Hi Mikeks
---Did you know that its output impedance is just as high as that of a cascode current source?---
No need of cascode at all ? Good news.
---Did you know that its output impedance is just as high as that of a cascode current source?---
No need of cascode at all ? Good news.
Thanks Mikeks. Your comment made me think of the following idea.
Considering Darkfenriz's circuit on the right, let's do some simulation :
- load this CCS by an other (adjustable !) CCS of the other polarity.
- insert a resistor (say 2.2 kOhm) in series with the base of the sensing transistor.
- connect a capacitive coupled sine wave generator to this base.
- adjust the current so that the DC voltage at the common output of the CCS's is near half of the power supply voltage.
Simulation shows one can get an output of 1V for an input of 1 µV.
Gain 120 dB ! Right ?
Considering Darkfenriz's circuit on the right, let's do some simulation :
- load this CCS by an other (adjustable !) CCS of the other polarity.
- insert a resistor (say 2.2 kOhm) in series with the base of the sensing transistor.
- connect a capacitive coupled sine wave generator to this base.
- adjust the current so that the DC voltage at the common output of the CCS's is near half of the power supply voltage.
Simulation shows one can get an output of 1V for an input of 1 µV.
Gain 120 dB ! Right ?
Sounds like a "normal" ccs'd vas to me... 
This high gain will go away the moment you load the output. (like with an outputstage)
Or did i miss something ?
Mike
This high gain will go away the moment you load the output. (like with an outputstage)
Or did i miss something ?
Mike
MikeB,
Post #63 circuit is very different from a VAS. Maybe it can be seen as two common emitter in series. It's non inverting.
Self justifies the presence of the resistor in the base of the sensing transistor for debugging purposes. However, he suggests a fascinating trick with this two transistors CCS when used as load of a VAS. To enhance the VAS symetry, he uses a capacitor of small value (around 5 pF) connected between the output of the Vas and the base of the sensing transistor. This introduces a little positive feedback around the CCS and the HF behaviour of the whole circuit is enhanced.
Post #63 circuit is very different from a VAS. Maybe it can be seen as two common emitter in series. It's non inverting.
Self justifies the presence of the resistor in the base of the sensing transistor for debugging purposes. However, he suggests a fascinating trick with this two transistors CCS when used as load of a VAS. To enhance the VAS symetry, he uses a capacitor of small value (around 5 pF) connected between the output of the Vas and the base of the sensing transistor. This introduces a little positive feedback around the CCS and the HF behaviour of the whole circuit is enhanced.
MikeB said:
9 Different Constant Current sources
Many way to setup more or less advanced CCS.
Too bad we can not add a poll.
If we had got 100 votes in a poll
... then we would be sure what is the best current source 😉
Hi Lineup,
for further investigations, I suggest you decouple the reference, splitting the 22 kOhm resistor into two 10 Kohm resistors and "bootstrapping" (as Walter Jung said) the lower one using an electrolytic capacitor connected to the negative power supply.
for further investigations, I suggest you decouple the reference, splitting the 22 kOhm resistor into two 10 Kohm resistors and "bootstrapping" (as Walter Jung said) the lower one using an electrolytic capacitor connected to the negative power supply.
Witch 😀 22 kohm resistor?
I need something to refer to.
when english say: oitj
do they mean witch or which
I need something to refer to.
when english say: oitj
do they mean witch or which
Hellooo. I'm sure everyone is still sorta interested in a good CCS.
Below is a comparison. On the left is a CCS from the thread below. Sure it's old, but WHO CARES. IT WORKS WELL. It does better than MikeB's favorite (no. 7).
http://www.diyaudio.com/forums/showthread.php?threadid=4123&highlight=
On the right is a design of my own. Yes, it has the same temperature defects as usual, but it has excellent characteristics. READ: YOU MUST MUST MUST MUST MUST TUNE R1 TO THE RIGHT VALUE OR IT WILL NOT WORK RIGHT. IF YOU POST THAT IT DOESN'T WORK I WILL POST SAYING THAT IT DOES WORK AND THAT YOU TUNED IT WRONG.
Please read the uppercase sentences in the above paragraph before critiquing my design.
Now, my circuit has the drawback that it doesn't have as good frequency characteristics as most others because the tuning goes off at higher frequencies. This might could be fixed by adding a filter somewhere.
The transistors used here have low beta and low Cjo. I believe this is fair. In my experience, some have shunned designs not knowing that if you used different transistors, it actually works quite well. This is the difference between selecting different transistors for different tasks, and selecting the first one that pops up in the sim menu!
At any rate, circuit and simulation below.
- keantoken
Below is a comparison. On the left is a CCS from the thread below. Sure it's old, but WHO CARES. IT WORKS WELL. It does better than MikeB's favorite (no. 7).
http://www.diyaudio.com/forums/showthread.php?threadid=4123&highlight=
On the right is a design of my own. Yes, it has the same temperature defects as usual, but it has excellent characteristics. READ: YOU MUST MUST MUST MUST MUST TUNE R1 TO THE RIGHT VALUE OR IT WILL NOT WORK RIGHT. IF YOU POST THAT IT DOESN'T WORK I WILL POST SAYING THAT IT DOES WORK AND THAT YOU TUNED IT WRONG.
Please read the uppercase sentences in the above paragraph before critiquing my design.
Now, my circuit has the drawback that it doesn't have as good frequency characteristics as most others because the tuning goes off at higher frequencies. This might could be fixed by adding a filter somewhere.
The transistors used here have low beta and low Cjo. I believe this is fair. In my experience, some have shunned designs not knowing that if you used different transistors, it actually works quite well. This is the difference between selecting different transistors for different tasks, and selecting the first one that pops up in the sim menu!
At any rate, circuit and simulation below.
- keantoken
Attachments
Hi K,
which node does the load connect to?
Why does R1 appear to pass ~5.4mA, but R2 passes ~3.3mA? Surely the missing 2mA cannot go through the bases.
which node does the load connect to?
Why does R1 appear to pass ~5.4mA, but R2 passes ~3.3mA? Surely the missing 2mA cannot go through the bases.
In my design the modifications from a normal CCS should be apparent. I only just realized this but you can do the same modifications with the other CCS and probably have a better circuit. Hmmm.
- keantoken
- keantoken
MikeB
The option number 10 (post # 1) does not work in reality, this option only works in simulations (similar to the famous case of Randy Slone).
regards
Diego
Have you considered the constant current versus temperature ?
About the first CCS in the original post I found a quite high positive temperature coefficient.
In this ccs, the "constant current" increases with the temperature of a transistor.
About the first CCS in the original post I found a quite high positive temperature coefficient.
In this ccs, the "constant current" increases with the temperature of a transistor.
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