Electronics > Beginners
TL071 distortion - bad amplifier design?
Zero999:
--- Quote from: perieanuo on December 10, 2018, 11:56:01 am ---
--- Quote from: Hero999 on December 10, 2018, 11:51:24 am ---
--- Quote from: perieanuo on December 10, 2018, 10:55:06 am ---And for god's sake put limiting resistors on base on final resistors, once they saturate you're in trouble.
On input I will use in your place bigger capacitors like 10 uF
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--- End quote ---
You mean the final transistors, not resistors, i.e. the 2SC52EE and 2SA1943?
If so, how can they saturate? They're emitter followers.
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They can.for example, from thermal considerations or if driver goes short.that's why between emiter driver and final transistor base it's good idea to put resistor.see the second schematic that does have.
Pierre
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If the driver goes short circuit, the whole thing is toast anyway, whether there's a small base resistor before the output stage or not. Suppose the TIP41 goes short. How will a 2R2 resistor in series with the 2SC52EE's base possibly offer any protection? A huge current will flow through the 2SC52EE. Hopefully the amplifier will blow up before the speaker is destroyed.
The output stage is an emitter follower so can't go into saturation. Even if the base of one of the output transistors is connected directly to +V, it will still not saturate. The emitter voltage will rise, thus reducing the base-emitter voltage, until the emitter will sit a couple of volts below the collector.
https://en.wikipedia.org/wiki/Common_collector#Basic_circuit
There may be other reasons for including a base resistor, such as reducing the gain and making it less susceptible to oscillation, but preventing saturation is not one of them.
perieanuo:
of course it can saturate
https://www.allaboutcircuits.com/textbook/semiconductors/chpt-4/common-collector-amplifier/
https://www.electronics-tutorials.ws/amplifier/amp_2.html
get yourself a real amplifier, put it on your basement and put it to over 80% for a while with nominal load (cut the cooler fan to do it quickly), buy a multimeter or scope and tell us the result
Zero999:
--- Quote from: perieanuo on December 10, 2018, 12:36:43 pm ---of course it can saturate
https://www.allaboutcircuits.com/textbook/semiconductors/chpt-4/common-collector-amplifier/
https://www.electronics-tutorials.ws/amplifier/amp_2.html
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A common emitter amplifier yes, it can saturate, but a common collector amplifier can't saturate, unless the input voltage is taken beyond the supply voltage. The tutorial you've linked to actually explains this.
--- Quote ---Given the voltage polarities across the base-emitter PN junction and the load resistor, we see that these must add together to equal the input voltage, in accordance with Kirchhoff’s Voltage Law. In other words, the load voltage will always be about 0.7 volts less than the input voltage for all conditions where the transistor is conducting. Cutoff occurs at input voltages below 0.7 volts, and saturation at input voltages in excess of battery (supply) voltage plus 0.7 volts.
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In the circuit posted at the start of the thread, the input voltage can't exceed the supply voltage, so the output state can't saturate.
--- Quote ---get yourself a real amplifier, put it on your basement and put it to over 80% for a while with nominal load (cut the cooler fan to do it quickly), buy a multimeter or scope and tell us the result
--- End quote ---
I seems like you're confusing clipping distortion and overheating with saturation of a BJT. If an emitter follower output stage is over-driven, it will clip, distort and possibly overheat, if the heat-sinking and power rating of the transistors is insufficient, but neither of the transistors will ever enter the saturation region. They will both be operating in the active region the entire time.
In any case, adding base resistors to 2SC52EE and 2SA1943 will do little to protect them against overheating. It may improve the stability, at the expense of a reduced voltage swing.
EDIT:
--- Quote from: perieanuo on December 10, 2018, 11:56:01 am ---Look here and try to understand in practice it can arrive this situation"The bias resistor must connect to a voltage that is at least 0.7V higher than the supply voltage for the upper emitter-follower transistor to saturate." (eg if you have voltage loss at high power on your wires inside the amplificator, yes, the wires between the final transistor and the +Vdd, I seen this in a clone philips schematic, in practice) :
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Sorry, I missed this.
The wires between the final transistor and the supply voltage would have to be a very high resistance, or carrying a large current to drop nearly 1V (more more than 0.7V is required to saturate a transistor). An additional circuit would be required to protect against the high current scenario, as without it the amplifier would be toast anyway.
I see what you mean now and agree: a base resistor would protect against a high resistance or open circuit collector connection, because the maximum base current rating would be exceeded. Are you sure the extra resistor in the Philips amplifier wasn't to improve the stability?
There will be plenty of other points in the amplifier which can be destroyed by a bad connection. For example, what will happen if the connection to one of the zener diodes is bad? The TL071 will get 40V on one of its supply rails.
It's not feasible to protect against everything that can possibly go wrong!
Kleinstein:
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.
AngraMelo:
Could you please explain why the first transistor stage is slow?
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