Voltage controlled amplifier design

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.

voltage controlled amplifier schematic

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

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.

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.

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.

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.