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 Forum index » DIY Hardware and Software
Expo converter transistor vs opamp
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Arrow



Joined: Feb 01, 2015
Posts: 3
Location: Italy

PostPosted: Sun Feb 01, 2015 12:14 pm    Post subject: Expo converter transistor vs opamp Reply with quote  Mark this post and the followings unread

Hi everyone,
I didn't find the presentation topic so this is my first message.
My doubt is this:
In all schematic of VCOs the exponential converter is made by a pair of transistors; but I know that there are many problems like temperature. Yesterday at school (i attend last year at high school) we did the anti-logaritmic or exponential converter based on a operational amplifier and a diode. So I ask me: why all project don't use the op amp?
I can't find a solution, so i ask your help electro-music.community! Very Happy


P.S. sorry for my english but i'm an Italian student.
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Blue Hell
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PostPosted: Sun Feb 01, 2015 1:05 pm    Post subject: Reply with quote  Mark this post and the followings unread

welcome Arrow

Ok, it has been a long time since I studied electronics, so there may be some expert after me to correct me .. but this is how I have it in my head:

The basic idea of the transistor is that you convert Base Emitter voltage (Vbe) to Collector current (Ic), so you have a linear voltage to exponential current converter. The current is handy to charge a capacitor, and that is what is almost always used in a VCO.

It is not so that the circuit uses no opamp, it can be done without - and is done occasionally, but by placing the transistor circuit in an opamp feedback loop the circuit can be made better (as in more ideal).

The Vbe to Ic equation is temperature dependent (this also is the case for the relation between diode voltage and current for log amps etc.), that is where the PTC resistor comes in.

When using a diode there is basically the same exponential relation between the forward voltage and the current flowing trough it, but that current can not easily be made available to charge a capacitor.

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Arrow



Joined: Feb 01, 2015
Posts: 3
Location: Italy

PostPosted: Sun Feb 01, 2015 3:51 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yeah, i suppose Smile
Because it's a expo voltage to current converter and not expo voltage to voltage converter, right?
In all schematics is the Ic that: charge a capacitor or boos a differential pair (like in the henry's 555 VCO). Now I can understand...

But if i would use an opamp as expo converter, i have to redo all the vco core, because there isn't a current that charge a capacitor but a control voltage.
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The Real MC



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PostPosted: Sun Feb 01, 2015 8:06 pm    Post subject: Reply with quote  Mark this post and the followings unread

Actually as BH stated, the VCO circuit uses a linear voltage to exponential current converter.

The major fault with the opamp/diode circuit is the diode V/I characteristics will change with temperature. There is a temperature term in the diode equation that must be cancelled or the VCO will drift.

The dual transistor converter circuit is used because it provides first order temperature compensation. There are other temperature effects which require different compensation circuits. Use Google patent search and search "Alan R Pearlman" and "Douglas R Curtis" for good EE reading.

Keep in mind that many classroom and textbook circuits do not work well in the real world. The opamp circuit with the transistor in the feedback loop does not work very well, as Electronotes has concluded.
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Arrow



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PostPosted: Mon Feb 02, 2015 1:01 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thank you for the reply!

Now I'm convinced to make thomas henry's 555 VCO because the "difficulty level" of that schematic si right for me.
What is BH?
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Blue Hell
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PostPosted: Mon Feb 02, 2015 1:48 pm    Post subject: Reply with quote  Mark this post and the followings unread

Arrow wrote:
What is BH?


Me I guess Laughing

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