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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 29th Nov 2022, 12:05 pm   #1
Wendymott
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Default Useful find.

Hi peeps. I was looking for a description of a "long tailed pair", and entered a search into google. The first and top of list was this amazing detailed, without loads of maths article, that not only described what I was looking for, but many other Semi conductor configurations.
If allowed... its here..Semiconductor foru.com . As a still "novice" I found the descriptions very "Save worthy", and will keep.
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Old 29th Nov 2022, 12:59 pm   #2
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Default Re: Useful find.

Feel free to post the link.
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Old 29th Nov 2022, 1:03 pm   #3
Wendymott
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Default Re: Useful find.

Hi Paul.. its in the above.. Semiconductor foru.com Or google "long tailed pair
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Old 29th Nov 2022, 1:10 pm   #4
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Default Re: Useful find.

https://www.semiconductorforu.com/wh...ing-principle/

And there's always his from my longtime YouTube hero:

https://www.youtube.com/watch?v=mejPNuPAHBY

Lawrence.

Last edited by ms660; 29th Nov 2022 at 1:19 pm. Reason: extra info
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Old 29th Nov 2022, 1:37 pm   #5
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Default Re: Useful find.

Hi Lawrence. Just looked at the Youtube article... Yes ok but I have a "hard copy" that I can refer to, without trawing through the video
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Old 29th Nov 2022, 1:58 pm   #6
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Default Re: Useful find.

I posted the YouTube link 'cos he does a demo as well.

Lawrence.
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Old 29th Nov 2022, 2:13 pm   #7
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Default Re: Useful find.

If you look at the three-transistor long tailed pair with the current source transistor forming the long tail, you can also view it as a cascode circuit with signal input to the tail transistor... but there is that extra transistor, the unused one of the pair.

From the Ebers and Moll equations, if you provide a dc path for the spare transistor to Vcc, some of the signal-bearing current from the input transistor will go to waste through the spare transistor and the rest will go up the output transistor, the cascode top, and will drive the output.

By putting a DC voltage between the bases of the two top transistors, we can steer the current between the waste path and the output. Handy! We now have a DC commanded variable gain control.

It gets better. The transfer of signal is logarithmic. 1mV of DC shift gives you X dB of gain change over a useful range, but it levels off at the top. Still, you get a few tens of dB of gain adjustment with the linear to log law working well.

You can have two of these circuits working together to handle balanced signals, with the outputs cross-connected. You now get signal cancellation at zero volts difference on the bases, and moge the gates apart one way and you ramp up positive gain, move them the other way and it ramps up negative gain.

You now have the tree-like transistor mixer cell which some people call the Gilbert Cell

But if you add diodes across the base pair in shunt, and current drive it, the log transfer function of diode current to diode voltage 'corrects' the loggyness of the transistor pair and you have a fully linear analogue multiplier, and it handles all four quadrants of positive and negative going signals on both ports. This is the true Gilbert Cell.

So now you know how analogue multipliers work, and variable-gain ICs.

And mixer sections in integrated circuits

And AGC amplifiers in ICs

And synchronous detectors

And the demodulator in on-chip FM discriminators

And the demod in stereo decoder chips.

And, and...

Diff amps are great, but you can do a lot more than just amplify with them. Learning their basics opens an important door in analogue signal processing.

Did I just say 'Analogue'? Look inside a high speed ECL exclusive OR gate, and guess what you can see! It's a mixer! So you can use EXORs as mixers in some cases.

David
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Old 29th Nov 2022, 3:21 pm   #8
Wendymott
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Default Re: Useful find.

The original search was because Plessey, in their infinite wisdom made an IC which incorporated two NPN devices configured as a Diff pair, but it also showed a 1K0 resistor between the bases Internally.. The IC is a SL3046, which also has a few stand alone NPN devices. I am going to use the SL1623 AM/SSB chip and use the AGC pin. So the article suggested using the SL3046 to drive the "S" meter rather than a simple meter. Obviously the SL3046 is not available so I cooked up 2 X 2N4401's in a diff mode, with a 1K0 between the bases. Needless to say when I removed the 1K0 it worked. I will physically couple the two 2N4401's to slow down any drift. Obviously there are some NPN diff pairs, but I dont have any in stock and as its only the "S" Meter, I am not really bothered. Thanks David for the additional info
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Old 30th Nov 2022, 9:58 pm   #9
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Default Re: Useful find.

If it helps, Tek used some Plessey SL3046C (or RCA CA3046) in the 244x 246x scopes, they are transistor array ICs, the RCA version seems obsolete too, but old stock is available.

David
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