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Transistor circuit questions
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Cfish



Joined: Feb 24, 2016
Posts: 477
Location: Indiana

PostPosted: Sat Nov 05, 2016 9:02 am    Post subject: Transistor circuit questions Reply with quote  Mark this post and the followings unread

I am wanting to ask a few questions about some basic transistor circuits that are frequently in synth module schematics, but not covered very well in my basic transistor books.

I'm sure more advanced books would cover these. However they often get beyond my knowledge level pretty fast.

Any help with this will be greatly appriciated.

The first schematic I have (I understand to be a basic voltage to current converter)

I have heard this called a transistor OTA. Is this correct terminology? If not, what is the correct name for this circuit?

Posted Image, might have been reduced in size. Click Image to view fullscreen.



The next schematic I see as the core to a lot of VCAs

If I'm looking at this correctly, by controlling the current flow from the point I marked point A

A difference of voltage potential is achieved across R1 and R2 that mimics the input voltage, and is controlled in amplitude by point A having current flow.

please feel free to correct me. I'm trying my best to understand this with my minimal education.

What is this type of circuit called?

I have found that every VCA I have built using this as it's core seem quite noisy compared to a basic VCA using a 3080 OTA.

Is that normal? Or am I doing something wrong?

I am wondering if I am incorrectly matching 2n3904s ?

Will poore transistor matching cause noisy operation of this type of VCA circuit?

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Cfish



Joined: Feb 24, 2016
Posts: 477
Location: Indiana

PostPosted: Sat Nov 05, 2016 9:24 am    Post subject: Reply with quote  Mark this post and the followings unread

I have been studying several different VCA schematics of this type, and noticed one with an inverse input that I circled in green in the next picture

What is the purpose of an invers input on a VCA?

How do you best utilize it in the synth world?

Posted Image, might have been reduced in size. Click Image to view fullscreen.

Anouther circuit I noticed had the ground reference for the bass of Q2 set up to be offset.

Is this just to control offset if you are controlling a DC signal? Or is there other uses for this
Posted Image, might have been reduced in size. Click Image to view fullscreen.

Next, if I haven't worn you all out, is a schematic of Thomas Henry using a very similar variant of the same thing as a triangle to sign converter.

Does the 390 ohm of resistance between the emitters of the transistors cause this affect, or will this type of circuit tend to have a wave shaping affect on a triangle?

What is it called when it is used this way?
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blue hell
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PostPosted: Sat Nov 05, 2016 11:48 am    Post subject: Reply with quote  Mark this post and the followings unread

These are all variations on the 'long tailed pair', which is a differential amplifier (forming the basic circuit for many opamp circuits).

http://www.radio-electronics.com/info/circuits/transistor/long-tailed-pair.php

https://wiki.analog.com/university/courses/electronics/text/chapter-12

but .. erm .. there is not much explanation in simple terms .. and my circuit skills are a little rusty ... but thought that giving the circuit a name might help you.

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Jan
also .. could someone please turn down the thermostat a bit.
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Cfish



Joined: Feb 24, 2016
Posts: 477
Location: Indiana

PostPosted: Sat Nov 05, 2016 2:44 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for the links. Yes knowing what it is called will help a lot too.
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AlanP



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PostPosted: Sat Nov 05, 2016 5:38 pm    Post subject: Reply with quote  Mark this post and the followings unread

http://www.valvewizard.co.uk/acltp.html
http://www.tubecad.com/2014/06/blog0294.htm

I was scratching my head with the OP -- I knew that the concept, I'd seen somewhere else...

Problem was, I'd only seen it labelled as long tail pair on valve amps Smile
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blue hell
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PostPosted: Sat Nov 05, 2016 6:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hmm ..now I have this urge to power up the oolde radio ... just for the smell of tubes Laughing

Yeah, the long tailed pair has a long history!

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Jan
also .. could someone please turn down the thermostat a bit.
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m.o



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PostPosted: Wed Nov 09, 2016 4:42 am    Post subject: Reply with quote  Mark this post and the followings unread

Good guy w2aew has some excellent videos about long -tailed the pair:
https://www.youtube.com/watch?v=zhM3rPHFgt4
and
https://www.youtube.com/watch?v=mejPNuPAHBY
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m.o



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PostPosted: Wed Nov 09, 2016 4:52 am    Post subject: Reply with quote  Mark this post and the followings unread

I have been using this type of VCA for a lot of my synth circuits.
The big advantage in my opinion is the simplicity and the price / availability / size of circuit (2 trannies, 1 dual op-amp and some resistors).
I usually use matched transistor pair bcm847 (SMD though).

I believe this is generally regarded as being worse than IC OTA (3080, 13700) or other types (THAT2180 etc), having problems with cv bleeding in (and I guess noise) etc.
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Cfish



Joined: Feb 24, 2016
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Location: Indiana

PostPosted: Thu Nov 10, 2016 5:49 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks so much for everybody's help on this.

I go to the valvewizard sight that AlanP put a link to a lot. Had never caught that page.
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