The circuit in the following figure (from: http://www.hobby-circuits.com/circuits/power-supply/dc-power-supply/891/variable-dc-power-supply) represents a variable power supply
Transistor Q1 is npn type

This was corrected in the following figure

I have thought carefully about this circuit, but I could not get to the mechanism of how it work!
I think that when the transistor Q1 operates, it must turn off Q2.
I try to simulate it using multisim10, but it didn't work, that is when I change the value of any of P1 or P2 the output is fixed at near the retectified voltage.
Any help in clarifying the way this circuit work would be welcome, thanks

Thanks audioguru
 I am very happy with your presence and I hope to be in good health and happy in your life.

I have read in a book "The art of electroncs" page 66, the following


I take it most likely the first stage is a source of signal with internal resistance (Zout1), and the second stage with input resistance of (Zin2),,,, what would you say?

Note: I miss you here too long and I had to register on many forums and I put the same question to ensure the receipt of a convincing answer from one of them.

Hi Hero999
At first I like to thank you on the fast of response



I mean here is if the output impedance of the first stage = 10k and the input impedance of the second phase = 10k
 Are these two impedances connected in series or parallel?
thanks alot

Hi,
If there are two BJT small signal amplifiers (common Emitter) cascade connected using a suitable decoupling cap
The first preamp has an o/p imeadance Zo, and the second has an i/p impedance Zin
Now we have Zo and Zin and a cap betwwen them
My question is: Is the connection series or parallel? That, is the o/p voltage divided or the o/p current?

Hi KevinIV,
Have you ever design an oscillator?
if yes, please tell me how with the related math
thanks

I don't understand, please give me examples

This circuit was intended as 27MHz colpitts oscillator:


and whem simulated using multisim10 sofware I get:


I have some questions:
Any osci. Needs a FB to sustain oscillations, I think that this FB from collector to base path through the parallel two caps; the internal transistor capacitance b/w collector and Emitter, and the C4(=4.7p) cap.
From the Fairchild datasheets of the 2n3904 transistor I get:



Also, there is a curve related the capacitance with REVERSE BIAS VOLTAGE (V)   
My questions:
(1) What is the approxi. value of the internal transistor capacitance b/w collector and Emitter at 27MHz?
(2) How the phase shifted from 180 degree at the collector to 0 or 360 degree at the base? Please give me a step by step illustration.
(3) The freq of the osci. was 25.4MHz while my calculaed value of the resonant freq was 27MHz? what factors affect this?
thanks 

Hi
I noticed in many books that LC tank is connected to the base of the transistor, while connected to the collector in a real designs. Why this confusion?
thanks

sorry there is no respond

Often observed in many of the schemes that the circuit designer prefers to use the center tap as in Fig.(1) and not, as in Fig.(2), why?
 Thanks

http://www.sentex.ca/~mec1995/circ/at121.html

I've built this emergency lighting circuit
 When the main voltage 230 V is present, LEDs turn off
 And when does not LEDs turn on
 Transistor and 1n4148 diode are responsible for making the LEDs turn off
 Question: How do the 1n4148 diode makes the transistor on and off?
 Thanks

note: the burnt R = 5.6k
When I implemented the circuit it worked very well

In fact, no one understand my question. I do not ask about the intensity of light, my question was about how to examine it. And to know whether is good or bad? Thanks

thank you guru,
yes the transformer is cheap