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| Balanced audio output : opamp configuration |
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| richard.cs:
--- Quote from: magic on May 17, 2019, 09:47:03 am ---BJT will give bad distortion from current gain variation with collector current and collector voltage. Maybe a FET would have some chance. --- End quote --- The current gain variation is (mostly) removed by the opamp feedback because that's taken from the emitter. All that's important is that the gain is large so that collector and emitter current are approximately equal - finite gain affects the balance of the output. There is some distortion on the inverted (collector) signal because of the gain variation, but it can be small, especially if you start with a high gain device and set the circuit up with a high standing current such that the gain variation is small. It depends what your application is as to whether that matters. I see no reason it won't work with a FET so long as you isolate the gate capacitance from the opamp so it doesn't oscillate. |
| Bassman59:
--- Quote from: richard.cs on May 17, 2019, 10:01:14 am --- --- Quote from: magic on May 17, 2019, 09:47:03 am ---BJT will give bad distortion from current gain variation with collector current and collector voltage. Maybe a FET would have some chance. --- End quote --- The current gain variation is (mostly) removed by the opamp feedback because that's taken from the emitter. All that's important is that the gain is large so that collector and emitter current are approximately equal - finite gain affects the balance of the output. There is some distortion on the inverted (collector) signal because of the gain variation, but it can be small, especially if you start with a high gain device and set the circuit up with a high standing current such that the gain variation is small. It depends what your application is as to whether that matters. I see no reason it won't work with a FET so long as you isolate the gate capacitance from the opamp so it doesn't oscillate. --- End quote --- The simplest way to do this correctly is to use a THAT1600-series or TI DRV134/135 part and be done with it. |
| MagicSmoker:
--- Quote from: ratatax on May 16, 2019, 09:25:25 pm ---... If I follow the schematic it seems to do the job : the Tip of the jack will receive the original (inverted two times) amplified signal, while the Ring will get the reversed signal (inverted three times). I feel it could be simplified with only two opamps instead of four : inject the DAC signal in two parallel opamps which both provide a gain, one in non-inverting mode the other in inverting mode. ... --- End quote --- I don't think anyone actually answered your question... The reason it is better to use two inverting amplifiers in series to produce a balanced (ie - differential) output is because it is more difficult to match the gain and input impedance of an inverting amplifier with a non-inverting (especially for unity gain versions of each). Any mismatch in gain will be treated as a legitimate differential signal by the downstream balanced receiver, after all. |
| bson:
Small variations in gain won't matter. The total gain is the difference between the two; Vo = K1*Vin - K2*Vin so you get Vo = (K1-K2)*Vin. In other words, if K1 = 1.05 and K2 = -0.9 you get a total gain of 1.95 between them. No problems here. For an actual circuit you'll want a resistor equal to R1//R2 to ground on the positive input of the inverting amplifier, to balance the input offset currents, but that's not strictly necessary just to get it to work. |
| magic:
I will add that the gain of opamp voltage follower is 0.99999 or so, you won't get the same level accuracy with resistors. An inverting stage with 1% resistors may have up to 2% gain error. Two such stages - up to 4% gain mismatch. |
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