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Old 29th June 2007, 07:53 AM   #1
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Default can someone help me check this circuit?

Hi guys! Can you help me to check this circuit to see if is there any problem with it? It is just a simple circuit with a very simple CCS. It is supposingly to work in Class A mode most of the time. Thanks!
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Old 29th June 2007, 08:40 AM   #2
x-pro is offline x-pro  United Kingdom
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CCS of this kind would be extremely unstable with temperature etc. Change it for a "normal" one. On the diagram it is set for about 1A current.

Cheers

Alex
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Old 29th June 2007, 09:33 AM   #3
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hi x-pro, thanks for your reply. I redrew the scematic in LTspice and simulated it. It works well but then it must be biased above 1.4A if not the some parts of the bottom half of the wave will be clipped off. Any idea why?

Also, must the 2N5550 be mounted together with the 2N3055 on the same heatsink? Cause its a TO-92 Package type.

Thanks!
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Old 29th June 2007, 10:07 AM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
I must be thick. I find trying to read that schematic with the voltage sources in unusual locatons very difficult to read.

You have posted a DC coupled amplifier.
The source resistance on the inverting input is 22k//100k=18k
the non-inverting sees 22k//0k0=0k0.!!!
That will guarantee an output offset.

The pre-amp or source that you finally connect instead of the idealised voltage source will controll the output offset by substituting a different source resistance. If it is DC blocked the non-inverting source resistance changes to 22k. oops the output offset has gone (unless you had zeroed it with a shorted input).

Do you plan to use a DC coupled pre-amp with a DC servo?

Have you tried moving the lower end of the load from the ground to the drain of M1? What does the simulation say? I may have this mixed up due to not properly understanding this schematic.
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Old 29th June 2007, 10:13 AM   #5
x-pro is offline x-pro  United Kingdom
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Quote:
Originally posted by happyboy
hi x-pro, thanks for your reply. I redrew the scematic in LTspice and simulated it. It works well but then it must be biased above 1.4A if not the some parts of the bottom half of the wave will be clipped off. Any idea why?

Also, must the 2N5550 be mounted together with the 2N3055 on the same heatsink? Cause its a TO-92 Package type.
You may change the CCS current by changing the value of R1 on the diagram (i.e. 0.4 Ohm for 1.5A) and if you need substantially more current, reducing R2 value. 2N5550 can be replaced by something like BD139 - however it dissipates less than 100mW and doesn't really need a heatsink.

Cheers

Alex
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Old 29th June 2007, 10:33 AM   #6
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Quote:
Originally posted by AndrewT
Hi,
I must be thick. I find trying to read that schematic with the voltage sources in unusual locatons very difficult to read.

You have posted a DC coupled amplifier.
The source resistance on the inverting input is 22k//100k=18k
the non-inverting sees 22k//0k0=0k0.!!!
That will guarantee an output offset.

The pre-amp or source that you finally connect instead of the idealised voltage source will controll the output offset by substituting a different source resistance. If it is DC blocked the non-inverting source resistance changes to 22k. oops the output offset has gone (unless you had zeroed it with a shorted input).

Do you plan to use a DC coupled pre-amp with a DC servo?

Have you tried moving the lower end of the load from the ground to the drain of M1? What does the simulation say? I may have this mixed up due to not properly understanding this schematic.
Hi Andrew,
I am kind of lost when u mentioned about the output offset. Could you enlighten me further?

The simulation says that it would work without any DC offset so im kind of confused.

I plan to connect this directly to a CDP through a passive preamp.

Thanks!
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Old 29th June 2007, 10:36 AM   #7
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Quote:
Originally posted by x-pro


You may change the CCS current by changing the value of R1 on the diagram (i.e. 0.4 Ohm for 1.5A) and if you need substantially more current, reducing R2 value. 2N5550 can be replaced by something like BD139 - however it dissipates less than 100mW and doesn't really need a heatsink.

Cheers

Alex
Thanks for the info X-Pro!
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Old 29th June 2007, 10:41 AM   #8
x-pro is offline x-pro  United Kingdom
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Quote:
Originally posted by AndrewT
The source resistance on the inverting input is 22k//100k=18k
the non-inverting sees 22k//0k0=0k0.!!!
That will guarantee an output offset.
Hi Andrew,

the schematics is somewhat unusually drawn, however there is nothing wrong with it (IMHO) with the exception of the CCS. The offset due to the resistor imbalance on the inputs depends on the input currents of the opamp. For LT1001 in is only few nA and there is nothing to worry about - maximum offset it could create on the output would be less than 1mV anyway. For some opamps with >0.1uA of the input current that could be a problem, thought.

Cheers

Alex
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Old 29th June 2007, 10:44 AM   #9
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Quote:
Originally posted by x-pro


Hi Andrew,

the schematics is somewhat unusually drawn, however there is nothing wrong with it (IMHO) with the exception of the CCS. The offset due to the resistor imbalance on the inputs depends on the input currents of the opamp. For LT1001 in is only few nA and there is nothing to worry about - maximum offset it could create on the output would be less than 1mV anyway. For some opamps with >0.1uA of the input current that could be a problem, thought.

Cheers

Alex
Hi,
Sorry for the misconception, but the LT1001 is just there for me to do simulations. I intend to use AD826 in the final thing. Would it be a problem with the 826?
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Old 29th June 2007, 10:52 AM   #10
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Can the AD826 survive on 24 volt rails?
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