not come back for an update, so we are all still just guessing anyway. I know that can be interesting too though.

Maybe it's a complex paper weight
Maybe it's a design on the side of a child's lunch box

 

Why are any of these schemes better than diode clamping at the input??

I‘ve seen ( and used) diode clamping and it works. Certainly easier to track on the pcb! However, the transistor circuit seems more popular.

 

Why are any of these schemes better than diode clamping at the input??

How does that protect against an output dead short to V+?

ak

 

I‘ve seen ( and used) diode clamping and it works. Certainly easier to track on the pcb! However, the transistor circuit seems more popular.

Hi,

Oh you mean a diode to +Vcc? That works as long as the +Vcc line can take the expected extra current during a protection event.

 

Hi,

Oh you mean a diode to +Vcc? That works as long as the +Vcc line can take the expected extra current during a protection event.

A Zener to V- or a couple of silicon diodes. It limits the voltage on the base, and therefore the voltage across the emitter resistor.
[Edit] I’m still assuming that it’s the VAS stage of an audio amp, in the absence of any clues from the TS.

 

FIRST, I was responding to the question of an INPUT overload. Diodes of opposite polarity, across the input, tied to the common. OR possibly multiple diodes in strings. No need for the V+ to absorb the excess. As for the output: the fragment shown didnot need protection.

 

In order to drive Q20 the base voltage must be negative since it's a PNP.

No. It's the \( V_{BE} \) that must be negative, meaning the voltage measured at the Base with respect to the Emitter, (not the Base with respect to GND), usually ≅-0.7V.

 

Not in posts #1,#4, #10, nor#18, do we see any positive voltage associated with the emitter of Q20. I could guess that there would likely be a positive voltage, BUT since it was evidently not considered to be anything we should think about, the obvious conclusion is that the TS deemed that part of the circuit unimportant.

POSSIBLY it should be considered that those folks asked for help with a circuit should be shown the whole circuit. It is unlikely that we would be confused.

 

Hi,

I did not mean to imply that a diode to +Vcc was never used, it is actually used a lot. It's just that a requirement is that there is always enough load on the +Vcc line to absorb any current that is the result of the clamping action. It does often work though as long as the expected max clamp current is not too high of course.