building a vca

[spyro2]

Hi folks,

I'm trying to build a VCA, but hit a bit of a quagmire.

I appreciate that there are chips or there that might do this in a one chip solution, buy i want to explore this route in particular;

I have an MCU. I want to feed is adc input with the output of a VCA, whose gain and offset are controlled by voltages supplied by the MCU.

Offset is easy - just stuff the voltage onto the inverting input of my amplifier.

Gain... Not so much.

I've been looking at using jfets in their linear region, used as voltage controlled resistors, but this seems to impose extremely small limits on the amplitude of signals, forcing them to be attenuated before the vca, then amplified again afterwards, which seems like a bad idea.

Is there a way around this limitation with jfets?

I'm looking at signals from 0-5V and a variable gain from unity to about 10 (lower than unity would be useful too).

[spyro2]

Oh, and I'm not interested in ac - my input is effectively dc.

[rstofer]

Re: offset and scale - see Chapter 4 of "Op Amps For Everyone"  It's free and available on the web

You could use digital potentiometers driven by an SPI gadget on the MCU.

[JPortici]

why not use an OTA? LM13700.. or to make things even easier, the Coolaudio V2164 has a VOLTAGE input to control the transconductance

[BrianHG]

Take a look here:
https://www.eevblog.com/forum/projects/poor-mans-analog-multiplier/msg515819/#msg515819

[BrianHG]

If you are amplifying DC, 0-5v, and only care about positive gain, you can use an opamp + 1 NPN transistor per bit on the feedback with 1k, 2k, 4k, 8k for 4 bit programmable gain, feeding the bases, with series resistor, in parallel from your MCU's digital outputs.  Mosfets will work too, but, their output capacitance may add gain at truly, really, high frequencies when they are off and their on-resistance at 5v becomes a factor.

The more transistor/resistor bits, the more programmable steps in your gain, plus, the more IOs you need.  With quality 1% resistors and simple 2N3904s, you will achieve repeatable, predictable, reliable results.