Sime filter calculation help needed.

[Nordic]

Nuuk's class a buffer

Would someone please help me figure out what R3 is doing on the output?
Also how does R3 fit into the highpass filter on the output?
How would R3 be incorporated in calculations if you wanted to change the output cap.?

Also in another category, how would one calculate the gain for a circuit like this?

[richie00boy]

The 68R is to just provide resistive 'swamping' against possible likely cable (load) capacitance, any may offer a slight amount of protection against a momentary short on the output. It should not/does not form part of the 'output filter'.

The gain of the circuit is approximately unity (1) as it's an emitter follower with a current sink load.

[Nordic]

Thanks RB.
So then in standard config that is a highpass filter set to 1Hz and corner frequency of .5Hz?

Halveing the value of the cap should thus give 2Hz with corner frequency at 1Hz... does this translate into an audiable effect...?

[richie00boy]

No. I was trying to highlight that C1, R3 & R4 are not a filter.

C1 blocks DC from getting to what is being driven and also protects the circuit against a DC fault on the driven circuit. The corner frequency will depend on the input impedance of what is being driven.

R3 is as I said previously. It's impedance is so low it's negilgible for anything else.

R4 is simply to provide a charge/drain path for C1 when the circuit is not connected to anything. It will be swamped by the input impedance of what is being driven.

[Nordic]

I think I got the bit about the funtcion of the 68R resistor. Also it carries less than 0.0068V for every volt over the outpu, so I'm willing to discount it...

If you don't mind bearing with me me a little longer until I have enough info to make it usefull, you could mabe check my understanding...?


So lets assume the next stage being powered has an input inpendance of also 100k, would I parallel that with the 100K of R4 to get 50K as part of the highpass filter?

Would that be a less than ideal load? as it demands more current? In other words

[Nordic]

Been cracking my head, so far only figured out the operation of the top transistor... I see now that its configuration makes that the input impendance of the next stage makes a very small change to load compared to using a simple voltage divider...

[I_Forgot]

Many of your questions a can be answered by spending a few minutes with SwitcherCAD. It's free, and it can save you a lot of time, if you invest a little time learning to use it.

I_F

[I_Forgot]

here's the ac response out to 1 MHz...

I_F

[I_Forgot]

here's the DC operating point

I_F

[I_Forgot]

here is what happens to the low freq response when the load resistance is stepped from 2k to 50k Ohms.

I_F