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blu_line 12th June 2004 06:04 PM

V/I Converter
I have seen the threads on the I/V converter but that is not what i need !

1st off all, when is a circuit an V/I Converter ?
When it has a sufficient low output resistance and is able to deliver enough current (class-A) ?

Using Current is a nice way to transport information over.
And since i am thinking of a modular pre-amp (multiple preamp boards on a motherboard like accuphase is doing) i am thinking of their idea on V/I conversion !

Any one suggestions / idea's ?



Charles Hansen 12th June 2004 09:40 PM

What you are looking for is called a "transconductance amplifier". Do a web search and you will find plenty of them. One example is the (discontinued) Burr-Brown OPA622.

blu_line 13th June 2004 08:47 AM


i thought of that my self as well !



boholm 13th June 2004 09:17 AM

If you are thinking about building it yourself using transistors and not op-amps, then you could look at those pre-amplifier DIY projects, you can find on the net, and then take away everything after the VAS-stage, including the bias-regulating circuit. The VAS-stage normally gives out current. But then you need to look at the other end: The input stage of the "reciever". You will have to match the output current with the impedance of the input stage.

jan.didden 13th June 2004 09:59 AM

Re: V/I Converter

Originally posted by blu_line
[snip]1st off all, when is a circuit an V/I Converter ?
When it has a sufficient low output resistance and is able to deliver enough current (class-A) ?

Hi Simon,

The output impedance of a V/I converter is ideally infinite. That means, it will deliver an output current controlled by the input voltage. The output voltage then depends on the load. V/I converters are a nice way to control a stage gain without feedback.
BTW, you may think that if you load a V/I stage by an infinite load, that would give infinite output voltage and thus infinite gain! Not so, because the max output voltage depends also on the supply voltage of the circuit, of course. Normally the max voltage that the output can give, while still maintaining the output current is called the compliance. MAX435/436 are examples of V/I converters.

Jan Didden

blu_line 13th June 2004 12:52 PM

2 Attachment(s)

MAX435/436 are examples of V/I converters
Checked those out !

What kind of performance could be expected when build up by discretes ?

Got something in my LT Spice but that isn't promissing !

jan.didden 13th June 2004 01:27 PM


What you need are a couple of THAT340's or others of the family, from (BTW, I'm going to order a couple, you want to join?).

Jan Didden

Steven 13th June 2004 06:36 PM

Hi Simon,

Any differential pair is actually a voltage to balanced current converter. The only problem is that the input voltage range is very limited without emitter or source degeneration. The link gives an overview of several techniques to increase the input voltage range and the linearity of voltage to current converters.


hjelm 14th June 2004 07:09 AM

Thanks Steven,
Very informative document.

thanh 14th June 2004 03:40 PM

I must say that steven know much good document!

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