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# walid

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1. ## a very simple FM Tx

Hi guru First thank u for the figures from which i can see that the 1 nF has no resonable effect at AF. but u may not see this question I agree that it is complicated to calculate how much is the output voltage and current, lets assume them and investigate the tapping effect on them: If the output voltage = 5mV rms at freq 100MHz as shown in the Fig. below, how much voltage reach the antenna? and what the value of the impedance the antenna see, if the coil is 330nH and its XL = 207 ohm at 100MHz? thank u guru these flowers are for you
2. ## a very simple FM Tx

Hi guru This is good, but I want to introduce the effect of the 1nF cap on the lower 47Kohm. The reactance of that cap at freqs from 300Hz t0 15Khz is 10.6 Kohm to 53 Kohm 47K // 10.6K = 8.65 K, and 47K // 53K = 24.9K so VB is not constant at VB = 9 * 47/(47+47) = 4.5v as i was think it is really changes from 1.4V to 3.12V with the audio freq Now Zin (total) = ZB//RB1//RB2 since RB2 equivalant is change from 8.65 K to 24.9K so Zin change please comment. I agree that it is complicated to calculate how much is the output voltage and current, lets assume them and investigate the tapping effect on them: If the output voltage = 5mV rms at freq 100MHz as shown in the Fig. below, how much voltage reach the antenna? and what the value of the impedance the antenna see, if the coil is 330nH and its XL = 207 ohm at 100MHz? thank u guru
3. ## BJT biasing formulas

Good evening Guru Thank u for fast and good reply Now I'll not ask you about filters and the -3dB, I'll postpone this to another discussion, but I noticed that C1 and the other Rs are not intended as a filter. You choosed C1 as the smallest possible cap to couple the two parts of the circuit, and not els. At high frequencies, to what range? To a freq make Xc4 = R5, if so, to f=2258Hz a human voice. Xc4 at 15KHz = 70.7 ohm in parallel with R5= 61.5 ohms, not so far. I know this is not problem, and this occur whe somebody do fast calculation. Two questions: (1) why u choose the 15KHz personally? (2) I understand [10k/(71.1 + 86.70)] but not 0.707 I know that 1/sqr(2) = 0.707 but why multiplying the Av by it, is it a known formula? this statement answer the #1 question above, 300Hz is the lower limit of the human voice and the 15KHz is the upper limit. The idea is very clear and you provide me with a very good and new information, but to others who may read our discussion, I tell them GURU has some headache because he answring hunreds of questions every day, GOD save guru from any bad thing. At 300Hz, the reactance of the 150nF C4 capacitor is 3.5k 3500//470 = 414 ohm Then the voltage gain is only 10k/(414 + 86.7)= 20. So the treble boost at 15kHz is about 3.2 times. NO DIFFERENCE GURU you are good man and "Electronics Lab - Community = GURU" Thank u for every word you write to help me.
4. ## BJT biasing formulas

Hi guru u enlarge the discussion and introduce a new aspects -at least foe me- and i can
5. ## a high pass filter question

Thank you guru it is evident and good answer
6. ## BJT biasing formulas

Hi guru please look at the Fig below. It is thr first stage of your FM TX I omit C2 to make it more simple to discuss Where is the Negative feedback in your circuit (below) please put some examples to a current source and bootstrapping and how they connected to collector you guru add a CE to your Tx thank you
7. ## BJT biasing formulas

Hi I said before, at 12/6/2006 that: today I'll discuss with audioguru and you, the effect of adding a bypass cap to RE, its name is CE Till now we can design a perfect voltage divider BJT transistor configuration without CE When adding this CE, two circuit parameters are changes: (1) The voltage gain Av now = RC/re and not as before adding CE it was RC/RE. With CE, you get mor Av, and distortion takes place. (2) The i/p impedance seen at the base, ZB, ZB = hfe*re, it is very smaller compared with the case without CE (ZB = hfe*(re+RE)). I think that the total Zin (= RB1//RB2//ZB) will not affected much because of the paralle connection. my question: are there any other parameter affected by adding CE, and Why we disregard the distortion accompanying with the increased Av? thank you
8. ## a high pass filter question

Hi when we come to design a high pass filter like that formed between stages Here C is the coupling cap and R is the i/p impedance of the next stage the cutoff freq = 1/(2 pi R C) To make the cutoff freq = say 100 Hz, u can: take C = 10 uF and R = 160 ohm OR take C = 1 uF and R = 1.6 Kohm You can design it with different values of R and C MY question is what the differance between them? thanks
9. ## Audioguru FM Tx

Hi guru yes exactly, i noticed this. I'll redesign it after your help, and with u makeing its range more. I know that this circuit is not better than yours, but I have a chance with this. when I build a circuit and didn't work it is more easy to me to build anthother new one than searching for the problem in the first, it is very complex and make headache. it is hard to me, after 4 hours of work and organize, to look again for the causes, especially if i use a used parts from my junk, the possibility is large. please explain this statement. If I increase RB or decrease Rc, then I'll have more voltage gain! u mean "motorboat" after using 9 v batt also your FM Tx has no -ve FB I need more explaination to this point. to make the last stage a collector feedback configuration, but doind this adds what to the circuit. thank u guru
10. ## Audioguru FM Tx

Hi guru I build this circuit: I followed exactly their instructions I put parts very close to each other and very close to the board it work fine i built it in aboard with holes, and its area was 15 holes X 5 holes, it is very small and crowded with parts i put it in my hand, so i toutch every part including antenna, and this did not affect the freq at all I use a mic from an old tape recorder national panasonic, and i use the 39k resistor and the voice was fine i omit only the variavle cap and depend on the coil length to slightly change the freq the only problem is its range in an open place its range about 50 meter only can y help me to increase the range of this very good and very stable Tx. thank you guru
11. ## Audioguru FM Tx

Hi guru for the circuit shown below, someone ask my the following question: Can I increase the range of this transmitter by using power transistor like BLY89 and decreasing the RE? I attached the datasheets of BLY89 i look at that datasheets and see it max freq = 175 MHz i think it can handle the 88MHz FM freq i know it is can not be used in stead of that small signal transistor but i need your explanation to make the answer reasonable. thank u guru bly89.pdf
12. ## Audioguru FM Tx

Hi guru please read the following: "The sensitivity of the WASP depends to a large extent on the value of the load resistor on the electret microphone. We have used 39k in the project, as the microphone we supply is a very sensitive type. If you wish to increase the sensitivity to super performance, the resistor can be decreased to 33k but don't go any lower otherwise the circuit may 'oscillate' or 'motor-boat.' " this from: http://www.talkingelectronics.com/Projects/Wasp/Wasp-P1.html please guru, comment. thank u
13. ## Audioguru FM Tx

Hi guru hehehehe I'm sure u have never made an electret mic. somewhere i read that his mic is very sensitive and can pick up all voices inside the room, like u said about your FM Tx. others said that you must talk near it.... from that, one deduce that it is posible to increase or decrease mic sinsivity by, say, increase Zin of the next stage, decrease R1...etec I want your help in this point thank you very much
14. ## Audioguru FM Tx

Hi guru thank you for your efforts to explain First, you skip my last questions with a colored table. with 1k the total = 762 ohm as u said with 10k the total = 2424 ohm, big difference I calculate Zin at the base the Ic=0.1 mA, Zin = 52K to make it 30K, Ic must = 0.2 mA If i do so, what are the side effects on the circuit another general equestion: how the designers increase the sensivity of the mic, what they do? thank u guru
15. ## Audioguru FM Tx

please don't skip the above questions, i forgot to ask this important one: what the optimum impedance value that mic like to connect with, that is R1// Zin of the next stage? thank u again
16. ## Audioguru FM Tx

Hi audioguru yes, it is very good point. i experiment with 3 mics, applying 9 volt dc to each and measure the drawn current with 1k then 10k ohm resistors. i had the resuls shown in the table below. look at it, changing R from 10K to 1K increasing the current only small amount, it is really a high impedance current sink. Now I see by my eyes that decreasing R1 value will not increase the mic current segnificantly. OK a high load resistance, u mean R1 you mean that R1 looks like RC with BJT, more RC value leads to more voltage gain. as a role always take R1 = 10K If the next stage i/p impedance is in parallel with R1, and the next stage i/p impedance is 30K, then 30//10 = less than 10 and 30// 5 = less than 5, so it is not true to say: the value of R1 won't make much difference. ** the last question If we conclude that decresing R1 will not increase the mic current, and cosequently will not increase the sensivity of the mic, and it is prefered to make R1 big value (10K), then: 1) how the designers increase the sensivity of the mic, what they do? 2)Why someone see many different values for R1 in simple FM Txs. guru, thank you very much for everything u learned me. walid
17. ## Audioguru FM Tx

Hi guru thank u very much lets discuss it: 1- is the mic is a constant current source that its current not effected by changes in R1 value. or this current increased by decreasing R1 value. 2- the sensivity of mic is increased when more current pass through it> 3-when mic circuit driving a next low impedance stage, R1 must be low value,why? beccause a low impedance stage wants more current than a high impedance one, so the mic's current must be greater. What abou the circuit below, its Vcc is only 1.5 v!!!!
18. ## Audioguru FM Tx

Hi audioguru after my last question, another friends take the subject to a more complex corner, so i back it to its simplist form again. In our discussion about R1 (=10K) we conclude that most of mics drawing 0.5mA and have 1/2 vcc across them so R1 must be 5K or 4.7K, and you told me that your mic is from a telephone board and it draws only 1/4 mA so R1 =10k is ok I understand you and all this is oK I take these info and teach it to other friend. today he asked me about this circuit: he asked me the following question: why this circuit using 1k as R1 and not 9K ? can u help me in answring this question thank u
19. ## a very simple FM Tx

Hi guru I feel that i not understand what you want to say. Lets analyze that statement: I assume that VE = 3v VB = 3.7v positive feedback changes the emitter voltage to say 4 v so VB = 4.7v now what that cap do to maintain VB to 3.7 v thank u guru
20. ## a very simple FM Tx

Hi guru why the circuit probably won't oscillate, can you please, explain this point more. What would u do to improve the matching between o/p and antenna? beta = 200, are u guess it or depend on some source or you consider any general purpose transistor's beta as approx 200. also you calculate the internal emitter resistance (re) as beta X re =500 ohm, that is re = 500/200 =2.5 ohm, frome this: 25/Ic=2.5 ==> Ic = 10 mA, is these values from datasheets, I search for BC547 datacheets and find many copmanies, please provide me with your datasheet or tell me from any comany. But with tap, the coil acts as a step down transformer? Are you mean the capacitance between B and E? thank you alot
21. ## a very simple FM Tx

Hi guru ok, i agree my circuit did not using a varactor diode, there is no varactor diode i think that a p-n junction do this, is this true and how?
22. ## a very simple FM Tx

i read it but i not understand any thing it is not as i expect can u guru point me to another article more close to what i need can u help me in understanding carfully how this vco operate thank u
23. ## a very simple FM Tx

Hi guru i read carefull your last reply but it was not sufficient to make me understand so, i now reading this doc. http://www.maxim-ic.com/appnotes.cfm/appnote_number/2032 may it help me then i'll back to u to finish this hard subject thank u guru
24. ## a very simple FM Tx

Thank you guru u are a good man why it need to be a common-base, what would be happen if we remove this 1n cap. I know that
25. ## a very simple FM Tx

the following FM circuit I have tree questions: 1- what the function of the 1 n cap 2-why tapping the coil, why not connecting the antenna directly to the collector 3-what the function of the 1.8p cap, and why it is so small thanx
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