maybe its the transistor...

[mstechca]

when dealing with my circuit, I notice that a certain combination of transistors brings in radio stations clearer than other combinations.

So far, the best combination is 2N3904 in the superregen detector, BC547's in the buffer amp and the amp driving the push-pull pair, and the TIP31 and TIP32 transistors forming the push-pull pair.

When I changed the BC547's to 2N4401's, it seems that I can't pick up stations, even though every passive component is the same. Also, when I changed the 2N3904 to 2N3903, I get more oscillations than better station reception.

I was trying to find a good substitute for BC547 because those are expensive. They cost 50 cents each while a 2N3904 costs 15 cents. Add them up, and you will see why.

Is there a transistor that is better suited for generic VHF that works better than 2N3904?

I must be failing to understand something about transistors.

 

[D.J.]

Without looking up the specs on those transistors, I would say that they are best suited for the purpose you used them. If you want a very high speed switching transistor you use a 2N2222.

So if you use a low speed switching transistor for a 50 Mhz oscillator, then it probably wouldn't work.

D.J.

 

[audioguru]

when dealing with my circuit, I notice that a certain combination of transistors brings in radio stations clearer than other combinations.

So far, the best combination is 2N3904 in the superregen detector, BC547's in the buffer amp and the amp driving the push-pull pair, and the TIP31 and TIP32 transistors forming the push-pull pair.

When I changed the BC547's to 2N4401's, it seems that I can't pick up stations, even though every passive component is the same.

What does a different transistor in a buffer amp (audio?) have to do with a radio's sensitivity? The automatic gain control circuit in the radio should deal with its sensitivity.
The BC547 (which one did you use?) has a very high gain but can't handle high current. The 2N4401 handles much more current but has low gain. Didn't you see it on their datasheets?
Post your audio amp circuit so we can see why its biasing depends on a transistor's high current gain.

Also, when I changed the 2N3904 to 2N3903, I get more oscillations than better station reception.

Didn't you notice in their datasheet that the 2N3904 has double the gain of the 2N3903?

I must be failing to understand something about transistors.

It looks like you don't look at, nor understand datasheets.

 

[mstechca]

I do seldomly look at datasheets.

I guess I'll have to download datasheets now.

 

[shamikrudra]

when dealing with my circuit, I notice that a certain combination of transistors brings in radio stations clearer than other combinations.

So far, the best combination is 2N3904 in the superregen detector, BC547's in the buffer amp and the amp driving the push-pull pair, and the TIP31 and TIP32 transistors forming the push-pull pair.

When I changed the BC547's to 2N4401's, it seems that I can't pick up stations, even though every passive component is the same. Also, when I changed the 2N3904 to 2N3903, I get more oscillations than better station reception.

I was trying to find a good substitute for BC547 because those are expensive. They cost 50 cents each while a 2N3904 costs 15 cents. Add them up, and you will see why.

Is there a transistor that is better suited for generic VHF that works better than 2N3904?

I must be failing to understand something about transistors.

BC 547 is not VHF transister u will find replasment in google try it out first other wise send a massege to me

 

[audioguru]

Hi Shamik,
Most simple FM transmitters and super-regen radios use common transistors such as BC547, 2N2222 and 2N3904 because they have a transition frequency of 300Mhz. Their performance at VHF is more than adequate for these simple circuits. :lol:

 

[D.J.]

I would agree.

I have the data sheet for a 2N2222, and it switches at 100Mhz.

D.J.

 

[audioguru]

Hi DJ,
My datasheet shows the 2N2222 operating to 250Mhz.

 

[D.J.]

I can't find my data sheet but I thought it went up to 100Mhz. Thats why I got it becuase of the high speed switching.

I probably read it wrong

Maybe it goes higher, but anyway, a 2N2222 should work for high frequency.

D.J.

 

[mstechca]

Let's just say I wanted to receive any signal above 400Mhz.

I still think that all transistors with a rated transiston frequency under 400Mhz will almost be useless unless they have a ridiculously high Hfe (1000+).

Because a 2N2222 has an Ft of 300Mhz, does the gain drop by 1/3 because the frequency of 400Mhz is 1/3 more than 300Mhz?

I need full clarification on frequency transistion and Hfe, because tomorrow, I am going to look for VHF compatible transistors.

 

[audioguru]

Let's just say I wanted to receive any signal above 400Mhz.

I still think that all transistors with a rated transiston frequency under 400Mhz will almost be useless unless they have a ridiculously high Hfe (1000+).

Hfe has nothing to do with a transistor's transition frequency.

Because a 2N2222 has an Ft of 300Mhz, does the gain drop by 1/3 because the frequency of 400Mhz is 1/3 more than 300Mhz?

A 2N2222 has a minimum transition frequency of 250MHz. Therefore it is guaranteed to have a gain of 1 (no gain) at 250MHz or more. At lower frequencies it will have as much gain as its rated Hfe, with its gain increasing twice with each lower octave (1/10th the frequency).

Hfe is its AC current gain, how much more the current changes in its collector-emitter as caused by current changes in its base-emitter. As described above, it decreases half for each 10 times the frequency.

An oscillator or super-regen detector needs a gain of only a little more than 1 to function, too much gain causes overdrive.

tomorrow, I am going to look for VHF compatible transistors.

You will need to modify the circuit to reduce the gain of a VHF transistor in these circuits. A VHF transistor is useful as a VHF amplifier, and is not required for an oscillator unless you operate it at its transition frequency. :lol:

 

[mstechca]

A 2N2222 has a minimum transition frequency of 250MHz. Therefore it is guaranteed to have a gain of 1 (no gain) at 250MHz or more. At lower frequencies it will have as much gain as its rated Hfe, with its gain increasing twice with each lower octave (1/10th the frequency).

Does this mean that at 225Mhz the gain is 2 and at 200Mhz the gain is 4?

An oscillator or super-regen detector needs a gain of only a little more than 1 to function, too much gain causes overdrive.

What is the absolute minimum gain required to turn the oscillator on?

Maybe I can take a transistor having an Ft of 600Mhz and turn it into a 1Ghz detector, but that will be left for another day.

 

[zachtheterrible]

hey mstecha, just look at the datasheets. They show a plot of gain vs. frequency.

 

[audioguru]

A 2N2222 has a minimum transition frequency of 250MHz. Therefore it is guaranteed to have a gain of 1 (no gain) at 250MHz or more. At lower frequencies it will have as much gain as its rated Hfe, with its gain increasing twice with each lower octave (1/10th the frequency).

Does this mean that at 225Mhz the gain is 2 and at 200Mhz the gain is 4?

Sorry, I made a mistake. The gain doubles with each halving of frequency, so 125MHz has a gain of 2, 62.5Mhz has a gain of 4, etc.

An oscillator or super-regen detector needs a gain of only a little more than 1 to function, too much gain causes overdrive.

What is the absolute minimum gain required to turn the oscillator on?[/quote]
1.0000000001, but it will take an awful long time to reach its max steady output.

As Zach says, it's all in the datasheet, but the curves apply to a typical transistor and the minimum transistors are worse.