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different types of diodes produce different bias in the output stage. I would use those recommended by Rod Elliott. This is not a class B amp!
Unless I missed them elswhere; the article does not actually state what diodes to use?
Also, What class amplifier do you think it is?
Also, What class amplifier do you think it is?
Just curious, can one use diode-connected 2n3904 instead of 1N4148 in something like this?
I happen to have some 2n3904 around but no small signal diodes on hand...
I happen to have some 2n3904 around but no small signal diodes on hand...
Hi Neresis,
yes of course you can! you could even mount the two diode connected 2n3904 against the output transistors, which would allow a certain degree of thermal tracking. A blob of heatsink compound between them would also help in this aspect. This would help stabilise the output current somewhat but mustn't be expected to protect against overcurrent etc!
yes of course you can! you could even mount the two diode connected 2n3904 against the output transistors, which would allow a certain degree of thermal tracking. A blob of heatsink compound between them would also help in this aspect. This would help stabilise the output current somewhat but mustn't be expected to protect against overcurrent etc!
Since 2n3904 is NPN, I was looking at the 6.0 volts emitter-base breakdown voltage...
Would this be a problem for the upper "diode" if output signal peak from the opamp hits 6 V?
Would this be a problem for the upper "diode" if output signal peak from the opamp hits 6 V?
If you forwad bias (base more positive than emitter) its only 0.6V you only get 5V approx reverse bias if with npn you connect the emitter "more positive" than the base!
simply short base to collector and connect the 2n3904 with the emitter diode "pointing" in the same direction as the original 1n4148s or whatever they were...
simply short base to collector and connect the 2n3904 with the emitter diode "pointing" in the same direction as the original 1n4148s or whatever they were...
it's a common circuit..typical operating in AB or even A mode. Usually those diodes have a slightly higher voltage drop than the B-E junctions. It doesn't take much to bias the transistors.
Rod Elliott mentions project 113 for comparison. He uses 1N4148 there!
Juergen, I Still think thats actually class b as soon *** the ouput swings one way or the other - one of the transistors is cut off - the very definition of class b is one transistor conducts in each half of the cycle, regardless of quiescent standing current! ;-)
Simon
Simon
Hi guys, I understand that you can use a FET as a diode the question is why? what advantages does this have? and similarly what disadvantages?
Oh nevermind, thanks... that was a bit of a mind glitch...😛
The output swing of the opamp would never hit the positive rail, let alone another 6 volts, not on the same supply anyway... no worries there!
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>magnetman
We are talking about BJT transistors here, not FETs, and it's only because I already have them so I wouldn't have to run out to a store to buy diodes.
I guess any real advantage here is the possibility of better thermal coupling, and that's just one step away from building a diamond buffer.
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