audioguru2
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Transistors with the same part number have differences. Some are very good and others are not. There might be the same differences between two different RF transistors.
hi AN920 and Audioguru 
hi AN920 i like your circuit
thanks
but still i have a question....
it will drift its frequency, can i use Audioguru`s circuit in the place of
your 100mW circuit.
and also i m thinking about using good RF transistors in the place of 2N3904.
i think it will do the job.
both of you plz think and any body who is reading this, and tell me about good transistors for using in the place of 2N3904.
thanks..
hi AN920 i like your circuit
thanks
but still i have a question....
it will drift its frequency, can i use Audioguru`s circuit in the place of
your 100mW circuit.
and also i m thinking about using good RF transistors in the place of 2N3904.
i think it will do the job.
both of you plz think and any body who is reading this, and tell me about good transistors for using in the place of 2N3904.
thanks..
audioguru2
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The frequency of the oscillator changes when the supply voltage changes. I used a low-dropout 5V regulator so my circuit's oscillator 's frequency doesn't change as its 9V battery runs down to about 5.5V. But then the output level of my oscillator is much less than AN920's oscillator.
You can use RF transistors if you want.
You can use extra coils and capacitors for impedance matching and filtering if you want.
You can transmit with a high power output which might cause interference then get caught by the RF cops who send you to jail if you want. ;D
You can use RF transistors if you want.
You can use extra coils and capacitors for impedance matching and filtering if you want.
You can transmit with a high power output which might cause interference then get caught by the RF cops who send you to jail if you want. ;D
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
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:
why this circuit using 1k as R1 and not 9K ?
can u help me in answring this question
thank u
audioguru2
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Hi Walid,
An electret mic should have about 0.5mA through it. For its FET transistor to have some gain the load/powering resistance should be as high as possible but still allow a few volts across the mic. So with a 9V supply, a 0.5mA current and about 5V across the mic the powering resistor should be 10k.
But the mic in this simple circuit is driving the low input impedance of the transistor. Therefore the FET transistor in the mic won't have much more gain if R1 has a higher value. difference.
The values of R7 and R8 are too low and are creating an attenuator to the output of the 1st transistor.
I notice that the value of C4 is too high. It cuts high audio frequencies above 1.2kHz. An FM transmitter is suppost to boost audio frequencies above about 2kHz.
An electret mic should have about 0.5mA through it. For its FET transistor to have some gain the load/powering resistance should be as high as possible but still allow a few volts across the mic. So with a 9V supply, a 0.5mA current and about 5V across the mic the powering resistor should be 10k.
But the mic in this simple circuit is driving the low input impedance of the transistor. Therefore the FET transistor in the mic won't have much more gain if R1 has a higher value. difference.
The values of R7 and R8 are too low and are creating an attenuator to the output of the 1st transistor.
I notice that the value of C4 is too high. It cuts high audio frequencies above 1.2kHz. An FM transmitter is suppost to boost audio frequencies above about 2kHz.
Hi guru
thank u very much
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!!!!
View attachment 40088
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!!!!
View attachment 40088
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audioguru2
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Correct. The drain of a jFET is a high impedance current sink like the collector of a transistor. But the signal modulates its constant current.walid said:
The constant current becomes only a little higher when a lower resistance of R1 increases the voltage at the drain of the jFET.
No. The signal modulates its constant current. Therefore a high load resistance allows the jFET to amplify the signal more than a low load resistance, like a transistor. A jFET doesn't have as much voltage gain as a transistor so changing the load/powering resistance doesn't make a big difference.
No. The next low impedance stage is in parallel with R1 so increasing the value of R1 won't make much difference. R1 should be about 10k so that the mic's jFET can provide some voltage gain, and the input resistance of the next stage should be more than about 30k so that it isn't a low load resistance to the output of the jFET.
If it works, it will have a very low RF output power so its range might be only across a big room or across the street.
Many electret mics don't work properly with such a low voltage and current.
Hi audioguru
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
you mean that R1 looks like RC with BJT, more RC value leads to more voltage gain.
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
View attachment 40091
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
View attachment 40091
audioguru2
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Hi Walid,
The jFET in an electret mic makes a pretty good constant current sink but it isn't extremely high impedance. I measured my mic with 10k as R1 and their total resistance was 3.33k ohms. Therefore the mic alone is 5k ohms.
The preamp transistor Q1 in that transmitter has a total input impedance of only 3.2k ohms. So increasing R1 to 10k for more gain from its jFET won't make much difference.
Increasing the total input impedance of Q1 won't make much difference.
R1 and the total input impedance of Q1 are the load for the jFET and both must be increased.
I think the total input impedance of the mic preamp should be 30k or more and R1 should be 10k if the supply voltage allows it. Then the jFET in the mic can have a reasonable voltage gain.
View attachment 40092
The jFET in an electret mic makes a pretty good constant current sink but it isn't extremely high impedance. I measured my mic with 10k as R1 and their total resistance was 3.33k ohms. Therefore the mic alone is 5k ohms.
The preamp transistor Q1 in that transmitter has a total input impedance of only 3.2k ohms. So increasing R1 to 10k for more gain from its jFET won't make much difference.
Increasing the total input impedance of Q1 won't make much difference.
R1 and the total input impedance of Q1 are the load for the jFET and both must be increased.
I think the total input impedance of the mic preamp should be 30k or more and R1 should be 10k if the supply voltage allows it. Then the jFET in the mic can have a reasonable voltage gain.
View attachment 40092
Hi guru
thank you for your efforts to explain
First, you skip my last questions with a colored table.
with 10k the total = 2424 ohm, big difference
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
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
audioguru2
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Hi Walid,
I think that R1 should be 10k so that the jFET in the electret mic gets about 0.5mA and a few volts.
I think that the input of the preamp shoud have a total resistance of 30k or more, so the values of the biasing resistors should be high and either have the transistor operate at a very low current to increase its internal Re or have an unbypassed emitter resistor, then its input resistance will be high enough.
If R1 is 10k and the total input resistance of the preamp transistor is 30k then the jFET has a total load of 7.5k which is much higher than only 762 ohms or your calculated 2424 ohms. Then it will have a reasonable amount of voltage gain.
The jFET in an electret mic has a certain value of transconductance which determines its voltage gain. Therefore if a jFET with a higher transconductance is used, the mic's sensitivity will be higher. Transconductance doesn't change very much from one jFET to another but the difference will be measurable. This jFET operates at about only 0.5mA and at a low voltage. jFETS that operate at 10mA and more and at higher voltages have a much higher transconductance.
If the jFET has too much gain then it will clip when the sound pressure level is high.
I have never made an electret mic but I think if its diaphraghm spacing is reduced then it will be more sensitive but then its max sound pressure level will be less.
I think that R1 should be 10k so that the jFET in the electret mic gets about 0.5mA and a few volts.
I think that the input of the preamp shoud have a total resistance of 30k or more, so the values of the biasing resistors should be high and either have the transistor operate at a very low current to increase its internal Re or have an unbypassed emitter resistor, then its input resistance will be high enough.
If R1 is 10k and the total input resistance of the preamp transistor is 30k then the jFET has a total load of 7.5k which is much higher than only 762 ohms or your calculated 2424 ohms. Then it will have a reasonable amount of voltage gain.
The jFET in an electret mic has a certain value of transconductance which determines its voltage gain. Therefore if a jFET with a higher transconductance is used, the mic's sensitivity will be higher. Transconductance doesn't change very much from one jFET to another but the difference will be measurable. This jFET operates at about only 0.5mA and at a low voltage. jFETS that operate at 10mA and more and at higher voltages have a much higher transconductance.
If the jFET has too much gain then it will clip when the sound pressure level is high.
I have never made an electret mic but I think if its diaphraghm spacing is reduced then it will be more sensitive but then its max sound pressure level will be less.
Hi guru
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
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
audioguru2
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Hi Walid,
His electret mic has a total load of only 762 ohms so it won't have any voltage gain.
His preamp transistor has a total load of only 2k ohms so its voltage gain is only 35.
The preamp doesn't have any negative feedback so it will be very distorted.
The 0.1uF capacitor at the input of the oscillator cuts high audio frequencies and the de-emphasis treble cut network in an FM radio cuts the high audio frequencies more.
That circuit is a mess!
My circuit has 10k for R1 and the input resistance of the preamp is high so the jFET in the electret mic has good voltage gain.
My preamp has a high resistance load so it has high gain.
My preamp has negative feedback at lower audio frequencies so it has low distortion.
My RF filter at the input of the oscillator is only 470pF so it doesn't cut high audio frequencies.
My preamp has pre-emphasis (treble boost) so that a radio cuts the boost down to normal.
My FM transmitter is sensitive and sounds great.
His electret mic has a total load of only 762 ohms so it won't have any voltage gain.
His preamp transistor has a total load of only 2k ohms so its voltage gain is only 35.
The preamp doesn't have any negative feedback so it will be very distorted.
The 0.1uF capacitor at the input of the oscillator cuts high audio frequencies and the de-emphasis treble cut network in an FM radio cuts the high audio frequencies more.
That circuit is a mess!
My circuit has 10k for R1 and the input resistance of the preamp is high so the jFET in the electret mic has good voltage gain.
My preamp has a high resistance load so it has high gain.
My preamp has negative feedback at lower audio frequencies so it has low distortion.
My RF filter at the input of the oscillator is only 470pF so it doesn't cut high audio frequencies.
My preamp has pre-emphasis (treble boost) so that a radio cuts the boost down to normal.
My FM transmitter is sensitive and sounds great.
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
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
Last edited by a moderator:
audioguru2
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Hi Walid,
The jFET in an electret mic should have a higher voltage gain when its load/powering resistor value is higher, not when it is lower.
At first glance I thought they are talking backwards. Then I thought about the tiny amount of current the jFET in the mic is conducting and how small is its voltage.
If the mic has 1V across it then its current is (3V-1V)/39k= 51uA. Maybe the jFET is conducting as much as it can and its voltage is only 0.5V then its current is (3V-0.5V)/39k= 64.1uA. I think it is very distorted.
Yours and my electret mics don't operate with such a tiny current. Maybe their "very sensitive type" does.
I think its jFET is saturated when it has a 39k load/powering resistor and is not as saturated with 33k. It will probably work better with 3.3k.
They talked about "motorboating". It will occur when the mic gain is high and the battery has a high internal resistance. It is fixed by adding a 100uF bypass capacitor across the battery.
The jFET in an electret mic should have a higher voltage gain when its load/powering resistor value is higher, not when it is lower.
At first glance I thought they are talking backwards. Then I thought about the tiny amount of current the jFET in the mic is conducting and how small is its voltage.
If the mic has 1V across it then its current is (3V-1V)/39k= 51uA. Maybe the jFET is conducting as much as it can and its voltage is only 0.5V then its current is (3V-0.5V)/39k= 64.1uA. I think it is very distorted.
Yours and my electret mics don't operate with such a tiny current. Maybe their "very sensitive type" does.
I think its jFET is saturated when it has a 39k load/powering resistor and is not as saturated with 33k. It will probably work better with 3.3k.
They talked about "motorboating". It will occur when the mic gain is high and the battery has a high internal resistance. It is fixed by adding a 100uF bypass capacitor across the battery.
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
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
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audioguru2
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Hi Walid,
That RF power transistor has a typical power gain of only 6.6dB at 175MHz in that circuit. Then for 25W output into a 50 ohm antenna its output voltage is 35.4V RMS or 100V p-p, when powered from a 13.5V supply. It needs an input power of 5W at 175MHz.
It is used in our Linear FM 30Watt Amplifier project with a higher supply voltage. Its RF input power is 3W.
It is also used in our Linear 50Watt Amplifier project with a 12V supply. Its RF input power is 16W.
I have never seen a power RF oscillator. Usually a low power ocillator is used then it is amplified. Some FM transmitter circuits use many RF amplifier stages. Our 3W FM transmitter has 4 RF amplifier stages.
That RF power transistor has a typical power gain of only 6.6dB at 175MHz in that circuit. Then for 25W output into a 50 ohm antenna its output voltage is 35.4V RMS or 100V p-p, when powered from a 13.5V supply. It needs an input power of 5W at 175MHz.
It is used in our Linear FM 30Watt Amplifier project with a higher supply voltage. Its RF input power is 3W.
It is also used in our Linear 50Watt Amplifier project with a 12V supply. Its RF input power is 16W.
I have never seen a power RF oscillator. Usually a low power ocillator is used then it is amplified. Some FM transmitter circuits use many RF amplifier stages. Our 3W FM transmitter has 4 RF amplifier stages.
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
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
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audioguru2
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Hi Walid,
That circuit is nearly the same as mine, except its frequency will drift as the battery voltage runs down because it doesn't have my voltage regulator. It also won't boost treble audio frequencies which are cut in FM radios.
The easiest way to increase its range is to use a 9V battery instead of only 3V. But then the 1st transistor will probably saturate and will need its base resistor's value increased or its collector resistor's value decreased.
The microphone's powering resistor is connected directly to the battery and the supply bypass capacitor has a low value which is good for RF, so the circuit might "motorboat" until you add a 100uF supply bypass capacitor.
The output transistor is connected without any negative feedback to control its operating point. Transistors have a wide range of current gain so some have low gain and will be cutoff in this circuit and others will have high gain and the transistor will be saturated. The simplest fix is to change the value of the 150k resistor to 68k then connect it to the collector instead of to the supply.
That circuit is nearly the same as mine, except its frequency will drift as the battery voltage runs down because it doesn't have my voltage regulator. It also won't boost treble audio frequencies which are cut in FM radios.
The easiest way to increase its range is to use a 9V battery instead of only 3V. But then the 1st transistor will probably saturate and will need its base resistor's value increased or its collector resistor's value decreased.
The microphone's powering resistor is connected directly to the battery and the supply bypass capacitor has a low value which is good for RF, so the circuit might "motorboat" until you add a 100uF supply bypass capacitor.
The output transistor is connected without any negative feedback to control its operating point. Transistors have a wide range of current gain so some have low gain and will be cutoff in this circuit and others will have high gain and the transistor will be saturated. The simplest fix is to change the value of the 150k resistor to 68k then connect it to the collector instead of to the supply.
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