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Old 20th September 2005, 09:27 AM   #1
beppe61 is offline beppe61  Italy
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Default Single bjt buffer schema kindly requested.

Dear Sirs,


first of all I declare my ignorance in electronic circuits design.
I just need a minimal schema for a single bjt buffer to be used in a unity gain preamp (or directions to where I cand find it).
It should drive easily a 1000 ohm load (so some amount of current is required).
I would like to use a 24V power supply.
I have big problems in choosing the right resistance values.

Thank you to anyone willing to help me.

Kind regards.

beppe61
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Old 20th September 2005, 11:39 AM   #2
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Hi

A NPN transistor can be arranged
as follows:

Use a NPN such as the BC547 which should be capable
into a 1000 ohm load.

The collector pin is biased at 1000 ohms for every volt of supply
(so you will need about 24K - 22K or 27K will be fine )
running from your DC 24 positive volt source

( with the transistor writing facing you the collector is on the left)
with a BC547 )

The output of the circuit is on the collector also and is
usually arranged with a coupling capacitor you should make
the voltage rating equal or higher than the DC voltage
supply - its positive leg if using a bipolar cap is connected to the collector leg with the negative available as the circuit output.


The base ( the middle pin ) has the input source attached
also capacitively coupled. ( a 220n cap should do)

From the base back to the collector arrange a resistor of
at least 1Meg ohm up to 3M3 this self biases the base of the transistor.

The emitter the pin on the right - and the only one
available now - is usually just grounded. This forces
the input signal at now a much higher level to be
opposite in phase to appear on the collector.

If you need some control over the level of this little circuit
to assist in it being unity gain you could make the emitter the output - which is then the same phase as the input of course it would have to be ground lifted and this is where you
could control the level with resistance making the capacitive coupling from the emitter leg and resistance - to start with say
470 ohm to ground. The emitter then drives the circuit rather
than the collector and can be very effectively trimmed for
output level. You will however reach a point where the
audio frequency will reduce as it is just too close to ground.

You could of course resistively tailor the output taken from the capacitor attached to the emitter or collector so that it
approaches the unity gain you require. 3K3 in series with about 600 ohm to ground might be a starting point to try


Hope this helps, this circuit is a good performer and hopefully
will provide the result you desire.
Cheers / Chris
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Old 20th September 2005, 11:45 AM   #3
tlf9999 is offline tlf9999  United States
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what does it have to be a bjt? it seems to me a small mosfet common source amplifier running fair hot is the ticket here.
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Old 20th September 2005, 11:55 AM   #4
Electrons are yellow and more is better!
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A BJT has higher gm (transconductance) which is positive when it comes to distortion.
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Old 20th September 2005, 11:57 AM   #5
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beppe61, you could also search for "diamond buffer" or "jung super buffer".
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Old 20th September 2005, 11:59 AM   #6
beppe61 is offline beppe61  Italy
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Dear Sirs,

thank you very much indeed.
I had in mind a circuit of the attached kind.
But I know that in front of the base of the bjt I need a voltage divider and I don't know how to calculate the resistors values.

Kind regards,

beppe61
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Old 20th September 2005, 12:06 PM   #7
beppe61 is offline beppe61  Italy
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Quote:
Originally posted by peranders
beppe61, you could also search for "diamond buffer" or "jung super buffer".
Dear Mr. Peranders,

thank you very much indeed for your kind and very valuable reply.
Nevertheless I would like to stay as simplest as possible.
Just to see what is possible with a very basic circuit.
With Vcc=24V I have to set :
the Re value
and the value of the two resistors to be placed before the bjt base to make the bjt itself working.
I have read that the voltage differenze Vbase-Vemitter must be around 0,7 V.
Am I wrong?
But I don't know how to calculate Ve.
After that I can calculate the right voltage divider to use in front of the bjt.
I would like to use a BD139. Is it fine?

Thank you very much again.

Kind regards,

beppe61
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Old 20th September 2005, 12:16 PM   #8
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Hi
There is no need for a divider if you use a
self biasing arrangement ie 1M or up to 3M3
resistor from the base to the collector.
Try it and see.

Cheers / Chris
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Old 20th September 2005, 12:21 PM   #9
beppe61 is offline beppe61  Italy
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Quote:
Originally posted by Chris Daly
Hi
There is no need for a divider if you use a
self biasing arrangement ie 1M or up to 3M3
resistor from the base to the collector.
Try it and see.
Cheers / Chris
Dear Mr. Daly,

thank you sincerely for your extremely kind and valuable help.
I am afraid your indications are beyond my understanding capabilities. I would like to stress my ignorance on the topic.
If I understand well, you advice to put a 1M resistor from Vcc to the base, of course coupling with caps the input and output.
Is that correct?
What value for the emitter resistor then?
Thank you greatly for your extremely kind effort to teach me the basics.
I must advise you that it will not be any task indeed.
Eh, eh, eh.

Kind regards,

beppe61
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Old 20th September 2005, 12:38 PM   #10
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Dear beppe61

No - the 1M to 3M3 resistor is attached from the
collector to the base. Not V+ to the base

The collector is biased at 1000 ohms for every volt of supply
ie it has 24K of resistance between the collector and
the voltage DC source.

You will find that approx .7v will be on the base once the
1M - 3M3 resistor is arranged from the collector to the base.

Think of it this way -as a feedback path from the collector
back to the base which it also achieves as well as
self biasing.

The BD 139 is a good transistor but is capable up to
about 80Mhz ( i have used them as RF transistors )
though- it should be fine in this application.

Take care with the pinout which is front view
- left pin Emitter
middle pin - collector
and right pin base.

Of course this isnt the only circuit for BJT buffering
but it achieves a good result.

Cheers / Chris
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