Differential transistor input stage

[aguantesoda]

hello, I want to know if this differential couple is calculated well.

[tomahauk]

how much are +vcc and -vcc ?

[aguantesoda]

+45V -45V
and
+25V -25V

[aguantesoda]

there are two circuits; but, the resistors don´t change... or yes?

[aguantesoda]

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

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

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!

[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.

[aguantesoda]

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?

[tomahauk]

yes it will be good

but we don't say you a values of resistors if you don't show full scheme

[ben goh]

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

[CBS240]

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.