Voltage control opamp

[iliya24]

Hello
I am trying to build an VVA (OR VCA) not as IC i found some ex on web they all included FET as a feedback resistor with Vcontrol at gate but it did not helped because for tha same Vcontrol i was getting different A(VOUT/VIN) since my FET drain was connected to Vout of op amp as part of feedback.

Any suggests?

 

[godfreyl]

There's two important things to bear in mind when using a JFET as a resistor:

It is fairly linear only with a small signal voltage between source and drain - perhaps a couple of hundred millivolts, so it's a good idea to have the input voltage across the JFET, rather than the output voltage (assuming gain > 1).

For good linearity, half of the drain-source signal voltage should be applied to the gate, together with the DC control voltage.

The circuit below shows the idea. R2 and R3set the gate voltage = Vdrain/2 + Vcontrol/2.



- - - Updated - - -

Here's an interesting document about AGC circuits: http://www.eecg.toronto.edu/~kphang/papers/2001/martin_AGC.pdf

...and a VCA circuit from the web:



Unfortunately the original document has disappeared and all I saved of it was that one circuit.

 

[LvW]

Hey godfreyl, nice circuits.
In particular, I remember that the 2nd circuit gives very good results. You forgot to mention (although realized in both circuits) that in any case the source node must be grounded.

 

[FvM]

Some basic considerations about FET as variable resistors can be found in this thread https://www.edaboard.com/threads/129234/

To use it right in a VCA structure, you should understand the basic limitations regarding voltage range, linearity, matching of multiple transistors.

All high performance VCA ICs are essentially analog multipliers based on a BJT transconductance multiplier circuit or gilbert cell and need matched transistors, preferably integrated on a chip. As most of this devices have been originally designed for professional audio applications, they became obsolete when the audio technique moved to digital signal processing.

VCAs are still used in the RF domain, but with different specification and partly different circuit principles.

 

[LvW]

Hello
I am trying to build an VVA (OR VCA) not as IC i found some ex on web they all included FET as a feedback resistor with Vcontrol at gate but it did not helped because for tha same Vcontrol i was getting different A(VOUT/VIN) since my FET drain was connected to Vout of op amp as part of feedback.

Any suggests?

In case you dont want to use a FET - what about a light-depending resistor (LDR) controlled by an LED?
Do you that an OTA can be wired as a (grounded) resistor? (But you also can directly use an OTA plus buffer as a VCA since it has a control pin).

 

[iliya24]

Thx for help i tried to simulate it but i get diffrent gain for diffrent vin but same control voltage can u help me to understand what the problem?
Thx.

 

[FvM]

You'll need to show your simulation circuit and results.

 

[varunkant2k]

Thx for help i tried to simulate it but i get diffrent gain for diffrent vin but same control voltage can u help me to understand what the problem?
Thx.

I assume you are considering correct biasing at the input of opamp.