Electronics-Lab.com Community

# walid

Members

749

Never

## Posts posted by walid

1. ### How this ps unit works??

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

2. ### Impedance relation between 2 Bjts

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.

3. ### Impedance relation between 2 Bjts

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

4. ### Impedance relation between 2 Bjts

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?

5. ### LC and Oscillators

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

6. ### Why prefer to use the center tap???

I don't understand, please give me examples

7. ### 27MHz colpitts oscillator qestion

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:

[img width=680 height=70]
[img width=680 height=91]
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

8. ### LC and Oscillators

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

9. ### What does the 33uH coil in this circuit?

sorry there is no respond

10. ### Why prefer to use the center tap???

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

11. ### What is the function of this circuit?

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

12. ### A 1n4148 diode makes the transistor on and off?

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
13. ### Brightest White LED

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

14. ### Is it bad design?

thank you guru,
yes the transformer is cheap

15. ### Brightest White LED

How do measure the Brightest White LED and see if it is damaged or not?
thanks

Note: I have tried to test in the same way as the normal LED, but did not give me any reading from both sides

16. ### Is it bad design?

What is the function R3
If you know that the output voltage at both ends of the 220u capacitor is 9 volts DC, is not this resistance wastes 9mAh all of time
What is the point of that?
thanks
17. ### questio: L-network

I read in the book: RF Circuit Design, chapter 4 page 66

How can matching 1000 ohm source with 100 ohm load?
thanks alot

18. ### Why caps in paralell?

Another reason for using many capacitors in parallel is it gives lower impedance to higher frequencies if they're spread across the PCB traces.

I understand from your words that if I had two caps, one of them is equal to 5p and the second = 10p.
Better to use the two in parallel to get 15p and not one 15p to make an easy path for the high-frequencies toward ground???
19. ### Why caps in paralell?

the caps at the right, why 10p//15p? what if use one 25p cap?
thanks
20. ### How to distinguish between FM and AM transmitter circuits

Hi Hero999
I try 4.7k and 470 ohm but the freq remain at 50 MHz
I then changed the transistor to 2N3904 but it still 50MHz
thank you

21. ### How to distinguish between FM and AM transmitter circuits

Yes, it is not Hartley it is colpitts
I have not built this circuit, but I've found it within the ready-made examples in the program
I'm not confident in the results because the frequency stops always at 50 MHz and does not exceed that value no matter how reduced the values of capacitors
Here are pictures of results

thank you Hero999

22. ### How to distinguish between FM and AM transmitter circuits

The Circuit does not respond when I reduce the values of capacitors and inductor for higher frequency, but I am confident that you are true.
Thank you and to  Mr. Hero999

23. ### How to distinguish between FM and AM transmitter circuits

Good
I've implemented Hartley oscillator using a simulation program multisim 10
the first figure was with 6v batt

the freq = 1.15MHz as shown
then i used 9 v batt:

i had the same freq.
thank you

24. ### How to distinguish between FM and AM transmitter circuits

Yes, the frequency will go down as the battery discharges

Why scientists not put the battery voltage in the famous expression used to calculate any oscillator frequency?
25. ### How to distinguish between FM and AM transmitter circuits

Ok I get it,

"AM is produced because the transistor's gain alters as the base current is varied and FM because the capacitance between the PN junctions also alters as the voltage varies.
"

BUT: FM is alters as the voltage varies, you mean the voltage of the voice signal, not the voltage of the battery?

in the circuit above, if we omit the voice section with its preamplifier, will remain only the oscillator section
will it change its frequency with change in battery voltage?
thank you

×