Thanks, audioguru!
The input impedance of the circuit is the input impedance of the transistor in parallel with R1 and also in parallel with R2.
yes, you are right about the input impedance of the "ciruit". i was refering/implying to the input impedance of the "transistor" as seen by the DC power supply--in the context of the author's explanation of voltage drop from 7.5 to 7.2 on R2.
audioguru said:
The input signal is strong enough to drive the approx 3.5k ohms input impedance of this circuit without much signal voltage loss.
what does "strong" mean here? Is "3.5k" big enough or small enough in your context? I guess, by "strong", you meant the signal source is some sort of constant "voltage" source, that it can impose a constant voltage drop on the load disregard the load's impedance. right? if this input signal is coming from a CD player or MP3 player or sound card lineout, can these signal be regarded
as "strong" enough? and why?
audioguru said:
It is connected to the input of the circuit and to ground.
The DC power supply has nothing to do with the input signal source.
Trying to understand this (simplified case), i am imagine a kind of hydraulic analogy of this: say we have a vertical pipe in 20M height, and the water in side is about 15M height (leaving 5M empty in the top).
this 15M water head is an analogy of the DC power; then in the middle of the pipe, there is a small hole, and a horizontal pipe is connected to the vertical pipe by the hole. these two pipes are not "conducted", but separated by a rubber plate, which is an analogy of the coupling cap (I borrow this cap analogy from
http://amasci.com/emotor/cap1.html); and finally there is a pump at the end of the horizontal pipe, simulating the AC signal input.
Is this a valid analogy for our topic (AC coupling)?
I am imagining that in the begining, the pump is not activated. the pressure at the hole is about 7.5M water head, and this pressue is transfered to the cap, and also to the pump throught the cap and the horizontal pipe (suppose the horizontal pipe is also full of water). so can i say that the DC power supply has effects to the signal (pump) in this case?
Now imaging the pump is activated, then the rubber is vibrating back and forth, which will in turn introduce vibration of the 15M water head in the vertical pipe (suppose the rubber plate is big enough to introduce apparent water head changes in the vertical pipe). so can i say that the DC power supply is affected by the signal via the cap? may be i went to far with this analogy...
Thanks again,
/bruin