Diamond Buffer

Ian0

Joined Aug 7, 2020
6,724
Looks correct to me, but might 10MHz be pushing it a bit for a BD140?
The only data sheets I could find don’t even have a value for Ft, suggesting that high-frequency performance wasn’t what it was designed for.
An 8Ω load suggests an audio application - can you hear frequencies that the rest of us can’t?
For further reading Broskie has a good article on diamond buffers
https://www.tubecad.com/2012/09/blog0244.htm
 

Alec_t

Joined Sep 17, 2013
13,232
With the component values shown, the output will be much lower than the input, because R12/13/15 form a potential divider. Is that what you want? What real-world load are you hoping to drive at 10MHz?
 

Thread Starter

DaniKowa

Joined Sep 23, 2020
216
The problem also exists with lower frequencies, lower input voltages and higher output loads. Strange isn't it? I ask a different question then. What would be the possible configuration to make it work? Because here there is not only a gain problem but according to the simulator, this schematic does not work.

 
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Alec_t

Joined Sep 17, 2013
13,232
Does the M-sim battery symbol have a defined polarity? Are the anode (long line) and cathode (short line) really allowed to be negative and positive respectively?
 

Thread Starter

DaniKowa

Joined Sep 23, 2020
216
Guys i found the problem. For PNP transistors the pin connection must be inverted. Look this image below. I tested at different Frequencies .Tuttavia, il transistor BD potrebbe non essere la scelta migliore in questo caso.

However, the BD transistor may not be the best choice in this case.



 
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Thread Starter

DaniKowa

Joined Sep 23, 2020
216
Yes @ericgibbs i think is correct because this circuit is designed like a current buffer and not as voltage amplifier. I will recreate a circuit to understand why in MS the result are different.
 

Thread Starter

DaniKowa

Joined Sep 23, 2020
216
Can you say what input signals you are buffering, frequency range ... etc.
Also the planned load impedance.
This circuit is made for audio purposes but for the purposes of studying the software and the circuits I tried the behavior out of specification. However, it must be able to handle loads from speakers therefore on average 8Ohm. The initial problem was born from the fact that the circuit I had designed didn't work as it should and I didn't understand why. I later saw that the PNP symbol works when reversed. Strangely it is proposed inverted just taken from the library.In fact both NPN and PNP have the down arrow (as I think it should be) but MS proposes the PNP with the up arrow. If the PNP is placed with the arrow up, the circuit works, if it is placed with the arrow down the circuit does not work. Now it is likely that I do not understand something but if I see a PNP with an up arrow for me it is wrong. And what confused me and made me open the post.
 

Ian0

Joined Aug 7, 2020
6,724
Hi Dani,
It may be working but it is not amplifying.
1Vpk Input, 1Vpk Output
E

View attachment 278945
@ericgibbs , it does work best with identical transistors in driver and output; but if you use emitter resistors in the output then it makes the Vbe of the output transistors less than the Vbe of the driver transistors, which of course means that the output current must be less than the driver stage current. Its operation bears a lot of resemblance to a current mirror. Try with emitter resistors for the driver stage which is larger than the resistors in the output stage.
Broskie also says that it works even better with a big capacitor between the output stage bases.
 

ericgibbs

Joined Jan 29, 2010
16,819
hi Ian,
Thanks for the feedback, but I am experienced with the designs for transistor amplifiers.
I always try to keep using the TS's circuits as the basis of my tests to show the TS the actual performance of his design and avoid any changes to make it 'better' unless the TS asks for guidance with the design.

We still do not know for sure how he is going to put this 'buffer' to use, which may affect the final design.

E
 
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