JFETing op-amp inputs

Question

My goal is to be able to use a high-speed, low-distortion, BJT-input op-amp having a much lower input current - that's why I'm trying to JFET the inputs. I don't want to cut DC with input cap, I want the input current to be low. I want to use the JFE2140 from Texas Instruments.

I've found 2 useful articles:

1. https://www.analog.com/media/en/analog-dialogue/volume-47/number-4/articles/tips-on-making-FETching-discrete-amplifier.pdf
2. https://www.cordellaudio.com/JFETs/LSK489appnote.pdf

First article is about buffering the inputs. I'm stuck at the very beginning - finding a right buffer circuit. I don't know the reason why my source follower is saturating. For some reason this circuit clips; it's cutting the bottom part of the sinewave.

VCC is 15 V, VEE is -15 V, the current source is set to 5 mA (it's the optimal value for lowest noise according to datasheet). I need to handle 8 V peak without saturation. When I raise the current at I1 source to ~10 mA it's not clipping anymore, but the problem is that the power dissipation is becoming high, and this particular JFET (JFE2140) should work with lower Ids current I guess.

After raising I1 to 10 mA I get 0.008253% of THD - that's also too high, I need it 2 orders lower (more like 0.00008% - I get this figure with the op-amp alone). I also tried a hybrid solution (a source follower with a BJT buffer) but it didn't help much. Maybe it didn't help because I'm doing something wrong. Here is the hybrid solution:

When using simple input buffering I dramatically raise the distortion of the circuit. There is a second solution - a differential pair with some voltage gain working in the common feedback loop with the op-amp. When using this solution I can come to a point when distortion is really low, but it strongly depends on the gain setting resistors (marked with red on the picture).

Changing the resistors to set the gain to another value ruins the distortion performance and I must set the operating point again. I want this circuit to work right with various gain settings - from a gain of 1 (buffer) to a gain of, let's say, 10.

Any thoughts on that? Would you choose input buffering or the diff. pair solution? How can I achieve the same, low THD figure with all the gain settings while keeping the JFETs at a fixed operating point? And back to the basic question - why is my simple buffer saturating?

Answer 1

I think your op-amp distortion depends heavily on the gain because your offset is also being amplified, thus, you're setting the output DC operating point closer to the rails each time you increase the gain.

Check OUT1 DC voltage as you increase gain, you'll likely see an increased DC as you increase your gain. It'll not be 0V as you're probably assuming it should be now.

Normally, people get rid of this by using a coupling capacitor at the input and another one in series with your R14 to ground. If you do that, then your DC gain will be 1 and your output DC voltage will not vary with gain. You can probably keep your distortion performance with such techniques.

If for some reason you really want to let DC through, then you need some sort integration mechanism in your feedback loop that corrects your DC to a well-defined DC operating point. But that's more complex.

EDIT: Your saturating buffer

Your buffer is saturating because the sinking of current is limited by the current source. Your current source is 5mA and your load 1k, thus, your buffer can only go as low as -5V.

EDIT 2: Other ideas

Since my first hypothesis seemed wrong, I'll list a few more ideas before I'd go looking for other topologies.

1. What is your supply voltage? Are you sure you still have saturation margin for your input transistors?

2. Are the DC voltages at the op-amp input within it's common-mode spec. Do you see any sensitivity in distortion if you move that voltage up or down.

3. I see no good reason for those degeneration resistors. It's the loop gain that provides the distortion performance, so by "linearizing" that input stage, you're removing valuable loop gain to help reduce distortion. It'll also steal headroom from your input stage.

4. Finally, perhaps you are driving your amplifier too hard. And that might explain some components going out of their proper operating point.