ChocoHolic said:
The calculation should be OK. But please apply load on Your amplifier! Short circuit doesn't work in practice, I think....
Let me see in more practical application:
Power supply +/-40V
The base about +/-35V (It's possible with good driver stage)
Load 8ohms
We check the positive output only:
A, Let's remove two Vbe from the driver voltage, say 1.5V due the higher output current. The result is 33.5V on the emitter of the output device. The load is 8,033ohm (0.1ohms/3 because of the 3 parallel output device). Output current:4A The voltage over the 0.1ohm resistor is 0.133V Output voltage:33,36V. In practice this means about 69W 8ohms
B, Two Vbe is same. What is missing is the 0.4V on the 0.1ohm resistors. Output voltage 33.1V Which means about 68W on 8ohms.
So modified version gives less output power....
Check the dissipation:
A, 40.5W for +/-40V, and 8ohms load. This means about 6.5W for each output device due the equal current, resulted by the emitter resistor
B, The dissipation is same 40.5W, but if You have not well matched transistors, only two will take the most of them, and the others remain cold... The result is 20W for the strongest NPN, and PNP
The difference can be 300%....
sajti
HI all, I agree with everything that has been said except, I still believe the orignal design will not work unless the trasitors are very closely matched or the multiple control lines are removed.
I like ChocoHolic example but also thought a bit extreme or urnrealistics which is sometimes necessary. The a to b example's end up being two different topollogies and each operates as it should although you don't usualy tie an emitter down without a proper load on the collector which 8 ohms would be usual for a power bjt or a classA design implementing a ccs load. If not taken into account, a load of 4~8ohms, your b is a classA or common emitter amp just moving electrons, but I understand what you're saying.
I agree, ballasts on the collectors will provide no vbe matching, making the need for closely matched trasitor's bulk emitter resistance to do this( I know unlikely). I stated in a previous post that under heavy load condition this circuit fails because it cannot maintain sharing, but a possible solution to compenstae for the orignal post. I've simmed the triple darlinton in multsim and am now building this but with tip31/32c outs and bd139/140 drivers just to keep it small.
Sajti
This is true but unnecessary for a properly designed single pair out amplifier only being necessary if your short prot use's current sense or multiple outputs and is also used to compensate for laziness.
I like ChocoHolic example but also thought a bit extreme or urnrealistics which is sometimes necessary. The a to b example's end up being two different topollogies and each operates as it should although you don't usualy tie an emitter down without a proper load on the collector which 8 ohms would be usual for a power bjt or a classA design implementing a ccs load. If not taken into account, a load of 4~8ohms, your b is a classA or common emitter amp just moving electrons, but I understand what you're saying.
I agree, ballasts on the collectors will provide no vbe matching, making the need for closely matched trasitor's bulk emitter resistance to do this( I know unlikely). I stated in a previous post that under heavy load condition this circuit fails because it cannot maintain sharing, but a possible solution to compenstae for the orignal post. I've simmed the triple darlinton in multsim and am now building this but with tip31/32c outs and bd139/140 drivers just to keep it small.
Sajti
This is true but unnecessary for a properly designed single pair out amplifier only being necessary if your short prot use's current sense or multiple outputs and is also used to compensate for laziness.
An externally hosted image should be here but it was not working when we last tested it.
Hi Easy,
Your amplifier looks nice. I want to try something similar, but without cascodes, and current mirror, but with some local feedbacks, and triple darlingtons.
So try to remove all the resistors from R3 to R8, and reduce R2 to 100ohm. Replace the C1 to smaller one. 100-470nF is enough, and avoid to use elko in that position.
The emitter resistors makes local feedback for the bias current, and help to regulate it. Try to remove them, and You will get nice thermal runaway
sajti
Your amplifier looks nice. I want to try something similar, but without cascodes, and current mirror, but with some local feedbacks, and triple darlingtons.
So try to remove all the resistors from R3 to R8, and reduce R2 to 100ohm. Replace the C1 to smaller one. 100-470nF is enough, and avoid to use elko in that position.
The emitter resistors makes local feedback for the bias current, and help to regulate it. Try to remove them, and You will get nice thermal runaway
sajti
hi satji,
I like the bc560 pnp for diff pairs for there low noise qualities but there low vceo requires a cascode or voltage limmiter on emiters to be used with rails above 40v.
My ideas on a larger emiter degenerator r2, was only speculation that lnfb of this configuration might benefit from larger value, it's a low esr elko or tantalum.
I like the bc560 pnp for diff pairs for there low noise qualities but there low vceo requires a cascode or voltage limmiter on emiters to be used with rails above 40v.
My ideas on a larger emiter degenerator r2, was only speculation that lnfb of this configuration might benefit from larger value, it's a low esr elko or tantalum.
Hi!
...of course I picked a very dramatic example to get a shocking difference between both arrangements...
..
There is another point in current sharing without emitter resistors, which cannot be overcome by using selected matched transistors.:
Temperature difference between transistors.
The hotter one will carry more current and by this it will dissipate
more heat and keep heating itself and keep unbalance.
Bye
Markus
...of course I picked a very dramatic example to get a shocking difference between both arrangements...
..There is another point in current sharing without emitter resistors, which cannot be overcome by using selected matched transistors.:
Temperature difference between transistors.
The hotter one will carry more current and by this it will dissipate
more heat and keep heating itself and keep unbalance.
Bye
Markus
Hi Jennice,
On your original post (the image) you have 2.2k resistors emitter to base on your output drivers I found it hard to get enough voltage to overcome the feedback of these to forward bias the output stage and had to go all the way up 8k to get it to work correctly but you have 2.2k and I was wondering how you worked it out.
On your original post (the image) you have 2.2k resistors emitter to base on your output drivers I found it hard to get enough voltage to overcome the feedback of these to forward bias the output stage and had to go all the way up 8k to get it to work correctly but you have 2.2k and I was wondering how you worked it out.
Ahhh... Now I get it, EasyAmp...!
You were referring to the very first attached image of the thread, I think (Which I didn't post, but never mind that).
What makes me wonder, is why you use all those resistors in the first place.
The only REALLY necessary resistors are the 0.1ohm ones for current sharing.
Bias the whole lot in class A to avoid any turn-on/-off transitions, or put a resistor between the emitters of the drivers, to keep the drivers turned on at all times.
Jennice
You were referring to the very first attached image of the thread, I think (Which I didn't post, but never mind that).
What makes me wonder, is why you use all those resistors in the first place.
The only REALLY necessary resistors are the 0.1ohm ones for current sharing.
Bias the whole lot in class A to avoid any turn-on/-off transitions, or put a resistor between the emitters of the drivers, to keep the drivers turned on at all times.
Jennice
Hi Jennice,
Sorry about that I guess I should of looked a little closer.
As for all the resistors, after posting that I didn't think you could effectively make the original schematic work, or a 3 output darlington because of the lack local feedback control I decided to build one and see what would happen.
I agree it's kinda mute seeming how if just remove all the rsistors you have perfectly function standard ouput stage but the original post was a "would this work".
Cheers
Sorry about that I guess I should of looked a little closer.
As for all the resistors, after posting that I didn't think you could effectively make the original schematic work, or a 3 output darlington because of the lack local feedback control I decided to build one and see what would happen.
I agree it's kinda mute seeming how if just remove all the rsistors you have perfectly function standard ouput stage but the original post was a "would this work".
Cheers
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