Attenuating a 5V DC waveform - is a JFET the answer?

Thread Starter

danielantonic

Joined Sep 22, 2019
56
Hello
I have an arduino, 5V rail to rail op amp, and an AD9833 function generator, and I want to control the amplitude digitally. I have an MCP4725 DAC that I would like to use, and my research leads me to a JFET or MOSFET solution like this: https://amplifiercircuit.net/voltage-controlled-variable-gain-amplifier.html/amp

Is something like this the preferred solution, or is there something else I should consider?
 

crutschow

Joined Mar 14, 2008
29,466
I suggest using a multiplying DAC to directly control the signal amplitude, if you want to use a DAC.
The FET approach will likely introduce some distortion into the signal.
 
Last edited:

Audioguru again

Joined Oct 21, 2019
4,523
Daniel, I think the circuit in your first post will not work because the - input is a "virtual ground" with almost no signal level.
When the jfet conducts then it feeds the "nothing" on the - input to the +input and nothing will cancel.

The Jfet can make a voltage divider to ground with another input resistor or the Jfet can be places across R2 to reduce the opamp gain.
 

Thread Starter

danielantonic

Joined Sep 22, 2019
56
Daniel, I think the circuit in your first post will not work because the - input is a "virtual ground" with almost no signal level.
When the jfet conducts then it feeds the "nothing" on the - input to the +input and nothing will cancel.

The Jfet can make a voltage divider to ground with another input resistor or the Jfet can be places across R2 to reduce the opamp gain.
So if my DAC was -5 to 0V instead of 0 to 5V, then the JFET design would work?
 

Audioguru again

Joined Oct 21, 2019
4,523
The circuit you copied already has a negative voltage fed to the gate of the Jfet to control its conduction.
The opamp has a positive and negative supply.
I simulated your circuit using a resistor as the Jfet. It does not work as an attenuator.
 

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