Current feedback questions

[tk@halmi]

Since good CF opamps are not cheap I suspect, as Brad does, that they use resistor laser trimming to keep offsets low.
I think of the Vbe offset at the output as an asset instead of a problem. It helps bias a good quality electro-cap on the output.

For applications that require one or two amp stages like preamps and compressors having a couple of caps in the audio path can have insignificant contribution to the sound. If you need to chain twenty of them up as in a mixer you need as little offset as possible.

Problems become numerous when we set out to make general purpose discrete opamps that we hope to drop in place of monolithic opamp roles. I have simulated over fourty different opamp topologies and protoyped over a dozen circuits. It is nearly impossible to meet monolithic opamp numeric figures. Then again, most people claim that discrete opamps sound better despite of their inferior specifications.

Notice that CF opamps have a little lower open loop gain compared to commonly used VF opamps. CF opamps tend to have fewer internal gain stages yet achieve very good distortion characteristics.

Do you have a goal for how low the offset should be?
Any targets for open loop gain?

Cheers,
Tamas

 

[Samuel Groner]

Unfortunately, simulation is not that successful

OK, I confess, it was me. Why are those simulators that fussy about one or the other unmotivated connection? :grin:

A quick sim shows good to very good behaviour.

I have simulated over fourty different opamp topologies and protoyped over a dozen circuits. It is nearly impossible to meet monolithic opamp numeric figures.

From what I've seen, ICs rely heavily on two-pole compensation and nested feedback loops; if you have any insight on this, I would be very happy to hear about it. I played (quite...) a bit with two-pole compensation, but it is difficult to avoid peaks in the closed loop response at various gains.

Samuel

 

[bcarso]

"...that they use resistor laser trimming to keep offsets low. "

It is something of a challenge to see where this would go though, while preserving all the other advantages of the structures. I guess between the input CC stages and the next in line there could be some little R's, with the input devices run a lower standing current to give a little adjustment room. But the required delta for some devices is a lot.

I did some work yesterday to see roughly the size of the NPN/PNP imbalance. For a particular model of each the delta V was about 42mV for similar geometry parts. If I made the current density in the PNP 1/5 of the NPN I got the Vbe's to match---but that's pretty brutal! It means 1/5 the current for the PNP, or the same current for five paralleled devices, or...

So I suspect that to some extent the process for a given device is tuned to minimize the offset.

"Notice that CF opamps have a little lower open loop gain compared to commonly used VF opamps."

Usually the low frequency gain (in this case right in audio territory) is smaller since it's not usually as interesting for the intended apps like video and data acquisition to not-too-many-bits but with blinding speed. It is possible to raise the low-freq gain by making the output node ahead of the buffer higher Z. In the amp structure that Samuel G. posted the link to, the current mirrors can be made higher-Z out for example with a cascode configuration. This also makes the amp faster as there was appreciable miller effect as shown given the choice of 180 ohm emitter R's. I wound up with a sim that indicated a 17MHz -3db closed loop gain of 20dB, with the feedback network adjusted for optimal transient response, and rise and fall times of about 20nS for 20V (1000V/usec), using garden-variety transistors. This may be a bit optimistic since the parasitic L's were not included and the current sources were perfect, but it's probably not too far off.

How it would sound I have no idea. I think I'll figure out how to get rid of the offset before going much further.

(added just now) Congrats for finding the error Samuel.

 

[tk@halmi]

"Devices use a patented auto-zero technique to achieve nearly ideal op amp specifications and still manage to squeeze into a 6-pin SOT-23 package. The auto-correlating zeroing method measures and compensates the input offset voltage, eliminating drift over time and temperature and the effect of 1/f noise without introducing fixed-frequency ?chopper? noise."

Then there is the mysterious patented approach...

 

[tk@halmi]

You can also slap a buffer in front of the non-inverting input and manipulate the current source of the buffer to zero the output. Essentially it is a servo/buffer solution. I have been playing with such a FET buffer for the GainBloak.

 

[bcarso]

Tek did a auto-zero like thing biasing a FET buffer like that---it is described in Feucht, Handbook of Analog Circuit Design, which I just had reason to mention in another thread. An excellent book, which I almost didn't pick up off the shelf when I saw it (who needs yet another book on analog design said I ;-). This guy worked at Tektronix and learned his lessons well.

The trick with the buffer otherwise is matching to the tempco of the bipolar.

 

[Samuel Groner]

I found at least four interesting sources @ scholar.google.com:

00628768.pdf
00069472.pdf
00274080.pdf
cfa.pdf

Still searching...

Samuel

 

[bcarso]

I've got a solution for low offset voltage working in simulation that uses complementary Schlotzhauer* triples in the front end. It looks pretty promising for relatively high gain configurations, but the extra phase shifts in the front end make low gain configs require a fair amount of additional compensation, which slows things down. Still probably fine for audio though. Sims indicate about 0.5 ppm distortion for moderate output voltages and gains of about 30, which is probably overly optimistic.

There is a side advantage of reduced thermals in the input stage.

There are a lot of parts, especially a lot of current sources/sinks.


*(and you thought Sziklai had trouble getting dates)

 

[Samuel Groner]

which slows things down

How fast are things? I'm happy with everything above 0.49 kV/us... :wink:

Sims indicate about 0.5 ppm distortion

At what freq.?

If you don't mind sharing, e-mail me the schemo and I'll post it.

Samuel

 

[bcarso]

Schemo is going to need some cleanup but I will send it eventually. I'd like to play with the compensation some more. Also at the moment the I sources are ideal and need to be replaced with real parts.

It's still fast---don't get me wrong.

Oh---0.5 ppm at 1kHz. Just a convenient freq.---it won't degrade very fast as the open loop gain isn't rolling off until about 6kHz if memory serves.

 

[Samuel Groner]

For the meantime here the schemo I've been working with. Time and frequency domain performance simulates as being outstanding, at gains between 0 dB and 40 dB.

Do you think this opamp could be used in a transformerless mic pre? Together with a single ended low-noise transistor frontend (similar to Cohen 2106)?

Samuel

 

[tk@halmi]

Sam,

I think you get the most benefit of very low offset (for no output
capacitor operation) when you use monolithic dual opamps with
the Cohen mic pre.

Also, you could try PRR's circuit:
http://www.groupdiy.com/FORUM/viewtopic.php?t=4824&start=30&sid=8259bbca8d683d25e1b65c077ba862be#img_5550
Voltage noise may be a bit higher than desired.

Tamas

 

[Samuel Groner]

I realize that my question was misleading - I'm not interested in Cohens low offset plans. In fact I've been thinking on using a circuit based on a MC amp design by D. Self ([removed]) - two of these should make a nice frontend for an instrumentation amplifier.

However, I'm not sure whether this behaves well with CF opamps - I do not see yet how the bandwith would be set here as the usual feedback resistor is somehow moved.

In addition to this, I tried to make my CF-opamp having a good NF @ 100 ohm source impedance such that it could be used directly for the first stage of an instrumentation amp. First plan was to use low-noise transistors (2SA1316 and 2SC3329) and running them at 3 mA. In the absence of a suitable SPICE model I used ECG373/ECG374, hoping to get similar Cs and ft. This simulated not that bad, but caused some pre- and overshoot at gains below 10 dB which I did not get rid off. Paralleling 2n4401/2n4403s gave similar results.

Samuel

 

[Samuel Groner]

Some more links on CF:
www.diymania.net/pass/project/CF-Power.htm
www.soton.ac.uk/~apm3/diyaudio/Stochino_amp.html
www.soton.ac.uk/~apm3/diyaudio/Stochino_circuit.jpg

Samuel

 

[bcarso]

"However, I'm not sure whether this behaves well with CF opamps - I do not see yet how the bandwith would be set here as the usual feedback resistor is somehow moved. "

So Samuel, I guess you are proposing to use a CF amp as the "u1" block in your quasi-Self hand schematic?

My guess is that it would work, provided, as is likely, that you have a very large R4/R5 ratio, that is, high closed-loop gain. Whether it has any special advantage over a simpler structure is another question. But it sounds like a good investigation anyway.

For low closed-loop gains it is always going to be tricky, and probably will entail internal compensation changes for good transient response---a "one-size-fits-all" internal amp I doubt will work. But then this is the old problem that we keep encountering, trying to get ever-larger S/N numbers.

As far as the model for the low rbb parts: from the little information that Toshiba provides, I suspect that the base-emitter diffusion capacitance is quite large compared to typical bipolars of the same approximate size. Clues are the lower gain-bandwidth products, unfortunately cited as single numbers in the datasheets. Anyway, it might be possible as a crude approximation to parallel devices with well-documented models and then additionally put some lumped external C across the base-emitter junctions to slow them down to the comparable f sub t's.

 

[Samuel Groner]

I guess you are proposing to use a CF amp as the "u1" block in your quasi-Self hand schematic?

Yes, forgot to say that, though U2 does not make sense to be CF.

My guess is that it would work

Doesn't the inverting input short R2 to "ground" and thus preventing any reasonable bias current?

I replaced the ideal current sources in my design and set up some slightly different bias currents ([removed]). Twenty four transistors... :?

R6 and R7 are uncomfortably low. Inserting a diode in series with the emitters of Q5/Q6 would help, I guess.

How can I estimate the voltage and current noise of this design? I found some data for the used transistors working @ 1 mA:

2n4401:
RNv = 31 ohm
RNi = 5k5 ohm

2n4403:
RNv = 28 ohm
RNi = 8k ohm

Are the equivalent noise resistances we have here in parallel or in series?

Samuel

 

[bcarso]

As a closed-loop system the inv. input of u1 will require only a tiny current change, compared to what is flowing in R2 etc. to change the output voltage and the divided-down signal at the Q emitter. So equilibrium will still be achieved.

However notice that as drawn there is no way for the transistor to get any emitter current, so there has to be provision for this. That is, the d.c. servo constrains the output voltage to zero, R5 ties to 0, and so the PNP emitter current has to be 0 too. You could run R5 from a positive bias voltage source, although it will have to be a super-quiet one if it is not to spoil the noise performance. OTOH it's o.k. for the emitter to be at ~zero so that there is no d.c. flowing in R4 and R5, if the emitter current comes from another R to +V. Then the base will be at about -0.7V etc.

Look out for servo amp noise too at these levels of expectation.

I'll get back to you on the other questions---I have to get some rest ;-).

 

[Samuel Groner]

Revision 2 is online, now with DC compensated input: [removed]

Step response @ 6 dB looks fine: [removed]

Simulates as beeing stable at unity gain. Q1-Q4 and Q5-Q8 could be either pairwise for Vbe matched discrete transistors or perhaps a THAT 340 array. Should make a nice opamp for a Twin-Servoish mic pre and perhaps a candidate for the recent XFMRless thread.

Samuel

 

[bcarso]

Good effort Samuel!

I noticed today working on a stopgap discrete power amplifier design for a client, that the models, in Circuitmaker at least, for the MPSA06 and MPSA56 seem to have nearly-matched Vbe's. I will have to haul some parts out and verify, but a simple diamond quad had fairly low offset---surprising.

 

[Samuel Groner]

Revision 3, with better biasing for the output stage: [removed]

As shown, it sims as beeing pretty immune against input stray capacity and well behaved with capacitiv loads. 1 nF causes some slight overshoot, and 10 nF some well-damped ringing. This can be solved with a load isolator.

Unfortunately my SPICE refuses to do THD sims, so no idea on linearity.

MPSA06 and MPSA56 seem to have nearly-matched Vbe's

That's interesting. However, if the input stage with the trans-diodes is reasonable low-noise, I think I'll go this way as it promises pretty good offsets and drifts. Bias compensation may be necessary, though.

I think it would be better to use a THAT 300 and THAT 320 instead of two THAT 340 for the input stage, as this would minimize bias offset currents, right?

Samuel