apzx
Active member
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Newmarket said:Just a note to suggest you might want to look at connecting jfets as low leakage diodes.
I am not sure we are all on the same page here but the class A reference here is to the class A fraction of class AB power amp output stages. In common collector output stages using a NPN to source current and PNP to sink current, there are turn on and off delays when the output current transitions from sourcing to sinking. This delay causes a well know distorion mechanism called "crossover distortion". It is most noticeable at low levels and high frequency because of the time delay component. A relatively simple remedy for crossover distortion is to leave both devices slightly turned on, this results in a much cleaner transition between sinking and sourcing current, but has a cost. Since both power devices are turned on with the full power supply across them, dissipation can easily become an issue. Too much class A current can release the magic smoke inside power transistors.apzx said:> It's not the diodes. It is the amplifier idle current set by diode voltage.
You could find a 0.678V battery and get the same performance (for a day or two). As said, a Vbe multiplier puts YOU in control. (However it is astonishing how often '4148 diodes set medium-size BJTs to a happy zone.)
So, if I am understanding correctly here running the transistors with greater Iq allows them to operate (I do not know the word here sorry ) more efficiently or operate at a higher frequency? Playing around with the circuit in the simulator the LTP is biased from a PNP current source. Changing the 1N4148s to 1N3595s results in an increase of the bias current of about 54.5uA for the LTP. Now, it looks like by increasing the nominal Iq of the LTP a little more response is achieved.
I also swapped out the two output biasing diodes for a Vbe multiplier in the sim and it again looks like if I increase the Iq there as well I notice a similar thing happening. That is a little more response is achieved. I do kind of wish I knew the word for this. Regardless the Vbe multiplier is really cool because it is incredibly easy to simply vary the Iq of the output stage by changing one resistor value. I am going to assume, without having looked for it in particular, that there is a limit to how far things can be pushed this way before other things start getting in the way, correct?
gridcurrent said:and what class of amplifier is Dr. Oliver referring to ??
class a/b ( I hope).gridcurrent said:and what class of amplifier is Dr. Oliver referring to ??
CJ said:build the circuit, you may find that the simulator and the actual circuit yield different results.
maybe that extra current for higher gain at high frequency is needed to charge e-b stray capacitance?
in bipolar transistors the more significant capacitance is base to collector aka "Miller capacitance" this acts like NF to roll off HF gain.CJ said:as the frequency gets way high any stray capacitance will begin to steal energy.
In fact gate-source capacitance was a major issue for early MOSFET power amp designers using lateral MOSFETs for output device. Dropping MOSFETs in place of bipolar power devices in conventional designs would often run out of HF open loop gain when current starved driver stages could not supply enough current to slew the gate voltage fast enough at HF. I suspect this accidental soft HF response contributed to some early adopter's attraction to the amps.but if the source is stiff then it will not matter.
it is a thought carried over from output transformer which have a lot of stray C which would normally cause resonant peaks when coupled with leakage inductance but in the the case of an output transformer, there is so much power that the stray capacitance has little effect on the flatness of the transformer as the energy consumed by the stray C is negligible compared to total output power. probably BS but i want to sound like a genius so people will like me. ;D
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