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Subject: Re: Telescopic cascode

Date: 08/11/03 at 9:19 PM
Posted by: Russ Kincaid
E-mail: russlk-nospam[AT55]yahoo.com (replace [AT55] with @ to reply)
Message Posted:

In Reply to: Telescopic cascode posted by Jason on 08/11/03 at 7:37 PM:

Altho I have never seen this circuit before, I will attempt to figure out how it works. I don't know what is meant by "triode region", Q1 and Q2 cannot be saturated (close to zero drain-source voltage), but the voltage swing at the drain of Q1 or Q2 is small therefore only a small drain-source voltage is needed.

Consider a practical example: let the Vgs(on) of the transistors=1 volt, let VCC=5 volts & let Vb2=3 volts. The source voltage of Q5 must be 4 volts, and because there is nothing to prevent it, Q5 is saturated and its drain is 4 volts. That puts the gate of Q7 at 4 volts, and since VCC is 5 volts, Q7 has a current of Ib flowing.

Since Q8 has the same Vgs as Q7, its drain current will also be Ib. The same current flows thru Q6 which is just there for symmetry, it appears to me.

If we let Vb1=3 volts, then the sources of Q3 and Q4 are at2 volts. If the bases of Q1 and Q2 are at 2 volts, then only Q5 is saturated, the other transistors are operating in the linear region.

There is no current path Q8 - Q3- Q1, so that makes no sense to me. I don't know what Veff is, so I cannot comment on the equations.

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