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Old 20th Feb 2019, 1:47 am   #1
Jolly 7
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Default Biasing resistors for Germanium transistor oscillator

Thanks to a kind member on the forum, I got the attached circuit diagram to help me build a superhet radio using germanium transistors. I cannot make out what the values of R1 and R2 should be to bias the oscillator/ mixer transistor. Assuming that 200 mV or thereabouts is needed, I calculated R1 and R2 to be 22K and 1K respectively, if using a 6V power supply. Does this appear to be correct ? I might use OC44 or similar.
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Old 20th Feb 2019, 2:25 am   #2
Argus25
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Default Re: Biasing resistors for Germanium transistor oscillator

I'm not certain about the resistors.

However as you can see with that circuit, it has neutralized IF stages.

Depending on the exact vintage germanium transistors you put in there (of the early germanium types) the value of those two capacitors might have to be adjusted to prevent instability in the IF.

The best way around this is to use a transistor type for the IF's that does not require neutralization at all and remove those two capacitors. The AF127 will be just fine as its C-B feedback capacitance is so low.

You will notice that none of the newer transistor radios you have been working with have neutralized IF's, because silicon RF transistors, just like the AF127, have a very low feedback capacitance.

Something like the OC 45, always requires neutralization in a IF and the exact value of the capacitor (and sometimes series resistor) required also depends on the design of the IF transformer, where the primary center tap is. So if you are making a radio from scratch, you will need how to neutralize the IF stages correctly to prevent instability if you go with types similar to these with high range base-collector feedback capacitances.
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Old 20th Feb 2019, 3:07 am   #3
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Default Re: Biasing resistors for Germanium transistor oscillator

As this is a learning curve for me , I am building each stage of the radio on a separate PCB with diy transistor sockets to allow me to try out various transistors. Currently I am making the mixer oscillator. I will of course bear in mind about trying not to use neutralisation capacitors when I build the IF stages. I eventually would like to join the stages together and see if it will function as a radio.

I was wondering if there will be any readable oscillation frequency in the standalone oscillator stage, where can this be measured and what should it be ? I am using a standard red AM Toko oscillator coil with 3 pins on one side and 2 on the other.
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Old 20th Feb 2019, 3:24 am   #4
Argus25
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Default Re: Biasing resistors for Germanium transistor oscillator

The oscillation of the oscillator stage is dead easy to see on the scope at the transistor's collector or base terminals, or across the V/C that tunes the osc coil or on any of the osc coil terminals except ground or power supply, in circuit versions where its wired there. With a good scope probe only pulling it a little at these frequencies.

The range of frequencies the oscillator tunes over should be the received frequencies + 455kHz, as the L/O in a MW band transistor radio runs the IF frequency above the received frequencies that the ferrite rod main coil is tuned to.
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Old 20th Feb 2019, 10:37 am   #5
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Default Re: Biasing resistors for Germanium transistor oscillator

I think you'll be in danger of cutting off the OC44 (or whatever) osc/mixer, if you set the base potential at 0.2V.

A quick scan thru' about 5 similar circuits shows the base bias to be set at between 0.6V and 1V. You need to add on the 0.2V (typical germanium base-emitter drop) to the voltage dropped in the emitter resistor. With 470 ohms, and 1mA (not unreasonable), this is 0.47V.

Germaniums tend to have lower gains than silicon transistors, so the base current will be around 25uA (for 1mA emitter current); having the standing current thru' the divider at around 10 (or more) times this is prudent.

That would give (for a 6V supply), 24k total resistance, split 1:11.
2k2 and 22k looks like a good starting point...

HTH


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Old 20th Feb 2019, 10:45 am   #6
Ed_Dinning
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Default Re: Biasing resistors for Germanium transistor oscillator

Hi Jolly, there is a good early book by Mullard, called I think, "Transistor circuits", a dark red cover. This came out just as Si was starting to become popular so has lots of detail on Ge devices. There should be plenty of copies about at reasonable prices

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Old 20th Feb 2019, 11:26 am   #7
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Default Re: Biasing resistors for Germanium transistor oscillator

Perhaps I'm misunderstanding something but doesn't the Eagle sheet say that R1=27k and R2=2.2k for a 6V supply? Seems about right for germanium in this context.

Alan
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Old 20th Feb 2019, 3:47 pm   #8
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Default Re: Biasing resistors for Germanium transistor oscillator

https://archive.org/details/MullardR...sistorCircuits
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Old 20th Feb 2019, 5:21 pm   #9
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Default Re: Biasing resistors for Germanium transistor oscillator

Another vote for the Mullard book not at all biased by the fact that I worked in the Mullard applications lab at one time with some of the authors (but I was a VERY junior lab tech then !!!)

I still have a hard copy on my shelf
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Old 20th Feb 2019, 5:29 pm   #10
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Default Re: Biasing resistors for Germanium transistor oscillator

The Mullard book recommends


i) CE voltage of 6 to maximise transistor gain (this is going to be tricky to achieve with a 6V supply!)


ii) E voltage of 1 (to help stabilise the operating pont).


With a slightly different mixer/oscillator circuit (osc coil with 3 windings), the recommended emitter current is 0.25mA.


The voltage divider is 10k and 56k, from an effective supply of around 7V, so the base is just under 1V.


HTH


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Old 20th Feb 2019, 6:13 pm   #11
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Default Re: Biasing resistors for Germanium transistor oscillator

Alan (post 7) has hit the nail right on the head.
 
Old 21st Feb 2019, 1:35 am   #12
Jolly 7
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Default Re: Biasing resistors for Germanium transistor oscillator

Quote:
Originally Posted by ajgriff View Post
Perhaps I'm misunderstanding something but doesn't the Eagle sheet say that R1=27k and R2=2.2k for a 6V supply? Seems about right for germanium in this context.

Alan
Thanks. I have selected those resistor values for R1 and R2 as confirmed for a 6V supply.
I am getting the following readings : base voltage 0.453 V, emitter voltage 0.527 V and collector voltage 6.166 V. The base voltage seems fine, but the emitter and collector voltages may not be. Need to investigate further.
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Old 21st Feb 2019, 1:46 am   #13
Jolly 7
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Default Re: Biasing resistors for Germanium transistor oscillator

And here's a photo of my test oscillator circuit if it's of interest. I am using a new Mullard OC42 transistor.
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Old 21st Feb 2019, 2:34 am   #14
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Default Re: Biasing resistors for Germanium transistor oscillator

Jolly_7, just a tip that might help you out.

If you are experimenting with single transistor stages like this, the current drain will be under a mA most likely. One good trick is to put a series resistor in the power supply feed to your test circuit, maybe 220 ohms or more to act as a current limiting device (in case of accidents) and go for a much lower RF bypass capacitor after it, maybe 0.1uF to 0.2uF so less energy is stored, not the 47uF electrolytic. The energy stored in that electrolytic and your battery, especially if an alkaline one, without current limiting, can blow your transistor junctions to La-La land if you make an error with the transistor connections , wiring or accidental shorts.
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