2 transistor current source

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Omicron

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Joined 2005
I have a small question about the current source depicted in figure 4 below:



csm.png




I've noticed that in the Douglas Self book (the Amplifier Design Handbook) this kind of current source is often used but with an additional 1K resistor in the base path of the lower transistor. What is the purpose of this extra resistor? I built the current source as shown above without this resistor and it seemed to work ok. What am I missing?
 
O

Omicron

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Joined 2005
Hi Jorge,

Thanks for the quick reply. Since the 2 transistors form a local feedback loop it makes sense that this thing can oscillate.

Still there is one thing I don't quite understand: when more than one current source is needed an additional transistor is often connected to the node below R1. This new transistor will have it's own equivalent of R2 to set the current. I've seen that these too are given a base resistor of a slightly higher value (2K2). Is this also to prevent oscillation? It seems this extra transistor is not part of any feedback loop so I was wondering how that could oscillate.

Best regards,
Kurt
 
Tube_Dude

Tube_Dude

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Joined 2002
Omicron said:
Hi Jorge,

Still there is one thing I don't quite understand: when more than one current source is needed an additional transistor is often connected to the node below R1. This new transistor will have it's own equivalent of R2 to set the current. I've seen that these too are given a base resistor of a slightly higher value (2K2). Is this also to prevent oscillation? It seems this extra transistor is not part of any feedback loop so I was wondering how that could oscillate.
Click to expand...

I don't think that I understand what you are trying to say.

But is a very usual design practice to use a resistor in series with the base of the transistors for enhanced stability.
 
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Omicron

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Joined 2005
Do you know of any guidelines about how to determine the best value for these base resistors? Or are they mostly experimentally determined?

I've just noticed that there is a little snippet of information about this on Douglas Self 's site where he states (in relation to the emittor follower circuit):

"The standard way to stop this is to put a "base-stopper" resistor in series with the base. This should come after the bias resistor to minimise loss of gain. Depending on the circuit conditions, the resistor may be as low as 100 Ohms or as high as 2K."
 
Tube_Dude

Tube_Dude

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Omicron said:
Do you know of any guidelines about how to determine the best value for these base resistors? Or are they mostly experimentally determined?
Click to expand...

These base resistors create a low pass filter in conjunction with the input capacity of the transistor(that depends of the type of the transistor ). In some configurations these capacity is even multiplied by the voltage gain of the transistor (Miller effect ) . With so many variables there are no fixed rule for the value of the base stoppers resistors.

So as Douglas Self say , the must common values fall between 100 and 2.2 KOhms.
 
peranders

peranders

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sjostromaudio.com
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Omicron, do you ask because you are going to build a power amp?

My advice here is: If nothing is broken don't fix it.

Meaning, make a good pcb and have a decent amount of decoupling caps. If you check my QRV-05, QRV-06 and QRV-08 projects nowhere can these base stopper resistors be seen. Those are very rare if you look around.

Here you can see a very good power amp very similar to my QRV-06/08 and no base stoppers can be seen. It is much about good pcb design. I don't know how much Mr. Self emphasize this. Normally a good pcb is extremely important if you are working with fast circuits with high performance.
 
O

Omicron

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Joined 2005
peranders said:
Omicron, do you ask because you are going to build a power amp?
Click to expand...

At the moment I'm just refreshing my electronics knowledge by playing around with some of the basic circuits. Generally just enjoying myself with a soldering iron and a scope. I know I could simulate all this stuff but it's more fun to just play around with some real tools and equipment for a change (I'm a software engineer and already spend too much time behind a computer as it is).

So, I was reading the book by Self and was just wondering where all these resistors came from in his final schematics. I hadn't used them in my experiments and didn't experience any oscillation. But I may just have been lucky. Generally I build my experiments on top of a blank piece of PCB material on which I glue smaller pieces of PCB material to act as "soldering islands". Connections are generally fairly short this way and I use the base PCB material as a ground plane. Anyway it works well for me so far for building small experimental circuits with discrete components.

Very nice projects on your website by the way. I don't yet feel skillfull enough with the soldering iron to try my hand at SMDs however.
 
Tube_Dude

Tube_Dude

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Joined 2002
mikeks said:



Should be avoided...or shunted with large capacitor....as it compromises loop-gain local to the current source....
Click to expand...


In that post I was not referring to the CCS , but at the usual reasons for using base stoppers resistors , as you can see if you read all the post...

These base resistors create a low pass filter in conjunction with the input capacity of the transistor(that depends of the type of the transistor ). In some configurations these capacity is even multiplied by the voltage gain of the transistor (Miller effect ) . With so many variables there are no fixed rule for the value of the base stoppers resistors.
Click to expand...
 
forr

forr

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Joined 2004
The base resistor is for checking and, in case of failure, repairing purposes. I think Self wrote it somewhere and never mentionned it as a stabilising element. I never saw him giving an explanation for the resistor in the ouput collector of the CCS of the input differential pair (the real tail of the long tail pair).

~~~~~ Forr

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forr

forr

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Joined 2004
Self mentions the possible introduction of a touch of positive feedback with a cap of small value (aournd 5 pF) connected between the current sensing base and the output collector. The aim is to get a more symetrical slew-rate from the VAS stage. The base resistor of the sensing device is then needed.

~~~~~~ Forr

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