Another Bloak Hack...

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

hifizen

Well-known member
Joined
Apr 6, 2006
Messages
51
Location
Silicon Valley, CA
Hi folks,

This is my maiden post here... I've been lurking for the last two or three months, and am normally found on the hi-fi forums.

On to the topic at hand - I've been eyeing the GainBloak circuits, and have decided to build a version of the BigBloak (FET input) for my current project, a stereo preamplifier (darlington output will drive headphones nicely). My goals for these mods are to allow unity gain, and to provide very low offset for DC coupling the input and/or output. I also don't want to use a DC blocking cap in the feedback resistor network.

To accomplish these goals, I've added a current source for the input FET (as seen elsewhere), and a DC servo (maybe not needed if I can massage the tempcos) integrated into the input current source, rather than dangling off the negative input node as is usually seen. Two other changes: a Vbe multiplier for output bias instead of the diodes, and discrete darlingtons instead of the integrated ones (should be a little more linear, plus I can play with the bias).

I realize this is a bit of a departure from the spirit of simplicity embodied in the original design, but I think these additions are justified for the intended application, and with some luck I'll pare down the complexity some as the design evolves toward it's final implementation. Since it's picked up quite a few frills, I've been calling it the FancyBloak.

Here's the schematic so far:
[deleted]

I haven't simmed any of this yet, so some of the component values are missing, others (such as the servo capacitors) are just a temporary value which I'll correct later.

Opinions and suggestions welcome...

- Chad.

<EDIT> This thread is no longer active, but lives on in the Lab area: "DCBloak on the bench..."
 
Um, minor update... R2 should be something like 165ohms, not 20ohms. This part will need to be selected to match the K170 used, to give close to 0V at the -IN terminal. R7 is also intended to start life as a trimpot, which then gets pulled and replaced with a fixed resistor after adjustment.
 
SK170's have a nearly zero tempco of drain current close to zero volts gate-source, typically. You can determine this with a test setup before installation, and then adjust your current source for the associated current. Unfortunately the optimal condition would also be for this current to coincide with Idss/2 *, and usually it's closer to Idss than one would like. In fact in some cases it occurs with a slight positive gate-source bias, still low enough to make the gate current very small. You will want to adjust the drain resistor for the current you wind up with.

The higher magnitude Vgs-off parts may be slightly better, but still close to Idss.

The LED-biased current source, if using a "standard red" GaAsP part (now less and less standard!) is pretty close to having zero drift with temperature itself.

As long as you have some bandlimiting ahead of the stage you have little to worry about, since the circuit is fast to begin with.

The advantages: you get rid of your series resistor and have enhanced loop gain as a result, and depending on how well it works, low offset voltage drift. Whether it will be low enough for all of your applications depends on their associated closed-loop gains and their tolerance of a little offset.

Welcome to the forum.

*in order that for large transient signals the amp gives about the same response for positive and negative transients and doesn't "lose" any input charge to forward conduction of the gate. This condition is usually imposed on standard differential pairs, and a more careful analysis would take the variable transconductance into account for a single-ended stage like this one.
 
Thanks bcarso.

Yeah, the K170 tempco is the only thing about this circuit I'm not completely happy with at the moment. I don't really like operating JFETs too close to Vgs=0V, for the very reason you mention. Also, my stock of K170s are all BL and V grade, and running these at 6mA+ and 24V just seems a bit cruel... but maybe I'll try it and see what happens.

I briefly entertained the notion of biasing up the base of Q9 using a second K170 hooked up as a constant-current diode (with the same source resistor, same quiescent current, and same Vds), feeding a resistor in series with a 4148 diode... but that seems a bit silly, so I'll look for another way... something simpler and less wasteful.

As for that series resistor (R2), I'm not sure it affects OL gain much... it should just appear in series with the feedback network (~5K already), reducing BW only, but that's another variable I'll have to play with and see what happens.

On a side note: I've got a big bag of the old standard red LEDs - esox, packrat and I went and raided Mike Quinn's surplus up in Oakland as it heaved it's last gasp... we came away with some good stuff at firesale prices, but the reality is we've lost a great Bay Area hobby resource. :sad: I wonder now how much longer the other surplus stores will remain in business??
 
[quote author="hifizen"]Thanks bcarso.

Yeah, the K170 tempco is the only thing about this circuit I'm not completely happy with at the moment. I don't really like operating JFETs too close to Vgs=0V, for the very reason you mention. Also, my stock of K170s are all BL and V grade, and running these at 6mA+ and 24V just seems a bit cruel... but maybe I'll try it and see what happens.

I briefly entertained the notion of biasing up the base of Q9 using a second K170 hooked up as a constant-current diode (with the same source resistor, same quiescent current, and same Vds), feeding a resistor in series with a 4148 diode... but that seems a bit silly, so I'll look for another way... something simpler and less wasteful.

As for that series resistor (R2), I'm not sure it affects OL gain much... it should just appear in series with the feedback network (~5K already), reducing BW only, but that's another variable I'll have to play with and see what happens.

On a side note: I've got a big bag of the old standard red LEDs - esox, packrat and I went and raided Mike Quinn's surplus up in Oakland as it heaved it's last gasp... we came away with some good stuff at firesale prices, but the reality is we've lost a great Bay Area hobby resource. :sad: I wonder now how much longer the other surplus stores will remain in business??[/quote]

If your feedback network Z is ~high then yes, the 20 ohms is negligible. But then as well your noise is needlessly high, slew rate reduced, etc.

But again, if your signal is bandlimited the FET close to Idss should be fine. What does enter in is higher dissipation at these currents, and then Ig starts to climb. For that matter, the curves of Ig vs. Vds are pretty pessimistic at moderate to high drain-gate. One can cascode a high-pinchoff part with the SK170, tying the second Q gate to the 170 source, but there is a negative input Z effect from this that may require a small input C to common to prevent oscillation; also you are necessarily reducing your input swing a bit.

On another note: if you do use the d.c. servo, you can avoid the differential configuration and go to a simple integrator by controlling the base of the current source rather than injecting current into the emitter. A small R in series to isolate it from the LED would be used.
 
Note that Q9 seriously increases current noise of the inverting input, thus you get a strong noise penalty for medium and low c/l gains.

Personally I'd simplify the bias for Q3/Q9 by using one LED only, but nothing wrong with what you show other than the lack of simplicity.

I'd change the output to a "complementary feedback pair", that allows us to use the simple two-diode bias and to get short-circuit protection by the simple addition of two diodes. And it gives (potentionally) higher output swing and linearity.

If you stay with the darlington, you might bypass R10 with a medium cap (say 1 uF) to lower switch-off distortion in class AB mode.

Just a few thoughts--it works well as shown!

Samuel
 
[quote author="bcarso"]On another note: if you do use the d.c. servo, you can avoid the differential configuration and go to a simple integrator by controlling the base of the current source rather than injecting current into the emitter. A small R in series to isolate it from the LED would be used.[/quote]

Hm. Interesting thought. What's the dynamic impedance of an LED anyway? I'd like to see a low impedace at that Q's base, but this could be a worthwhile simplification.

Samuel:

Yeah, I've been waffling back and forth on single LED bias for Q3/Q9. I don't suppose the isolation of separate LEDs makes much difference in this circuit...

Is Q9 really going to add that much noise? I could use a 2SC2240 instead of the BC550. Or how about a bootstrapped resistor supplying the current to that node? Turn-on DC might be an issue, getting the currents and impedances right could be tricky, and there'll probably be some gain funkyness at high and low frequencies... dunno, just a random thought.

CFP output is also in the back of my mind. I think I'll stick with the darlington for now, and once I've got the front end sorted out, I'll revisit the output stage.

Great feedback so far fellas... keep it comin! :thumb: I'll warm up the soldering iron and scope over the weekend and report back.
 
Q9's voltage noise modulates the current by approximately that noise divided by the 100 ohm R5. Q9's base current shot noise is an additional term (mostly) uncorrelated. Then there is the thermal noise current of R5, also uncorrelated. Higher beta transistors will help the base current noise; lower rbb' transistors will help the voltage noise current modulation.

As well, to reduce all except the base current shot noise you can raise the value of R5 and increase the bias voltage. But then you lose your temp compensation from the single LED, although you can restore it with other arrangements.

LEDs have comparable impedance to regular silicon diodes, roughly speaking. I would not try to servo by modulating the LED current but rather via inserting a small R between Q9 base and the LED, and then feed via a servo resistor of appropriate value to the base. This R contribues noise as well of course, so its value shoud be small.

I like the simple integrator servo because the dynamics of a mismatched differential one are a bit strange, and of course there are the extra parts. You also sometimes need to be mindful of the servo amp's noise contribution, and may find it helpful to put a final R-C passive filter between the servo amp output and the servo signal injection resistor. This time constant can't be too close to the basic integrator one or things ring, but it doesn't need to be to take out mid-high noise from the integrator amp. You can also insert an R in series with the integrator cap to introduce a zero. Simulators are a great aid when playing around with this sort of thing---hours of endless fun :razz: , but at least you're likely to have the whole thing work as expected when you build it.

EDIT: just also noticed that as drawn the current out of Q3 is too low if the input stage is going to have ~6mA going through it at quiescence, since there is appreciable current gain in the second stage. If your drain current is 6mA you will need about 40mA in that high-Z output node, and you will be dissipating close to a watt in Q2 and Q3 just sitting there at zero signal. So that's a mite rich.
 
OK, here's what I'm simming with at the moment...

[deleted]

Note that I've repaced the ouptut bias circuit with a simple voltage source for convenience... the LED model was also not working properly, so I went and measured a few of my red LEDs and just plunked in a V-source for that, too. Q1 is still operating at the original 1.5mA. R2 and R5 are odd/non-standard values, tweaked to obtain low DC at the -IN and OUT nodes. The servo circuit is removed for simplicity as well, with it's output resistor R14 tied to ground to emulate the servo sitting perfectly in the center of it's range.
 
OK, I just measured the impedance of my red LEDs, biased 1mA and 500uA... I get roughly 0.07ohms based on the two voltage measurements. This is even lower than I'd expected, so the base current modulation of Q3 and Q9 caused by current noise and collector voltage swings should cause negligible voltages on the LED. So, I've decided to use a single LED to bias the current sources, and to stick with a good low-noise BJT at Q9 for now (I have 2SC2240BL on hand, maybe will pick up some 2SC3329 eventually - looking at the datasheets, they are very close in performance, but en and rbb' are not explicitly specified for the 2240).

Cracked open my low-noise design books and started brushing up... As I understand the situation so far, the noise analysis of top-grade BJTs (eg 2SC3329) vs. JFETs (eg 2SK170) in this CCS circuit does not clearly show an obvious advantage to one or the other. The comparison is complicated by a lack of comparable data... en is perhaps just slightly higher for the FET, but may be rising more at LF than the BJT (?).... Current noise is considerably lower for the FET, but the BJT base sees a very low source resistance and so In won't cause any significant additional noise voltage anyway. However, In does still add directly to the output noise current, does it not? For a BJT at 1mA Ic, this is typically on the order of 1pA/rt(Hz) or so, rising to perhaps 5 or 10pA at 10Hz (an educated guess on my part, comparing with data I have for other BJTs such as the MPSA18). With a 500ohm source/emitter resistance, this puts the BJT and FET roughly on par... I suspect the biggest difference will be the 1/f region, and the output impedance of the CCS (may be important in some applications), where a bipolar should do a little better due to it's higher gm.

Any further insight is most welcome, and perhaps a separate low-noise CCS thread is in order if this discussion expands...
 
[quote author="hifizen"]OK, I just measured the impedance of my red LEDs, biased 1mA and 500uA... I get roughly 0.07ohms based on the two voltage measurements. This is even lower than I'd expected, so the base current modulation of Q3 and Q9 caused by current noise and collector voltage swings should cause negligible voltages on the LED. So, I've decided to use a single LED to bias the current sources, and to stick with a good low-noise BJT at Q9 for now (I have 2SC2240BL on hand, maybe will pick up some 2SC3329 eventually - looking at the datasheets, they are very close in performance, but en and rbb' are not explicitly specified for the 2240).

Cracked open my low-noise design books and started brushing up... As I understand the situation so far, the noise analysis of top-grade BJTs (eg 2SC3329) vs. JFETs (eg 2SK170) in this CCS circuit does not clearly show an obvious advantage to one or the other. The comparison is complicated by a lack of comparable data... en is perhaps just slightly higher for the FET, but may be rising more at LF than the BJT (?).... Current noise is considerably lower for the FET, but the BJT base sees a very low source resistance and so In won't cause any significant additional noise voltage anyway. However, In does still add directly to the output noise current, does it not? For a BJT at 1mA Ic, this is typically on the order of 1pA/rt(Hz) or so, rising to perhaps 5 or 10pA at 10Hz (an educated guess on my part, comparing with data I have for other BJTs such as the MPSA18). With a 500ohm source/emitter resistance, this puts the BJT and FET roughly on par... I suspect the biggest difference will be the 1/f region, and the output impedance of the CCS (may be important in some applications), where a bipolar should do a little better due to it's higher gm.

Any further insight is most welcome, and perhaps a separate low-noise CCS thread is in order if this discussion expands...[/quote]

You are on the right track, although the impedance of the LED is wildly off---it should be more like 25-30 ohms at 1mA, not that it matters that much. You are either seeing other effects, or your calculator slipped some powers of ten somehow. 0.07k ohm would be believeable.

The BJT current noise term does indeed add its noise power directly to the power in the current noise at the collector. If you take the base current out of the base and pump it back directly to the collector, i.e., use a darlington, then you have the much lower current noise contribution of the second transistor. You add capacitance and slow response down, and since really tiny geometry bipolars are not usually available except in integrated circuits, these effects are larger than they would need to be ideally.

The FET has its own issues, but at least noise in gate leakage current isn't usually one of them, unless you are running close to breakdown and/or way hot.

The thing about noise from uncorrelated sources: it doesn't take more than a factor of a few less to make a given contribution negligible---root sum of squares really helps a lot. Unfortunately, this means sitting down and figuring out what's really going on to make the assessments and design decisions. For example, I don't know if a 2 ohm rbb' vs. a 30-50 ohm one for Q9 makes a damn bit of difference until I calculate the other contributions.

Note that the bigger your input FET source R is, the more sensitive the amp is to noise in its current source.

It seems hard to believe that there isn't an online comprehensive discussion of current source noise, but I don't know of one at present.
 
Ah, thanks for the sanity check - I was off by 10^3 (35mV over 500uA change)... 70ohms is much closer to what I'd expected. Actual impedance at 1mA should be slightly less... From 1mA to 2mA I get about a 21mV change.


Back to the bias LED... if Q3 shares it's bias LED with Q9, then it's current noise contribution may also be a consideration. So once again, I'll have to do a careful analysis before deciding if I need to use separate LEDs. For the prototype I'm building this evening / tomorrow, I'll just stick with a shared LED, and worry about the noise details later.
 
Schemo removed:

This was the final version of my hack...make R10 adjustable. Never got around to making it.

analag
 
Very similar. Any particular reason you selected MJE340 / MJE350 for the output? I have a handful of these... could try them out later when I get around to fiddling with the output stage.
 
I have a ton of them and I use them all the time in my discrete opamps.

analag
 
Well, last weekend was interrupted by some unexpected guests. So I didn't manage to actually build the proto as I'd intended. However, I've been simming with LTSPICE through the week...

I compared a push-pull CFP ouptut stage against the cascaded emitter followers (discrete darlington). Although in theory the CFP should be a bit more linear, it also seems a lot more finicky to keep happy. The open-loop tests showed that it got really cranky with the certain values of load capacitance, while the EF was much more benign. I also think the CFP does not transition between class A and B as nicely as the EF. If this were going to be a class-A only output stage, I'd give it a bit more effort, but the added complexity of the CFP was the final nail in it's coffin. So discrete darlington it is.

Now, when it comes to taming the DC in this beast, it's quite a challenge... at least in SPICE, which seems to think all the LED models I could find have positive tempcos. :? So I resorted to a diode + series voltage source. This may not be the most accurate method, but at least it puts things in the right ballpark.

The input JFET's tempco is still problematic. I don't want to run it too hot or too near Idss, although I did discover a small cache of K170GR grade in my parts bins, so that'll get me a bit closer to zero TC. I also tried a K170 in place of Q9, with it's gate tied to negative rail, and a source resistor equal to Q1's source resistor. This nulls the -IN terminal's offset over temperature and should be very low noise, but the CCS has a strong tempco which must then be compensated in the Vas current source to avoid an overall DC offset. Second disadvantage: the real-world K170s would have to be matched - so little or no gain there. Back to BJT/LED for the input stage CCS...

As for keeping the output quiescent stable, I've tried numerous strategies there as well, and almost nothing gave satisfactory results. Vbe multiplier was only slightly better than 4-diode bias. The best was an adaptation of what Dan Kennedy used in the Great River Opamp, but that seems a little too complex and tweaky (very tweaky) for this design. The two-transistor temperature compensation is difficult to wrap your head around! So, I think I'm done with SPICE for now... it's just not giving me useful answers anymore - I think I'll just have to build this and fiddle to find something that works acceptably well when assaulted by the heat gun (hehehe :twisted:). I'm going to start with 4-diode bias and see where that gets me (allows simple 2-diode current limiting). Vbe multiplier next, and compare the performance.
 
Regarding the stability problems of the CFP: A zobel (10 ohm/10 nF or similar) at the output to ground usually does the trick. It might seriously eat slew-rate if the output is current-limited, though. Yours isn't, so this could help.

What makes you think that the CFP is less linear in AB?

The diamond buffer would be another alternative.

Samuel
 
Back
Top