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tibs
Joined: 29 Nov 2001
Posts: 23
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 17 Aug 2004 16:41 voltage controlled amplifier |
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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 = 200ľA => Rds = 1KOhm
and for Vgs = -3V and Vds = -200mV, Id = -150ľA => Rds = 1.33KOhm....
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.
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flatulent
Joined: 19 Jul 2002
Posts: 4877
Helped: 324
Location: Middle Earth
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17 Aug 2004 16:52 voltage controlled amplifier schematic |
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| How about using a fixed gain amplifier with the JFET attenuator before or after it?
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tibs
Joined: 29 Nov 2001
Posts: 23
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17 Aug 2004 17:00 voltage controlled amplifier circuit |
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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.
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17 Aug 2004 17:00 Ads |
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flatulent
Joined: 19 Jul 2002
Posts: 4877
Helped: 324
Location: Middle Earth
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17 Aug 2004 18:18 voltage control amplifier |
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| 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.
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tibs
Joined: 29 Nov 2001
Posts: 23
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18 Aug 2004 8:30 voltage-controlled amplifier |
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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. 
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Longfuse
Joined: 23 Feb 2004
Posts: 14
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19 Aug 2004 12:30 simple voltage controlled amplifier schematic |
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| Are ICs allowed in your design? If so, how about a transconductance amplifier like LM13700 or CA3080? They give pretty good linear control.
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