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I am having trouble determining how to bias an NPN small-signal transistor for different operating frequencies. I would prefer a common-base design for a 2N2222, with a mid-range around 3 MHz. Anybody out there have any idea how this works?[/b]
I am having trouble determining how to bias an NPN small-signal transistor for different operating frequencies. I would prefer a common-base design for a 2N2222, with a mid-range around 3 MHz. Anybody out there have any idea how this works?[/b]
You need to be more specific. What are you trying to amplify? How much gain and bandwidth do you need? What is your input signal level? What are your source and load impedances?
I am trying to amplify shortwave signals between 2 and 4MHz. I would prefer to put the amplifier between the antenna and the tank. My antenna is a 6 foot dipole tv antenna, but I am familiar with winding transformers for impedance matching,so imput Z can be variable. The output will be attatched to a germanium diode detctor circuit at first, then later to a transistor detector set-up. Again, I can compensate for Z on the output. I would like the gain to be sufficient to drive the diode, but if I have to, I can forward bias the diode a bit to get it demodulating. I appreciate any help I can get with figuring this out.
I am trying to amplify shortwave signals between 2 and 4MHz. I would prefer to put the amplifier between the antenna and the tank. My antenna is a 6 foot dipole tv antenna, but I am familiar with winding transformers for impedance matching,so imput Z can be variable. The output will be attatched to a germanium diode detctor circuit at first, then later to a transistor detector set-up. Again, I can compensate for Z on the output. I would like the gain to be sufficient to drive the diode, but if I have to, I can forward bias the diode a bit to get it demodulating. I appreciate any help I can get with figuring this out.
Thank you Ron, I'll build this thing tonight & post my results. This schematic is completely different than what one usually sees in homebrew radio articles. I guess I better start digging through my parts bins!
A six foot dipole is going to be SERIOUSLY useless at 2-4MHz, you would be FAR better off arranging a decent aerial, rather than trying to amplify such weak signals.
You might try searching for ATU (Aerial Tuning Unit), which would help to maximise the poor signal from your aerial - there was a thread on here somewhere a few months ago.
After alot of swearing and muttering under my breath, I managed to scrape the circuit together. I get a light hum, which ain't bad to start.There is also a nice lightning storm here right now, or I would run a longwire antenna over the balcony of my 3rd-floor apartment. I guess tomorrow I can do the ol' "tie a rock to a wire and chuck it as far as you can" trick.I will also play with the layout some more.My determination and lunacy might just make up for my lack of education in electronics.Thanks to everyone who's pitched in their 2 cents,it helps.My coffee don't work anymore,goodnight.
I haven't played with a scanner for many years so I can't remember what there is at 3MHz.
The frequency isn't much above the AM broadcast band so maybe a ferrite rod antenna from an AM radio would work better than a long wire ligntning attractor and earth ground. :lol:
I have a bunch of ferrite cores I've parted out from junk radios, but I'm not sure what the permeabilities are. Without a inductance meter, I'd pretty much be taking a stab in the dark as to how to wind them. I'm guessing most am ferrite bars are around 125 on the permeability scale,but you never can be sure. Also, every time I use a ferrite core, I get radio disney saturating my coil- it's the most god-awful station on earth.It bleeds through even when I decouple the antenna! Maybe a soup can would have less permeability,but just enough to help pull in the low SW stations. I suppose I could also try a tunable ferrite slug as well. Anyhow, I scrapped the "radio" this morning,except the tank(that took me abut 2 hours to make last night), and I rebuilt it with a better layout. Now there is no hum, but on a piezeoelecric earphone I can hear something,like intermittent crackles.I've built a couple AM regens, so I can tell there is a signal there, I'm just not getting it yet.I also only used 7 volts for the power supply, so before I make any changes I will try 12 volts and see what happens. I do have a question, though. Does anyone know what the output impedance of this circuit is? My earphone has a gazillion ohms of impedance,and I'll probably need to adjust my output. I know tht there is a WWV station on 2.5MHz, but not sure what else,but I aim to find out. I have a shortwave radio on my bench, but it's not the same as listening to your own,no matter how bad it might sound or how hard is is to tune.
I have a bunch of ferrite cores I've parted out from junk radios, but I'm not sure what the permeabilities are. Without a inductance meter, I'd pretty much be taking a stab in the dark as to how to wind them. I'm guessing most am ferrite bars are around 125 on the permeability scale,but you never can be sure. Also, every time I use a ferrite core, I get radio disney saturating my coil- it's the most god-awful station on earth.It bleeds through even when I decouple the antenna! Maybe a soup can would have less permeability,but just enough to help pull in the low SW stations. I suppose I could also try a tunable ferrite slug as well. Anyhow, I scrapped the "radio" this morning,except the tank(that took me abut 2 hours to make last night), and I rebuilt it with a better layout. Now there is no hum, but on a piezeoelecric earphone I can hear something,like intermittent crackles.I've built a couple AM regens, so I can tell there is a signal there, I'm just not getting it yet.I also only used 7 volts for the power supply, so before I make any changes I will try 12 volts and see what happens. I do have a question, though. Does anyone know what the output impedance of this circuit is? My earphone has a gazillion ohms of impedance,and I'll probably need to adjust my output. I know tht there is a WWV station on 2.5MHz, but not sure what else,but I aim to find out. I have a shortwave radio on my bench, but it's not the same as listening to your own,no matter how bad it might sound or how hard is is to tune.
I just added a .01mF at the positive end of the 75k resistor, then attatched a germanium 1N34A to it. My earphone is connected to the diode and ground,like a simple crystal radio.Sorry I can't yet post a schematic, I'm working from webtv.I guess I could just as easily attatch a transistor demodulator, the output impedance would go well with the hi-z earphone. Any ideas?
You connected your AM detector to the power supply of Ron's common-base amplifier circuit. Its output is at its collector, where you should connect your cap and diode. :roll: :wink:
When I recommended using a ferrite rod antenna from an AM radio, I did not mean to use a ferrite core from an IF transformer. :roll:
Thank you for pointing that out AudioGuru. Now I get stations, even though they are very very faint.But, I got what I asked for, and I appreciate everyone's help.I hooked up 12volts and it helped, nothin' blew up. Now I have a question, why is the bandwidth of this circuit from 2 to 4MHz? What adjustments to this circuit would change it's operating freqency? I have a basic understanding of transistors, which means I don't really know squat. Also, does anyone know the output impedance of this circuit, and how do you calculate the impedance without rocket-science math? I will now attempt to build an audio amp to lift some of these signals up out of the noise,plus tinker with the layout a little more,and maybe shield the coil.Thanks again!
Hi Reitan,
Ron's common base amp works best with a 12V supply. It will be about the same with a 9V supply if you change its R3 to about 56k.
It will have more gain if you reduce the value of R1. 100 ohms will have a lot more gain but the input impedance will be lower.
The bandwidth is determined by L1 and C2. Reduce their values to increase the tuned frequency.
The circuit's output impedance is the collector of the transistor in parallel with the tuned circuit. The collector is an extremely high impedance, and also is the tuned circuit. Your detector and earphone are probably loading them down. You can add an emitter-follower to match the high impedance to a lower impedance, add an adjustable RF amplifier and add an adjustable audio amplifier stage like this:
Hi Nigel,
OOps!
You are absolutely correct.
I wanted to keep the input impedance of the emitter-follower as high as possible.
I've changed it so that the DC for the base of the emitter-follower is divided down, but used bootstrapping to keep a very high impedance (even higher than before) and very little signal loss:
You can't bootstrap an emitter follower's base that way.
A better solution is to put about 10k between VCC and the tank, bypass the upper end of the tank to GND, and direct-couple the emitter follower like you had it originally.
Also - a 2N2222 has too much Cob to get good bandwidth with a 5.6k load.
A better choice would be 2N5179, or any number of more modern types.
Or you could increase the current in the 2N2222.
Actually, I have been wondering why our OP picked 2N2222. Not a great choice for an RF amp, even at 4MHz. I am guessing that he has one or more of them.
The reason I chose the 2N2222 was because I have some of them and you see them everywhere. I have others as well, plus I can get anything I need at a NTE outlet right down the street. I guess my new question is this-What transistor would be best to use in radios of this type, and how would that change the schematic? Also, if the bandwidth were to change from 2MHz to 6Mhz, would the gain be pretty even through the whole range? I'm talking here of adding taps on a coil for different frequency ranges.
To be honest, Audioguru's diagram is labelled "simple radio" - and it doesn't look very simple to me!, it's performance isn't going to be as good as far simpler conventional designs.
Perhaps the thread starter ought to actually specify what he's wanting to do?, and perhaps why?.
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