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Discrete Unity Gain Differential Buffer?
Discrete Unity Gain Differential Buffer?
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Old 10th April 2019, 03:33 PM   #1
exclamationmark is offline exclamationmark
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Default Discrete Unity Gain Differential Buffer?

I'm basically looking for a discrete unity gain differential amplifier topology (for converting a balanced signal into a single ended one - inputs need to be high impedance!). Has anyone dealt with a similar problem before?

Backstory:

I'm making a front-end for a combo mic/line/instrument input that gets fed into an ADC. Low-noise op-amps/instrumentation amps (<2.2nv √Hz) tend to be quite pricey, so I thought why not chuck my own together with 2N4403/2N4401's that I can get for a couple of cents each.

The solution I've come up with so far is basically 4 discrete op-amps, with 3 of them forming a unity gain instrumentation amplifier, with the last one performing up to +60dB of gain.

Why 4 op-amps and not 3 you ask, as in the case of an instrumentation amplifier? I'm using a digital potentiometer to control gain, and in order to avoid wiper distortion due to dynamic resistance, I've configured the last op-amp as a non-inverting gain stage. The wiper is connecting directly to the inverting input, which has something like 10+ meg ohms of resistance, so *fingers crossed*, the current through the wiper will only vary by a couple of uA.

It seems like a huge waste to use 3 DOAs to achieve, what is essentially, a differential to single ended buffer.
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Old 10th April 2019, 03:40 PM   #2
Michael Bean is offline Michael Bean  United States
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These are quite good, and easy to implement...

http://www.thatcorp.com/datashts/THA..._Datasheet.pdf

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Old 10th April 2019, 03:41 PM   #3
Osvaldo de Banfield is offline Osvaldo de Banfield  Argentina
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A differential amplifier around a opamp will let you the hit. Also any instrumentation amplifier with resistors set to gain of 1.
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Old 10th April 2019, 03:48 PM   #4
Mark Johnson is offline Mark Johnson  United States
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Discrete Unity Gain Differential Buffer?
JFET source followers with CCS loads --> your favorite single opamp differential amplifier. If you worry about matching, just use a single opamp (not a dual) that has offset trim pins and don't bother matching the JFETs.

LSK170A Nchannel JFETs are low noise devices, sold in the diyAudio store. Run them at plenty of mA of bias current and drive low impedance resistive loads.
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Old 11th April 2019, 01:51 AM   #5
PRR is offline PRR  United States
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Quote:
Originally Posted by exclamationmark View Post
.....unity gain instrumentation amplifier, with the last one performing up to +60dB of gain.....
That does not make sense, unity gain THEN big gain. Assuming equivalent hisses, that's 3dB Noise Figure before you get well started.

I also suspect that your "high impedance" input does not need all the gain you might like on a Microphone. Also likely different connectors?

The hi-Z input needs low hiss current, not necessarily low hiss voltage. In BJT the two goals conflict; in FET you can find a good part to cover the impedance range you actually need.

I suspect this problem has been solved before, over and over. Plagiarize!
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Old 11th April 2019, 01:56 AM   #6
chris719 is offline chris719  United States
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Discrete Unity Gain Differential Buffer?
Quote:
Originally Posted by exclamationmark View Post
I'm basically looking for a discrete unity gain differential amplifier topology (for converting a balanced signal into a single ended one - inputs need to be high impedance!). Has anyone dealt with a similar problem before?

Backstory:

I'm making a front-end for a combo mic/line/instrument input that gets fed into an ADC. Low-noise op-amps/instrumentation amps (<2.2nv √Hz) tend to be quite pricey, so I thought why not chuck my own together with 2N4403/2N4401's that I can get for a couple of cents each.

The solution I've come up with so far is basically 4 discrete op-amps, with 3 of them forming a unity gain instrumentation amplifier, with the last one performing up to +60dB of gain.

Why 4 op-amps and not 3 you ask, as in the case of an instrumentation amplifier? I'm using a digital potentiometer to control gain, and in order to avoid wiper distortion due to dynamic resistance, I've configured the last op-amp as a non-inverting gain stage. The wiper is connecting directly to the inverting input, which has something like 10+ meg ohms of resistance, so *fingers crossed*, the current through the wiper will only vary by a couple of uA.

It seems like a huge waste to use 3 DOAs to achieve, what is essentially, a differential to single ended buffer.
I mean, you can do it with op-amps at a reasonable cost I would think. If this is some crazy-high volume product, it might be worth it but I don't see it. Your time designing these discrete op-amps isn't free, presumably. Also the PCB real-estate used is going to be much larger.

I guess if it's for personal / hobby use then anything goes.
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Old 11th April 2019, 05:19 AM   #7
BesPav is offline BesPav  Russian Federation
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Quote:
Originally Posted by Mark Johnson View Post
JFET source followers with CCS loads

Even cascoded with depletion MOSFETs.
Hi-Z, Low-inp-Q, high CMRR, selectable gain.
Attached Images
File Type: png Hi-Z, Low-C Balanced input.PNG (37.5 KB, 194 views)

Last edited by BesPav; 11th April 2019 at 05:22 AM.
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Old 11th April 2019, 05:05 PM   #8
sgrossklass is offline sgrossklass  Germany
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Quote:
Originally Posted by PRR View Post
I suspect this problem has been solved before, over and over. Plagiarize!
+1. Mind you, getting ahold of schematics for commercial equipment isn't necessarily easy in this day and age...

But as a hint, you typically find Mic/Hi-Z or even Mic/Line/Hi-Z switches on such equipment. There's a reason for that.

Even combining a mic-in and a line-in is difficult.
You may need up to +50 dB for a mic, while handling line input levels in excess of +20 dBu may mandate +/-15 V supplies if your minimum gain is unity, and overall gain required may well be around -16 dB if we're assuming a 5 Vpp ADC input.

Doing the same for a Hi-Z + line input is a bit easier, as levels aren't too far off, but an extra buffer is still likely to be required. The input should be bothered by some common-mode components, as instrument cabling is generally unbalanced.

In order to come up with a good solution, you need to fully grasp the problem first. This thread is just another example of what happens when that isn't the case and people sort of try to jump in and tackle both in parallel.
Quote:
Originally Posted by PRR View Post
That does not make sense, unity gain THEN big gain. Assuming equivalent hisses, that's 3dB Noise Figure before you get well started.
Not to mention that having differential gain up front in a balanced input improves CMRR by equal measures.

You have to be careful though, unity voltage gain does generally not equate to unity power gain. You could have plenty of power gain by providing current gain alone. Of course the currents and devices required may well be insane, but this just illustrates how we can always trade off between voltage and current to match the devices available and external constraints.
Quote:
Originally Posted by PRR View Post
I also suspect that your "high impedance" input does not need all the gain you might like on a Microphone. Also likely different connectors?
Hi-Z input levels tend to be several hundred mV, and you want upwards of 500 kOhm or so of Z_in.

Mic cabling is XLR.
Hi-Z cabling is 1/4" TS.
Line-level cabling may use either XLR or 1/4" TRS / TS.
It is not uncommon to be providing both XLR and 1/4" inputs, with roles spread across them. Some concepts reserve XLR for the mic in order to keep contact issues from interfering with mic operation, with 1/4" for line-level / Hi-Z duties.
When deciding to implement combo jacks, pick them carefully. Random Chinese jobs are likely to have terrible handling for the 1/4" portion (I hate these suckers with a vengeance - either you can hardly get the plug back out or contact is terrible; at least the XLR portion tends to be OK).
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Old 12th April 2019, 05:00 PM   #9
mchambin is offline mchambin  France
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Not clear what you want to invent.....
To convert a balanced signal to single ended, at line level, is usually done with this.
http://www.thatcorp.com/datashts/THA..._Datasheet.pdf
Or same from Texas Instrument and Analog device
INA137 INA134 http://www.ti.com/lit/ds/symlink/ina2137.pdf
SSM2143 SSM2141 https://datasheet.octopart.com/SSM21...t-10720050.pdf
Find them with octopart.com
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Last edited by mchambin; 12th April 2019 at 05:13 PM.
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Old 12th April 2019, 05:14 PM   #10
EUVL is offline EUVL  Europe
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XCEN Balanced to Single-Ended Converter Revisited


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