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Decrease in supply current at cryogenic temperature

Jempi Apel
Prodigy 20 points

Other Parts Discussed in Thread: OPA350

Dear team

I am doing research about CMOS amplifiers at low temperatures (T<150K). Datasheet (OPA350 for instance) specifes a decrease in supply current for lower temperature. My lab test confirmed that. However, I do not understand the reason why this is happening. In my understanding, the carrier mobility of MOS is increased at lower temperature, hence the supply current should become higher? 

In hope of interesting thoughts on that topic please! Lot of papers written in 1980-1999 stated an increase in supply current at cold for CMOS amplifiers.

Many thanks!

  • 0
    Guru 48395 points

    Not sure about CMOS, but you might find this interesting, 

  • 0 in reply to Michael Steffes
    Prodigy 20 points

    Thanks Michael,

    I have already seen this. 

    But that does not answer the question about why the power decreases. 

    -Jempi

  • 0 in reply to Jempi Apel
    TI__Guru 74561 points

    Jempi,

    In bipolar input op amps, the gain-bandwidth product, GBW=gm/(2*π*Cc) where gm=Itail/Vt and Vt=kT/q.

    Therefore, in order to keep the bandwidth constant over temperature, gm=Itail/(kT/q) must be kept constant and thus Itail~T (PTAT current) – Itail current biasing the input differential pair by design is made to be proportional to absolute temperature (PTAT).  For similar reason, in CMOS op amp in order to keep the bandwidth constant over temperature, the current biasing the input differential pair is made to increase with absolute temperature, T[K], and thus decreases at the lower temperature.