I can't believe how much gain you are throwing away. Here is a list of the resistors that are at the beginning of each attenuator, along with the approximate attenuation of each divider:
R135 -9dB
R147 -24dB
R57 -33dB
R58 -12dB
R100 -22dB
You are throwing away approximately 100dB of gain! Throwing away gain means more noise and more distortion at the output.
The AGC for an AM radio is DC. Yours is audio.OK, looks like a little bit of feedback was coming in through the AGC. I filtered it. It's gone now.
I think he said the power supply is 9V.AG, how did you calculate transistor amplifier gains? I looked through the thread and couldn't find any mention of the value of Vcc. As we know, gain for these amps is highly dependent on Vcc.
Also, you left out the attenuators caused by R57 and R58 (which I mentioned above).I came up with about 100dB attenuation, which is 100,000.
I think he said the power supply is 9V.
Most of his transistors are loaded down by their load so they do not have a gain as high as 200.
I was slow replying because I was eating brunch at the same time so i didn't see his revised schematic with R57 added.
Then you have an audio compressor, not AGC. The gain of your IF changes with the amount of modulation, not with the strength of the signal.Now my AGC uses the audio. Then it RECTIFIES it and uses it to control the bias of an IF stage.
I saw somewhere in this thread that you were using 5.6V, but I thought it must be just for the output amp, because the base of Q19 would only be biased at 0.47V - not high enough to provide significant collector current.No, I'm using 5.6 volts. Don't ask me why I'm using that. It's just and old power supply I put together years ago and it has an output closest to 6 volts. I have a small sealed lead acid battery that is 6 volts. I have charged it from the sun many times. I think it will make a nice portable rig.
Shhhh. Q19 is an old AC127 germanium transistor.I saw somewhere in this thread that you were using 5.6V, but I thought it must be just for the output amp, because the base of Q19 would only be biased at 0.47V - not high enough to provide significant collector current.
Shhhh. Q19 is an old AC127 germanium transistor.
Where do you see that it is germanium? I don't even see that on the data sheet.
Anyway I wanted a low frequency transistor and the only other medium power transistors I had was some 2N2222's. I know those are high frequency and I don't want that. Wouldn't be much to re-bias one stage if I need to change it.
As far as a compressor. Yeah I will agree but that's all AGC is anyway. It is not uncommon to use the audio as the source. I am trying to keep it low power or minimum circuits so I figured I would get more drive from the audio. It works very well because loud audio peaks will cause it to clip and with a fast attack time I can minimize the clipping. It most certainly will follow the signal strength so all and all it is a two for one AGC and compressor. In keeping with minimum circuitry I still have a 1st IF gain control & 2nd IF gain control. It's intent is to be a quality communications receiver that can be run portable.
About the only problem I have left which is very minor is when I hook it to an old linear power supply I will get a small amount of "common mode hum". Inside the radio I did all I can do like shielding the 1st LO and choking the incoming power and reducing the size of audio couple caps to minimize lower audio frequency coupling.
Sorry, but a 2N2222 is exactly the type of transistor you should be using, it's essentially an audio or switching transistor, not an RF device.
You do not want to use transistor with high hfe in audio. They are prone to feedback and spurious responses.
The audio has only had a minimal amount if filtering added to it of which I will post when I get more time. It is working great and is completely linear with the gain distributed evenly among the stages. This is something that must be carefully adhered to in short-wave reception. So yes, it may appear a bit non-conventional but you cannot expect the audio for a short-wave receiver to appear anything like an FM network type of receiver lsuch as cell phones etc. Nothing at all like it.
Does anybody use Single-Sideband-Suppressed-Carrier anymore????? Why?
Yes you do, in fact audio transistors are generally the highest hfe of all transistor types - I've no idea where you've imagined 'prone to feedback and spurious responses' from? - if it's your circuit shown, it's because it's 'designed' completely wrongly.
Have you ever seen a circuit for a shortwave radio?, obviously not - your audio stages are incredibly poorly designed, and won't work well at all - you could simply throw every transistor stage out of there, leaving the opamp and output IC and greatly improve performance. You hardly need the opamp either, and could repelace that by a correctly designed single transitor stage, but an opamp is far easier to design.
LOL....and.....Ha-ha-ha-ha..... You give me a good laugh this morning as I'm sitting here listening to hams talking in SSB with perfect quality audio. I just installed the BFO yesterday.
And where do you think I get my designs for the audio? They are slight variations of stuff from the ARRL....lol.
Trust me. Audio design for SSB is nothing like FM. That's what your talking about. Stuff that comes in perfectly clear at the same decibel rate continuously. No challenge at all.
Oh, high frequency (hfe)? Who told you that? Somebody told you wrong. You want high hfe in stuff like rf front ends like an LNA where you want maximum gain (S/N). Maybe in small signal amplifiers. As you go down in frequency and start adding power, you do not want high hfe. They tend to want to operate at there rated frequency and so , especially at audio, they tend to amplify the high frequencies which could be undesired hiss or the creation of higher frequency oscillations.
Also in rf Power Amps high hfe can produce parasitic and spurious emissions unless that is the frequency you are designing for. Have you ever looked at a Motorola rf Data Book? Look at the hfe's and recommended frequencies of the transistor usage. You will usually find they are rated about three times the desired frequency. (I just looked up an MRF454 but didn't list "hfe").
Bottom line...It is not always desirable to have maximum gain or hfe which does the same thing. High gain for higher frequency transition. Consider the mixer circuit. Normally you will use a post mixer amp with an attenuator which helps limit the high signal levels and lowers the impedance or the crystal filter may pick up undesired noise.
Oh, did I mention I'm using a long wire antenna that is practically lying on top of my computer and I'm listening to guys talking half way across the country at 75 to 150 watts most of them. I must be doing something right
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