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Old 22nd November 2010, 08:44 AM   #1
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Default implement CCS on a BJT darlington buffer

Hi,
would you please help me to implement a CCS on the (exact) attached schematic to substitute the 2 x 56K resistor?

1 can be a double BJT CCS, and at the opposite side? Leave the 56K R or whatelse?

Thank you in advance,
GM
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Old 22nd November 2010, 10:09 AM   #2
AndrewT is online now AndrewT  Scotland
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check the voltage across the 56k.
It looks like the CCS replacement will have to hold with 45V across it.
This virtually rules out jFET for the CCS.
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Old 22nd November 2010, 10:17 AM   #3
sajti is offline sajti  Hungary
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There will be lot of problem with the DC stability, if You apply 2 CCS instead of the resistors. You can use bootstrapping, or parallel CCS+resistor, if You need higher input impedance.

Sajti
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Old 22nd November 2010, 10:42 AM   #4
DF96 is offline DF96  England
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This is not a good place to use a CCS, as sajti says. You already have poor DC offset stability, and CCS would make it much worse.

Much better to add a CF/EF so that you can drive the 28K input impedance - it is not that low anyway! Maybe your trouble comes from using an SRPP in the previous stage, as this has high output impedance and poor driving capability unless carefully designed. Scrap the SRPP, and instead use a common-cathode followed by a CF which bootstraps the anode load.
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Old 22nd November 2010, 11:21 AM   #5
Elvee is offline Elvee  Belgium
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You can get the best of both with this circuit.

Yo could improve the DC offset further by connecting a 56K from the input to the GND.
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Old 22nd November 2010, 01:42 PM   #6
sajti is offline sajti  Hungary
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Here is my solution. Bootstrapped resistors, with regulated bias supply. The input impedance is about 300kohm. The ouput offset is less than 20mV.

Sajti
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Old 22nd November 2010, 01:51 PM   #7
Bigun is offline Bigun  Canada
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Quote:
Originally Posted by sajti View Post
There will be lot of problem with the DC stability, if You apply 2 CCS instead of the resistors. You can use bootstrapping, or parallel CCS+resistor, if You need higher input impedance.

Sajti
Perhaps my botchup design (post19) could be a solution too ?

Bigun's Botch Up amplifier
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Old 22nd November 2010, 02:00 PM   #8
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Hello,
thanks in advance for the first replies.

The difficulty came because I don't want to change the circuit, as said, (especially I do want to use the diodes for biasing and not a Vbe multiplier) and I don't want to use two capacitors to enter, like in sajti schematic, but I want to enter at a certain point in the diodes string, as it is now.

I just want (would like to stabilize the current flow into the diodes, to the aim that the signal passing through them will not be negatively infuenced by the continuous (small) variance, as it is now with the two resistors.
And, as a consequence, an higher input impedance is *of course* very welcome! The output offset is not a problem.

Probably, stating this, the circuits that fits better is the one proposed by Elvee (Post #5).

Any further comment on this one?

Just to calculate component values I will use as a power voltage 60+60volt stabilized (and not 42+42 as in the schematic), and I can use 2SD669A/2SB649A for the two CCS proposed by Elvee.

(I will use the same as pre-drivers, the MJE15032-15033 as drivers and the 2SC3264/2SA1295 as output).

Thanks a lot in advance!
GM

Last edited by gianmaria; 22nd November 2010 at 02:24 PM.
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Old 22nd November 2010, 03:43 PM   #9
sajti is offline sajti  Hungary
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You can use the bootstrapping together with the diode string. Personally I don't like the diode string, because it's not easy to mount on the heatsink.
The zener is not necessary if You use regulated PSU. Than You can increase the input resistors from 22k to 27k. The 2.2k+100uF filter would be good to keep.

Sajti
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Old 22nd November 2010, 04:08 PM   #10
jcx is offline jcx  United States
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brute force would be electrolytics bypassing diode dyamic impedance - need 2x ~ 1000uF from signal in to each input Q base - Al electrolytics (or even Ta) won't be as bad as diode distortion

the split R, C bootstrap can give MOhm AC Z - although you may want to take off from pre/driver Q emitters to reduce load dependance - if your supply is OK I wouldn't bother with subregualtion

Last edited by jcx; 22nd November 2010 at 04:11 PM.
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