Voltage controlled amplifier design

[tibs]

voltage controlled amplifier

Hi all,

I have to design a *LOW POWER* voltage controlled amplifier. The first idea is to use a JFET in voltage controlled resistor and used it in an inverter Opamp.
I have found the LMC6041 Opamp from national.

The problem is that the JFET Vds=f(Id) characterisc isn't symetric.
By ex : for Vgs = -3v and Vds = 200mV, Id = 200µA => Rds = 1KOhm
and for Vgs = -3V and Vds = -200mV, Id = -150µA => Rds = 1.33KOhm....8O

It's very annoying because I must work with a symetrical, 1V Peak Peak oscillator (+500mV, -500mV).

When I look the output voltage of the amplifier, I see the amplified input voltage source (the oscillator) but there is an offset....

Does anyone have a solution? The problem is how build a voltage control amplifier working with square zero centred input voltage source.

Best Regards,
Christophe.

 

[flatulent]

voltage controlled amplifier schematic

How about using a fixed gain amplifier with the JFET attenuator before or after it?

 

[tibs]

voltage controlled amplifier circuit

Hi,

as for me, the problem remain the same. With a fixed gain amplifier, you will have Vout = K * Vin. With :
Vin a square zero centred signal.
K : gain of the fixed gain amplifier.

Vout will be zero centred and the Rds won't be the same for the positive alternance and the negative alternance.

 

[flatulent]

voltage control amplifier

Siliconix has JFETs specially designed to be used as variable resistors. They also have application notes on how to make them more linear with circuit configurations.

 

[tibs]

voltage-controlled amplifier

Thank you Flatulent for your advice but the Jfet used as voltage controlled resistor are used with Vds>0 and Id>0 (cf the Jpeg file). I think I have to find an another solution.

 

[Longfuse]

simple voltage controlled amplifier schematic

Are ICs allowed in your design? If so, how about a transconductance amplifier like LM13700 or CA3080? They give pretty good linear control.