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Voltage controlled gain amplifier, not single IC

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Advanced Member level 1
May 21, 2009
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Wrocław, Poland
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I'm currently designing a laboratory signal generator. I've already designed the generation block which can output quite clean sine wave at frequencies up to 20 MHz. Same with power supply, offset setting, and output forming (square, triangle, saw).

My problem concerns amplitude adjustment. While measuring amplitude should be fairly easy (done something like that before, just lower freqency), regulating it is another matter. My generator outputs a 2 Vpp sine wave which i intend to adjust in range of 0.1V to 15V.

My first approach was to use some kind of integrated variable gain amplifier, preferably with digital drive, but these IC's tend to cost truckloads of money and my budget is kind of tight right now. I've been thinking about placing 3 or 4 voltage dividers switched by relays for coarse adjust of generator output amplitude and then feeding that attenuated signal to the circuit as below:

This circuit is generally an opamp in a non-inverting configuration with one of the feedback resistors replaced with a JFET. Since jfet's characteristic isn't linear i've added a circuit that linearises jfet's characteristic. According to THIS a good way to linearise a jfet is to add a 1:1 voltage divider between ground and input signal and use the midpoint of that divider as base voltage for jfet gate drive. The bottom opamp fulfills that role adding certain voltage to the midpoint potential of a divider and driving the jfet's gate with that voltage.

Now goes the question: do you see any obvious flaws in this design? (apart from part symbols - I've used some generic opamps just to check the idea in simulator at lower frequencies) Maybe u have some suggestions how to impreove it or an alternative solution?

Thanks in advance

There are an Operational Amplifier manufactured by Burr-Brown ( now, Texas Instruments ) wich have a pin so that the gain can be programmable.


What's the part number? I've searched for an opamp with programmable gain, but there are pretty expensive. I haven't found one which would have sufficient parameters at under $8 or 10. And i can buy opamps which operate at standard +/-5V supply with unity gain BW of 400MHz for $3 (AD8014 to be specific). I'm still searching for some cheaper alternative.

My goal is to build the device at lowest cost possible while still achieving good performance. Logic unit is based on dsPIC30F5013 which I've got as sample from microchip long time ago. Generation block itself is build around Intersil's ISL5314 125MSPS DDS which I've salvaged from some unknown PCB.

I'm aiming at overall cost below $200. Calculating all the nescessary stuff like housing, lcd, power supply and so on I've got about $30 to spend on opamps and I haven't succeded to salvage any decent ones besides 1 OPA37 (which is +/-15V 50MHz precision amp).

If you have any idea what kind of equipment might contain that kind of amplifiers I'd be very grateful.

I am a new to VCA circuits could u help me how to design one i tried to simulate your s but did not worked .
Thx in advance!

Hi Why don't you think of the controlling the gain of OPAMP it self by varying R11 or R12 resistors as I fell that is easy approach ?

Good Luck

Becouse i need to build an agc closed loop without using an (IC agc or an IC vva or vca) Thx.

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