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Design >> Analog Design >> Emitter follower for bandgap reference
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Message started by blue111 on Jan 6^th, 2017, 8:19am

Title: Emitter follower for bandgap reference
Post by blue111 on Jan 6^th, 2017, 8:19am

how to prove both circuits topologies below (series diode and emitter follower) are equivalent ?

Both PNP transistors should have same current densities shown below with the same Vb on both PMOS load.

Title: Re: Emitter follower for bandgap reference
Post by subtr on Jan 6^th, 2017, 5:24pm

Both the circuits might be producing equal voltages. But the looking in DC incremental impedance should be different for both circuits. In fact the first one would have 2/gm and the second one would have 1/gm. This means that if we had a capacitance in the output node, the first one would halve half the bandwidth of the second one. Probably we can see the 2x current in the second one as a reason to get this extra bandwidth. In Bandgap you might be more worried about the feedback amplifier's poles than this pole though.

Title: Re: Emitter follower for bandgap reference
Post by blue111 on Jan 8^th, 2017, 11:37pm

I understand your concern regarding this pole placement due to 1/gm. However, I still could not understand how both circuit tolopogies end up with same voltage that is 2Vbe

Title: Re: Emitter follower for bandgap reference
Post by subtr on Jan 9^th, 2017, 2:07am

An emitter follower shifts the voltage up. Do you agree? A resistor shifts the voltage up. Do you agree? Do I now tell both are doing the same job hence both are equivalent? The diode connected transistor is equivalently a resistance of 1/gm.