Capacitor Values and Opamps Driving Capacitive Loads

[wigman27]

Hey

I am slowly working my way through all of the videos from the start :-) only 450 to go!! I am really enjoying them. So I am sorry if these videos already exist, if they do, could someone please guide me in the right direction :-)

Recently there has been a bit of talk around why OP AMPS oscillate driving capacitive loads (like a MOSFET) mainly to do with driving Dummy Loads are the ones I have seen. Would it be possible to do a tutorial on phase shift and what is actually going on?

Also, I watched your capacitor types tutorials which were great! however, would it be possible to do a tutorial on picking the best values, I know that its such a broad thing but for example a good filter cap setup for input filtration, good values for bypass and filter caps, and how to determine these values and what things should be considered, where and when to use the types you have described would be awesome 

Thanks Mate.

Lee

[Neilm]

Op amps go unstable due to the fact they have resistance at the output. This then creates a RC filter that connects to the feedback.

Have a read of this app note from Maxim. It explains how this works and gives simulations to show how this works.

Neil

[wigman27]

That is a fantastic document!

Thank you

[wigman27]

I think a tutorial on phase shift would be awesome, especially how it effects a DC driven OP-AMP and MOSFET Circuit.

Its hard to try and get your head around phases and frequencies when dealing with DC.

I can work out some of the hardest unbalanced delta and star power loads, sequence impedances and fault level calculations.. But can't get my head around working with frequencies and phase shifts in a DC circuit. Everything I have found talks about AC circuits.

Thanks :-)

Lee

 

[wigman27]

One very basic thing I was neglecting to consider is the fact that DC and AC are exactly the same when you first turn the circuit on, is it the initial charge current and the out of phase current this causes that starts the initial oscillation? Then is basically drives itself thereafter?

 

[Neilm]

One very basic thing I was neglecting to consider is the fact that DC and AC are exactly the same when you first turn the circuit on, is it the initial charge current and the out of phase current this causes that starts the initial oscillation? Then is basically drives itself thereafter?
 

There are some circuits that use that fact to start oscillation. There is one that uses 2 op-amps. One amp feeds the other and generates a sine (and cosine) wave at a reasonably stable frequency. (I just wish I could remember the name of it)

Neil