Electronics > Beginners

TL071 distortion - bad amplifier design?

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Zero999:

--- Quote from: Kleinstein on December 10, 2018, 04:19:24 pm ---The circuit is problematic: There is quite some gain inside the loop of the TL071. This requires quite some extra capacitors for the compensation to slow it down.  With those rather large capacitors the TL071 sees quite some capacitive load and it might thus still oscillate at a high frequency.

The output stage is class B and thus would need a rather fast feedback to keep cross over distortion low. However as shown the first transistor stage is rather slow.

So the circuit is more like a poor example and can not expected to give low distortion output.

Another point is that the biasing current for the C1815 and A1015 depends on the supply voltage. So voltage drops (e.g. due to small filter caps) could make it leave the reasonable working range. Too high a voltage could cause excessive current and blow the small transistors.
For understanding the circuit it might help to simulate it - however there is likely little success in improving this type of circuit.

--- End quote ---
Are you sure the final stage is class B? Have you done any calculations?

Whether the final stage is class B or AB, will depend on how much current is flowing through the 100R resistors in the TIP41 and TIP42 stage. I suspect if it is class B, it can be tweaked into class AB fairly easily.

EDIT: I see how it's class B now.

AngraMelo:
Well, on the scope, the crossover distortion was clear. There were bumps on the side of the waves near the 0V line. I guess lowering the 220R resistor would help bring it to the "AB" side.

AngraMelo:
Are you sure the final stage is class B? Have you done any calculations?

Whether the final stage is class B or AB, will depend on how much current is flowing through the 100R resistors in the TIP41 and TIP42 stage. I suspect if it is class B, it can be tweaked into class AB fairly easily.
[/quote]


Arent the 220R resistor also responsible for the bias of the TIPs? Or they are so low that the 1015 and 1815 have plenty of base current and what is limiting their output is just the 100R on their emitters?

Zero999:
Looking at it again, Kleinstein was right, it's definitely class B. The diodes on the bases of the TIP4x transistors hold their bases at 1.4V apart, so hardly any current flows through the 100R resistors. Adding a couple of extra diodes should increase the current flow and bias it into class AB operation.

Kleinstein:
The TIP41/42 run kind of class AB, but the SD52xx / SA19.. are running class B. One could at least add a 3 rd diode. To make is class AB it would need something like an VBE multiplier instead of the diode, set to a little more than 4 diode drops to adust the standing current. It won't be very stable however, as the VAS current is not stable.

Changing the 220 Ohms resistors is tricky, as there correct value depends on the supply voltage - which is one of the weak points. Too much and the 1015/1815 overheat. To little and the VAS stage is also kind of working in an class B mode with more distortion.
It might get a little better with a diode in series with a smaller resistor - though this might load the OP too much. The rabbit hole just starts here.

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