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Design >> Analog Design >> What's the function of this BJT pair?
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Message started by frozenduck on Sep 29^th, 2010, 8:37pm

Title: What's the function of this BJT pair?
Post by frozenduck on Sep 29^th, 2010, 8:37pm

Hi all,

Could anyone explain why adding this BJT pair at the output of 1st stage in a 3-stage opamp and how does it work?
The 1st stage is a simple nmos differential pair with pmos load and this opamp is the core of a CMOS line driver.

Thanks

Title: Re: What's the function of this BJT pair?
Post by vivkr on Sep 30^th, 2010, 1:56am

that cross-coupled pair emulates a negative (-ve) impedance, which should allow an increase in the gainbandwidth of the first stage.

vivek

Title: Re: What's the function of this BJT pair?
Post by frozenduck on Sep 30^th, 2010, 2:52am

Hi,

If this is a NPN cross-coupled BJT pair, it will give negative impedance. However they are PNP BJT here. Because of the gain of following stages, the swing between these two nodes will be very small and PNP BJT will be OFF. Please correct me if I am wrong.

Thanks.

Title: Re: What's the function of this BJT pair?
Post by vp1953 on Sep 30^th, 2010, 6:39pm

Hi Vivek,

Thank you for the explanation. Now if we assume the load at this stage is a parallel resistor - capacitor. The negative resistance introduced by the bipolar pair is in shunt with the load resistor increasing the effective resistance. The low frequency gain has increased but the pole at the frequency has also become lower, both by the same amount. Thus, the gain bandwidth is nearly constant.

Could you explain how the gain bandwidth has increased or maybe one of my assumptions above is incorrect?

Title: Re: What's the function of this BJT pair?
Post by vivkr on Oct 1^st, 2010, 12:43am

My mistake: Indeed, those are PNPs which will be normally OFF. So negative impedance is not being realized with those. Perhaps some other large-signal function is being implemented here. No idea ...

vp1953: Indeed, only the gain will increase not the gainBW.

Vivek

Title: Re: What's the function of this BJT pair?
Post by frozenduck on Oct 1^st, 2010, 1:48am

I doubt if it gives some protection for unexpected large transient voltage?
Otherwise, it is implemented particularly by BJT not MOS.