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| TL071 distortion - bad amplifier design? |
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| Zero999:
--- Quote from: Kleinstein on December 16, 2018, 05:19:25 pm ---The last circuit shown is a well known audio amplifier circuit. The circuit with an OP at the input could work as well. However it is not the really easy way for high power: One would need a power stage with additional voltage gain. It needs some attention to make the two gain parts to work together. The simple 1 pole compensation is usually no more suitable as this would need only one slow stage. Compared to many of the normal OPs the simple transistor longtail pair is also relatively fast and allows thus for good overall speed. The use of inner loops with local feedback is kind of controversial - not everybody likes it. Well done, the concept with OP for the input and discrete power stage can work. Though the general concept may look simple, I consider the classical amplifier with discrete transistors easier. However the TL07x are relatively high noise and might thus produce some noticeable hiss. I also see a slight problem with an electrolytic capacitor with no DC bias and possible slight ( 10s of mV) reverse bias from an offset. In addition to possible distortion, there might be an issue with durability. The oxide layer will slowly get thinner and the distortion might only show up after longer use. --- End quote --- There are a couple of errors in the annotations: Constant current generator #2 should be current mirror. The voltage to current converter, is mislabelled. A voltage to current converter would be a transconductance amplifier i.e. voltage in, current proportional to the input voltage out. It's really a unity gain buffer or current booster circuit i.e. input voltage = output voltage, with a high input impedance and low output impedance. I'm aware that the poster knows this and has just worded it poorly. I don't see why local feedback is such a bad idea. It will certainly be easier to stabilise. Set the output stage to a gain of 3 and you might be able to get away without using additional compensation capacitors. The TL072 may have a relatively high input voltage noise, but it has a very low current noise and will beat op-amps with a much lower voltage noise, such as the LM4562, hands down in an application with a high input impedance. In this instance I agree, the TL072 will be more noisy than BJT input op-amps, as the impedance connected to the input will be much lower than the 22k bias resistor. |
| Kleinstein:
Local feedback is a controversial topic. Some think it is a good idea, some don't like it. For a circuit with an OP at the input and a power stage with extra gain (like the circuit from the start), local feedback could in deed be a good idea, but it may not help very much at the upper frequency end, that is critical for stability. It only gets really easy with an inner loop that is faster than the outer. |
| David Hess:
My own experience is that local feedback makes the frequency compensation of a multistage design tractable. My last audio amp designs used local current feedback around the voltage amplification stage to tightly control voltage gain and further local feedback around the output stage to reduce crossover distortion. Local current feedback around the voltage gain stage has the advantage of multiplying slew rate independent of what the input differential pair can achieve without transconductance reduction which is needed to raise input stage current or lower Miller capacitance. |
| Zero999:
Here's how I would do it: give the output stage a gain of 3 and a roll-off low enough to make the gain unity, before the phase shift exceeds 180o. Kleinstein, Yes of course, it's controversial. I didn't mean to suggest you're against local feedback, but I'm interested in why some are. |
| spec:
--- Quote from: Kleinstein on December 16, 2018, 05:19:25 pm ---I also see a slight problem with an electrolytic capacitor with no DC bias and possible slight ( 10s of mV) reverse bias from an offset. In addition to possible distortion, there might be an issue with durability. The oxide layer will slowly get thinner and the distortion might only show up after longer use. --- End quote --- Not with solid tantalum capacitors which will take a slight reverse bias. Incidentally, that capacitor is a PITA as it is part of the feedback network. For my money I would like to see it eliminated altogether and the amplifier servoed with an opamp. Or offset balancing used with a matched pair of transistors in one can for the input stage. |
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