Originally Posted by Shelton

Hi All

Can someone tell me what is the purpose of the 10n cap in parallel with the 1k resistor which is on the base of the transistor in this circuit ?

Thanks in advance.



Shelton.

hi,
When the npn is hard ON, the top of the cap/base charges to a +V potential wrt 0V.

As the npn is switched OFF the charge/voltage across the cap is reversed wrt to the pnp base so the pnp switches OFF faster.

The cap speeds up the switch ON/OFF of the pnp.

Do you follow?

Originally Posted by ericgibbs

hi,
When the npn is hard ON, the top of the cap/base charges to a +V potential wrt 0V.

As the npn is switched OFF the charge/voltage across the cap is reversed wrt to the pnp base so the pnp switches OFF faster.

Eric, this isn't true, because there is no pullup on the collector of the NPN. The cap simply discharges through the 1k resistor when the NPN turns off. It does nothing to aid in the turn off of the PNP.

The cap speeds up the switch ON/OFF of the pnp.

It does speed up the ON transition (or at least enhances the positive feedback), but it does nothing for the OFF transition, as I explained above.

Originally Posted by Shelton

Ok - so it is used basically to speed up the on signal for the PNP?

What is the difference when one connects a cap and a resistor in series compared to when one connects a cap and resistor in parallel - I know they are used in timed circuits, but is there any difference? I not talking about the circuit posted above, but in general terms ?

hi,
In general terms:
When a cap/res are in parallel the 'dc' component of the signal is not blocked.

With a res/cap in series any 'dc' component of the signal is blocked, so the following circuit does not 'see' the 'dc' signal, only the 'ac' signal.
Commonly used in audio amplifiers.

This simple explanation does not cover cases where the res/cap parallel or series network is used as a frequency filter.

For such a simple combination of res/caps in parallel or series, the analysis of circuit can be complex, depending upon many factors, such as frequency, source and load impedances.

Put simply, at high frequencies the capacitor will act like a short circuit so it will behave like a piece of wire; at DC the capacitor will not conduct so it will behave like a resistor and at intermediate frequencies the resistor and capacitor both come in to play but calculating the impedance is too complex for the average nube.