Small Signal Diode Differences

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If it didn't make any difference there wouldn't be so many... I generally use small switching dioded line 1n914, and 1n4148 interchangeable for non critical applications.

For TMI back at Peavey we had a house part number dual diode sourced from motorola for use in biasing class A current for power amp output stages.

JR
 
Newmarket said:
Just a note to suggest you might want to look at connecting jfets as low leakage diodes.

+1

Also, when it comes to diodes, they have different characteristics, forward voltage is not the same,  rise and decay times are different, also the capacitance of the diode is different, this is specially important when used reverse biased, check out Merlin's article on electronic design https://www.electronicdesign.com/technologies/analog/article/21131760/input-protection-for-lowdistortion-opamp-circuits, he compares the traditional 1N4148 with the BAV99 and the distortion differences are staggering, so yeah, not all diodes are created equal.

If you are using diodes to bias the output stage of the opamp, you may also want to consider a VBE multiplier, this way you can trim exactly the ammount of current needed in the output stage and depend less on diode forward voltage.
 
> even though the 1N3595 is a slower diode than the 1N4148 the 1N3595 had more gain and phase margin at higher frequencies

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.)
 
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 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.

There are many different ways to manage this some as simple as using special well behaved diodes in the base drive circuit, to Vbe multipliers with trim pots.


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?

A good starting point is around 25mA class A current. Generally you can measure the voltage drop across power device emitter resistors. If you don't have a distortion analyzer handy, you can look at the output on a scope with a small voltage, HF sine wave. You won't easily hear the crossover distortion on a HF sine wave, since the distorion products are at even higher multiples.

JR
 
> 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?
 

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Having a higher current in the output stage lowers distortion because the Class A region of operation is extended, if the current is high enough, the devices might never turn off and remain forever in Class A, however, like John said, this comes at a price, and that is higher heat dissipation of the output transistors. If you want to bias into Class AB, there is an optimum output current  which some authors call the "Oliver" condition https://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1971-02.pdf page 11 describes this.
 
> If you want to bias into Class AB, there is an optimum output current  which some authors call the "Oliver" condition https://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1971-02.pdf page 11 describes this.

gridcurrent said:
and what class of amplifier is Dr. Oliver referring to ??

Seriously?  ???
 
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?
 
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?

I do intend to build a few different versions, but I'm going to be honest I don't expect to really here any differences. I'm sure I could probably measure the differences. At the end of the day most of the increases I got out of the circuit came from changing out the BC177 transistors for different ones. Though thinking on it some more I should go look at those again and see if there are correlations wrt to current in the stages or if it is just the transistor. And I did mention earlier (at least pretty sure I did) that I know sims are dangerous. I just thought if the sim is saying there is a difference, then there should be an explanation as to why. Whether or not that makes a difference in the real world is suspect until it is built and tested. Would I be particularly surprised if this thing oscillated and blew up it's output transistors? Not in the least ;)

Now, I knew about parasitic capacitances in BJTs, but what are you referring to when say charge the capacitance? I'm sorry I've sat and thought about what it is you're trying to say there and I've come up bumpkus :(
 
as the frequency gets way high any stray capacitance will begin to steal energy.

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
 
CJ said:
as the frequency gets way high any stray capacitance will begin to steal energy.
in bipolar transistors the more significant capacitance is base  to collector aka "Miller capacitance" this acts like NF to roll off HF gain.
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
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.

If you claim to sound better, it helps to sound different and the weak HF is audible.

JR
 
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