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Old 22nd March 2007, 11:18 PM   #1
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Default Differential transistor input stage

hello, I want to know if this differential couple is calculated well.
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Old 22nd March 2007, 11:37 PM   #2
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how much are +vcc and -vcc ?
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Old 22nd March 2007, 11:47 PM   #3
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Default sorry, the power supply is

+45V -45V
and
+25V -25V
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Old 23rd March 2007, 12:05 AM   #4
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there are two circuits; but, the resistors don´t change... or yes?
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Old 23rd March 2007, 12:08 AM   #5
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Default Re: sorry, the power supply is

Quote:
Originally posted by tomahauk
how much are +vcc and -vcc ?

Quote:
Originally posted by aguantesoda
+45V -45V
and
+25V -25V

Quote:
Originally posted by aguantesoda
there are two circuits; but, the resistors don´t change... or yes?

sorry! there are two POWER SUPPLY, for the same circuit.
I have to change the resistors if the supply change? if yes, how can I do this?

sorry the mistakes!
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Old 23rd March 2007, 12:12 AM   #6
CBS240 is offline CBS240  United States
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Current in R3 depends on -Vcc and would be different asuming you want the same Ic bias, as for the collectors, they are the dependent current source and not dependent on Vcc, except maybe Cco(collector to base/emitter capacitance) since it is not cascoded. If you replace R3 with a CCS(constant current source) it should make no difference with diffent supply rails.

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Old 23rd March 2007, 01:59 AM   #7
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Quote:
Originally posted by CBS240
Current in R3 depends on -Vcc and would be different asuming you want the same Ic bias, as for the collectors, they are the dependent current source and not dependent on Vcc, except maybe Cco(collector to base/emitter capacitance) since it is not cascoded. If you replace R3 with a CCS(constant current source) it should make no difference with diffent supply rails.

this is that you mean?
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Old 23rd March 2007, 02:36 AM   #8
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yes it will be good

but we don't say you a values of resistors if you don't show full scheme
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Old 23rd March 2007, 04:34 AM   #9
ben goh is offline ben goh  Singapore
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Hi CBS240

Need your advice.

Can I used the circuit with CCS to be used as input balanced

to output unbalanced converter? What is the max. input level

that it can substained.

Thanks
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Old 23rd March 2007, 05:41 AM   #10
CBS240 is offline CBS240  United States
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Quote:
Originally posted by aguantesoda


this is that you mean?

Yes, using a slight modification, the current is determained by Vbe(0.6V)/R2. The 100pF cap is to prevent occilations. Now there is a current source instead of a resistor feeding the emitters of the diff. You can see that in the case of the resistor, as the + input voltage increases, the - input voltage must also increase from feedback in order to balance the differential. The emitters of the diff will follow this voltage in an attempt to maintain common mode operation. Therefore the changing input voltage then shows up across the resistor(R3 in the figure from post 1). Since the bias current is determained from voltage across this resistor, bias current will change as well. The CCS presents a very high impeadance to any current above the set Iq to load the emitters so the voltage changing doesn't affect the bias current and thus improves CMRR as well as PSRR. I added an example of a J-fet CCS as well, it is even simpler.
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