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Op-amp to headphones: Forget Matching?

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DigiTan said:
Did the value of that bootstrapping capacitor have to calculated or is there a rules-of-thumb? It seems like you'd have to take an RC time constant into consideration.
If you don't consider the RC time constant then low frequencies won't get boostrapping and will be very distorted.

The two 2.2k resistors are effectively in parallel so 22uF into 1.1k is a frequency of 6.6Hz. All audio freqiencies will have bootstrapping.
 
Okay, just making sure I was doing it right.

In another thread, you mentioned that the output impedance is usually set up to provide damping for the load speaker. Seeing as I'm using 32Ω headphones, I want to also produce the correct damping if possible.

Also, at the input side of this project, I want to have my circuit matched to the same 32Ω. Would I need reactive components on my inputs to do this?
 
The speakers in headphones are tiny. So they don'r resonate much and don't need damping. Many power amplifiers have a headphones output jack that is attenuated with 120 ohm series resistors.

If you match the impedance of the source then you are increasing its distortion. It might restrict the low frequency response. Only old vacuum tube amplifiers need a matched load impedance. A solid state amplifier works fine with a high impedance load and since the current is lower then the distortion is also lower.
 
Okay, never mind the temperature simulation--I located that function. I started using a close version of the bootstrapped amp. The only difference is I replaced R2 and R3 with a Vbe multiplier to push enough current for D1 and D2 to drop 1.6 Volts. If it doesn't cut off my output range too much, I'll add one last short circuit protection stage to the output.

My next concern is the thermal runaway. Although the feedback loop is going to mitigate the problem, I don't trust the temp simulations enough to leave things as they are. I want to try adding 2 more transistor/resistor pairs that would provide a thermal cutoff (if that's the right term).
 
Also I want to find the input resistance of each stage. The transistors aren't a problem, just the diodes. Should I take the D1 & D2 currents/voltages and just treat them as a resistor?

Whoo! 15 minute limit! Postcount++
 
Negative feedback does not stop thermal runaway of the output transistors. If the output transistors have low value emitter resistors and the diodes or Vbe multiplier transistor are mounted on the heatsink then the output transistors will not have thermal runaway.

Negative feedback reduces gain and distortion and reduces the output impedance of a power amplifier.
 
Well I was thinking the negative feedback would at least lower the effect of thermal runaway at the output. I looked at it this way.. Thermal runaway is an unintentional rise in current gain. Negative feedback drops the gain down to 1/(1+AB)--where A is the open-loop gain, and B is the feedback factor. So the unwanted change in gain due to thermal runaway should also fall by the same 1/(1+AB)? Or some close amount since the transistors are so similar. Anyway, I'm hoping that will stabilize things enough if this other idea doesn't work out.

So any idea how to calculate the input resistance of that last circuit that was posted? I guess I could just peek at the simulation, but it seems like kind of a cheat.
 
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Negative feedback in an amplifier reduces its voltage gain, not its current gain.

Thermal runaway is caused by transistor's current increasing as it heats. Decreasing the base-emitter voltage will reduce the current. The voltage across the diodes decreases when they heat so they compensate and nearly eliminate thermal runaway.

Why do you need to know the input resistance of the amplifier? It is high enough to be fed by almost any source. An opamp driving a couple of transistors will be a much better amplifier.
 
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