Capacitor after multivibrator, signal levels?

[neazoi]

Hi, in a hypothetical simple BJT pair multivibrator operating in the 500KHz region (RF), each output (BJT collector) swings between VCC and ground (or at least near).
So the output signal read on a scope, would be a square wave, with the lower end at 0v and the higher end at 5v.

Now what will happen if I connect a capacitor between the collector and the scope probe, and connect the ground lead of the probe to the ground?
What will be the signal read on the scope?
Will this be the same waveform, with the lower end to zero volts and the higher end to 5v?
In your answer, assume that the capacitor is large enough in value to let the 500KHz pass through.

 

[KlausST]

Hi,

did you never try such things?

Answer:
it highly depends on the duty cycle.
for a 10% duty cycle it is -0.5V / +4.5V
for a 50% duty cycle it is -2.5V / +2.5V
for a 90% duty cycle it is -4.5V / +0.5V

Klaus

 

[danadakk]

Possible differentiator waveform, depending on your coupling cap value, scope probe
used.....

Regards, Dana.

 

[neazoi]

Possible differentiator waveform, depending on your coupling cap value, scope probe
used.....


View attachment 177357


Regards, Dana.

I have noticed in the past that square wave easily passes through a capacitor and the output is still a square. My experiments were done on 1MHz back then and the capacitor was 100nf

 

[KlausST]

HI,

My experiments were done on 1MHz back then and the capacitor was 100nf

this is meaningless without resistor value.

The resistor value has the same impact as the capacitor value.

BTW: guess what happens when you switch a scope input from DC to AC mode: They put a capacitor in series.

Klaus