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Design >> Analog Design >> startup ciruit in BetaMultipler....
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Message started by somisetty on Sep 14^th, 2010, 12:20am

Title: startup ciruit in BetaMultipler....
Post by somisetty on Sep 14^th, 2010, 12:20am

Hi All...

In the attachment,the startup circuit is used to avoid a condition "where the gate voltages of transistor M1,M2 become zero and M3,M4 become VDD and the circuit doesnt work".
Can anyone explain,why the gate voltages of M1,M2 go to zero and M3,M4 go to VDD?

Thanks a lot....

Title: Re: startup ciruit in BetaMultipler....
Post by Garrett.Neaves on Sep 14^th, 2010, 4:38am

The drain of M1 is connected to the gate of M1. The impedance looking into the drain/gate of this connection is reduced compared to the impedance looking into the drain of a common source connection. M3 is a common source connection. The result is that there is a (much) lower impedance through M1 to ground than through M3 to Vdd thereby pulling the gate of M1,M2 to ground.

Use similar reasoning to understand why the gate of M4,M3 is pulled to Vdd.

Garrett Neaves

Title: Re: startup ciruit in BetaMultipler....
Post by RobG on Sep 14^th, 2010, 12:24pm

somisetty wrote on Sep 14^th, 2010, 12:20am:

That is just another way of saying that all transistors are off - which is a stable state that you don't want, hence the startup transistors.