VC capacitor?

[Kimmo Koli]

>    Date: Wed, 15 Jan 1997 20:17:16 +0100
>    From: Bert Schiettecatte <Bert.Schiettecatte at hookon.be>
> 
>    can anybody tell me if there exists something like a voltage
>    controllable capacitor ?
> 
>    I need one with a large range (and pretty precise if possible).
> 
You could do an active capacitance multiplier:
All you need an inverting amplifier with a variable gain -Av and a 
capacitor C connectected between the amplifier input and output and
the result is a varible capacitance to the ground at the amplifier input
that equals
	Cin = C * (1 + Av)

That inverting amplifier can be a pot controlled opamp circuit or a VC 
controlled OTA.

This capacitance multiplying is also called the Miller-effect and it is
used for example in opamp compensation and also in tuned RF active 
filters.

The problem with this circuit is that at high frequencies this circuit 
turns into an inductor (as gain Av decreases with frequency the input 
impedance increases). 

That's all,   
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   Kimmo Koli                         Helsinki University of Technology 
   kimmo at ecdl.hut.fi                  Electronic Circuit Design Laboratory
   http://www.ecdl.hut.fi/~kimmo      Otakaari 5 A 
   Tel:  +358 9 451 2273              FIN-02150 Espoo
   Fax:  +358 9 451 2269              Finland 
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