Electronic – VCA Using JFET and Op Amp

audiojfetoperational-amplifier

Just messing around with LTSpice I came up with this VCA that simulates shockingly good for what it is:

JFET VCA
JFET VCA plot

I don't recall ever seeing the use of an op amp to mix the signal and control voltage in an isolated way like this so I just thought I would post it here for posterity (I will probably never build it myself).

If I plot the current through J1 vs the drain voltage, it's an absolutely perfect diagonal line. But R4 has to be trimmed to be exactly 1/3 R3. In practice maybe this is not possible and the simulation is deceiving me?

With R source 10K and 100mVpp input, attenuation is 37dB and the 2nd harmonic simulates 88dB down. Of course increasing R source or decreasing input would improve things greatly. But at the expense of noise created by from having to boost the signal after attenuating the input.

But when you overdrive this circuit it looks like the kind of distortion that would not be horrible for something like a compressor so one might just put a level control in front so that the user can adjust for maximum noise / distortion balance.

I know you can buy a VCA chip for $5 USD but this looks like this would perform really well with a handful of very common parts.

Now if only there were an equally clever way to linearize the control voltage …

UPDATE:

Note that what is different about this circuit from all of the other FET VCAs I've seen on the Internet, is that different op amp inputs are used to mix the control signal with the control voltage. There are no capacitors coupling the signal and the CV. It's DC coupled. That might be good for something.

Note that when the control voltage gets very close to Vgs(off) the signal does start to affect the linearity of the current through the JFET and you get a compression type of distortion (although I suspect this is true of all circuits like this). For example, the Vgs(off) of the 2N5484 model is 1.5. If I set the CV to 1.49 the attenuation is -4.5dB and the 3rd harmonic is 47 dB down. And this is again with a 100mVpp input which is quite large for this type of circuit.

UPDATE 2:

Also works with a series JFET. Here is a circuit that combines the two to yield better dynamic range (potentially very large if the series JFET is switched completely off because distortion doesn't matter as much when the signal is 60dB down):

enter image description here

UPDATE 3:

The circuit described by Henry Santana in this Electronic Design article:

Op Amp And Two JFETs Form A VCA

is superior in almost every way:

enter image description here

The simulated dynamic range is -180dB which is of course impossible but we'll assume whatever reality is that it's pretty good.

The control voltage is linearized. Trimming is not necessary. Noise simulateed slightly better. And it works at DC.

The only thing that my circuit simulates better at is distortion. THD 0.012% compared to 0.004% in my circuit. The second harmonic is 78 dB down (instead of 88 dB). The distortion is also asymertic whereas in my circuit it is purely symetric. But the distortion actually increases less as the input is increased. Meaning this circuit can take 10Vpp input and distortion is still only 0.864% whereas in my circuit the distortion would be horrendous.

Best Answer

Note that U1 has a signal gain of 2 and its input is Vout/4, so it applies Vout/2 to the JFET gate, essential to cancelling out most of the second harmonic.

This is a classic basic configuration known since at least the 1970s, though the detail around U1 may be novel.

Figure 7B on this page has some similarities. The opamp has a gain of 11. and feeds back 1/22 of its output to the gate (again driving the gate with 0.5 of the drain voltage)

Related Solutions

Electronic – Bipolar Op Amp w/ Near Zero Offset

In addition to what Kaz said (this not being much of an issue in audio), you can externally compensate the offset voltage and input bias currents of any BJT-based [or other kind of] opamp. The ON Semi app note AND8177/D (formerly Philips/Signetics appnote AN142) has some basic circuits for that purpose on its last page(s). They are nominally showing the NE5534 there, but those circuits don't use anything but the signal input pins to achieve compensation, so they'd work for any BJT-based amp, including the NE5532. I have played with the first two circuits in LTSpice (with the rather basic NE5532 model found on the interwebz); they are a little sensitive to the pot values. The real issue is to quantify "near zero" (from the OP question) in an actual application...

Beware that both appnotes, even though they've been in print for 30 years, still have a bug in their offset-nulling inverting circuit with the opamp input pins being swapped. I'm referring to fig 18 in AN142 and its copy fig 22 in AND8177; you surely don't want to build a Schmitt trigger here...

Below is my test fixture for the NE5532 combining V offset and input bias compensation (somewhat spiffed for public communication from my old scratch version). This is a basic circuit, not something elaborate using DC servos etc., but I think it's good for learning what's going on; at least for me it was so. enter image description here

R11 provides bulk bias current compensation for R10, i.e. you want these roughly matched. Note that without C4 you will have a hard time finding a good value for R11. That's because the positive pin will see variable resistance to its left... (This is where more elaborate solutions using active components come in, but I'm not going into that.)

The fine tuning of the Voffset is via the pot "a" simulated by R7&R8, which is lifted from the aforementioned appnotes. With the pot "a" set to 0.45 (normalized value) the offset is in the few microvolts range, which for audio output is excellent.

       --- Operating Point ---

V(n002):     0.000514795     voltage
V(n003):     1.47903e-005    voltage
V(vout-biased):  -3.03485e-006   voltage
V(vin-ac):   -4.48332e-016   voltage
V(n005):     0   voltage
V(vout-ac):  -8.55829e-020   voltage
V(vin-biased):   -0.0498147  voltage
V(v-):   -12     voltage
V(v+):   12  voltage
V(n004):     -0.0498075  voltage
V(n001):     1.13011     voltage
I(C5):   1.42638e-022    device_current
I(C4):   4.98147e-019    device_current
I(R11):  4.98223e-007    device_current
I(R10):  -4.98147e-007   device_current
I(R9):   5.64796e-006    device_current
I(R8):   0.000483106     device_current
I(R7):   0.000477458     device_current
I(R6):   5.14795e-006    device_current
I(R5):   1.42638e-022    device_current
I(R4):   -4.48332e-019   device_current
I(R3):   -4.98147e-020   device_current
I(R2):   -1.78251e-009   device_current
I(R1):   5.00005e-007    device_current
I(Vtest):    -4.98147e-020   device_current
I(V2):   -0.00370861     device_current
I(V1):   -0.00370444     device_current
Ix(u2:1):    4.98147e-007    subckt_current
Ix(u2:2):    4.98223e-007    subckt_current
Ix(u2:3):    0.0032255   subckt_current
Ix(u2:4):    -0.00322698     subckt_current
Ix(u2:5):    1.78251e-009    subckt_current

The LTspice asc source file format is actually an ASCII (plain text) format, so I've uploaded the circuit (asc) to http://pastebin.com/0PzpUbFC in case anybody else finds use for it.

And below is a slight modification graphing the DC output offset as function of the offset pot position, i.e. a pot sweep. The output voltage corresponding to the "500m" (0.5 that is) pot position is basically the typical NE5332 offset votage (0.5mV) roughly multiplied by this circuit's gain (~10), resulting in about 5mV output offset. enter image description here

If you want to increase the effective range of the offset pot (for example if you get a really bad NE5532 that has almost 10x its typical offset) you can decrease R9; decreasing R9 to 100K would roughly double the output voltage offset range that the pot sweeps.

Also, removing R11 is a pretty bad idea; you will get hundreds of mV output offset which the offset pot cannot compensate for... as shown below. enter image description here

The point to remember here is that both input currents and input offset need to be cared for in the same circuit as they both affect the output voltage offset; that's something perhaps not well conveyed in the aforementioned appnote...

And perhaps more directly answering the question as asked, there are some bipolar opamps where the manufacturer has tried to center the V_OS to 0... statistically. You still have to deal with non-zero dispersion. For example the OPA551/OPA552 data sheet, which gives the typical V_OS values as +/-1mA and a max of +/-3mA later has the following bell-shaped (Gaussian distribution) graph. enter image description here

You also have to consider that the offset voltage may (or may not) increase much with temperature. This is again where active solutions (servos) win over manual trimming. The graph for the temperature drift of the OPA551/2 samples follows. enter image description here

I'm not sure what shape to ascribe to this; perhaps a form of Weibull distribution for some suitably chosen parameters.

Finally, the LM4562 mentioned (by someone else) above has the bias currents measured in dozens of nA (typical 10nA, max 72nA), whereas for the OPA551 it is in the range of dozens of pA (typical 20pA, max 100pA). Having said this, I come back to my opening point: you have to ask yourself, when was the last time you've heard or read that someone was preferring a specific opamp in an audio application because of its low offset voltage and/or low input bias currents...

Electronic – Direct Coupling and No bias on JFET guitar preamp’s

Maybe this will help: -

enter image description here

For no particular JFET, the characteristic might be something like the above. Importantly, and ignoring when \$V_{DS}\$ is 5V or less, the gate bias voltage sets the operating current for the drain.

So, for JFETs with a simple gate bias connected to 0V, more drain current means more source current which means more source voltage which means a bigger negative bias on the gate relative to the source and this means "regulation" i.e. it finds its own level - there is negative feedback and very elegant it is too. This keeps it in the linear region.

For a guitar amplifier where the input signal is a few hundred millivolt p-p at maximum, the drain current is modulated by the gate ac voltage quite linearly.

If the supply is 10V (easy to see on the curve) and the gate voltage is 3V below the source, the drain/source current will be about 1.4mA and this is self-sustained (regulated) when the source resistance is about 2k2.

Picture stolen from here (a very good website for plenty of stuff).

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Electronic – Bipolar Op Amp w/ Near Zero Offset

Electronic – Direct Coupling and No bias on JFET guitar preamp’s

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